The rotator phases of neat and hydrated 1-alcohols

We report an x-ray scattering study of the rotator phases in both dry and hydrated 1-alcohols CH3-(CH2)(n-1)-OH(12 less than or equal to n less than or equal to 26). Hydration was accomplished by using both fixed composition and controlled relative humidity. A hexagonal phase and two tilted phases are reported in the temperature versus chain length and temperature versus humidity phase diagrams. Hydration tends to drive the system from one tilted rotator state to the other, and then to the untilted hexagonal phase. The hexagonal phase shows a six-layer periodicity illustrating the crossover between trilayer alkane behavior and bilayer surfactant behavior. Hydration and chain-length mixing increase the temperature range of the rotator phase, The rotator phase will take in about 1 H2O per 2 alcohols which is twice the saturated water content of the liquid phase. This is shown to explain the unusual rise in the melting temperature with hydration. (C) 1996 American Institute of Physics.