Alkyl polyglucoside microemulsion phase behavior

We review several key elements of alkyl polyglucoside (CmGn) microemulsion phase behavior. The low solubility of CmGn surfactants in oils such as alkanes makes producing CmGn microemulsions and subsequent study of their phase behavior difficult. Increasing the solubility of CmGn in oil is therefore helpful for the systematic study of CmGn-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 (CiEj) produces a region of the three-phase body (a ‘chimney’) that is independent of temperature; thus CmβG1 are not completely soluble in the co-oil formed of alkane and CiEj at higher temperatures. Then, through a novel approach using oxygenated ether oils (CkOC2OCk), microemulsions are formed with water, CkOC2OCk, and CmβG1 and the phase behavior studied as a function of temperature and composition. Increased CmβG1 solubility in the more hydrophilic ether oils produces patterns of phase behavior in water–CkOC2OCk–CmβG1 mixtures that are identical to those observed in water–alkane–CiEj mixtures. Using the water–ether oil–CmβG1 mixtures as a base case, the role of CmGn surfactant structure in setting CmGn microemulsion phase behavior is explored. The solubility of the α-d anomer (n-alkyl α-d-glucopyranosides, CmαG1) 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–CkOC2OCk–CmαG1 mixtures. Adding CmG2 compounds to water–CkOC2OCk–CmβG1 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–CkOC2OCk–CmβG1 mixtures dramatically reduces the total amount of surfactant needed to form a single-phase microemulsion.