Dynamic Determination of Vapor/Water Interface Adsorption for Volatile Hydrophobic Organic Compounds (VHOCs) Using Axisymmetric Drop Shape Analysis: Procedure and Analysis of Benzene Adsorption

Abstract

A rapid and accurate method has been developed to characterize equilibrium aqueous solution surface tension variations due to adsorption of volatile hydrophobic organic compounds (VHOCs) at the vapor/water interface. The procedure involves contacting an initially solute-free pendant water drop with an advecting air stream containing water vapor and slowly increasing VHOC vapor concentration at atmospheric pressure. Changes in aqueous surface tension, induced by interfacial solute adsorption, are measured by Axisymmetric Drop Shape Analysis-Profile (ADSA-P) with digital image processing. Bulk vapor-phase solute pressures (concentrations) are concurrently quantified using sample loop gas chromatograph injection. “Rapid” equilibrium between vapor-phase and interface-phase solute concentrations allows correlation of transient outputs, producing aqueous surface tension versus vapor-phase solute pressure isothermal relationships. Equilibrium aqueous solution surface tension/vapor-phase solute pressure isotherms for the test solute benzene were measured in duplicate at 285.2, 291.2, 298.2, 303.2, and 315.2 K. Isotherms were interpolated with a mathematical form combining a nonideal two-dimensional equation of state and the Gibbs relative interface excess equation to estimate interfacial adsorption parameters. Comparison of limiting vapor-phase/interface-phase distribution coefficients and corresponding thermodynamic calculations to previously published values shows good agreement. Additional sensitivity analysis, including variations in flow rate and method of solute contact, confirm the general applicability of the presented method for measuring volatile solute vapor/water interface adsorption isotherms.