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Adsorption Dynamics of Aqueous 1-Octanol Solutions at the Vapor / Liquid Interface

Andrew Prpich1, Mohammad Elias Biswas2, and Pu Chen1. (1) Chemical Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada, (2) Chemical and Biomolecular Engineering, John Hopkins University, Baltimore, MD 21218

The adsorption dynamics of aqueous 1-octanol solutions were investigated through experimental measurement of dynamic surface tension by Axisymmetric Drop Shape Analysis-Profile (ADSA-P) of a pendant drop. Simultaneous adsorption from both sides of the vapor / liquid interface was considered by introducing aqueous 1-octanol solution to the surrounding environment, as well as the bulk droplet. The influence of the presence of a surfactant vapor phase on the resulting dynamic surface tension profiles was studied by varying the concentration of 1-octanol in both the droplet and environment solutions. It was determined that the direction of the dynamic surface tension profile (increase or decrease) is dependent on the direction of the concentration gradient between the bulk droplet solution and the surrounding environment solution. A modified adsorption isotherm was derived based on the classic Langmuir isotherm, accounting for adsorption from both sides of the vapor / liquid interface. The modified adsorption isotherm was combined with the Gibbs equation to create a new surface equation of state capable of predicting the initial and final surface tension as a function of the droplet solution and environment solution concentrations. The equilibrium parameters in the surface equation of state were determined for the 1-octanol system by performing non-linear regression on the model using experimental data from a central composite design (CCD). The dynamic surface tension profiles from select cases of the aqueous 1-octanol system were also measured using a modified Wilhelmy plate method for comparison with the results from ADSA-P. Furthermore, it was observed that the dynamic surface tension profiles of two other structurally similar components (1-butanol and 1-octylamine) demonstrated similar trends to that of the aqueous 1-octanol system.