Phase-transfer energetics of small-molecule alcohols across the water-hexane interface: molecular dynamics simulations using charge equilibration models

Abstract

We study the water-hexane interface using molecular dynamics (MD) and polarizable charge equilibration (CHEQ) force fields. Bulk densities for TIP4P-FQ water and hexane, 1.0086±0.0002 and 0.6378±0.0001 g/cm(3), demonstrate excellent agreement with experiment. Interfacial width and interfacial tension are consistent with previously reported values. The in-plane component of the dielectric permittivity (ɛ(||)) for water is shown to decrease from 81.7±0.04 to unity, transitioning longitudinally from bulk water to bulk hexane. ɛ(||) for hexane reaches a maximum in the interface, but this term represents only a small contribution to the total dielectric constant (as expected for a non-polar species). Structurally, net orientations of the molecules arise in the interfacial region such that hexane lies slightly parallel to the interface and water reorients to maximize hydrogen bonding. Interfacial potentials due to contributions of the water and hexane are calculated to be -567.9±0.13 and 198.7±0.01 mV, respectively, giving rise to a total potential in agreement with the range of values reported from previous simulations of similar systems. Potentials of mean force (PMF) calculated for methanol, ethanol, and 1-propanol for the transfer from water to hexane indicate an interfacial free energy minimum, corresponding to the amphiphilic nature of the molecules. The magnitudes of transfer free energies were further characterized from the solvation free energies of alcohols in water and hexane using thermodynamic integration. This analysis shows that solvation free energies for alcohols in hexane are 0.2-0.3 kcal/mol too unfavorable, whereas solvation of alcohols in water is approximately 1 kcal/mol too favorable. For the pure hexane-water interfacial simulations, we observe a monotonic decrease of the water dipole moment to near-vacuum values. This suggests that the electrostatic component of the desolvation free energy is not as severe for polarizable models than for fixed-charge force fields. The implications of such behavior pertain to the modeling of polar and charged solutes in lipidic environments.

FIG.1

Density profiles for the water-hexane system. z = 0 is the center of mass of the system.

FIG.2

In-plane dielectric permittivity, ε_∥, of water and hexane as a function of longitudinal position relative to the center of mass of the system. The total system Gibbs dividing surface is indicated by the dashed vertical line.

FIG.3

Orientational profiles for the hexane and water as functions of longitudinal position relative to the center of mass of the system. S_zz is defined by Equation 21 and θ is the angle between the normal of the interface and the water permanent dipole moment. The total system Gibbs dividing surface is indicated by the dashed vertical line.

FIG.4

Dipole moments of the hexane-water interfacial system as functions of longitudinal position relative to the center of mass of the system. The total system Gibbs dividing surface is indicated by the dashed vertical line.

Fig.5

A) Average magnitude of the total induced dipole moment of water. B) Average magnitude of the z-component of the induced dipole moment of water. Profiles are shown as functions of the longitudinal position relative to the center of mass of the system. The total system Gibbs dividing surface is indicated by the dashed vertical line.

FIG.6

Average angle between the fixed dipole moment and total dipole moment of water in the water-hexane interfacial system as a function of longitudinal position relative to the center of mass of the system. The total system Gibbs dividing surface is indicated by the dashed vertical line.

FIG.7

A) Interfacial potentials for the water-hexane system. Shown are the individual contributions from the hexane and the water as well as the total potential. B) Quadrupole and dipole components of the water contribution to the interfacial potential of the water-hexane system.

FIG.8

Profiles of the free energy for transferring water and hexane across the interface are shown as functions of longitudinal position relative to the center of mass of the system. The total system Gibbs dividing surface is indicated by the dashed vertical line.

FIG. 9

Profiles of the free energy for transferring methanol, ethanol and 1-propanol across the interface are shown as functions of longitudinal position relative to the center of mass of the system. Bulk water is centered at the origin; bulk hexane is centered at 25 Å.

FIG.10

Dipole moment profiles of methanol, ethanol and 1-propanol in the water-hexane system as functions of longitudinal position relative to the center of mass of the system. The total system Gibbs dividing surface is indicated by the dashed vertical line.