Physical−Chemical Properties of the <i>n</i> -Octyl β- <scp>d</scp> -Glucoside/Water System. A Phase Diagram, Self-Diffusion NMR, and SAXS Study

Abstract

The n-octyl β-d-glucoside/water binary phase diagram (temperature vs concentration) and the aggregation parameters of the individual phases have been determined. n-Octyl β-d-glucoside forms four different phases together with water. At temperatures below 22 °C, there is an isotropic solution region from neat water extending up to almost 60 wt % n-octyl β-d-glucoside. As the concentration is further increased, three liquid crystalline phases form in the following order: hexagonal, cubic, and lamellar. At high surfactant concentrations (>93 wt %) the lamellar phase is in equilibrium with hydrated crystals. The hexagonal phase disappears at temperatures slightly higher than 20 °C. The solution region has been investigated with 1H-NMR to deduce the self-diffusion coefficients of both n-octyl β-d-glucoside and water. From these results it has been possible to draw conclusions about the surfactant aggregation behavior at dilute concentrations and when the n-octyl β-d-glucoside concentration is increased. The water diffusion in the n-octyl β-d-glucoside/water system has been compared with the diffusion process of water in a glucose solution, and it has been possible to interpret the data in terms of water diffusion in a glucose solution with some additional obstruction effects from the micellar hydrocarbon cores. The liquid crystalline phases have been examined by means of small-angle X-ray scattering and analyzed in terms of repetition distances and area/head group in the hexagonal, cubic, and lamellar phases. An important result of this study is the fact that the area per glucose unit in the surfactant is an almost invariant quantity across the phase diagram.