A lyotropic inverse ribbon phase in a branched-chain polyoxyethylene surfactant: pressure effects

A centred-rectangular lyotropic ribbon phase, rarely observed in inverse (type II) systems, has been found in the branched-chain polyoxyethylene surfactant tetradecyloctadecyl-tetraoxyethylene ether (C14C16EO4) in excess water. This phase is stabilised by the application of hydrostatic pressure. The ratio of the 2-D cell parameters, b/a, is observed to be less than root 3 (1.732) over the range of temperatures and pressures studied. The constructed pressure-temperature phase diagram shows that, at high temperatures or low pressures, the inverse ribbon phase converts into an inverse micellar cubic phase of spacegroup Fd3m, and at the opposite extreme, a lamellar gel phase was formed. The lattice parameters of the inverse ribbon phase were found to vary with pressure, with the structure becoming increasingly distorted away from 2-D hexagonal symmetry (b/a = root 3) with increasing pressure.