Adsorption of volatile hydrophobic organic compounds at the vapor/water interface

Abstract

Aqueous solution surface tension as a function of vapor-phase solute pressure isotheims were measured at atmospheric pressure for single and binary component benzene, methyl-substituted benzene (i.e., methylbenzene, 1,2-dimethylbenzene, 1,3- dimethylbenzene, 1,4-dimethylbenzene, 1,3,5-trimethylbenzene), and trichloroethene adsorption. Solute-induced surface tension variations were quantified using a dynamic adsorption protocol in conjunction with Axisymmetric Drop Shape Analysis-Profile (ADSA-P) applied to pendant drop tensiometry. For single component adsorption, isotherms were measured at temperatures of 285.2K, 291.2 K, 297.2 K, 298.2 K, 303.2 K, and 315.2 K, for vapor-phase solute pressures ranging from zero to near/at saturated vapor pressure. For binary solute experiments, three intermediate constant vapor-phase mole ratio isotherms were developed for each solute pair (i.e., benzene and each of the five methyl-substituted benzenes) at temperatures of 285.2 K, 291.2 K, 298.2 K. Results for single component adsorption studies indicate that for a given vapor-phase solute pressure, interface-phase solute activity increases with molecular size (mass) among the benzene homologues. Similarly, compounds are more strongly adsorbed at the vapor/water interface as the system temperature decreases. Ideal standard free energy, enthalpy, and entropy changes of adsorption, calculated from limiting isotherm data, suggest specific solute-water interactions and a perturbation of the interface-phase water structure on adsorption. Further analysis of binary solute isotherms indicates that interface-mixing is well described by a two-dimensional application of Raoult's law, implying ideal interface-phase solute-solute interactions. Consideration of the twodimensional second virial coefficients suggests that interface-phase solute molecules engage in attractive interactions, with greater interactions for larger molecular sizes.