Three-component microemulsions formed using pH-degradable 1,3-dioxolane alkyl ethoxylate surfactants

A series of three nonionic surfactants which undergo acid-catalyzed hydrolysis, O-[(2,2-dialkyl-1,3-dioxolan-4-yl) methoxy] –O′-methoxy poly(ethylene glycol), or cyclic ketal (CK), were synthesized. The surfactants shared similar HLB values (9.2–9.8) but differed in the relative length of their two alkyl tails. Water/CK/isooctane microemulsion systems formed by each of the three surfactants also shared similar phase behavior, physical properties, and surfactant hydrolysis kinetics. The apparent critical microemulsion concentration for three CK surfactants, 0.060–0.075 g g−1 (in both water and isooctane), and the efficiency, the minimum amount of surfactant required to totally mix water and oil into single microemulsion phase, 0.35–0.40 g g−1, are significantly higher than values possessed by regular, linear, alkyl ethoxylate surfactants. The underlying causes for the high values were investigated. Due to the broad distribution of its ethoxylate head group size, CK surfactants were distributed among all phases within two- and three-phases systems, with CKs possessing shorter ethoxylate groups partitioning to the oil phase and CKs with larger ethoxylate groups to the aqueous phase. Small-angle neutron scattering experiments suggest that CK exists as monomers or very small pre-micellar aggregates in isooctane at 12% and as ellipsoidal aggregates in water at 2.5–25%. Surface pressure versus surface area measurements of CK surfactant monolayers at the water-air interface demonstrate two differences from linear alkyl ethoxylate monolayers: the occurrence of a collapse at moderately low surface pressure (30 mN m−1) and high surface area per molecule (0.59 nm2) and the inability to undergo dissolution when subjected to a constant, moderately high surface pressure (25 mN m−1). The results suggest that CK molecules are not able to pack as efficiently as linear alkyl ethoxylates at interfaces, and are strongly anchored to monolayers in aqueous media.