Water at Hydrophobic Surfaces:  When Weaker Is Better

Abstract

The upsurge of interest in the nature of water adjacent to hydrophobic liquids is due in part to the growing appreciation for its unique characteristics for supporting chemical synthesis, nanoparticle assembly, oil remediation, and a host of other chemical separation processes. 1,2 The important characteristics of these interfaces that lend themselves to these applicationssmolecular orientation, polarity, interfacial charge and electric fieldssall stem from the disruption of the bulk water hydrogen-bonding network. [3][4][5] Water molecules seem to adapt to hydrophobic neighbors by rearranging themselves to maximize available hydrogen-bonding opportunities and minimize unfavorable dipole interactions. 6 Using equilibrium molecular dynamics simulations, we have discovered that this adaptation follows trends associated with the molecular properties of the hydrophobic liquid neighbor. Our studies reveal that the degree of water structuring in the immediate vicinity of the oil-water junction is highest when the hydrophobic phase is the least polar, that polar organics result in wider interfacial regions, and that the maximum extent of water molecule orientation does not occur at the Gibbs dividing surface.We have used the Amber 7 package 7 to perform the molecular dynamics simulations. A cubic box, 40 Å on each side and containing 2135 POL3 7 water molecules, was minimized and equilibrated for 200 ps. Temperature was controlled by weak coupling to a heat bath at 300 K; molecular geometries were constrained using the SHAKE algorithm; long-range interactions were limited to 8 Å using the particle mesh Ewald technique. A separate box the same size was prepared, either empty (to simulate the air-water interface) or with carbon tetrachloride, dichloromethane, or chloroform. The organic models and associated point charges were taken from the literature. [8][9][10][11] The number of organic molecules (Table 1) was selected to reproduce the bulk densities of the liquids at room temperature. The hydrophobic liquids were minimized and equilibrated in a similar fashion to the bulk water box. Hydrophobic-aqueous interfaces were then prepared by joining an equilibrated water box with an equilibrated hydrophobic box to create a 40 × 40 × 80 Å 3 system that was then subject to further energy minimization and equilibration. The dynamics of each system were then followed for 10 ns, and we recorded atomic coordinates every 50 fs. The results we describe are therefore based on ensemble averages of 200 000 configurations for each system. Density profiles from the simulations were fit to a hyperbolic tangent profile (solid lines in Figure 1) to obtain the position of the Gibbs dividing surface. For all subsequent analyses, care was taken to align the Gibbs surfaces for each aqueous-hydrophobic system studied.Order parameters 2,12-14 are formulated as a measure of the extent to which the water molecules tend to orient with respect to the lab frame coordinates (Figure 2). The parameter S 1 ) 0.5〈3 cos 2 θ -1〉 quantitatively describes the d...

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