Determination of interfacial tension and contact angle of dense non-aqueous phase liquid waste mixtures.

Abstract

Groundwater supplies are threatened by the pervasiveness of dense non-aqueous phase liquid wastes in the subsurface. Most studies treat these wastes as pure solvents, rather than as the mixtures they actually are. To predict the distribution and migration of these wastes in the subsurface, the waste's interfacial properties must be understood. The interfacial tension and contact angle on quartz were studied in tetrachloroethylene (PCE)/water systems containing selected combinations of octanoic acid (OA), dodecylamine (DDA), Aerosol OT (AOT), and alkyl polyoxyethylene glycols (C₁₂E₆, C₁₂E₉, and C₁₄E₆). In the OA/DDA mixtures, the interfacial tension was lower than simple addition prediction and the contact angle was increased around neutral pH despite OA itself does not change contact angle. Speciation distribution modeling suggested that the protonated DDA and deprotonated OA formed a neutral complex, whose formation as a function of pH was almost in phase with the synergistic surface activity. This suggested that this complex could be responsible for the positive synergistic interfacial behaviors. In the mixed AOT/C₁₂E_x and mixed nonionic surfactant systems, interfacial tension was higher than the prediction based on simple addition. This negative synergism was attributed to the steric repulsion and repulsive interaction between the hydrophobic tails. The contact angle increases with increasing C₁₂E₆ concentration and decreasing aqueous pH and showed a negative synergism upon the addition of AOT, which by itself does not change wettability. The addition of a second nonionic surfactant either had little impact or increased the contact angle. These results can be explained by the variation of surfactant adsorption on quartz and at the PCE/water interface. Different values of contact angle associated with the order of exposure were observed in the nonionic surfactant system and were attributed to irreversible adsorption on quartz. Characterization of the interfacial properties of two real wastes showed that the dry cleaning PCE waste behaved like the model system containing nonionic surfactants and the degreasing TCE waste behaved like the system containing anionic surfactants, a supposition which was substantiated by analysis of the chemical composition of the two wastes.