Probing Additive Loading in the Lamellar Phase of a Nonionic Surfactant: Gibbs Ensemble Monte Carlo Simulations Using the SDK Force Field

Abstract

Understanding solute uptake into soft microstructured materials, such as bilayers and worm-like and spherical micelles, is of interest in the pharmaceutical, agricultural, and personal care industries. To obtain molecular-level insight on the effects of solutes loading into a lamellar phase, we utilize the Shinoda-Devane-Klein (SDK) coarse-grained force field in conjunction with configurational-bias Monte Carlo simulations in the osmotic Gibbs ensemble. The lamellar phase is 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. We study both the unary adsorption isotherm and the effects on bilayer structure and stability caused by n-nonane, 1-hexanol, and ethyl butyrate at several different reduced reservoir pressures. The nonpolar n-nonane molecules load near the center of the bilayer. In contrast, the polar 1-hexanol and ethyl butyrate molecules both load with their polar bead close to the surfactant head groups. Near the center of the bilayer, none of the solute molecules exhibits a significant orientational preference. Solute molecules adsorbed near the polar groups of the surfactant chains show a preference for orientations perpendicular to the interface, and this alignment with the long axis of the surfactant molecules is most pronounced for 1-hexanol. Loading of n-nonane leads to an increase of the bilayer thickness, but does not affect the surface area per surfactant. Loading of polar additives leads to both lateral and transverse swelling. The reduced Henry's law constants of adsorption (expressed as a molar ratio of additive to surfactant per reduced pressure) are 0.23, 1.4, and 14 for n-nonane, 1-hexanol, and ethyl butyrate, respectively, and it appears that the SDK force field significantly overestimates the ethyl butyrate-surfactant interactions.

Challenge to Reconcile Experimental Micellar Properties of the CnEm Nonionic Surfactant Family

“…Although attempts − have been made with varying degrees of success, all atom simulations are currently too expensive for a thorough investigation of the micellar behavior. Some of the computational demand can be addressed by the use of coarse-grained modeling approaches such as with united atom or beaded string models such as used with dissipative particle dynamics. − These approaches employ models with reduced complexity in the number of particles and smoother interaction potentials, both of which allow larger dynamical time step sizes in the simulations. The reduced number of particles per molecule allows much larger simulations to be run.…”

Nonane and Hexanol Adsorption in the Lamellar Phase of a Nonionic Surfactant: Molecular Simulations and Comparison to Ideal Adsorbed Solution Theory

“…However, there is a loss in accuracy compared to united-atom or all-atom force fields. Although the saturated vapor pressure at T = 300 K of C9 for the SDK model ( p C9 0 = 0.609 ± 0.003 kPa) is very accurate with a deviation of less than 2%, the saturated vapor pressure of C6OH for the SDK model ( p C6OH 0 = 3.04 ± 0.01 kPa) is overpredicted by a factor of 22 . That is, judged solely by the vapor pressure, the SDK model for C6OH behaves like 1-propanol (for which the deviation would be less than 5%) .…”

“…To obtain the binary loading isotherms, simulations were run in the N W N C10E3 N C9 N C6OH p C9 p C6OH p meso T osmotic Gibbs ensemble with three simulation boxes, where the subscript “meso” refers to the surfactant/water mesophase, at T = 300 K, p meso = 101 kPa, and 25 combinations of p C9 and p C6OH , the pressures for the C9 and C6OH reservoirs. Based on differences in the unary loadings, p C9 ranged from 0.04 to 0.64 p C9 0 and p C6OH ranged from 0.02 to 0.32 p C6OH 0 . Since the compressibility factors of the saturated vapor phases are near unity ( Z C9 > Z C6OH > 0.997), intermolecular interactions were turned off in the two reservoir boxes.…”

“…Based on differences in the unary loadings, p C9 ranged from 0.04 to 0.64 p C9 0 and p C6OH ranged from 0.02 to 0.32 p C6OH 0 . Since the compressibility factors of the saturated vapor phases are near unity ( Z C9 > Z C6OH > 0.997), intermolecular interactions were turned off in the two reservoir boxes. Molecules in all three boxes were allowed to translate and, with the exception of the one-site water model, rotate and undergo conformational changes.…”

“…In previous work, we utilized Gibbs ensemble Monte Carlo (GEMC) simulations to examine the solute loading and spatial distribution separately for n -nonane (C9) and 1-hexanol (C6OH) additives into a 50/50 w/w triethylene glycol mono- n -decyl ether (C10E3)/water system, forming a lamellar mesophase with a surfactant bilayer . An important finding of our prior simulation study is the different spatial loading preferences for the C9 and C6OH molecules.…”