Effect of alkyl sulfates on the phase behavior and microstructure of alkyl polyglucoside microemulsions

The effects of adding alkyl sulfate surfactants to water-n-alkyl β-D-glucopyranoside (C_mβG₁) and water-alkyl ethylene glycol ether (C_kOC₂OC_k)-C_mβG₁ mixtures are systematically explored. In a water-C₁₀βG₁-sodium decyl sulfate (SDeS) mixture, miniscule amounts of SDeS (SDeS:C₁₀βG₁ molar ratio of 0.0025) cause the upper miscibility gap of the water-C₁₀βG₁ mixture to vanish. Adding small amounts of alkyl sulfates to water-C_kOC₂OC_k-C_mβG₁ mixtures increases the surfactant efficiency, shifts the single-phase microemulsion region to higher temperatures, and shrinks the three-phase region. These phenomenological phase behavior observations are explained in terms of electrostatic effects introduced by the addition of an ionic surfactant to nonionic micelles and monolayers. Small-angle neutron scattering (SANS) measurements from several D₂O-C_kOC₂OC_k-C _mβG₁-alkyl sulfate mixtures containing equal amounts of D₂O and C_kOC₂-OC_k are analyzed using a model for bicontinuous microemulsions. The values obtained from this analysis show that the monolayer spacing of D₂O-C_kOC₂OC_k-C _mβG₁-alkyl sulfate mixtures grows with increasing ionic surfactant concentration, and this increased spacing accounts for the observed increase in surfactant mixture efficiency.