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_kOC₂OC_k)-n-alkyl polyglucoside (C_mG_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_kOC₂OC_k-C₈βG ₁-NaClO₄ mixtures the solution microstructure dissolves. With increasing alkyl chain length of the surfactant (m), water-C_kOC₂OC_k-C_mG_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_iE_j mixtures. Exchanging the α-D (C_mαG₁) for β-D (C_mβG₁) form reduces the degree of correlation between the water and oil domains in water-C_kOC₂OC_k-C_mG₁ 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_iE_j mixtures. Analysis of the Porod regime of the scattering spectra of several water-C_kOC₂OC_k-C_mG₁ mixtures shows that the average headgroup area of the C_mG₁ 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_mG₁ molecules.