Aggregate-based sub-CMC solubilization of hexadecane by surfactants

Abstract

Solubilizaiton of hexadecane by two surfactants, SDBS and Triton X-100, at concentrations near the critical micelle concentration (CMC) and the related aggregation behavior was investigated in this study. Solubilization was observed at surfactant concentrations lower than CMC, and the apparent solubility of hexadecane increased linearly with surfactant concentration for both surfactants. The capacity of SDBS to solubilize hexadecane is stronger at concentrations below CMC than above CMC. In contrast, Triton X-100 shows no difference. The results of dynamic light scattering (DLS) and cryogenic TEM analysis show aggregate formation at surfactant concentrations lower than CMC. DLS-based size of the aggregates (d) decreases with increasing surfactant concentration. The zeta potential of the SDBS aggregates decreases with increasing SDBS concentration, whereas it increases for Triton X-100. The surface excess (Γ) of SDBS calculated based on hexadecane solubility and aggregate size data increases rapidly with increasing bulk concentration, and then asymptotically approaches the maximum surface excess (Γ_max). Conversely, there is only a minor increase in Γ for Triton X-100. Comparison of Γ and d indicates that an excess of surfactant molecules at the aggregate surface has a great impact on surface curvature. The results of this study demonstrate the formation of aggregates at surfactant concentrations below CMC for hexadecane solubilization, and indicate the potential of employing a low-concentration strategy for surfactant applications such as remediation of HOC contaminated sites.