Adsorption of benzene and methyl-substituted benzenes at the vapor/water interface. 2. Single-component VHOC adsorption

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, interface-phase 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 x 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.