Intermediate phases of surfactant - water mixtures

Abstract

Intermediate phases in two non-ionic polyoxyethylene surfactantiD20 systems - nonaethylene glycol mono (1 1-oxa-14,18,22,26-tetramethylheptacosyl) ether (C30E09) and hexaethylene glycol n-hexadecyl ether (C15E05) have been studied. Both systems have a rhombohedral mesh phase with the space group symmetry R3m, and model calculations show this phase to consist of hexagonally arranged pores in a surfactant bilayer. The formation of this phase is driven by the need to introduce curvature into the interface as the temperature decreases and hydration of the ethylene oxide headgroups
increases.
In the C30E09 system a small angle neutron scattering study on a 42 wt.% sample has allowed the epitaxial relationship between the lower temperature hexagonal (H1) phase and the mesh intermediate phase, with the (10) planes of the H 1 phase becoming the (001) planes of the mesh phase.
The concentration interval of 48 - 62 wt.% of the C 1 6E06 system has been studied using optical microscopy, 2H nuclear magnetic resonance, and small angle scattering in order to establish the detailed phase behaviour. On cooling from the
lamellar phase, the phase sequence observed is defected lamellar, mesh intermediate, Ia3d cubic, and hexagonal plus gel. On heating from the hexagonal plus gel phase region, the intermediate and cubic phase observed upon cooling is replaced by an Ia3d cubic phase region. The mesh intermediate forms when the water filled defects of the higher temperature defected lamellar phase correlate between the surfactant layers. The C16E06 system has been perturbed by the inclusion of either the shorter chain surfactant hexa-ethylene glycol mono n-dodecyl ether (C12EO6) or the oil octadecane (C 1 81-138). The addition of C12E05 promotes aggregate structures with more curved interfaces, and the intermediate mesh phase is eliminated in favour of the Ia3d cubic phase. The addition of C181-138 swells the surfactant bilayers and promotes flatter
interfaces, but does not stabilise the intermediate phase, as only small additions of oil suppress this phase.