Understanding solute uptake into soft micro-structured materials, such as bilayers and micelles, is of specific interest in the pharmaceutical, agricultural, and personal care industries. Currently, there are continuum-solvation methods available that can rapidly estimate the amount of a solute that will load into these soft micro-structured materials. However, these methods do not give any insight into the structural orientation and positioning of the solutes at a molecular level. To obtain this insight, we utilize the Shinoda-Devane-Klein (SDK) force field in conjunction with configurational-bias Monte Carlo simulations in the osmotic Gibbs ensemble. One simulation box contains a lamellar system comprised of a bilayer formed by triethylene glycol mono-n-decyl ether (C10E3) surfactants surrounded by water with a 50/50 surfactant/water weight ratio. This simulation cell is allowed to undergo volume and shape fluctuation, by changing either the z cell length independently or the x and y cell lengths in a coupled fashion and is found to have a stable lamellar phase in agreement with experiment. The other simulation box is a vapor-phase reservoir containing solute molecules at a constant applied pressure, and only solute molecules are allowed to transfer between the two simulation boxes. We study both the adsorption isotherm and structural effects of n-nonane, 1-hexanol, and ethyl butyrate on the bilayer at several different reduced pressures with respect to the saturated vapor pressure of the solutes. We observe that both ethyl butyrate and 1-hexanol load closer to the water and hydrophilic end of the surfactant, while n-nonane loads closer to the center of the bilayer, towards the hydrophobic end of the surfactant. These differences in the spatial distribution of the additive molecules lead to distinct changes in the structure of the bilayer. Our methods can be extended to study the binary and ternary loading of n-nonane, 1-hexanol, and ethyl butyrate into a C10E3 bilayer.

Gibbs ensemble Monte Carlo simulations for additive loading in surfactant bilayers

255th National Meeting (2018)

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