Molecular-scale Description of SPAN80 Desorption from the Squalane-Water Interface

TL;DR

This study uses molecular dynamics to analyze SPAN80 desorption at the squalane-water interface, revealing weak loading dependence and providing estimates for escape times into bulk phases, consistent with experimental observations.

Contribution

It provides a detailed molecular-scale understanding of SPAN80 desorption, including free energy profiles and escape times, which were previously not well characterized.

Findings

Surface tension is weakly dependent on loading.

Desorption free energy profiles are consistent across loadings.

Escape time into water is approximately 300 hours.

Abstract

Extensive all-atom molecular dynamics calculations on the water-squalane interface for nine different loadings with sorbitan monooleate (SPAN80), at $T=300$K, are analyzed for the surface tension equation of state, desorption free energy profiles as they depend on loading, and to evaluate escape times for absorbed SPAN80 into the bulk phases. These results suggest that loading only weakly affects accommodation of a SPAN80 molecule by this squalane-water interface. Specifically, the surface tension equation of state is simple through the range of high tension to high loading studied, and the desorption free energy profiles are weakly dependent on loading here. The perpendicular motion of the centroid of the SPAN80 head-group ring is well-described by a diffusional model near the minimum of the desorption free energy profile. Lateral diffusional motion is weakly dependent on loading. Escape times evaluated on the basis of a diffusional model and the desorption free energies are $7\times 10^{-2}$ s (into the squalane) and $3\times 10^2$ h (into the water). The latter value is consistent with irreversible absorption observed by related experimental work.