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 m βG 1 ) and water−alkyl ethylene glycol ether (C k OC 2 OC k )−C m βG 1 mixtures are systematically explored. In a water−C 10 βG 1 −sodium decyl sulfate (SDeS) mixture, miniscule amounts of SDeS (SDeS:C 10 βG 1 molar ratio of 0.0025) cause the upper miscibility gap of the water−C 10 βG 1 mixture to vanish. Adding small amounts of alkyl sulfates to water−C k OC 2 OC k −C m βG 1 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 D 2 O−C k OC 2 OC k −C m βG 1 −alkyl sulfate mixtures containing equal amounts of D 2 O and C k OC 2 OC k are analyzed using a model for bicontinuous microemulsions. The values obtained from this analysis show that the monolayer spacing of D 2 O−C k OC 2 OC k −C m βG 1 −alkyl sulfate mixtures grows with increasing ionic surfactant concentration, and this increased spacing accounts for the observed increase in surfactant mixture efficiency.