Phase diagram and thermodynamics of the n-octyl β-D-glucoside/water system

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.