Surface and Micellar Properties of a Long-Chain Nonionic Surfactant

Abstract

In order to study the surface-chemical behavior of a highly polyoxyethylated surfactant without the masking influence of species of low degrees of polyoxyethylation, a dodecanol adduct with an average of 30 ethylene oxide molecules, C12(EO)30, was stripped of low molecular weight species, up to C12(EO)6 or C12(EO)10, by molecular distillation. The components of highest molecular weight and polyethylene glycols were removed by ultrafiltration, leaving a product of rather narrow molecular weight distribution with the average composition C12(EO)28. Surface tension-concentration relationships, measured at 15, 25, 40, and 55°, were used to derive values for the following parameters: critical micelle concentration (CMC), plateau surface tension above the CMC, surface excess and limiting area per surfactant molecule adsorbed at the water/air interface, and free energy, enthalpy, and entropy changes of micellization. Reduced hydration of the polyoxyethylene portion of the surfactant with increasing temperature lowered its hydrophilic-lipophilic balance. This resulted in lower CMC and plateau surface tension values, smaller area per molecule at the water/air interface, and greater negative free energy of micelle formation. The heat and entropy change of micelle formation were constant and positive between 15 and 55°. This indicates that the latter is responsible for the formation of micelles. The corresponding increase in randomness was attributed mainly to water surrounding the hydrocarbon chains of single surfactant molecules and trapped in their coiled polyoxyethylene chains, which loses its contact with the surfactant molecules and hence its structure when they ag-gregate into micelles. These properties were compared with those of a surfactant of nearly the same average degree of polyoxyethylation but containing a fraction of low degree of polyoxyethylation.