Lyotropic Phases from Short Chain Monoalkylphosphate/ Water Systems. A Multinuclear NMR and X Ray Diffraction Study

Abstract

The phase diagrams of sodium monobutyl and monohexyl phosphate/water systems have been derived from NMR and X-Ray diffraction experiments. Isotropic, lamellar, hexagonal and gel phases have been observed over the considered concentration and temperature range (surfactant molar fraction X_M from 0.1 to 0.4 and -20[ddot]C < T < 100[ddot]C). They were delineated by <³¹P and water ²H NMR and identified by means of small angle X ray diffraction from which geometrical parameters have been derived. The hexagonal phase was shown to exhibit a strong tendency to orient in the magnetic field upon slow cooling from the isotropic phase. This macroscopic orientation remains indefinitely stable out of the magnetic field. Monobutyl and monohexyl phosphates exhibit similar phase diagrams which can be derived from each other by a simple shift along the concentration and temperature scales. Only the monobutyl derivative has therefore been extensively studied. Special attention has been paid to the hexagonal hase, since its orientation properties allow some particular experiments to be done. ²H NMR was used to monitor the orientation of water molecules in the hexagonal phase. The molecular order and equilibrium conformation of the surfactant molecules were derived from ³¹P chemical shift anisotropy, ²H quadrupolar splittings of the hydrocarbon chain, as well as by means of the unusually well resolved ¹³C -³¹P and ¹H - ³¹P dipolar couplings observed on oriented samples.