Molecular Order and Dynamics in Short Chain Alkylphosphate/Water Mesophases

Abstract

The phase diagrams of sodium mono and dibutylphosphate/ water mesophase have been investigated by ²H and ³¹P NMR as well as by X ray diffraction. In the case of monobutylhydro- genophosphate (MBHP) one successively obtains the hexagonal (H_I) and lamellar (L_α) phases on raising the amphiphile concentration, while the L_α phase only is observed for the dibu- tylphosphate (DBP). Upon cooling down the samples from the isotropic phase, the directors of the MBHP HI and DPB L_α phases become, respectively, oriented parallel and perpendicular to the magnetic field with a narrow angular distribution. The chain conformations as well as the molecular order parameters have been determined from the P chemical shift aniso- tropy (CSA) and deuteron quadrupolar splittings completed by the observation of the ³¹P-¹³C and ³¹P-H dipolar splittings in the case of the oriented MBHP HI phase. The rotamer populations about the 0-C and C-C bonds appear to be governed by the confinement of phosphoalkyl chains in restricted volumes. A quasiaxial reorientation of DBP in the L_α phase has been evidenced by ³¹P relaxation at 202.7 and 36.5 MHz where the CSA and dipolar mechanisms predominate, respectively. The former accounts for the marked angular dependence of T₁ observed at 202.7 MHz.