Intrinsic Structure of Hydrophobic Surfaces: The Oil-Water Interface

We investigate the water-oil interface using molecular dynamics simulations of realistic models of alkanes and water. The intrinsic density profiles are computed using a methodology that removes the smoothing effect of the capillary waves. We show that at 300 K the intrinsic width of the gap separating the oil and water phases spans little more than one water molecule diameter, and undergoes very weak short-ranged fluctuations, indicating that the water-oil interface is a rigid molecular structure at ambient temperature. Only near the drying transition (above 500 K for dodecane), the gap features uncoupled fluctuations of the oil and water surfaces, as expected in a typical drying structure. We find that the intrinsic structure of water next to the oil phase is remarkably similar to the bare water-vapor interface.

Figure 1

Intrinsic density profiles of water (oxygen atoms) and dodecane (all carbon pseudoatoms) for liquid-vapor (full lines) and liquid-liquid interfaces (dashed lines) at 300 and 400 K. The mean density profiles are presented for the lower $T$. The arrows signal the tail of the hydrocarbon layers in direct contact with the water surface.

Figure 2

Intrinsic orientational order parameters for (left) dodecane ($T(z)$, at two temperatures) and (right) water ($T(z)$, and the dipole $p(z)$ at the lower temperature). The order parameter for dodecane at 400 K has been shifted upwards to facilitate the comparison of the results. Full and dashed lines correspond to the liquid-vapor and liquid-liquid interfaces, respectively.

Figure 3

Inverse dispersion of the fluctuations of the water-dodecane interface as a function of the square of the wave vector. The lines represents a linear fit of the low-$q$ simulation data to Eq. (1).