| 1. |
- Ericsson, Caroline, et al.
(författare)
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Thermotropic phase behaviour of long-chain alkylmaltosides
- 2005
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 7:15, s. 2970-2977
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Tidskriftsartikel (refereegranskat)abstract
- The thermotropic phase behaviour and phase structure of crystalline and non-crystalline n-tetradecyl-beta-D-maltoside (C(14)G(2)) and n-hexadecyl-beta-D-maltoside (C(16)G(2)) have been investigated by means of differential scanning calorimetry and X-ray techniques. Upon lyophilisation, both compounds form a solid, lamellar phase comprising disordered head groups and hexagonally packed alkyl chains that are suggested to be tilted and interdigitated. This ordered lamellar phase melts into a metastable lamellar liquid crystal, which re-crystallises to a high-temperature crystalline polymorph comprising interdigitated, non-tilted alkyl chains. Remarkably, the high-temperature polymorph of C(14)G(2) has the same melting point as that of C(16)G(2), namely 105 degrees C for both surfactants. A low-temperature polymorph of anhydrous C(14)G(2) crystallises from water at room temperature, whereas the hemihydrate of C(14)G(2) crystallises at 6 degrees C from water, or from chloroform containing trace water. X-ray data suggest both these crystalline modifications to comprise interdigitated and tilted alkyl chains.
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| 2. |
- Kocherbitov, Vitaly
(författare)
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Driving forces of phase transitions in surfactant and lipid systems
- 2005
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 109:13, s. 6430-6435
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Tidskriftsartikel (refereegranskat)abstract
- In aqueous surfactant and lipid systems, different liquid crystalline phases are formed at different temperatures and water contents. The "natural" phase sequence implies that phases with higher curvature are formed at higher water contents. On the other hand, there are exceptions to this rule, such as the monoolein/water system. In this system an anomalous transition from lamellar to reverse cubic phase upon addition of water is observed. The calorimetric data presented here show that the hydration-induced transitions to phases with higher curvature are driven by enthalpy, while the transitions to phases with lower curvature are driven by entropy. It is shown that the driving forces of phase transitions can be determined from the appearance of the phase diagram using the approach based on van der Waals differential equation. From this approach it follows that the slope of the phase boundary should be positive with respect to water content if the phase diagram obeys the "natural" phase sequence. The increase of entropy, which drives the anomalous phase transitions, arises from the increase of disorder of the hydrocarbon chains.
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| 3. |
- Kocherbitov, Vitaly, et al.
(författare)
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Hydration of dimethyldodecylamine-N-oxide: Enthalpy and entropy driven processes
- 2006
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 110, s. 13649-13655
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Tidskriftsartikel (refereegranskat)abstract
- Dimethyldodecylamine-N-oxide (DDAO) has only one polar atom that is able to interact with water. Still, this surfactant shows very hydrophilic properties: in mixtures with water, it forms normal liquid crystalline phases and micelles. Moreover, there is data in the literature indicating that the hydration of this surfactant is driven by enthalpy while other studies show that hydration of surfactants and lipids typically is driven by entropy. Sorption calorimetry allows resolving enthalpic and entropic contributions to the free energy of hydration at constant temperature and thus directly determines the driving forces of hydration. The results of the present sorption calorimetric study show that the hydration of liquid crystalline phases of DDAO is driven by entropy, except for the hydration of the liquid crystalline lamellar phase which is co-driven by enthalpy. The exothermic heat effect of the hydration of the lamellar phase arises from formation of strong hydrogen bonds between DDAO and water. Another issue is the driving forces of the phase transitions caused by the hydration. The sorption calorimetric results show that the transitions from the lamellar to cubic and from the cubic to the hexagonal phase are driven by enthalpy. Transitions from solid phases to the liquid crystalline lamellar phase are entropically driven, while the formation of the monohydrate from the dry surfactant is driven by enthalpy. The driving forces of the transition from the hexagonal phase to the isotropic solution are close to zero. These sorption calorimetric results are in good agreement with the analysis of the binary phase diagram based on the van der Waals differential equation. The phase diagram of the DDAO-water system determined using DSC and sorption calorimetry is presented.
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| 4. |
- Kocherbitov, Vitaly, et al.
(författare)
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Hydration of MCM-41 Studied by Sorption Calorimetry
- 2007
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Ingår i: J. Phys. Chem. C. - : American Chemical Society (ACS). ; :2007, 111 (35)
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Tidskriftsartikel (refereegranskat)abstract
- Hydration of mesoporous silica MCM-41 was studied using the method of sorption calorimetry. By combining water sorption and nitrogen sorption experiments, we calculated the density of silanol groups on the MCM-41 surface as 1.6 nm-2. Comparison of capillary condensation regimes of water and nitrogen showed that the apparent density of water confined in MCM-41 pores is ca. 0.88 g/cm3. The pore diameter calculated using a combination of X-ray and sorption data is 39 Å. Calculations based on application of the Kelvin-Cohan equation on the water sorption data are in reasonable agreement with this value. The sorption calorimetric results show that the capillary condensation of water in the pores is driven by enthalpy; the entropic effect is negative. A mechanism of hydration that involves formation of small unfilled cavities adjacent to pore walls can be used to explain the observed enthalpy end entropy effects. Comparison of sorption and desorption data indicates the presence of air trapped in pores when hydration is performed by mixing MCM-41 with liquid water. The heat effect of pre-capillary condensation adsorption of water on hydroxylated MCM-41 is much more exothermic compared to the original calcined material.
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| 5. |
- Kocherbitov, Vitaly, et al.
(författare)
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Hydration of microcrystalline cellulose and milled cellulose studied by sorption calorimetry.
- 2008
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Ingår i: Phys. Chem. B.. - : American Chemical Society (ACS). ; 112:12, s. 3728-3734
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Tidskriftsartikel (refereegranskat)abstract
- The hydration of two different polymorphs of microcrystalline cellulose (cellulose I and II), as well as the hydration of amorphous cellulose was studied using water sorption calorimetry, gravimetric water vapor sorption, nitrogen sorption, and X-ray powder diffraction. Amorphous cellulose was prepared by means of ball-milling of microcrystalline cellulose (MCC). Whereas X-ray data showed the untreated MCC to consist of cellulose I, the amorphous cellulose was found to recrystallize into cellulose II after contact with water or water vapor at relative humidities (RHs) above 90%. Sorption isotherms show an increase of water sorption in the sequence cellulose I < cellulose II < amorphous cellulose. The enthalpy of water sorption becomes more exothermic in the same sequence. The specific area of cellulose is dramatically higher when calculated from the water adsorption than when calculated from nitrogen adsorption. A proposed mechanism of water sorption by MCC implies the adsorption of water molecules at solid-solid interfaces, i.e., between neighboring microfibrils, which explains the observed difference between water and nitrogen. The Brunauer-Emmett- Teller (BET) model is therefore not appropriate for the description of the hydration of cellulose. Rather, the Langmuir model represents a more accurate description of water sorption by MCC at low RH. At higher RH, the water adsorption competes with capillary condensation. The thickness of microfibrils, as calculated using the fitting of the sorption isotherm of MCC with the Langmuir equation, is about 4 nm. This value compares favorably with literature data.
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| 6. |
- Kocherbitov, Vitaly, et al.
(författare)
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Hydration of Thermally Denatured Lysozyme Studied by Sorption Calorimetry and Differential Scanning Calorimetry
- 2006
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Ingår i: J.Phys.Chem.B.. - : American Chemical Society (ACS). ; 110, s. 10144-10150
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Tidskriftsartikel (refereegranskat)abstract
- We have studied hydration (and dehydration) of thermally denatured hen egg lysozyme using sorption calorimetry. Two different procedures of thermal denaturation of lysozyme were used. In the first procedure the protein was denatured in an aqueous solution at 90 °C, in the other procedure a sample that contained 20% of water was denatured at 150 °C. The protein denatured at 90 °C showed very similar sorption behavior to that of the native protein. The lysozyme samples denatured at 150 °C were studied at several temperatures in the range of 25-60 °C. In the beginning of sorption, the sorption isotherms of native and denatured lysozyme are almost identical. At higher water contents, however, the denatured lysozyme can absorb a greater amount of water than the native protein due to the larger number of available sorption sites. Desorption experiments did not reveal a pronounced hysteresis in the sorption isotherm of denatured lysozyme (such hysteresis is typical for native lysozyme). Despite the unfolded structure, the denatured lysozyme binds less water than does the native lysozyme in the desorption experiments at water contents up to 34 wt %. Glass transitions in the denatured lysozyme were observed using both differential scanning calorimetry and sorption calorimetry. Partial molar enthalpy of mixing of water in the glassy state is strongly exothermic, which gives rise to a positive temperature dependence of the water activity. The changes of the free energy of the protein induced by the hydration stabilize the denatured form of lysozyme with respect to the native form.
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| 7. |
- Kocherbitov, Vitaly, et al.
(författare)
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Hydration of trimethylamine-N-oxide and of dimethyldodecylamine-N-oxide: An ab initio study
- 2007
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Ingår i: Journal of molecular structure. Theochem. - : Elsevier BV. - 0166-1280. ; 808:1-3, s. 111-118
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory (B3LYP) calculations of the hydration of trimethylamine-N-oxide (TMAO) and a surfactant dimethyldodecylamine-N-oxide (DDAO) are reported. Hydrogen-bonded complexes of DDAO with up to three water molecules and of TMAO with up to five water molecules are studied. Interactions of both TMAO and DDAO with water give rise to the binding energies that are much higher than that in the water dimer. A significant charge transfer from amine oxide to water is observed. Energies of water-water interactions increase in the presence of amine oxides. The hydration number of TMAO in the hydrogen bonded complexes is shown to be three. The energies of amine oxide-water interactions in the hydrogen bonded complexes have been analyzed using differential energetic parameters analogous to the parameters measured in calorimetric experiments.
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| 8. |
- Kocherbitov, Vitaly
(författare)
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Properties of Water Confined in an Amphiphilic Nanopore
- 2008
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Ingår i: J. Phys. Chem. C. - : American Chemical Society (ACS). ; 112, s. 16893-16897
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Tidskriftsartikel (refereegranskat)abstract
- Molecular dynamics simulations of water and nitrogen confined in a model amphiphilic nanotube were performed. The nanotube has a diameter of 4 nm and consists of hydrophobic atoms and regularly placed OH groups. The results show that the density of water close to the nanotube walls is lower compared to the density in the center of the nanotube. The hydrogen bonded network of water molecules is practically intact compared with the bulk water. The simulation confirms that the experimentally observed low formal density of water in the nanopores (0.88 g/cm3) is due to formation of small unfilled cavities adjacent to the pore walls. Nitrogen molecules are localized primarily in the unhydrated cavities. The presence of nitrogen molecules is not the main reason for the decrease of water density in the nanotube.
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| 9. |
- Silva, Claudia L, et al.
(författare)
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Stratum corneum hydration: phase transformations and mobility in stratum corneum, extracted lipids and isolated corneocytes
- 2007
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Ingår i: Biochimica et Biophysica Acta - Biomembranes. - : Elsevier BV. - 0005-2736. ; 1768:11, s. 2647-2659
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Tidskriftsartikel (refereegranskat)abstract
- The outermost layer of skin, stratum corneum (SC), functions as the major barrier to diffusion. SC has the architecture of dead keratin filled cells embedded in a lipid matrix. This work presents a detailed study of the hydration process in extracted SC lipids, isolated corneocytes and intact SC. Using isothermal sorption microcalorimetry and relaxation and wideline 1H NMR, we study these systems at varying degrees of hydration/relative humidities (RH) at 25 °C. The basic findings are (i) there is a substantial swelling both of SC lipids, the corneocytes and the intact SC at high RH. At low RHs corneocytes take up more water than SC lipids do, while at high RHs swelling of SC lipids is more pronounced than that of corneocytes. (ii) Lipids in a fluid state are present in both extracted SC lipids and in the intact SC. (iii) The fraction of fluid lipids is lower at 1.4% water content than at 15% but remains virtually constant as the water content is further increased. (iv) Three exothermic phase transitions are detected in the SC lipids at RH=91–94%, and we speculate that the lipid re-organization is responsible for the hydration-induced variations in SC permeability. (v) The hydration causes swelling in the corneocytes, while it does not affect the mobility of solid components (keratin filaments).
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