Free-energy analysis of solubilization in micelle

Abstract

A statistical-mechanical treatment of the solubilization in micelle is presented in combination with molecular simulation. The micellar solution is viewed as an inhomogeneous and partially finite, mixed solvent system, and the method of energy representation is employed to evaluate the free-energy change for insertion of a solute into the micelle inside with a realistic set of potential functions. Methane, benzene, and ethylbenzene are adopted as model hydrophobic solutes to analyze the solubilization in sodium dodecyl sulfate micelle. It is shown that these solutes are more favorably located within the micelle than in bulk water and that the affinity to the micelle inside is stronger for benzene and ethylbenzene than for methane. The micellar system is then divided into the hydrophobic core, the head-group region in contact with water, and the aqueous region outside the micelle to assess the relative importance of each region in the solubilization. In support of the pseudophase model, the aqueous region is found to be unimportant to determine the extent of solubilization. The contribution from the hydrophobic-core region is shown to be dominant for benzene and ethylbenzene, while an appreciable contribution from the head-group region is observed for methane. The methodology presented is not restricted to the binding of a molecule to micelle, and will be useful in treating the binding to such nanoscale structures as protein and membrane.

Free energy profiles for penetration of methane and water molecules into spherical sodium dodecyl sulfate micelles obtained using the thermodynamic integration method combined with molecular dynamics calculations

“…The ρ(r) obtained in the present study is in very good agreement with the results of other studies. 16,42,43 For instance, the peak position of the sulfate ion of the SDS micelle is almost the same as that calculated by Matubayasi et al 16 Furthermore, as in the previous studies, a low-density region is observed at the center of the SDS micelle in the present work. These results indicate that our calculations have been performed correctly.…”

“…In our previous work, the free energy of transfer, G, of methane from bulk water to the SDS micelle core rather than the free energy profile G(r) was calculated using the thermodynamic integration method with respect to the potential parameters. The free energy profile G(r) for methane solubilization was also evaluated approximately by Matubayasi et al 16 using distribution function theory combined with MD calculations. The results are also plotted in Fig.…”

“…However, few theoretical studies of solubilization phenomena starting from intermolecular interactions and statistical mechanics have been performed. To our knowledge, other than our own previous studies, a theoretical study of solubilization has been performed only by Matubayasi et al, 16 based on distribution function theory combined with MD calculations.…”

Solvation Thermodynamics of Organic Molecules by the Molecular Integral Equation Theory: Approaching Chemical Accuracy

“…The energy-representation method provides a universal computational and theoretical platform for calculation of thermodynamic parameters of solvation and has been already applied to a range of different systems: supercritical fluids, air-water interfaces, micelles, lipid membranes, proteins, and QM/MM systems. 129,[374][375][376][377][378][379][380][381][382][383]385 The approximation used in the current version of energy-representation method is simply PY-type and HNC-type ones over the energy coordinate, however, it is considered a new class of bridge functional since the coordi-nate (abscissa) of distribution function has been switched from distance to energy. The connection between spatial coordinate-based and energy-based approaches can be an interesting topic of investigation.…”

A molecular dynamics study of structure and dynamics of surfactant molecules in SDS spherical micelle

“…a variety of structure formations of the surfactant molecules in water [1][2][3][4]. Recently, several molecular dynamics (MD) calculations have been done for spherical micelles formed in water, which leads the researcher's interest to the structure and dynamics at a molecular level [5][6][7][8][9][10][11][12].…”