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Aqueous solution surface tension (pressure) as a function of vapor-phase trichloroethene (TCE) pressure isotherms were measured at atmospheric pressure at 287.2, 291.2, 297.2, 303.2, and 315.2 K using a flow-through vapor adsorption method. Solute (i.e., TCE)-induced surface tension variations were quantified using Axisymmetric Drop Shape Analysis-Profile (ADSA-P); vapor-phase TCE pressures were measured using automated gas chromatograph analysis. Surface tension reductions of 5-10% from neat water at saturated TCE vapor pressure were noted. Measured surface tension (pressure) isotherms were used to estimate vapor/water interface adsorption as a function of vapor-phase TCE pressure using the Gibbs relative interface excess (i.e., adsorption) equation and a nonideal two-dimensional equation of state. Complete isothermal adsorption profiles were nonlinear, with accelerated adsorption at increasing vapor-phase TCE pressures. Comparison to other studies of adsorption at infinite dilution (i.e., linear partitioning) and corresponding to thermodynamics (i.e., ideal equilibrium standard molar free energy, enthalpy, and entropy change) indicate good agreement. Estimates of TCE planar surface area were used to calculate the fraction of monolayer coverage at the vapor/water interface as a function of vapor-phase TCE pressure, which approached a maximum of 0.6-0.75 at saturated vapor pressure.

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Equilibrium isothermal relationships between aqueous solution surface tension and vapor-phase solute pressure of individual volatile hydrophobic organic compounds (VHOCs) (benzene, methylbenzene, 1,2-dimethylbenzene, 1,3-dimethylbenzene, 1,4-dimethylbenzene, and 1,3,5-trimethylbenzene) were measured employing a dynamic adsorption method, axisymmetric drop shape analysis-profile (ADSA-P), and gas chromatographic analysis. Systems were analyzed at atmospheric pressure in duplicate at 285.2, 291.2, 297.2, 303.2, and 315.2 K, for vapor-phase VHOC pressures up to saturated vapor pressure. Developed isotherms were fit with a mathematical form combining a nonideal two-dimensional equation of state and the Gibbs relative interface excess equation. Derived functions were used to quantify vapor/water relative interface solute excess, interfacephase activity coefficients, and ideal standard molar Gibbs free energy, enthalpy, and entropy changes of adsorption. Results indicate that molecular size dictates adsorption among the compounds considered, with generally increased interface excess and isotherm nonideality as molecular size increased. Calculated values for ideal free energy change of adsorption (at infinite dilution) were linear as a function of temperature and evenly spaced as a function of methyl substitution (approximately 2-3 × 10 3 J mol -1 more negative for each methyl substitution). Calculated values for enthalpy change were less negative than the corresponding values of liquefaction, suggesting specific interactions between solutes and interface-phase water molecules. Entropy changes, more negative than predicted for the loss of one translational degree of freedom, further support this conclusion.

A series of gas (vapor)-advecting water-unsaturated column experiments using a low organic content (f(oc)) silica sand was conducted to determine mass distributions of chlorinated-volatile hydrophobic organic compounds (C-VHOCs) in a natural sorbent system. C-VHOCs used were trichloroethene (TCE), tetrachloroethene (PCE), chlorobenzene (CB), and 1,3-dichlorobenzene (DCB). Four volumetric water contents (theta(w) = 0.07, 0.12, 0.17, 0.20) and several influent gas-phase C-VHOC (solute) concentrations were considered. The method of temporal first moments was applied to complete breakthrough curve data to determine total C-VHOC gas-phase retardation and associated gas-phase C-VHOC mass fraction. Results were compared to an equilibrium partitioning advective-dispersive formulation of total gas-phase retardation. Literature-derived values of Henry's law constants and independent measurements of gas/water interface areal extent and interface phase adsorption allowed quantification of C-VHOC mass fractions in the aqueous and gas/water interface phases. Unaccounted C-VHOC mass, derived from comparison of measured C-VHOC retardation to independent phase prediction, was attributed to solid-phase sorption. Results indicate that for all conditions tested, gas/water interfacial adsorption exhibited only a small effect on C-VHOC vapor retardation (accounting for < or = 10% of the total C-VHOC distributions). Solid-phase association was the dominant uptake mechanism, accounting for 46-91% of the total C-VHOC mass in the porous system. Evaluation of the solid-phase C-VHOC uptake results in terms of a modified form of the Dubinin-Radushkevich (DR) isotherm equation provided strong evidence supporting the mechanism of pore-filling in this natural, low f(oc) sorbent.