Density, orientation, solvation and adsorption effects on surface tension of mixtures

Abstract

The present article offers a novel analysis tool that potentially can be used to describe and characterize the process of interfacial adsorption of both simple molecules and complex molecules (surfactants, biomolecules, etc). This analysis tool was developed for evaluating the influence of the density, orientation, solvation and adsorption effects on the surface tension of mixtures. We propose a procedure that consists in split the excess surface tension function into three additive contributions. That is, a residual function (R) based on the concept of elastic energy density in which both the interfacial density and the axial orientation are taken into account, an enthalpic function (H) considering the solvation effects, and a contribution that includes the preferential adsorption (AD) effects. This approach is referred in this work as RHAD. The AD contribution was assessed using the Langmuir isotherm model. In order to evaluate the reliability of the RHAD method, simple molecular approaches describing each contribution were chosen. Some experimental systems were employed as binary mixtures models for the evaluation of the descriptive capability of RHAD. The experimental data for both complexes and simple binary mixtures were successfully reproduced. Thus, this methodology allowed us to quantitatively and qualitatively incorporate the influence of molecular parameters on the experimentally observed behavior of this important physical property. Also, the results shown that when one of the contributions domains over the others, the overall behavior of the surface tension of the binary mixture is mainly led by the difference between the surface tensions (π0) and the ratio of the molar volumes (V2/V1) of their pure components. Meaningful deviations from ideal behavior and the occurrence of aneotropy can be further interpreted employing RHAD. Finally, our results suggest that the adsorption process is better understood by considering the interfacial width.