| 1. |
- Kocherbitov, Vitaly, et al.
(författare)
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A desorption calorimetric method for use at high water activities
- 2004
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Ingår i: Thermochimica Acta. - : Elsevier BV. - 0040-6031. ; 411:1, s. 31-36
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a microcalorimetric technique to simultaneously measure water activity and enthalpy of mixing (differential heat of sorption) as functions of composition at high water activities. The instrument consists of a sorption vessel in a double twin microcalorimeter. A sample at high water activity is placed in one chamber of the vessel and an unsaturated salt solution is injected into another chamber. A tube connects the chambers and diffusion will cause the water in the sample to be desorbed and condensed in the salt solution. As the rate of diffusion is high when the sample is at high water activity it is possible to make detailed studies of phenomena taking place at high water activity. The method has been tested on the system dimethyldodecylamine oxide (DDAO)-water. (C) 2003 Elsevier B.V. All rights reserved.
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| 2. |
- Kocherbitov, Vitaly
(författare)
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A new formula for accurate calculation of water activity in sorption calorimetric experiments
- 2004
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Ingår i: Thermochimica Acta. - : Elsevier. - 0040-6031 .- 1872-762X. ; 414:1, s. 43-45
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Tidskriftsartikel (refereegranskat)abstract
- A sorption calorimetric method proposed by Wadsö and Markova allows determination of water activity. Calculation of water activity is based on Fick’s first law. The correct form of the Fick’s law takes into account not only vapour diffusion itself but also bulk flow of vapour. A new formula, which uses the correct form of the diffusion law for direct calculation of water activity, is proposed.
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| 3. |
- Kocherbitov, Vitaly, et al.
(författare)
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Glassy crystalline state and water sorption of alkyl maltosides
- 2004
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 20:8, s. 3056-3061
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Tidskriftsartikel (refereegranskat)abstract
- A differential scanning calorimetric and sorption calorimetric study of two alkyl maltosides, C(8)G(2) and C(10)G(2), was performed. In the dry state, C(8)G(2) and C(10)G(2) do not form solid crystals but undergo a glass transition upon temperature change. The glass is partly ordered and has the same lamellar structure as the liquid crystals formed by the two maltosides. To reflect the presence of the glass transition and the structure, the terms "glassy crystals" and "glassy liquid crystals" can be used. A mechanism of the relaxation of the glassy crystals based on the results of small-angle X-ray scattering experiments is proposed. Experiments on water sorption showed that the glassy crystals turn into lyotropic liquid crystals upon sorption of water at constant temperature. This isothermal glass transition can be characterized by water content and change of partial molar enthalpy of mixing of water. A method to calculate the phase diagram liquid crystals-glassy liquid crystals is proposed.
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| 4. |
- Kocherbitov, Vitaly, et al.
(författare)
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Lysozyme-water interactions studied by sorption calorimetry
- 2004
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 108:49, s. 19036-19042
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Tidskriftsartikel (refereegranskat)abstract
- Hydration of hen egg white lysozyme was studied by using the method of sorption calorimetry at 25, 40, and 50degreesC. Desorption calorimetric measurements were performed at 25 and 40degreesC. The activity of water and partial molar enthalpy of mixing of water were determined as functions of water content. Hydration of lysozyme occurs in four steps: slow penetration of water into the protein-protein interface; gradual glass transition, which occurs in every protein molecule independently of other molecules; further water uptake with disaggregation of protein aggregates and formation of a monolayer of water; and accumulation of free water. The amount of bound water found in desorption experiments is 420 water molecules per lysozyme molecule. Two hysteresis loops were found in the sorption isotherm of lysozyme. The small loop is caused by the slow penetration of water molecules into the protein-protein interface at very low water contents, while the large loop is due to the slow kinetics of aggregation of protein molecules upon desorption. The phase diagram of the lysozyme-water system is presented.
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| 5. |
- Kocherbitov, Vitaly, et al.
(författare)
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Phase diagram and physicochemical properties of the n-octyl alpha-D-glucoside/water system
- 2003
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084. ; 5:23, s. 5262-5270
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Tidskriftsartikel (refereegranskat)abstract
- Four experimental methods were used to study the phase diagram, as well as the thermodynamic and structural properties of the binary system n-octyl alpha-D-glucoside water in the temperature range 25-130degreesC. Sorption calorimetry allows one to determine the activity of water and enthalpy of mixing as functions of water content at constant temperature, while DSC scans temperature at constant composition and provides information on enthalpies of phase transitions. Therefore, the combination of the two calorimetric methods is a powerful too] to study composition-temperature phase diagrams. While calorimetry can be used to determine boundaries of the phases, NMR and SAXS methods are used to study their structures. A detailed phase diagram of the system is presented. A liquid crystalline cubic phase previously not reported in the system was found. The hydration in the system is endothermic, excluding the exothermic formation of hydrates. Using the sorption calorimetric method the lengths of the very short tie lines between the isotropic micellar and liquid crystalline phases were determined. Van der Waals's differential equation was used to calculate the slopes of t e phase boundaries. The parameters of the lamellar, cubic and hexagonal liquid crystalline phases were determined by means of SAXS. It was found that the area per surfactant headgroup in the liquid crystalline phases varied with composition.
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| 6. |
- Kocherbitov, Vitaly, et al.
(författare)
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Phase diagram and physiochemical properties of the n-octyl a D-glucoside/water system
- 2003
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 5:23, s. 5262-5270
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Tidskriftsartikel (refereegranskat)abstract
- Four experimental methods were used to study the phase diagram, as well as the thermodynamic and structural properties of the binary system n-octyl -D-glucoside/water in the temperature range 25-130°C. Sorption calorimetry allows one to determine the activity of water and enthalpy of mixing as functions of water content at constant temperature, while DSC scans temperature at constant composition and provides information on enthalpies of phase transitions. Therefore, the combination of the two calorimetric methods is a powerful tool to study composition-temperature phase diagrams. While calorimetry can be used to determine boundaries of the phases, NMR and SAXS methods are used to study their structures. A detailed phase diagram of the system is presented. A liquid crystalline cubic phase previously not reported in the system was found. The hydration in the system is endothermic, excluding the exothermic formation of hydrates. Using the sorption calorimetric method the lengths of the very short tie lines between the isotropic micellar and liquid crystalline phases were determined. Van der Waalss differential equation was used to calculate the slopes of the phase boundaries. The parameters of the lamellar, cubic and hexagonal liquid crystalline phases were determined by means of SAXS. It was found that the area per surfactant headgroup in the liquid crystalline phases varied with composition.
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| 7. |
- Kocherbitov, Vitaly, et al.
(författare)
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Phase diagram and thermodynamics of the n-octyl beta-D-glucoside/water system
- 2002
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 106:11, s. 2910-2917
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we describe the application of a sorption calorimeter to study the binary system n-octyl beta-D-glucoside/water at 25, 40, and 60 degreesC. At 25 degreesC we also carried out experiments with heavy water which showed similar results to those with normal water. The method used allows one to simultaneously obtain information about the activity of water and the enthalpy of mixing in binary systems. The obtained results show that the hydration in the system is endothermic and hence driven by entropy. We have also specified the concentration ranges and thermodynamic properties of the different phases in the system. The first step of isothermal hydration in the system when the lamellar phase is formed involves high endothermic effect and is similar to the transition observed during the heating of the pure surfactant. Forces between the layers of the lamellar phase in the next step of hydration have also been studied at three temperatures. Using the calorimetric method, we have detertrimed concentration ranges of the narrow two-phase regions of the transitions from lamellar to cubic and isotropic phases. We have used van der Waals differential equation to calculate temperature-composition slopes of the phase boundaries using the isothermal properties of the system.
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| 8. |
- Kocherbitov, Vitaly
(författare)
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Salt-saturated salt solution as a standard system for sorption calorimetry
- 2004
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Ingår i: Thermochimica Acta. - : Elsevier BV. - 0040-6031 .- 1872-762X. ; 421:1-2, s. 105-110
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Tidskriftsartikel (refereegranskat)abstract
- The method of sorption calorimetry allows simultaneous monitoring of water activity and the partial molar enthalpy of mixing of water. The way of calibration of sorption calorimeter affects the accuracy of the data obtained in sorption experiments. In order to improve the accuracy of the method one can use physico-chemical calibration instead of electrical calibration. The system for use in the calibration should keep its properties constant during the sorption of water, therefore a heterogeneous system can be used. The particular system suitable for this purpose is magnesium nitrate hexahydrate in equilibrium with its saturated solution. The enthalpy of dissolution of the hexahydrate in its saturated solution needed for the calibration has been determined by isothermal titration calorimetry at 25 and 40 degreesC. The titration results are in agreement with the calculations based on van der Waals differential equation. (C) 2004 Elsevier B.V. All rights reserved.
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| 9. |
- Sparr, Emma, et al.
(författare)
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The effect of bacteriorhodopsin, detergent and hydration on the cubic-to-lamellar phase transition in the monoolein-distearoyl phosphatidyl glycerol-water system
- 2004
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Ingår i: Biochimica et Biophysica Acta - Biomembranes. - : Elsevier BV. - 0005-2736 .- 0006-3002 .- 1878-2434. ; 1665:1-2, s. 156-166
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Tidskriftsartikel (refereegranskat)abstract
- The cubic phase of monoolein (MO) has successfully been used for crystallization of membrane proteins. It is likely that the transition to a lamellar phase upon dehydration is important for the crystallization process, and that the internal dimensions of the lipid phases (i.e., water pore diameter) are crucial for the inclusion and the diffusion of membrane proteins. In the present study, we investigated the cubic-to-lamellar phase transitions in the MO-water and the MO-distearoyl phosphatidyl glycerol (DSPG) systems. The MO-water system was investigated by means of isothermal sorption and desorption microcalorimetry. We show that the transition from cubic to lamellar phase induced by desorption is driven by entropy. At 25 degreesC, this occurs at a water activity of 0.98 with a transition enthalpy of 860 J/mol (MO). The phase behavior was also investigated in the presence of a small amount of the transmembrane protein bacteriorhodopsin (bR), and a detergent, octyl glucoside (OG), and it was shown that both bR and OG stabilize the lamellar phase. Analogous results were obtained for the MO-DSPG-water system. The latter system resembles the MO-water system in that a cubic-to-lamellar phase transition is induced by dehydration, although the structural properties of these phases are slightly different. Finally, we demonstrate that bR can be crystallized from a cubic phase of MO-DSPG-buffer. (C) 2004 Elsevier B.V. All rights reserved.
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| 10. |
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