| 1. |
- Amenitsch, H, et al.
(författare)
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Bile salts form lyotropic liquid crystals
- 2003
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - 0927-7757 .- 1873-4359. ; 213:1, s. 79-92
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Tidskriftsartikel (refereegranskat)abstract
- A reinvestigation of the phase diagrams relative to some conjugated and non-conjugated bile salts in water has demonstrated the formation of lyotropic liquid crystalline phases, in contradiction with generally accepted statements. The phase behaviour is complex and the phase diagrams are unusual, compared to most surfactants and lipids. In particular, coexistence of liquid crystalline phases with crystals has been obser ed. The formation of liquid crystalline phases requires very long equilibration times and the thermal stability of the lyotropic phases is moderate. The observed structure is tentatively assumed to be of the reverse hexagonal type. Structural relations with currently accepted models for the organisation of bile salts into micelles and solid form have been found. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 2. |
- Antunes, Filipe, et al.
(författare)
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A rheological investigation of the association between a non-ionic microemulsion and hydrophobically modified PEG. Influence of polymer architecture
- 2003
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - 0927-7757. ; 215:1-3, s. 87-100
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Tidskriftsartikel (refereegranskat)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. |
- Francisco, J D, et al.
(författare)
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Liquid crystalline properties and extractability of monoolein-water systems by supercritical carbon dioxide
- 2003
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - 0927-7757. ; 213:1, s. 69-78
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Tidskriftsartikel (refereegranskat)abstract
- The influence of phase structure on the extraction recovery of monoolein from monoolein-water systems by supercritical carbon dioxide was investigated. Two monoolein-water mi tures at concentrations of 9 wt% water and 18 wt% water, respectively, were studied. H-2-NMR was used to monitor changes of the phase structures in supercritical conditions. The results showed that the phase structure and the water-monoolein interactions play a role on the extraction yield. The monoolein recovery was higher in the 9 wt% water sample in which L-alpha phase was present compared to the 18 wt% water sample in which the L-alpha melted to yield a L-2 phase. When the phase structures were the same in both samples, the extraction recovery was determined by the solubility properties of the components of the system. The changes due to the presence of supercritical carbon dioxide were shown also to depend on the water concentration of the monoolein-water mixture besides the temperature. The 18 wt% mixture showed a L-2 phase structure in all the range of the temperature investigated while the 9 wt% water mixture showed phase changes from L-alpha phase to L-2 phase in the same temperature range. (C) 200 Elsevier Science B.V. All rights reserved.
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| 4. |
- Groth, Cecilia, 1972, et al.
(författare)
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Diffusion of water in multilamellar vesicles of dialkyl and dialkyl ester ammonium surfactants
- 2003
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - 1873-4359 .- 0927-7757. ; 228:1-3, s. 64-73
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Tidskriftsartikel (refereegranskat)abstract
- The NMR self-diffusion technique is used for measuring diffusion of water in highly concentrated multilamellar vesicle solutions. The signal intensity of water, i.e. the water echo-decay, is monitored down to very small intensities thus providing an accurate measure of how water is diffusing in the solution. It is noted that a large curvature is dominating the functional form of the echo-decay indicating the presence of a large number of multilamellar vesicles. It is also concluded that in order to measure accurately the volume fraction of water inside and outside vesicles the experimental time scale can be changed. From a multiexponential fit to the echo-decay the fraction fast and the sum of all slow components can be extracted. When the apparent fraction "vesicle water", P-vw(app), is plotted versus the experimental time scale the graph produced is a good representation of the difference in how fast water diffuses over the different vesicle membranes. From an extrapolation to "zero time" the true fraction of trapped water can be extracted, i.e. a quantitative measure of the volume fraction of vesicles at a certain concentration of surfactant.
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| 5. |
- Göransson, Anders, et al.
(författare)
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Mechanisms responsible for sub-micron particle deposition in a laminar wall jet
- 2002
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - 0927-7757. ; 211:2-3, s. 133-144
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Tidskriftsartikel (refereegranskat)abstract
- An experimental study was performed to investigate the dependency of particle size on the deposition rate. Polystyrene latex particles of two radii (0.23 and 0.38 mum) were deposited onto a glass surface covered with indium-tin oxide. Reduced deposition efficiency was observed for the larger particles, although an increase in the deposition efficiency was seen as that the diffusion boundary layer became thicker. A critical degree of surface coverage, Brit, was defined as the fraction of surface coverage at which the linear variation of flux with time ended. When the two particle sizes were compared, it was found that the value of Brit depended only on the wall shear stress. This result indicates that surface shielding is an important factor in the decline in particle flux. The deposition process was divided into two separate processes, described by a mass-transfer coefficient and an adhesion rate coefficient. It was found that the process was governed by mass-transfer during the initial period, but for higher degrees of surface coverage adhesion became the rate-determining factor. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 6. |
- Göransson, Anders, et al.
(författare)
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Modelling of sub-micron-sized particle deposition in a laminar wall jet - an investigation of the role of shielding
- 2002
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - 0927-7757. ; 205:3, s. 149-159
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Tidskriftsartikel (refereegranskat)abstract
- The deposition of polystyrene latex particles (0.23 mum in radius) onto a glass surface, covered with indium-tin oxide, has been studied using a wall-jet cell. The particle deposition could be followed in a single experiment at several positions, exposed to a wide range of wall shear rates. A deposition model including a shielding function was applied in order to describe the process. Three shielding functions obtained from the literature, which assumed different shielded areas, were compared. The size of the excluded area was shown to be dependent on the wall shear rate: a higher shear rate resulted in a larger area being blocked by deposited particles, thus preventing non-deposited, free-flowing particles from reaching the surface. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 7. |
- Karlson, L, et al.
(författare)
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Complex formed in the system hydrophobically modified polyethylene glycol/methylated alpha-cyclodextrin/water. An NMR diffusometry study
- 2003
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - 0927-7757. ; 228:1-3, s. 171-179
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Tidskriftsartikel (refereegranskat)abstract
- In aqueous solutions hydrophobically modified polyethylene glycol (HM-PEG) forms a transient polymer network held together by intermolecular hydrophobic associations. In the present investigation we have used NMR-diffusometry to study how the addition of methylated alpha-cyclodextrin (M-alpha-CD) influences the polymer network. The addit on of M-alpha-CD resulted in an increased mean self-diffusion of HM-PEG, DHM-PEG, which is referred to a degradation of the polymer network when hydrophobic associations are disrupted due to complex formation between the hydrophobic groups of HM-PEG and M-alpha-CD. Addition of small amounts of M-alpha-CD results in a dramatic increase in DHM-PEG. Upon further addition of M-a-CD the increase in DHM-PEG is less dramatic and at excess M-alpha-CD, DHM-PEG levels off and equals the mean self-diffusion coefficient for unmodified PEG with the same molecular weight. The suggested interpretation is that the addition of the first molecules of M-alpha-CD mainly reduces the probability for hydrophobic associations inter-connecting different clusters of polymer micelles whereas at higher M-alpha-CD concentrations a disengagement of the individual clusters into separate HM-PEG molecules becomes important. (C) 2003 Elsevier B.V. All rights reserved.
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| 8. |
- Oliviero, C, et al.
(författare)
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Dynamic phase diagram and onion formation in the system C10E3/D2O
- 2003
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - 0927-7757. ; 228:1-3, s. 85-90
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Tidskriftsartikel (refereegranskat)abstract
- The influence of the shear on the lamellar phase, L, of the system C10E3/D2O was studied along an isoplethal path (40 wt.% C10E3) in the temperature range 25-42degreesC. A dynamic phase diagram was determined by steady-state rheometry, where by shear action the lamellar phase was transformed into multilamellar vesicles (MLVs) ("onions"). The location of "onions" in the dynamic phase diagram, depends only on the temperature and the applied shear rate, and not on the shear history. The classical lamellar phase structure is stable at rest and at low shear rates. When exposed to higher shear rates, the lamellar structure is transformed into onions. The transition from lamellae to onions is shifted to higher shear rates with increasing temperature. In a range of shear rates in between the stable lamellae and stable onion structure, the transition is incomplete. The transformation of lamellae into onions appears to be governed by the imposed strain, in agreement with earlier studies. The effect of temperature can be understood from the general property of nonionic surfactants where the monolayer spontaneous curvature decreases with increasing temperature. (C) 2003 Published by Elsevier B.V.
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| 9. |
- Pacios, I E, et al.
(författare)
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Polymerization of N,N-dimethylacrylamide in Aerosol OT-water mixtures: from lamellae to segregation
- 2003
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - 0927-7757. ; 218:1-3, s. 11-20
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Tidskriftsartikel (refereegranskat)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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| 10. |
- Svensson, Anna, et al.
(författare)
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Phases and structures of a polyion-surfactant ion complex salt in aqueous mixtures: cationic hydroxyethyl cellulose with dodecylsulfate counterions
- 2003
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Ingår i: Colloids and Surfaces A: Physicochemical and Engineering Aspects. - 0927-7757. ; 228:1-3, s. 91-106
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Tidskriftsartikel (refereegranskat)abstract
- A polyion-surfactant ion complex salt, cat-HECDS, has been synthesised, consisting of cationic hydroxyethyl cellulose w th dodecylsulfate counterions. Ternary phase diagrams have been established for aqueous mixtures of cat-HECDS with either the conventional surfactant NaDS or the polyelectrolyte cat-HECCl. Such mixtures represent the simplest possible aqueous mixtures of oppositely charged polymer and surfactant where the proportions of polyion and surfactant ion may be varied. Phases and structures were investigated by visual inspection through crossed polarisers and by small-angle X-ray scattering (SAXS),The pure complex salt cat-HECDS was insoluble in water, but it could absorb up to 60 wt.% water. No crystalline order was detected at any water content. Dissolution of the complex salt in water occurred if sufficient amounts of either surfactant or polyelectrolyte was added. A smaller excess of surfactant (50% by charge) than polyelectrolyte (100% by charge) was needed to dissolve the complex salt in the dilute region. The efficiency of excess NaDS to dissolve the complex salt was attributed to hydrophobic interactions be ween the surfactant and the cat-HEC backbone. The free surfactant concentration at the onset of dissolution was in good agreement with the critical association concentration observed in previous gel experiments.The investigated phase diagrams were dominated by a large isotropic phase containing micellar surfactant aggregates without long-range order. The micellar aggregation number varied from similar to20 for aqueous cat-HECDS to similar to80 for mixtures rich in NaDS. A small fraction of added complex salt was enough to destroy the hexagonal phase formed in binary mixtures of NaDS and water. The absence of liquid crystalline phases containing significant amounts of complex salt was attributed to the stiff character of the polyion and its low charge density. In a ternary complex salt-surfactant-water system studied previously, where the complex salt contained a flexible polyion with a high charge density, liquid crystalline phases were found at all proportions of complex salt. (C) 2003 Elsevier B.V. All ri hts reserved.
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