Electron spin-echo studies of doxylstearic acid spin probes in microemulsions

Abstract

We have probed the molecular interactions in the surfactant–cosurfactant sheath of frozen water-in-oil (w/o), oil-in-water (o/w) and middle-phase microemulsions composed of water, sodium dodecyl sulphate (SDS), butan-l-ol, toluene and NaCl by performing two-pulse electron spin-echo modulation (e.s.e.m.) analyses for a series of x-doxylstearic acid spin probes with x= 5, 7, 10, 12 and 16. The microemulsions were prepared with deuterium labels in D2O, [2H10]butan-l-ol or [2H8]toluene. The data correlate well with the now accepted interface structure consisting of SDS and butanol molecules separating the water-rich and oil-rich domains. The surfactant sheath is free of water molecules but contains alcohol molecules in the head-group region and substantial amounts of toluene in most of the alkyl-tail region. The differences observed between the o/w, w/o and middle phases can be ascribed to changes in the lipophilic character of the systems.