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

Abstract

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 K. Fujimoto Science, Nagoya University, Japan (Received 14 October 2011; accepted 5 December 2011; published online 6 January 2012) The free energy profiles, G(r), for penetration of methane and water molecules into sodium dodecyl sulfate (SDS) micelles have been calculated as a function of distance r from the SDS micelle to the methane and water molecules, using the thermodynamic integration method combined with molecular dynamics calculations. The calculations showed that methane is about 6-12 kJ mol −1 more stable in the SDS micelle than in the water phase, and no G(r) barrier is observed in the vicinity of the sulfate ions of the SDS micelle, implying that methane is easily drawn into the SDS micelle. Based on analysis of the contributions from hydrophobic groups, sulfate ions, sodium ions, and solvent water to G(r), it is clear that methane in the SDS micelle is about 25 kJ mol −1 more stable than it is in the water phase because of the contribution from the solvent water itself. This can be understood by the hydrophobic effect. In contrast, methane is destabilized by 5-15 kJ mol −1 by the contribution from the hydrophobic groups of the SDS micelle because of the repulsive interactions between the methane and the crowded hydrophobic groups of the SDS. The large stabilizing effect of the solvent water is higher than the repulsion by the hydrophobic groups, driving methane to become solubilized into the SDS micelle. A good correlation was found between the distribution of cavities and the distribution of methane molecules in the micelle. The methane may move about in the SDS micelle by diffusing between cavities. In contrast, with respect to the water, G(r) has a large positive value of 24-35 kJ mol −1 , so water is not stabilized in the micelle. Analysis showed that the contributions change in complex ways as a function of r and cancel each other out. Reference calculations of the mean forces on a penetrating water molecule into a dodecane droplet clearly showed the same free energy behavior. The common feature is that water is less stable in the hydrophobic core than in the water phase because of the energetic disadvantage of breaking hydrogen bonds formed in the water phase. The difference between the behaviors of the SDS micelles and the dodecane droplets is found just at the interface; this is caused by the strong surface dipole moment formed by sulfate ions and sodium ions in the SDS micelles.

Molecular dynamics study of free energy of transfer of alcohol and amine from water phase to the micelle by thermodynamic integration method

“…In our previous works, [15][16][17][18] two extremes of solubilization of water and hydrocarbons were investigated. First, we performed molecular dynamics (MD) calculations and found that the free energy of transfer of a water molecule from bulk water to sodium dodecyl sulfate (SDS) micelle has a large positive value, +28 kJ mol −1 , and that the water molecule is seldom contained in the SDS micelle.…”

Effect of diffuseness of micelle boundary on the solute distribution upon solubilization

“…Figure 2 shows the potential of mean force W(R) as a function of the distance R of the solute center of mass from the micellar center. The hydrophobic solute is stabilized at the center of micelle compared to the aqueous region [1,7,11,12], and is more so when the restraint is stronger (k of Eq. (1) is larger).…”

“…Figure 3 shows the probability distribution function thus determined. When k = 0 and no restraint is applied to the micelle, the solute is most likely to stay in the outer portion of the hydrophobic-tail region (R ≈ 15Å) and distributes diffusely toward the center of micelle [7,11,12,28,29]. At k = 0, indeed, the potential of mean force presented in Figure 2 varies by ∼1 kcal/mol within the hydrophobic-tail region, and the probability in Figure 3 is larger in the outer portion due to the presence of the R 2 factor in Eq.…”

“…MD meets this necessity, and can determine the atomic distributions of water and of hydrophobic tail and hydrophilic headgroup of the surfactant under the used set of potential functions. When the micelle is small and spherical (on average), the headgroup occupies a diffuse region typically of severalÅ thickness [2][3][4][5][6][7][8][9][10][11][12]. The distribution of the headgroup at instantaneous (snapshot) configuration is not spherical, furthermore, and it becomes spherical only after the averaging over statistical ensemble and/or time.…”

Physical and functional properties of tunicate (Styela clava) hydrolysate obtained from pressurized hydrothermal process

“…Hydrolysis is important for chemical decomposition in which the compounds are spitted into smaller compounds by reacting with water. Hydrolysis of protein under controlled condition generates peptides, and thus the functional properties of a protein is improved (Fujimoto et al 2012). So, a proper hydrolysis technique at suitable and optimum conditions plays an important role in industrial bio-functional food preparations.…”