Surface and colloid chemistry of systems containing pure sugar surfactant

The effects of changes in temperature and electrolyte (NaCl) concentration on the distribution and aggregation of pure sugar surfactants (mainly decyl beta-D-glucoside) in hydrocarbon + water systems has been investigated. Distribution coefficients of surfactant monomers between water and oil have been obtained; the influence of NaCl on the distribution gives the salt effect on the standard chemical potential of the surfactant molecule in aqueous solution. The decyl chain is salted out (i.e. salt causes an increase in chemical potential of the chain), whereas it is found that the glucoside moiety is salted in, unlike ethyleneoxy groups on alkyl ethoxylates which are salted out by NaCl. In toluene + aqueous NaCl systems with decyl beta-glucoside present above its critical aggregation concentration (cmc), normal Winsor behavior is observed. The oil/water interfacial tension passes through a minimum at a temperature (the phase inversion temperature, PIT) around which emulsions invert from oil-in-water (at low temperature) to water-in-oil, in much the same way as for systems with ethoxylate surfactants. Three phases exist in equilibrium at temperatures close to the phase inversion condition. The PIT for the decyl beta-glucoside system with toluene is about 65 degrees C, and for dodecyl beta-glucoside with cyclohexane as oil it is 47 degrees C. Addition of NaCl lowers the PIT in both systems. It is shown how, from the effects of salt on molecularly dispersed surfactant molecules together with appropriate interfacial tensions, the effect of electrolyte on surfactant present in close-packed monolayers (both planar and curved, as in aggregates) can be obtained. The salt effect on headgroups is tentatively related to complexation of ions with carbohydrates in aqueous solution.