Alkyl polyglucoside microemulsion phase behavior

We review several key elements of alkyl polyglucoside (C_mG_n) microemulsion phase behavior. The low solubility of C_mG_n surfactants in oils such as alkanes makes producing C_mG_n microemulsions and subsequent study of their phase behavior difficult. Increasing the solubility of C_mG_n, in oil is therefore helpful for the systematic study of C_mG_n-based microemulsion formulations. To this end, the role of cosurfactants in producing microemulsions with water, alkanes, and n-alkyl β-D-glucopyranosides is first discussed. Adding C₁₀βG₁ to mixtures of water-alkane-ethoxylated alcohol surfactants (C_iE_j) produces a region of the three-phase body (a `chimney') that is independent of temperature; thus C_mβG₁ are not completely soluble in the co-oil formed of alkane and C_iE_j at higher temperatures. Then, through a novel approach using oxygenated ether oils (C_kOC₂OC_k), microemulsions are formed with water, C_kOC₂OC_k, and C_mβG₁ and the phase behavior studied as a function of temperature and composition. Increased C_mβG₁ solubility in the more hydrophilic ether oils produces patterns of phase behavior in water-C_kOC₂OC_k-C_mβG₁ mixtures that are identical to those observed in water-alkane-C_iE_j mixtures. Using the water-ether oil-C_mβG₁ mixtures as a base case, the role of C_mG_n surfactant structure in setting C_mG_n microemulsion phase behavior is explored. The solubility of the α-D anomer (n-alkyl α-D-glucopyranosides, C_mαG₁) in water is much less than that of the β-D surfactant, and these solubility boundaries extend to high surfactant and oil concentrations in water-C_kOC₂OC_k-C_mαG₁ mixtures. Adding C_mG₂ compounds to water-C_kOC₂OC_k-C_mβG₁ mixtures shifts the phase behavior to high temperatures, again demonstrating the extreme hydrophilic nature of the sugar headgroup. Finally, adding small amounts of ionic alkyl sulfate surfactants to water-C_kOC₂OC_k-C_mβG₁ mixtures dramatically reduces the total amount of surfactant needed to form a single-phase microemulsion.