Microstructure Properties of Alkyl Polyglucoside Microemulsions

Small-angle neutron scattering was used to characterize the length scales and interfacial properties of water−alkyl ethylene glycol ethers (C k OC 2 OC k )− n -alkyl polyglucoside (C m G n ) bicontinuous microemulsions systematically as a function of surfactant structure and oil hydrophobicity. Analysis of the scattering spectra in terms of a thermodynamic model shows that on approach to the tricritical point in water−C k OC 2 OC k −C 8 βG 1 −NaClO 4 mixtures the solution microstructure dissolves. With increasing alkyl chain length of the surfactant ( m ), water−C k OC 2 OC k −C m G n mixtures become increasingly structured. Conversely, increasing the surfactant headgroup length ( n ) makes the mixtures slightly less structured. The variations in solution microstructure with changes in m and n are not as large as those observed with changing surfactant structure in water−alkane−C i E j mixtures. Exchanging the α-D (C m αG 1 ) for β-D (C m βG 1 ) form reduces the degree of correlation between the water and oil domains in water−C k OC 2 OC k −C m G 1 mixtures. Increasing the alkyl chain length of the oil ( k ) produces increased structure in all mixtures studied, which is in contrast to the trend observed in water−alkane−C i E j mixtures. Analysis of the Porod regime of the scattering spectra of several water−C k OC 2 OC k −C m G 1 mixtures shows that the average headgroup area of the C m G 1 surfactants increases as the oil hydrophobicity ( k ) decreases. The increased surfactant headgroup area presumably results from increased oil solubility in water, which in turn allows for more contact between the oil and water at the interface and a larger distance between C m G 1 molecules.