Effect of spacer length on the micellization of cationic trisiloxane surfactants

Abstract

Siloxane surfactants, hydrophobic (poly)siloxane chain and hydrophilic polar headgroup linking with spacer ((CH2)3 or (CH2)3S(CH2)3), exhibit peculiarly chemical and physical properties. The aggregation behavior and surface properties of siloxane surfactant can be influenced by the hydrophobicity of spacer. Cationic trisiloxane surfactants ([Si3-C3-Min]Br, [Si3-C4-Min]Br, and [Si3-C5-Min]Br) with the same imidazolium group, hydrophobic heptamethyltrisiloxane, and spacer (different number of methylene) were synthesized. The effect of the spacer on micellization in aqueous solution was investigated. Compared with 1-methyl-3- trisiloxaneimidazolium chloride ([Si3Min]Cl), critical micelle concentration (CMC), effectiveness (πCMC), and minimum area occupied by a surfactant molecule (Αmin) values of the investigated surfactants decrease with increase the number of methylene in spacer, while, γCMC value increases as the length of spacer increase. The micellization process was enthalpy driven. The ΔHm0 value becomes more exothermic for [Si3-C5-Min]Br attributed to the increasing hydrophobicity of spacer. The ΔHm0 of [Si3-C3-Min]Br is less exothermic than that of [Si3-C4-Min]Br and [Si3-C5-Min]Br. Spherical aggregates with the range of 20–500 nm were formed in aqueous solution at 2 times CMC.