Impact of Multi-Interface Surfactant Adsorption on Wettability in Dense Nonaqueous Phase Liquid Systems

Abstract

The wettability, and hence the distribution and transport, of dense nonaqueous phase liquid (DNAPL) wastes in the subsurface is strongly affected by surfactants that are present in the wastes. To understand their impact, this work examined the dependence of the contact angle of quartz/tetrachloroethylene (PCE)/water systems containing the anionic surfactant Aerosol OT (AOT) or/and the nonionic surfactant hexaoxyethylene glycol mono-n-dodecyl ether (C₁₂E₆) on the surfactants' adsorption at both the PCE/water and silica/water interfaces. Results showed that C₁₂E₆ enhanced oil wetness at low pH. However, the system reverted to water-wet upon the addition of AOT, which by itself, did not alter the wettability of the system. To mechanistically explain such behavior, it was proposed that, based on measurements of adsorption onto quartz and calculations of surface excess using an equation-of-state approach, the increased oil-wetness in systems containing C₁₂E₆ is due to the entropy-driven attraction between the oxyethylene groups of the adsorbed C₁₂E₆ at both the PCE/water and water/silica interfaces. Addition of AOT reduced the adsorbed C₁₂E₆ at both interfaces, lowering the attraction between PCE and silica. Thus, a consideration of surfactant adsorption at both interfaces, and the subsequent interaction between the molecules sorbed at both interfaces, is critical to explaining the wettability behavior of waste DNAPLs.