(631a) Molecular Theory for the Hydrophobic Interactions and Absorption of Span80 at a Squalane-Water Interface

Molecular theory for the Hydrophobic Interactions and Absorption of Span80 at a Squalane-Water Interface

M. I. Chaudhari, S. B. Rempe, D. Asthagiri, L. Tan, L. R. Pratt
　

Abstract:
The principal result of LMF theory is outlined, then tested by obtaining radial distribution functions (rdfs) for Ar atoms in water, with and without attractive interactions distinguished by the Weeks-Chandler-Andersen (WCA) separation.
Change from purely repulsive atomic solute interactions to include realistic attractive interactions diminishes the strength of Ar-Ar hydrophobic bonds.
Since attractions make a big contribution to hydrophobic interactions, Pratt-Chandler theory, which did not include attractions, should not be simply comparable to computer simulation results with general physical interactions, including attractions.
The Ar-Ar osmotic second virial coefficients become more attractive with increasing temperature below T = 360K.
Ultimately, LMF theory does not accurately describe the numerical results for the effects of solute attractive forces on hydrophobic interactions in this case.
We extend this work to investigate the adsorption/desorption behavior of surfactant at the oil/water interface.
According to recent experimental work, TWEEN80 shows an irreversible adsorption behavior at a squalane/water interface when the surface tension is above 32mN/m and an alternative partial reversibility when the surface tension is below this critical value.
We use molecular dynamics to study of the squalane/water/surfactant interface, substituting SPAN80 for TWEEN80 as an initial step for simplicity. We apply parallel tempering calculations to obtain the surface tension of this interface.
The temperatures for the parallel tempering simulations range from 260K to 450K in order to achieve an exchange probability between neighboring temperatures around 20%.
We then further exploit a windows sampling strategy to study the free energy variation for desorption of a SPAN80 molecule from the interface at 300K.
Preliminary pulling results suggest that the observed irreversibility of the adsorption/desorption is due to kinetic sluggishness of the SPAN80 tail motion in the squalane phase.