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 √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 = √3) with increasing pressure.