Vapour–liquid equilibrium relationship between toluene and mixed surfactants

Abstract

Micellar partitioning of volatile organic compounds (VOCs) in surfactant solutions and its effects on vapour–liquid equilibrium is fundamental to the overall design and implementation of surfactant-enhanced remediation. Knowledge of the vapour–liquid equilibrium partitioning coefficients for VOCs, especially in contaminated soils and groundwater in which they exist, is required. Headspace experiments were performed to quantify the effect of three mixed surfactants, cetyltrimethyl ammonium bromide (CTMAB) with tetrabutylammonium bromide (TBAB), sodium dodecyl sulph ate (SDS) with Triton X-405 (TX405), and CTMAB with Triton X-100 (TX100), on the apparent Henry's constants (H c ) of toluene at temperatures ranging from 25 °C to 40 °C. The H c values were significantly reduced in the presence of all three mixed surfactants at concentrations above their critical micelle concentrations (CMC). Mixed micellar partitioning, showing effects on the vapour–liquid equilibrium of toluene, was primarily responsible for the significant reduction of H c in their mixed systems. The mixed surfactants CTMAB–TX100 had the greatest effect on H c above the CMC, followed by SDS–TX405, then CTMAB–TBAB. Mixed systems of CTMAB–TX100 decreased H c at concentrations significantly lower than the SDS–TX405 and CTMAB–TBAB concentrations, because of to the lower CMC of CTMAB–TX100. Vapour–liquid equilibrium data were also tested against the model (H c =H/(1+K(X−CMC)) that described the partitioning of VOCs in vapour–water–micelle phases. The correlation of H c with mixed surfactant concentrations (X) and CMC can be utilized as an effective tool to predict the H c by mixed surfactants.