| 1. |
- Dias, R, et al.
(author)
-
DNA-lipid systems. A physical chemistry study
- 2002
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In: Brazilian Journal of Medical and Biological Research. - 0100-879X. ; 35:5, s. 509-522
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Journal article (peer-reviewed)abstract
- It is well known that the interaction of polyelectrolytes with oppositely charged surfactants leads to an associative phase separation; however, the phase behavior of DNA and oppositely charged surfactants is more strongly associative than observed in other systems. A precipitate is formed with very low amounts of surfactant and DNA. DNA compaction is a general phenomenon in the presence of multivalent ions and positively charged surfaces; because of the high charge density there are strong attractive ion correlation effects. Techniques like phase diagram determinations, fluorescence microscopy, and ellipsometry were used to study these systems. The interaction between DNA and catanionic mixtures (i.e., mixtures of cationic and anionic surfactants) was also investigated. We observed that DNA compacts and adsorbs onto the surface of positively charged vesicles, and that the addition of an anionic surfactant can release DNA back into solution from a compact globular complex between DNA and the cationic surfactant. Finally, DNA interactions with polycations, chitosans with different chain lengths, were studied by fluorescence microscopy, in vivo transfection assays and cryogenic transmission electron microscopy. The general conclusion is that a chitosan effective in promoting compaction is also efficient in transfection.
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| 2. |
- Antunes, Filipe, et al.
(author)
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A rheological investigation of the association between a non-ionic microemulsion and hydrophobically modified PEG. Influence of polymer architecture
- 2003
-
In: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - 0927-7757. ; 215:1-3, s. 87-100
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Journal article (peer-reviewed)abstract
- ydrophobically modified polymers (HM-P) typically behave as thickeners in a wide range of systems. The thickening effect in an aqueous solution of this kind of polymer depends on intermolecular hydrophobic associations and also on chain entanglements if the polymer concentration is significantly above the overlap concentration. In the present investigation a rather short end-capped polymer has been investigated at concentrations that are significantly below the overlap concentration. Despite the rather low polymer concentration, polymer chains were connected into a three-dimensional network by using microemulsion droplets as cross-linking points. The simple structure of the solution simplifies interpretations of results since chain entanglements can be expected to be of low importance and only intermolecular hydrophobic associations have to be considered. In particular the rheological response is in most cases well characterized by one single relaxation time and, then, the solution can be rationalized withi the framework of the Maxwell model. We have found that the length of the polymer chain's hydrophobic end-groups, as well as the temperature, have a large influence on dynamics of the system, while the length of the hydrophilic mid-block has a relatively small significance. On the other hand, the connectivity in the system depends critically on the microemulsion concentration. Thus, a maximum was found in viscosity as a function of volume fraction, interpreted as being due to a decrease in crosslink lifetime. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 3. |
- Antunes, Filipe, et al.
(author)
-
Network formation of catanionic vesicles and oppositely charged polyelectrolytes. Effect of polymer charge density and hydrophobic modification
- 2004
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 20:11, s. 4647-4656
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Journal article (peer-reviewed)abstract
- In nonequimolar solutions of a cationic and an anionic surfactant, vesicles bearing a net charge can be spontaneously formed and apparently exist as thermodynamically stable aggregates. These vesicles can associate strongly with polymers in solution by means of hydrophobic and/or electrostatic interactions. In the current work, we have investigated the rheological and microstructural properties of mixtures of cationic polyelectrolytes and net anionic sodium dodecyl sulfate/didodecyldimethylammonium bromide vesicles. The polyelectrolytes consist of two cationic cellulose derivatives with different charge densities; the lowest charge density polymer contains also hydrophobic grafts, with the number of charges equal to the number of grafts. For both systems, polymer-vesicle association leads to a major increase in viscosity and to gel-like behavior, but the viscosity effects are more pronounced for the less charged, hydrophobically modified polymer. Evaluation of the frequency dependence of the storage and loss moduli for the two systems shows further differences in behavior: while the more long-lived cross-links occur for the more highly charged hydrophilic polymer, the number of cross-links is higher for the hydrophobically modified polymer. Microstructure studies by cryogenic transmission electron microscopy indicate that the two polymers affect the vesicle stability in different ways. With the hydrophobically modified polymer, the aggregates remain largely in the form of globular vesicles and faceted vesicles (polygon-shaped vesicles with largely planar regions). For the hydrophilic polycation, on the other hand, the surfactant aggregate structure is more extensively modified: first, the vesicles change from a globular to a faceted shape; second, there is opening of the bilayers leading to holey vesicles and ultimately to considerable vesicle disruption leading to planar bilayer, disklike aggregates. The faceted shape is tentatively attributed to a crystallization of the surfactant film in the vesicles. It is inferred that a hydrophobically modified polyion with relatively low charge density can better stabilize vesicles due to formation of molecularly mixed aggregates, while a hydrophilic polyion with relatively high charge density associates so strongly to the surfactant films, due to strong electrostatic interactions, that the vesicles are more perturbed and even disrupted.
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| 4. |
- Barreleiro, Paula, et al.
(author)
-
Mechanism of formation of DNA-cationic vesicle complexes
- 2003
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In: Faraday Discussions. - : Royal Society of Chemistry (RSC). - 1364-5498. ; 122, s. 191-201
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Journal article (peer-reviewed)abstract
- Cationic vesicles and DNA form complexes that are promising gene delivery systems. Despite the increasing number of publications on their morphology and structure, the mechanism leading to their formation is not yet understood due to a lack of kinetic data. In the present study the kinetics of the interaction between DNA and cationic vesicles were followed using stopped-flow turbidity and small-angle neutron scattering techniques. The neutron real-time experiments were performed on a high-flux diffractometer, the D22 at the ILL, using a stopped-flow set-up. Extruded mixed vesicles of dimethyldioctadecylammonium bromide (DODAB) with various amounts of dioleoylphosphatidylethanolamine (DOPE) were investigated at 25degreesC. The results show that the transition from unilamellar vesicles to a multilamellar structure upon DNA addition occurs in three steps. The first step, on the millisecond time scale, is currently not accessible to neutron scattering but was observed by stopped-flow turbidity and fluorescence experiments. The second step, on a time scale of seconds, corresponds to the formation of an intermediate with a locally cylindrical structure. As time progresses this unstable intermediate evolves to a multilamellar structure, on a time scale of minutes. An understanding of the mechanisms behind the DNA cationic vesicle complex formation event will allow the production of more homogeneous, efficient delivery systems in pharmaceutically acceptable forms.
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| 5. |
- Barreleiro, Paula, et al.
(author)
-
The kinetics of DNA-cationic vesicle complex formation
- 2003
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In: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 107:25, s. 6208-6213
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Journal article (peer-reviewed)abstract
- Stopped-flow fluorescence studies were performed to investigate the kinetics of formation of complexes between DNA and cationic vesicles. We followed the kinetics as a function of charge ratio and lipid vesicle composition. Binary mixtures of a saturated cationic lipid, dioctadecyldimethlammonium bromide (DODAB), and a zwitterionic lipid, dioleoylphosphatidylethanolamine (DOPE), were used to prepare small unilamellar vesicles at 25 degreesC. The binding followed first-order reactions and occurred stepwise. The results presented indicate the existence of a three-step mechanism for cationic vesicle "binding" to DNA: first DNA rapidly ads rbs onto the oppositely charged membrane surface (milliseconds), and thereafter, on a longer time scale (seconds), the formed DNA-lipid complex continues to aggregate and grow; a third step is also observed, which seems to be due to rearrangement of the complex with a further change of the DNA conformation. The time constant of each step was determined by the relative amounts of DNA and cationic vesicles and the composition of the lipid bilayer of the vesicles.
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| 6. |
- Bergh, Jonas, et al.
(author)
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Tailored fluorouracil, epirubicin, and cyclophosphamide compared with marrow-supported high-dose chemotherapy as adjuvant treatment for high-risk breast cancer : A randomised trial
- 2000
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In: The Lancet. - 0140-6736 .- 1474-547X. ; 356:9239, s. 1384-1391
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Journal article (peer-reviewed)abstract
- Background: Chemotherapy drug distribution varies greatly among individual patients. Therefore, we developed an individualised fluorouracil, epirubicin, cyclophosphamide (FEC) regimen to improve outcomes in patients with high-risk early breast cancer. We then did a randomised trial to compare this individually tailored FEC regimen with conventional adjuvant chemotherapy followed by consolidation with high-dose chemotherapy with stem-cell support. Methods: 525 women younger than 60 years of age with high-risk primary breast cancer were randomised after surgery to receive nine cycles of tailored FEC to haematological equitoxicity with granulocyte colony-stimulating factor (G-CSF) support (n=251), or three cycles of FEC at standard doses followed by high-dose chemotherapy with cyclophosphamide, thiotepa, and carboplatin (CTCb), and peripheral-blood stem-cell or bone-marrow support (n=274). Both groups received locoregional radiation therapy and tamoxifen for 5 years. The primary outcome measure was relapse-free survival, and analysis was by intention to treat. Findings: At a median follow-up of 34.3 months, there were 81 breast-cancer relapses in the tailored FEC group versus 113 in the CTCb group (double triangular method p=0.04). 60 deaths occurred in the tailored FEC group and 82 in the CTCb group (log-rank p=0.12). Patients in the CTCb group experienced more grade 3 or 4 acute toxicity compared with the tailored FEC group (p<0.0001). Two treatment-related deaths (0.7%) occurred in the CTCb group. Six patients in the tailored FEC group developed acute myeloid leukaemia and three developed myelodysplastic syndrome. Interpretation: Tailored FEC with G-CSF support resulted in a significantly improved relapse-free survival and fewer grade 3 and 4 toxicities compared with marrow-supported high-dose chemotherapy with CTCb as adjuvant therapy of women with high-risk primary breast cancer.
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| 7. |
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| 8. |
- Bilalov, Azat, et al.
(author)
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Mixing oil and water by a DNA-based surfactant
- 2004
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In: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 108:39, s. 15408-15414
-
Journal article (peer-reviewed)abstract
- The self-assembly behavior of a cationic surfactant (dodecyltrimethylammonium) with DNA as counterion in mixtures of water and decanol was investigated. The phase diagram was established and the different regions of the phase diagram characterized with respect to microstructure by H-2 NMR (D2O), small-angle X-ray scattering (SAXS), and other techniques. The DNA-cationic surfactant aggregate is virtually insoluble in water but shows a high solubility in decanol; in decanol, highly viscous solutions/rubberlike solid material is found. In the presence of both solvents, there is a formation of liquid crystalline phases. Both lamellar and reversed hexagonal phases are found, the former favored by higher water contents, the latter by higher concentrations of decanol. The liquid crystalline phases are in equilibrium, via two-phase and three-phase regions, with the essentially pure components.
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| 9. |
- Braem, Alan, et al.
(author)
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Influence of DNA adsorption and DNA/cationic surfactant coadsorption on the interaction forces between hydrophobic surfaces
- 2004
-
In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 20:15, s. 6407-6413
-
Journal article (peer-reviewed)abstract
- The forces between hydrophobic surfaces with physisorbed DNA are markedly and irreversibly altered by exposure to DNA/cetyltrimethylammonium bromide (CTAB) mixtures. In this colloidal probe atomic force microscopy study of the interactions between a hydrophobic polystyrene particle and an octadecyl-trimethylethoxysilane-modified mica surface in sodium bromide solutions, we measure distinct changes in colloidal forces depending on the existence and state of an adsorbed layer of DNA or CTAB-DNA complexes. For bare hydrophobic surfaces, a monotonically attractive approach curve and very large adhesion are observed. When DNA is adsorbed at low bulk concentrations, a long-range repulsive force dominates the approach, but on retraction some adhesion persists and DNA bridging is clearly observed. When the DNA solution is replaced with a CTAB-DNA mixture at relative low CTAB concentration, the length scale of the repulsive force decreases, the adhesion due to hydrophobic interactions greatly decreases, and bridging events disappear. Finally, when the surface is rinsed with NaBr solution, the length scale of the repulsive interaction increases modestly, and only a very tiny adhesion remains. These pronounced changes in the force behavior are consistent with CTAB-induced DNA compaction accompanied by increased DNA adsorption, both of which are partially irreversible.
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| 10. |
- Cárdenas, Marité, et al.
(author)
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DNA and cationic surfactant complexes at hydrophilic surfaces. An ellipsometry and surface force study
- 2004
-
In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 20:20, s. 8597-8603
-
Journal article (peer-reviewed)abstract
- The adsorption and formation of DNA and cationic surfactant complexes at the silica-aqueous interface have been studied by ellipsometry. The interaction between the DNA-surfactant complexes at the mica-aqueous interface has been determined by the interferometric surface force apparatus. Adsorption was as expected not observed on negatively charged hydrophilic surfaces for DNA and when DNA-cationic surfactant complexes were negatively charged. However, adsorption was observed when there is an excess of cationic surfactant, just below the point of phase separation. The adsorption process requires hours to reach steady state. The adsorbed layer thickness is large at low surface coverage but becomes more compact and thinner at high coverage. A long-range repulsive force was observed between adsorbed layers of DNA-cationic surfactant complexes, which was suggested to be of both electrostatic and steric origin. The forces were found to be dependent on the equilibration time and the experimental pathway.
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| 11. |
- Cárdenas, Marité, et al.
(author)
-
DNA compaction at hydrophobic surfaces induced by a cationic amphiphile
- 2003
-
In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 19:19, s. 7712-7718
-
Journal article (peer-reviewed)abstract
- Cetyltrimethylammonium bromide (CTAB) induces partially irreversible compaction of DNA-adsorbed layers on hydrophobic silica. Additionally, there is a synergistic increase in the adsorbed amount when both CTAB and DNA are present as compared to the surface excess concentration of either of the individual components. In this study of the DNA adsorption and DNA-CTAB coadsorption by ellipsometry, emphasis has been placed on the DNA molecular weight as well as its conformation (single and double stranded). The DNA molecular weight and conformation have a large effect on the surfactant-free DNA adsorption behavior but not on he mixed DNA-CTAB adsorption behavior. Comparison between interfacial and bulk complexation has been made where possible. The DNA-CTAB complexes at the interface are neutral despite the large excess of DNA in the bulk. The final structure of the adsorbed layer was found to be dependent on the history of complex formation and DNA size.
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| 12. |
- Cárdenas, Marité, et al.
(author)
-
DNA compaction by cationic surfactant in solution and at polystyrene particle solution interfaces: a dynamic light scattering study
- 2004
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In: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084. ; 6:7, s. 1603-1607
-
Journal article (peer-reviewed)abstract
- The effect of cationic surfactant on the compaction of DNA both in bulk solution and on polystyrene particles has been studied by dynamic light scattering. DNA undergoes a transition from a semiflexible coil to a more compact globule in aqueous solution as a consequence of cationic surfactant binding. Thus the hydrodynamic radius of DNA decreases from 107 nm to about 50 nm in the presence of cationic surfactant. Such compaction is also observed on DNA coated polystyrene particles. In this case, the hydrodynamic layer thickness of DNA coated particles is reduced from 36 to 5 nm in the presence of cetyltrimethylammonium bromide.
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| 13. |
- de Melo, J S, et al.
(author)
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Time-resolved and steady-state fluorescence studies of hydrophobically modified water-soluble polymers
- 2003
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In: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 107:46, s. 12605-12621
-
Journal article (peer-reviewed)abstract
- A comprehensive study of the photophysical behavior of poly(acrylic acids) randomly labeled with pyrene using steady-state and time-resolved fluorescence spectroscopy is presented. The influence of external factors, such as different solvents and pH in the aqueous solution, on the polymer photophysics has been investigated. These factors induce major changes in the polymer conformation, which are reflected in the fluorescence experiments. The random introduction of the hydrophobic pyrene groups along the macromolecule favors the coexistence of static (preformed dimers) and dynamic (excimers) monomer quenching phenomena together with a fraction of isolated monomers (not able to form excimer), as revealed by time-resolved and steady-state fluorescence data. In dioxane and methanol solvents, the global analysis of the fluorescence time profiles shows a rise time of similar to21-32 ns followed by a decay of similar to90-138 ns with an additional long r decay component with a low preexponential factor. This is consistent with the fact that in organic solvents such as methanol and dioxane, which are considered to be good solvents, the fraction of preassociated and isolated chromophores is highly reduced and excimer formation is essentially due to a dynamic mechanism. A kinetic scheme involving two types of monomers (M-A and M-B) and one excimer (E) is proposed. From the fluorescence decays it was possible to extract quantitatively the percentage of ground-state preformed dimers along with the percentage of isolated chromophores at room temperature. In addition, it is shown that the fraction of associated ground-state chromophores that can be excited is always larger than that of the isolated chromophores. The rate constants for excimer formation (k(a)), dissociation (k(d)), and deactivation (k(E)) were determined considering the absence and presence of preformed dimers. Additional photophysical and spectroscopic data consisting of wavelength shifts, peak- o-valley ratios and differences (obtained both from absorption and from excitation spectra collected at the monomer and excimer emission region), and the vibronic I-1/I-3 ratio in the pyrene monomer emission were found to be pH-dependent for the polymers in aqueous solution. Hence, by combining the results from steady-state fluorescence measurements with time-resolved fluorescence data, information is provided on how the chain conformation of the labeled PAA polymers changes depending on the solvent (water at different pH values or organic solvents). In water, the conformation changes from compact at low pH to an open polymer coil at high pH, whereas the polymers are in an extended state when in dioxane and methanol.
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| 14. |
- Dias, Rita, et al.
(author)
-
Compaction and decompaction of DNA in the presence of catanionic amphiphile mixtures
- 2002
-
In: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 106:48, s. 12608-12612
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Journal article (peer-reviewed)abstract
- The dissociation of DNA-cationic surfactant complexes with the addition of a negative amphiphile was studied by fluorescence microscopy (FM). The unfolding of DNA molecules previously compacted with cationic surfactant was shown to be dependent on the anionic surfactant chain length; lower amounts of a longer-chain surfactant were needed to release DNA into solution. However, we observed no dependence on the hydrophobicity of the compacting agent. The structures of the aggregates formed by the two surfactants, after the interaction with DNA, were imaged by cryogenic transmission electron microscopy (cryo-TEM). We found that it is possible to predict the structure of the aggregates the surfactants will form, such as vesicle phases, from the behavior of mixed surfactant systems, which brings new insight to this subject and several interesting opportunities for applications.
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| 15. |
- Dias, Rita, et al.
(author)
-
DNA and surfactants in bulk and at interfaces
- 2004
-
In: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - : Elsevier BV. - 0927-7757. ; 250:1-3, s. 115-131
-
Journal article (peer-reviewed)abstract
- Recent investigations of the DNA interactions with cationic surfactants and catanionic Mixtures are reviewed. Several techniques have been used such as fluorescence microscopy, dynamic light scattering, electron microscopy, and Monte Carlo simulations. The conformational behaviour of large DNA molecules in the presence of cationic surfactant was followed by fluorescence microscopy and also by dynamic light scattering. These techniques were in good agreement and it was possible to observe a discrete transition from extended coils to collapsed globules and their coexistence for intermediate amphiphile concentrations. The dependence on the surfactant alkyl chain was also monitored by fluorescence microscopy and, as expected, lower concentrations of the more hydrophobic surfactant were required to induce DNA compaction, although an excess of positive charges was still required. Monte Carlo simulations on the compaction of a medium size polyanion with shorter polycations were performed. The polyanion chain suffers a sudden collapse as a function of the concentration of condensing agent, and of the number of charges on the polycation molecules. C Further increase in the concentration increases the degree of compaction. The compaction was found to be associated with the polycations promoting bridging between different sites of the polyanion. When the total charge of the polycations was lower than that of the polyanion, a significant translational motion of the compacting agent along the polyanion was observed, producing only a small-degree of intrachain segregation, which can explain the excess of positive charges necessary to compact DNA. Dissociation of the DNA-cationic surfactant complexes and a concomitant release of DNA was achieved by addition of anionic surfactants. The unfolding of DNA molecules, previously compacted with cationic surfactant, was shown to be strongly dependent on the anionic surfactant chain length: lower amounts of a longer chain surfactant were needed to release DNA into solution. On the other hand, no dependence on the hydrophobicity of the compacting agent was observed. The structures of the aggregates formed by the two surfactants, after the interaction with DNA. were imaged by cryogenic transmission electron microscopy. It is possible to predict the structure of the aggregates formed by the surfactants. like vesicles, from the phase behaviour of the mixed surfactant systems. Studies on the interactions between DNA and catanionic mixtures were also performed. It was observed that DNA does not interact with negatively charged vesicles, even though they carry positive amphiphiles; however, in the presence of positively charged vesicles, DNA molecules compact and adsorb on their surface. Finally Monte Carlo simulations were performed on the adsorption of a polyelectrolyte on catanionic surfaces. It was observed that the mobile charges in the surface react to the presence of the polyelectrolyte enabling a strong degree of adsorption even though the membrane was globally neutral. Our observations indicate that the adsorption behaviour of the polyelectrolyte is influenced by the response given by the membrane to its presence and that the number of adsorbed beads increases drastically with the increase of flexibility of the polymer. Calculations involving polymers with three different intrinsic stiffnesses showed that the variation is non-monotonic. It was observed also that a smaller polyanion typically adsorbs more completely than the larger one, which indicates that the polarisation of the membrane becomes less facilitated as the degree of disruption increases. (C) 2004 Elsevier B.V. All rights reserved.
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| 16. |
- Dias, Rita, et al.
(author)
-
DNA interaction with catanionic vesicles
- 2002
-
In: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 106:48, s. 12600-12607
-
Journal article (peer-reviewed)abstract
- DNA-cationic liposome complexes as possible vehicles for gene delivery is currently an important issue. In this work, the interaction between DNA and thermodynamically stable, spontaneously formed, catanionic vesicles with a net positive charge is studied. A phase map was drawn for the aqueous system of DNA and positively charged vesicles, composed of CTAB (cetyltrimethylammonium bromide) and SOS (sodium octyl sulfate), and showed, as expected, a strong associative phase behavior with the formation of a precipitate. A two-phase region was observed over all the studied concentrations. For DNA-surfactant mixing ratios, [DNA]/ [S+] below 1.3 by charge, we found, by optical and electron microscopy, a coexistence between undisturbed vesicles and DNA-surfactant complexes. In samples with a higher excess of DNA, only DNA-vesicle complexes were observed, in solution. The structure of these complexes was studied by both small-angle X-ray diffraction (SAXS) and cryogenic transmission electron microscopy (cryo-TEM), and a short-range lamellar structure composed of amphiphile bilayers with DNA molecules ordered and packed between these stacks was found. This type of structure has already been mentioned in the literature as being the most frequently found structure for DNA-liposome mixtures and shows that the vesicles we used, having major advantages with respect to preparation and stability, have similar behavior and can thus be successfully used as model systems. We observed, however, an interesting difference in comparison with previously studied systems. Thus, the addition of DNA in excess to the samples leads not to the coexistence of DNA-vesicle complexes and DNA, as observed before, but to a probable inclusion of DNA in excess in the complexes and therefore to a coexistence of complexes and anionic surfactant micelles expelled from the bilayers.
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| 17. |
- Dias, Rita, et al.
(author)
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DNA-Surfactant interactions, compaction, condensation, decompaction and phase separation
- 2004
-
In: Journal of the Chinese Chemical Society. - 2192-6549. ; 51:3, s. 447-469
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Research review (peer-reviewed)abstract
- Recent investigations-of the interaction between DNA and alkyltrimethyl ammonium bromides of various chain lengths are reviewed. Several techniques have been used such as phase map determinations, fluorescence microscopy, and electron microscopy. Dissociation of the DNA-surfactant complexes, by the addition of anionic surfactant, has received special attention. Precipitation maps for DNA-cationic surfactant' systems were evaluated by turbidimetry for different salt concentrations, temperatures and surfactant chain lengths. Single-stranded DNA molecules precipitate at lower surfactant concentrations than double-helix ones. It was also observed that these systems precipitate for very low concentrations of both DNA and surfactant, and that the extension of the two-phase region increases for longer chain surfactants; these observations correlate well with fluorescence microscopy results, monitoring the system at a single molecule level. Dissociation of the DNA-cationic surfactant complexes and a concomitant release of DNA was achieved by addition of anionic surfactants. The unfolding of DNA molecules, previously compacted with cationic surfactant, was shown to be strongly dependent on the anionic surfactant chain length; lower amounts of a longer chain surfactant were needed to release DNA into solution. On the other hand, no dependence on the hydrophobicity of the compacting agent was observed. The structures of the aggregates formed by the two surfactants, after the interaction with DNA, were imaged by cryogenic transmission electron microscopy. It is possible to predict the structure of the aggregates formed by the surfactants, like vesicles, from the phase behaviour of the mixed surfactant systems. The compaction of a medium size polyanion with shorter polycations was furthermore studied by means of Monte Carlo simulations. The polyanion chain suffers a sudden collapse as a function of the condensing agent concentration and of the number of charges on the molecules. Further increase of the concentration gives an increase of the degree of compaction. The compaction was found to be associated with the polycations promoting bridging between different sites of the polyanion. When the total charge of the polycations was lower than that of the polyanion, a significant translational motion of the compacting agent along the polyanion was observed, producing only a small-degree of intrachain segregation. However, complete charge neutralization was not a prerequisite to achieve compacted forms.
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| 18. |
- Dias, Rita, et al.
(author)
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Modeling of DNA compaction by polycations
- 2003
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 119:15, s. 8150-8157
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Journal article (peer-reviewed)abstract
- In this work we study polycations as efficient compacting agents of a medium size polyanion by means of Monte Carlo simulations. The systems are characterized in terms of a conformational analysis in which shape, overall dimensions, structure factors, radial distribution functions, and the degree of accumulation of the compaction agent near the polyanion are taken into consideration. Results show that the degree of compaction depends on the size of the positive chais and their number. The role of electrostatic interactions is paramount in the compaction process, and an increase in the number of molecules of the compacting agent or in the number of charges of each molecule leads to collapse, which may be followed by some unfolding in situations of overcharging. Compaction is associated with polycations promoting bridging between different sites in the polyanion. When the total charge of the polycations is significantly lower than that of the polyanion, interaction produces only a small degree of intrachain segregation in the latter, allowing for significant translational motion of the compacting agent along the longer chain. However, complete charge neutralization is not mandatory to achieve compact forms. (C) 2003 American Institute of Physics.
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| 19. |
- Hansson, P, et al.
(author)
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Phase separation in polyelectrolyte gels interacting with surfactants of opposite charge
- 2002
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In: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 106:38, s. 9777-9793
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Journal article (peer-reviewed)abstract
- Macroscopic phase separation in covalent sodium polyacrylate (PA) networks following the absorption of cetyltrimethylammonium bromide/chloride (CTAB/C) from aqueous solutions is studied experimentally and theoretically. The gels are shown to consist of a solvent-swollen polyelectrolyte network (core), surrounded by a dense surface phase (skin) of polyion/surfactant complexes. The effect of core swelling on network structure and polyion/surfactant interaction in skins is discussed. It is demonstrated that the skin limits the swelling of the core. A model of the equilibrium swelling of phase separated gels is developed, taking into account the osmotic swelling of the core due to the presence of mobile counterions, the work of deformation of the core network, and the work of deformation of the skin. The last contribution is described using the theory of rubber elasticity. The core network is described using an empirical equation of state. The model is used to calculate the volume of gels after the absorption of various amounts of surfactant, Comparison with experiments shows that the agreement is satisfactory. The skin microstructure is investigated by means of small-angle X-ray scattering, optical birefringence, and time-resolved fluorescence quenching. The size, shape, and spatial organization of surfactant micelles is found to depend on the composition of the skin. Stoichiometric polyion/surfactant complexes (free from simple ions) form an ordered cubic structure (space group: Pm3n). The incorporation of bromide or chloride ions leads to a transition to hexagonal structure. The transition is related to the corresponding transition in complexes between linear PA and CTAB. Skins with cubic structure are found to be elastic and can be deformed at constant volume. The structural basis for the rubber-like behavior is discussed. The appearance of hexagonal skin microstructure is found to correlate with an anomalous swelling/deswelling pattern leading to the formation of "balloon gels".
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| 20. |
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| 21. |
- Ivanova, R, et al.
(author)
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Effect of pharmaceutically acceptable glycols on the stability of the liquid crystalline gels formed by poloxamer 407 in water
- 2002
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In: Journal of Colloid and Interface Science. - : Elsevier BV. - 1095-7103 .- 0021-9797. ; 252:1, s. 226-235
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Journal article (peer-reviewed)abstract
- The ability of poly(ethylene oxide)-poly(propylene oxide)-poly (ethylene oxide) (PEO-PPO-PEO) block copolymers (Poloxamers) to form "gels" (lyotropic liquid crystalline structures) in water is of interest to pharmaceutical applications. In such applications the presence of polar organic solvents is often desirable or required. The effect of such solvents on the stability of lyotropic liquid crystalline gels formed by PEO-PPO-PEO block copolymers was assessed by studying the phase behavior and structure in ternary isothermal (25degreesC) systems of pharmaceutical interest consisting of Poloxamer 407 (EO100PO70EO100), water, and one of the following solvents (referred to here collectively as "glycols"): glycerol, propylene glycol, ethanol, polyethylene glycol 400, and glucose. Small-angle X-ray scattering was employed to establish the structure of the liquid crystals obtained and to determine their characteristic length scales. The stability range of the liquid crystalline gel phases in the systems studied was found to vary with the glycol type. For example, the micellar cubic structure can accommodate about 0.85:1 parts glucose per part water (in terms of weight) and up to as much as 5.5:1 parts propylene glycol per part water. A correlation between the glycol effects on the stability of the liquid crystalline phases and glycol physiochemical characteristics such as octanol/water partition coefficient or solubility parameter is proposed.
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| 22. |
- Karlberg, Maria, et al.
(author)
-
Mixed solutions of hydrophobically modified graft and block copolymers
- 2004
-
In: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - : Elsevier BV. - 0927-7757. ; 236:1-3, s. 159-164
-
Journal article (peer-reviewed)abstract
- Phase separation temperatures and viscosities of dilute and semidilute aqueous mixtures of hydrophobically modified graft and block copolymers have been studied. The graft copolymer was hydrophobically modified ethyl(hydroxyethyl)cellulose (HM-EHEC), and the block copolymers were hydrophobically modified poly(ethylene glycol) (HM-PEG) with diblock (DB) or triblock (TB) structures, i.e., modified at one or both ends, respectively. For comparison, viscosity measurements were made also on mixtures of TB with HM-HEC, and on systems where one of the polymers was unmodified (i.e. HM-EHEC/PEG or TB/EHEC). All results indicate mixed hydrophobic associations between graft and block copolymers. Moderate viscosity enhancements were found on addition of HM-PEG to HM-EHEC, and the effects were larger for TB than for DB. The viscosity changes were quite sensitive to the concentration of HM-EHEC. (C) 2004 Elsevier B.V. All rights reserved.
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| 23. |
- Karlson, L, et al.
(author)
-
A rheological investigation of the complex formation between hydrophobically modified ethyl (hydroxy ethyl) cellulose and cyclodextrin
- 2002
-
In: Carbohydrate Polymers. - 0144-8617. ; 50:3, s. 219-226
-
Journal article (peer-reviewed)abstract
- The thickening effect of a hydrophobically modified polymer in an aqueous solution is dependent on intermolecular hydrophobic associations, and if the polymer concentration is significantly above the overlap concentration also on chain entanglements. In this investigation we have added different cyclodextrins (CD) in order to decouple hydrophobic polymer-polymer associations via inclusion complex formation with the polymer hydrophobic tails. Both size and hydrophobicity of the cavity of the CD-molecules were found to have an effect on the process. In addition, the influence of chemical structure of the polymer hydrophobic tails was investigated. Either a linear C-14-chain or a more bulky nonylphenol group was used. The viscosity as a function of CD-concentration first decreased strongly, and then attained a constant value. At excess CD the viscosity became virtually the same as in a solution of the unmodified parent polymer, provided that complex formation was not sterically bindered. This suggests that all hydrophobic links, originating from the hydrophobic modification process, which influence the theology could be deactivated. On the other hand, with combinations where the complex formation was hindered to a certain degree the initial decrease was less accentuated, and also, the viscosity leveled out at a significantly higher value. In an attempt to rationalize the data a simple model based on the assumption that each complex formed deactivates one theologically active link was used. In combination with the Langmuir adsorption model the number of complexes as a function of CD concentration could be obtained. This model also gave a value of the complex formation constant. Furthermore, in solutions where all hydrophobic links could be deactivated the results from the model suggested that all polymer hydrophobic tails were originally active in forming the network. (C) 2002 Elsevier Science Ltd. All rights reserved.
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| 24. |
- Karlson, L, et al.
(author)
-
Cyclodextrins in hydrophobically modified poly(ethylene glycol) solutions: Inhibition of polymer-polymer associations
- 2002
-
In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 18:23, s. 9028-9034
-
Journal article (peer-reviewed)abstract
- in an aqueous solution of a hydrophobically, end-modified poly(ethylene glycol) polymer, HM-PEG, the thickening effect is dependent on intermolecular hydrophobic associations and the formation of a network structure. In the present investigation cyclodextrin, CD, has been added to an aqueous HM-PEG solution and a decrease in Newtonian viscosity has been followed. The decreased viscosity refers to polymer-polymer associations becoming less numerous when complexes between polymer hydrophobic tails and CD become more frequent; CD-decorated polymer hydrophobic tails have no possibility of contributing to the network. It was found that deactivation of the first few hydrophobic tails has very large consequences for the viscosity. A termination of a fraction as small as 10% (or below) of the total amount of polymer hydrophobic tails may reduce viscosity to a level almost corresponding to that of the unmodified parent polymer. This can be understood by taking into account that a solution of a HM-PEG polymer is, expected to be inhomogeneous with large concentration fluctuations and that the viscosity is likely to be strongly decreased by reducing the probability of hydrophobic associations responsible for connecting different blusters. The effect of CD on rheology is very different for different architectures of hydrophobically modified polymer, in particular between graft copolymers and end-capped ones. This can be understood from the differences in network structure.
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| 25. |
- Kipkemboi, P, et al.
(author)
-
Phase behaviour and structure of amphiphilic poly(ethylene oxide)-poly(propylene oxide) triblock copolymers ((EO)(4)(PO)(59)(EO)(4) and (EO)(17)(PO)(59)(EO)(17)) in ternary mixtures with water and xylene
- 2003
-
In: Canadian Journal of Chemistry. - 0008-4042. ; 81:8, s. 897-908
-
Journal article (peer-reviewed)abstract
- The isothermal ternary phase behaviour and structure of amphiphilic copolymer-water-xylene systems was investigated for two poly(ethylene oxide)-poly(propylene oxide) (PEO-PPO) triblo k copolymers, (EO)(4)(PO)(59)(EO)(4) and (EO)(17)(PO)(59)(EO)(17), at 25degreesC. The phase boundaries were identified using H-2 NMR and inspection under polarized light. The microstructure and structural length scales in the lyotropic liquid crystalline phases were determined and characterized by small-angle X-ray scattering. The amounts and relative proportions of the selective solvents can modulate the resulting microstructures, although the phase sequence observed depends on the relative PEO-PPO block ratio. A variety of lyotropic liquid crystalline (LLC) phases are thermodynamically stable in the system with copolymer (EO)(17)(PO)(59)(EO)(17), both of the normal ("oil-in-water") and of the reverse ("water-in-oil") morphology. The high hydrophobic content of copolymer (EO)(4)(PO)(59)(EO)(4), however, favours reverse LLC structures with low interfacial curvature. No normal LLC phases are formed by this system.
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| 26. |
- Köping-Hoggard, M, et al.
(author)
-
Relationship between the physical shape and the efficiency of oligomeric chitosan as a gene delivery system in vitro and in vivo
- 2003
-
In: Journal of Gene Medicine. - : Wiley. - 1521-2254 .- 1099-498X. ; 5:2, s. 130-141
-
Journal article (peer-reviewed)abstract
- Background Chitosans of high molecular weights have emerged as efficient. nonviral gene delivery systems, but the properties and efficiency of well-defined low molecular weight chitosans ((.) 5 kDa) have not been studied. We therefore characterized DNA complexes Of Such low molecular weight chitosans and related their physical shape and stability to their efficiency as gene delivery systems in vitro and in vivo.Methods Individual complexes between six different chitosan oligomers (6-, 8-, 10-, 12-, 14- and 24-mers) and fluorescence-labeled T4 DNA were visualized and classified into six physical shapes using video-enhanced fluorescence microscopy. The effects of chitosan chain length, charge ratio (+/-) and solvent properties (pH and ionic strength) on the stability and structure of the complexes were studied. Gene expression in vitro and in vivo were studied using a luciferas reporter gene.Results Free DNA appeared as extended coils. Chitosan complexes had, a variety of physical shapes depending on the experimental conditions. in general, the fraction of complexes that had nonaggregated, globular structures increased with increasing chain length of the chitosan oligomer, increasing charge ratio and reduction of pH (from 6.5 to 3.5). A further increase in charge ratio for globular complexes or a further reduction in pH (to 2.5) increased the fraction of aggregates, indicating a window where pharmaceutically desirable globules are obtained. Gene transfection efficiencies in vitro and in vivo were related to the physical shape and stability of the complexes. Only the 24-mer formed stable complexes that gave a high level of gene expression comparable to that of high molecular weight ultrapure chitosan (UPC) in vitro and in vivo.Conclusions Chitosan oligomers form complexes with DNA in a structure-dependent manner. We conclude that the 24-mer, which has more desirable physical prope ties than UPC, is more attractive as a gene delivery system than the conventional high molecular weight chitosans. Copyright (C) 2002 John Wiley Sons, Ltd.
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| 27. |
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| 28. |
- Larsson, Britt, et al.
(author)
-
Blood supply and oxidative metabolism in muscle biopsies of female cleaners with and without myalgia
- 2004
-
In: The Clinical Journal of Pain. - : Ovid Technologies (Wolters Kluwer Health). - 0749-8047 .- 1536-5409. ; 20:6, s. 440-446
-
Journal article (peer-reviewed)abstract
- OBJECTIVES: Pathomechanisms of work-related myalgia are poorly understood. Myalgia is thought to be caused by excitation of nociceptors present in the muscular tissue but not in the muscle fiber itself. Muscle contraction in combination with hypoxia is known to excite nociceptors. Morphologic analysis can contribute to the knowledge of the excitation of nociceptors. This study thoroughly examines the morphology of the trapezius muscle's capillary supply and signs of disturbed oxidative metabolism to understand their role in work-related myalgia. METHODS: Surgical trapezius muscle biopsies were obtained from 25 female cleaners with long-standing work-related myalgia, 25 female cleaners without trapezius myalgia, and 21 healthy teachers. Enzyme and immunohistochemical stainings were performed to highlight fibers with aberrant intermyofibrillar patterns, indicating a disturbed oxidative metabolism (also known as moth-eaten fibers) and a disturbed capillary supply of different fibers. RESULTS: A significantly lower number of capillaries per fiber area in cleaners suffering from myalgia compared with cleaners without trapezius myalgia was found. Moth-eaten fibers were found in the 3 groups, but these fibers were significantly more prevalent in the groups of cleaners than in the healthy teacher group. CONCLUSION: This work indicates that the capillary supply of trapezius is affected in work-related trapezius myalgia. More studies are needed to understand possible mechanisms that would explain the occurrence of moth-eaten fibers.
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| 29. |
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| 30. |
- Larsson, Britt, et al.
(author)
-
The prevalences of cytochrome c oxidase negative and superpositive fibres and ragged-red fibres in the trapezius muscle of female cleaners with and without myalgia and of female healthy controls
- 2000
-
In: Pain. - 1872-6623 .- 0304-3959. ; 84:2-3, s. 379-387
-
Journal article (peer-reviewed)abstract
- The association of cytochrome c oxidase negative fibres (COX-negative) and ragged-red fibres (RR-fibres) with work related trapezius myalgia has been proposed. Hitherto studies have been small or without control groups. The aim of the present study was to investigate the prevalences of RR-fibres and COX-negative fibres in female cleaners with (n=25) and without (n=23) trapezius myalgia and in clinically healthy female teachers (n=21). The cleaners did mainly floor cleaning requiring monotonous loading on the trapezius muscle. A questionnaire covering background data and aspects of pain (prevalence, duration, intensity and influence on daily living) was answered. Biopsies were obtained from the trapezius muscle by an open surgical technique. The three groups did not differ in prevalence of COX-negative or COX-superpositive (i.e. type-I fibres with extremely strong brownish reaction in both the COX and SDH/COX stainings) fibres. The prevalence of COX-negative fibres was age dependent. Two subgroups of RR-fibres were present when stained for COX; COX-negative (73%) and COX-superpositive (26%) fibres. Forty-two percent of the COX-negative fibres were RR-fibres and 79% of the COX-superpositive were RR-fibres. A significantly (P=0.002) higher proportion of the COX-superpositive fibres in the cleaners were RR-fibres compared to the teachers. Multivariate regression analysis revealed that age, occupation as cleaner and a tender point in the trapezius were significantly associated with increased prevalences of RR-fibres; a cleaner with a tender point had a 4.35 higher prevalence of RR-fibres compared to a teacher without a tender point. No correlations between other pain related variables and prevalence of RR-fibres were noted. In conclusion, RR-fibres but not COX-negative or COX-superpositive fibres were correlated with cleaning work tasks and with a tender point in the trapezius.
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| 31. |
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| 32. |
- Lindman, Björn, et al.
(author)
-
Colloidal systems in DNA packaging
- 2003
-
In: Meeting on Self-Assembly - the Future. - 1586033824 ; , s. 374-386
-
Conference paper (peer-reviewed)abstract
- The interaction between DNA and cationic cosolutes- multivalent ions, cationic surfactants and cationic macromolecules- has direct biological implications and is also foreseen to have a number of applications, in particular for transfection and protection and purification of DNA. We have during the last few years established a broad physico-chemical research program on various mixed systems of DNA and cationic surfactants. This collaborative Lund-Coimbra project will be outlined, reporting recent findings as well as research plans. The research is largely based on our previous work on mixed polymer-surfactant systems and as a basis we will describe some important general features of those.
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| 33. |
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| 34. |
- Lindman, Björn, et al.
(author)
-
DNA-lipid systems. An amphiphile self-assembly and polymer-surfactant perspective
- 2002
-
In: Lipid and Polymer-Lipid Systems. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 0340-255X .- 1437-8027. - 9783540430018 ; 120, s. 52-63
-
Conference paper (peer-reviewed)abstract
- The interaction between DNA and oppositely charged surfactants has been investigated by several techniques, like fluorescence microscopy, electron microscopy, phase diagram determination, and ellipsometry. The phase behaviour is more strongly associative than that in previously studied systems. A precipitate is formed for very low amounts of surfactant and DNA. DNA compaction is a general phenomenon in the presence of multivalent ions and positively charged surfaces; because of the high charge density there are strong attractive ion correlation effects. The interaction between DNA and catanionic mixtures (i.e., mixtures of cationic and anionic surfactants) was also investigated. We observed that DNA compacts and adsorbs onto the surface of positively charged vesicles and that the addition of anionic surfactant can release free DNA back into solution from a compact globular complex between DNA and cationic surfactant. Finally, we investigated DNA interactions with polycations, chitosans with different chain lengths, by fluorescence microscopy, in vivo transfections assays and cryogenic transmission electron microscopy. The general conclusion is that a chitosan effective in promoting compaction is also efficient in transfection.
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| 35. |
- Marques, Eduardo, et al.
(author)
-
A calorimetric study of the gel-to-liquid crystal transition in catanionic surfactant vesicles
- 2002
-
In: Thermochimica Acta. - 0040-6031. ; 394:1-2, s. 31-37
-
Journal article (peer-reviewed)abstract
- Dilute solutions of catanionic vesicles formed by the mixed single-chained (sodium dodecylsulphate, SDS) and double-chained (didodecyldimethylammonium bromide, DDAB) surfactants have been investigated by differential scanning calorimetry. It is for the first time reported a gel-to-liquid crystal phase transition temperature, T-m, in this type of mixed vesicles. The SDS-rich vesicles (at X-SDS = 0.71) show a concentration-dependent T-m in the range 9-16 degreesC. Addition of salt is seen to have an effect on T-m similar to that observed with increasing surfactant concentration, both inducing a decrease in T-m. These results differ from those obtained for neat DDAB vesicles. The observed effects in the two types of vesicles are rationalised in terms of headgroup electrostatic interactions which may have influence on the chain packing and phase transition temperature.
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| 36. |
- Marques, E F, et al.
(author)
-
Self-organization of double-chained and pseudodouble-chained surfactants: counterion and geometry effects
- 2003
-
In: Advances in Colloid and Interface Science. - 1873-3727. ; 100-102, s. 83-104
-
Journal article (peer-reviewed)abstract
- Self-organization in aqueous systems based on ionic surfactants, and their mixtures, can be broadly understood by a balance between the packing properties of the surfactants and double-layer electrostatic interactions. While the equilibrium properties of micellar systems have been extensively studied and are understood, those of bilayer systems are less well characterized. Double-chained and pseudodouble-chained (or catanionic) surfactants are among the amphiphiles which typically form bilayer structures, such as lamellar liquid-crystalline phases and vesicles. In the past 10-15 years, an experimental effort has been made to get deeper insight into their aggregation patterns. With the double-chai ed amphiphiles, by changing counterion, adding salt or adding anionic surfactant, there are possibilities to depart from the bilayer aggregate in a controlled manner. This is demonstrated by several studies on the didodecyldimethylammonium bromide surfactant. Mixtures of cationic and anionic surfactants yield the catanionics, surfactants of the swelling type, and also show a rich phase behavior per se. A variety of liquid-crystalline phases and, in dilute regimes, equilibrium vesicles and different micellar shapes are often encountered. Phase diagrams and detailed structural studies, based on several techniques (NMR, microscopy and scattering methods), have been reported, as well as theoretical studies. The main features and conclusions emerging from such investigations are presented. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 37. |
- Miguel, Maria, et al.
(author)
-
DNA-cationic amphiphile interactions
- 2003
-
In: Colloids and Surfaces a-Physicochemical and Engineering Aspects. ; 228:1-3, s. 43-55
-
Journal article (peer-reviewed)abstract
- DNA shows strong interactions with cationic cosolutes and these have both biological and technological significance. We outline our research on various mixed system of DNA and cationic amphiphiles including the interaction of DNA with simple cationic surfactants as well as the interaction with catanionic mixtures and positively charged catanionic vesicles. An overview from phase behavior to microstructure will be presented. We will also address DNA compaction and decompaction phenomena in different systems. Finally, simulations on DNA confinement and interaction with cationic polyions are considered. (C) 2003 Published by Elsevier B.V.
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| 38. |
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| 39. |
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| 40. |
- Pacios, I E, et al.
(author)
-
Polymerization of N,N-dimethylacrylamide in Aerosol OT-water mixtures: from lamellae to segregation
- 2003
-
In: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - 0927-7757. ; 218:1-3, s. 11-20
-
Journal article (peer-reviewed)abstract
- The polymerizable N,N-dimethylacrylamide (DMAA) is added to the lamellar liquid crystalline system formed by the anionic surfactant Aerosol OT and water. Small angle X-ray scattering, gives the same bilayer thickness and hydrophilic group cross-sectional area as without DMAA, and an environment sensitive fluorescent probe indicates that DMAA is likely to be in the water layers between the AOT bilayers. This shows that the structure of the AOT bilayer in the lamellar phase is not affected by DMAA. However, DMAA induces instability of the lamellar phase, and a second phase, an isotropic solution, appears for larger DMAA contents The effect of DMAA in stabilizing this phase with respect to lamellae is much larger than that expected from a simple dilution with water. Furthermore, there is a combined influence of temperature and DMAA concentration on the coexistence of the lamellar and the isotropic phases. When an initiator is added and the monomer polymerizes the tendency to phase separate increases strongly. The lamellar spacing after polymerization is below the value predicted by the global AOT/H2O ratio. This indicates that the polymer segregates from the AOT phase and that the lamellar phase is osmotically compressed by the polymer rich phase. (C) 2003 Elsevier Science B.V. All rights reserved.
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| 41. |
- Pacios, IE, et al.
(author)
-
Equilibrium between poly(NN-dimethylacrylamide) and the lamellar phase of aerosol OT/water
- 2002
-
In: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 106:19, s. 5035-5041
-
Journal article (peer-reviewed)abstract
- A mixed amphiphilic system composed of the anionic surfactant Aerosol OT (AOT), in water forming a lamellar phase, to which is added a neutral noninteracting polymer, poly(N,N-dimethylacrylamide), is studied experimentally by SAXS, H-2 NMR, and microscopy, in a range of surfactant and polymer compositions. Addition of the polymer produces a decrease in the lamellar spacing, the decrease by the polymer being almost twice that produced by an equal volume of AOT. Microscopy reveals heterogeneity, but no macroscopic phase separation occurs. H-2 NMR detects that on increasing the polymer concentration some water is in an isotropic environment. It is inferred that the presence of the polymer induces a microscopic phase separation into a polymer-rich isotropic phase and a surfactant-rich lamellar phase, and this is tested theoretically by calculating the osmotic pressures in these two phases. In the lamellar phase, the effect of electrostatic, undulation, van der Waals, and hydration forces on the AOT bilayer is considered; in the isotropic phase, the osmotic contribution of the polymer is considered. These two pressures correlate well, supporting theoretically the hypothesis of the two phases in equilibrium.
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| 42. |
- Pacios, IE, et al.
(author)
-
In situ polymerization of N,N-dimethylacrylamide in aerosol OT-water: Modified lamellar structure and multiphase separation
- 2002
-
In: Macromolecules. - : American Chemical Society (ACS). - 0024-9297 .- 1520-5835. ; 35:20, s. 7553-7560
-
Journal article (peer-reviewed)abstract
- N,N-Dimethylacrylamide is polymerized in the lyotropic surfactant system containing Aerosol-OT (AOT) and water. The polymerization is thermally initiated with AIBN, which allows a homogeneous initiation in this very viscous mixture. The starting mixture contains a lamellar liquid crystalline phase and an isotropic phase in equilibrium. After the polymerization, new phases develop which appear to have a lamellar structure, even if originated from the initial isotropic phase. More phases appear after the polymerization, and they are lamellar, since the phase behavior shifts toward the lamellar region when the monomer is consumed. Once the polymer is formed in situ, it segregates from the lamellae and forms an isotropic microphase which does not macroscopically separate. The appearance of this polymer-rich phase modifies the structure of the lamellar mesophase by partially deswelling it. This gives a shorter lamellar spacing. The law that expresses how the AOT-H2O spacing is contracted by the polymer is deduced from the equilibrium between the two (lamellar and isotropic) microphases. All the macroscopic final phases contain polymer, although in different proportions. The molecular weight of the polymer is the same in all phases. The presence of AOT in the medium has no influence on the resulting tacticity, which is the same as that for a polymer obtained in pure water.
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| 43. |
- Pacios, IE, et al.
(author)
-
The effect of poly (N,N-dimethylacrylamide) on the lamellar phase of Aerosol OT/water
- 2002
-
In: Colloid and Polymer Science. - : Springer Science and Business Media LLC. - 0303-402X .- 1435-1536. ; 280:6, s. 517-525
-
Journal article (peer-reviewed)abstract
- The effect of a water-soluble uncharged polymer on the stability of the lamellar phase of the Aerosol OT (AOT)/water system is studied. The lamellar phase still exists when water is replaced by an aqueous solution of poly(N,N-dimethylacrylamide) (Rapproximate to4x10(2) Angstrom). Since the coil dimensions are (much) larger than the thickness of the water layers (d(w)approximate to51 Angstrom), the polymer molecules do not enter the lamellar phase. Instead segregation in small domains occurs, and in equilibrium with the AOT-rich phase another separate phase containing the polymer is formed. The polymer-rich phase exerts an osmotic pressure that reduces the water content in the AOT-rich phase, and by compression the repeat distance is reduced.
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| 44. |
- Pais, AACC, et al.
(author)
-
Polyelectrolytes confined to spherical cavities
- 2002
-
In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 117:3, s. 1385-1394
-
Journal article (peer-reviewed)abstract
- The effects of confining a polyelectrolyte solution containing a polyion and its counterions in spherical cavities of different sizes have been investigated by Monte Carlo simulations using a simple model system. Polyions of three different linear charge densities and counterions of three different valences have been examined. Structure, energy, and free energy properties of all systems have been determined as a function of the sphere radius. In nearly all cases, the free energy of confining the polyelectrolyte solution increases as the sphere radius is decreased. The free energy cost decreases as the linear charge density of the polyion is reduced and as the counterion valence is increased, although for completely different reasons. A strong consistency among the variations of the different structural and thermodynamic results on the linear charge density and counterion valence was found. The study provides information relevant for experimental systems of polyions confined to, e.g., viruses, vesicles, and zeolite cavities. (C) 2002 American Institute of Physics.
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| 45. |
- Persson, Gerd, 1970-
(author)
-
Amphiphilic Molecules in Aqueous Solution
- 2003
-
Doctoral thesis (other academic/artistic)abstract
- The aim of this thesis was to investigate amphiphilic molecules in aqueous solution. The work was divided into two parts. In the first part the effects of different counterions on phase behavior was investigated, while the second part concerns the 1-monooleoyl-rac-glycerol (MO)/n-octyl-β-D-glucoside (OG)/2H2O-system. The effects of mixing monovalent and divalent counterions were studied for two surfactant systems, sodium/calcium octyl sulfate, and piperidine/piperazine octanesulfonate. It was found that mixing monovalent and divalent counterions resulted in a large decrease in cmc already at very low fractions of the divalent counterion. Moreover, the degree of counterion binding for piperidine in the piperidine/piperazine octanesulfonate system was much higher than predicted, probably due to the larger hydrophobic moiety of piperidine. The effects of hydrophobic counterions were studied for eight alkylpyridinium octanesulfonates (APOS). The results were discussed in terms of packing constraints. The anomalous behavior of the 2H2O quadrupolar splittings in the lamellar phases was explained by the presence of two or more binding sites at the lamellae surface. The MO/OG/water system was studied in general and the MO-rich cubic phases in particular. When mixing MO and OG it was found that OG-rich structures (micelles, hexagonal and cubic phase of space group Ia3d) could solubilize quite large amounts of MO, while the MO-rich cubic structures where considerable less tolerant towards the addition of OG. The micelles in the OG-rich L1 phase were found to remain rather small and discrete in the larger part of the L1 phase area, but at low water concentration and high MO content a bicontinuous structure was indicated. Only small fractions of OG was necessary to convert the MO-rich cubic Pn3m structure to an Ia3d structure, and upon further addition of OG a lamellar (La) phase formed. Since the larger part of the phase diagram contains a lamellar structure (present either as a single La phase or as a dispersion of lamellar particles together with other phases), the conclusion was that introducing OG in the MO structures, forces the MO bilayer to become more flat. Upon heating the cubic phases, structures with more negative curvature were formed. The transformation between the cubic structures required very little energy, and this resulted in the appearance of additional peaks in the diffractograms.
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| 46. |
- Samoshina, Yulia, et al.
(author)
-
Adsorption of cationic, anionic and hydrophobically modifie polyacrylamides on silica surfaces
- 2003
-
In: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - : Elsevier BV. - 0927-7757. ; 231:1-3, s. 195-205
-
Journal article (peer-reviewed)abstract
- The adsorption of modified polyacrylamides (PAM) at the silica surface has been studied by ellipsometry. To study the influence of the type of modification of the polyacrylamide on the adsorption behaviour, we have used cationic, anionic and hydrophobically modified polyacrylamides with varying charge density and degree of hydrophobisation. The adsorbed amount and adsorbed layer thickness were obtained at a neutral pH and at a constant ionic strength (C-KCl = 0.1 M). The effect of surfactant present during the polymerisation process has also been investigated. Cationic modified polyacrylamides have been found to adsorb most strongly onto the silica surface and a large overcompensation of the surface charge has been observed in all cases. For cationic modified polyacrylamides having additional hydrophobic groups the adsorbed amount decreases with density of hy rophobic groups, and the adsorbed layer thickness is larger. Anionic modified polyacrylamides also adsorb onto the negative silica surface, though the adsorbed amounts are small. The presence of the surfactant Tween 80 leads to an increase in the adsorbed amount for the polymers which adsorb weakly in the absence of the surfactant. However, for the polymers, which have already shown a good ability to adsorb on silica, the addition of surfactant has no significant effect on the adsorbed amount. (C) 2003 Elsevier B.V. All rights reserved.
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| 47. |
- Schillén, Karin, et al.
(author)
-
Association of Napthalene-Labeled Poly(acrylic acid) and Interaction with Cationic Surfactants. Fluorescence Studies
- 2000
-
In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 16:26, s. 10528-10539
-
Journal article (peer-reviewed)abstract
- Water-soluble poly(acrylic acid) has been covalently labeled with a fluorescent hydrophobic chromophore, naphthalene (Np), randomly attached onto the polymer backbone with an amount of 3 mol %. The polymer, which is a new type of hydrophobically modified polymer denoted PAAMeNp-34, was investigated using steady-state fluorescence spectroscopy in aqueous solutions of different pH and in methanol. The fluorescence emission spectra of PAAMeNp-34 in water exhibit both Np monomer emission (with intensity IM) and Np excimer emission (with intensity IE). The excimer emission is mainly due to the association of Np groups, preformed in their ground electronic state as a result of the hydrophobic interaction. For a PAAMeNp-34 aqueous solution, the intensity ratio, IE/IM, decreases in the pH range where the electrostatic repulsive forces overcome the hydrophobic interactions between the Np groups and the polymer chain expands because of the intrapolymer repulsion between the negatively charged carboxylate groups. In methanol, the excimer emission is low because hydrophobic interactions are insignificant in this solvent. The interaction between PAAMeNp-34 and cationic surfactants of different alkyl chain length (dodecyl-, tetradecyl-, and hexadecyltrimethylammonium chloride) was also studied in dilute aqueous solutions at pH 3.0 and pH 6.8. The addition of surfactants perturbs the Np−Np interactions because of polymer−surfactant associations. This causes a detectable change in the fluorescence emission, which is followed with increasing surfactant concentration. From the onset of the change, the force that dominates the interaction between the polymer and the surfactants at different pH can be examined. At low pH, PAAMeNp-34 is uncharged and hydrophobic forces dominate the polymer−surfactant interaction. The photophysical properties of the system therefore show a clear dependence on the hydrophobicity (or chain length) of the surfactants. On the other hand, at pH 6.8, where the polymer is negatively charged, almost no or very little difference between the three surfactants is observed at the onset of fluorescence change, which indicates that electrostatic forces dominate the interaction at the lowest surfactant concentrations.
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| 48. |
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| 49. |
- Terada, Eiji, et al.
(author)
-
Adsorption of cationic cellulose derivative/anionic surfactant complexes onto solid surfaces. II. Hydrophobized silica surfaces
- 2004
-
In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 20:16, s. 6692-6701
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Journal article (peer-reviewed)abstract
- The effect of the anionic surfactant SDS (sodium dodecyl sulfate) on the adsorption behavior of cationic hydroxyethyl cellulose (Polymer JR-400) and hydrophobically modified cationic cellulose (Quatrisoft LM-200) at hydrophobized silica has been investigated by null ellipsometry and compared with the previous data for adsorption onto hydrophilic silica surfaces. The adsorbed amount of LM-200 is found to be considerably larger than the adsorbed amount of JR-400 at both surfaces. Both polymers had higher affinity toward hydrophobized silica than to silica. The effect of SDS on polymer adsorption was studied under two different conditions: adsorption of polymer/SDS complexes from premixed solutions and addition of SDS to preadsorbed polymer layers. Association of the surfactant to the polymer seems to control the interfacial behavior, which depends on the surfactant concentration. For the JR-400/SDS complex, the adsorbed amount on hydrophobized silica started to increase progressively from much lower SDS concentrations, while the adsorbed amount on silica increased sharply only slightly below the phase separation region. For the LM-200/SDS complex, the adsorbed amounts increased progressively from very low SDS concentrations at both surfaces, and no large difference in the adsorption behavior was observed between two surfaces below the phase separation region. The complex desorbed from the surface at high SDS concentrations above the critical micelle concentration. The reversibility of the adsorption of polymer/ SDS complexes upon rinsing was also investigated. When the premixed polymer/SDS solutions at high SDS concentrations (>5 mM) were diluted by adding water, the adsorbed amount increased due to the precipitation of the complex. The effect of the rinsing process on the adsorbed layer was determined by the hydrophobicity of the polymer and the surface.
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- Terada, Eiji, et al.
(author)
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Adsorption of cationic cellulose derivatives/anionic surfactant complexes onto solid surfaces. I. Silica surfaces
- 2004
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In: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 20:5, s. 1753-1762
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Journal article (peer-reviewed)abstract
- The effect of the anionic surfactant SDS (sodium dodecyl sulfate) on the adsorption behaviors of cationic hydroxyethyl celluloses (Polymer JR-125, JR-400, and JR-30M) and hydrophobically modified cationic cellulose (Quatrisoft LM-200) at silica surfaces in the presence of a 10 mM NaCl solution has been investigated by null ellipsometry. The adsorbed amount of LM-200 is found to be considerably larger than adsorbed amounts of other polymers. The rate of adsorption for the LM-200 is also lower than that for the Polymer JR series under comparable conditions. Electrostatic interaction is found to be the major driving force for the adsorption. The effect of SDS on adsorption was studied under two different conditions: adsorption of polymer/SDS complexes from premixed solutions and additions of SDS to the preadsorbed polymer layers. In all cases, associative binding of the surfactant to the polymer seems to control interfacial behavior, which depends on the surfactant concentration. Maximum adsorption was obtained at the surfactant concentration below the phase separation region, and the complex desorbed from the surface at a high SDS concentration above the critical micelle concentration. The reversibility of the polymer/SDS complexes adsorption upon rinsing with NaCl solutions was also investigated. When the premixed polymer/SDS solutions at high SDS concentrations (>5 mM) were diluted by salt solutions, the adsorbed amount increased sharply with precipitation of the complex. A larger amount of the JR-400/SDS complex precipitated during the rinsing process seemed to adsorb to a larger extent to silica than the corresponding LM-200/SDS complex.
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