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

Extensive all-atom molecular dynamics calculations on the water-squalane interface for nine different loadings with sorbitan monooleate (SPAN80), at T = 300K, are analyzed for the surface tension equation of state, desorption free energy proles 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. Specically, the surface tension equation of state is simple through the range of high tension to high loading studied, and the desorption free energy proles are weakly dependent on loading here. The perpendicular motion of the centroid of the SPAN80 head-group ring is well-described by a diusional 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 0.07s (into the squalane) and 300h (into the water). The latter value is consistent with irreversible absorption observed by related experimental work.