Physicochemical Studies on Surfactant Aggregation 1. Effect of Polyethylene Glycols on the Micellization of SDS

Abstract

Interfacial adsorption and micellization of sodium dodecylsulfate (SDS) in aqueous liquid and solid polyethylene glycols (PEG) of varying chain lengths was studied by surface tension, conductance, and spectrophotometric methods. Both groups of PEGs enhanced the critical micelle concentration (CMC) of SDS. Fraction of counter ion binding (β), determined from conductance data, showed PEG chain length dependence. Surface excess (Γ max), minimum area per surfactant molecule (Amin) and free energy change of adsorption () were evaluated from surface tension data, while the free energy change of micellization () were determined from the CMC values. Effects of liquid and solid PEGs were similar, although slight differences were observed individually. In case of solid PEGs, in addition to CMC another premicellization/preaggregation process was observed. β, Γ max, and values decreased while Amin value increased with the with the increase in PEG concentration in water. Liquid PEGs behaved like cosolvent while the solid PEGs behaved like polymer, whereby initially polymer-surfactant aggregate were formed followed by the enhanced micellization. Results suggested that addition of PEG lowered the dielectric constant/polarity of the medium, which effectively stabilized the surfactant molecules in the bulk, and hence changed the micellization parameters.