Structure and Depletion at Fluorocarbon and Hydrocarbon/Water Liquid/Liquid Interfaces

The results of x-ray reflectivity studies of two oil/water (liquid/liquid) interfaces are inconsistent with recent predictions of the presence of a vaporlike depletion region at hydrophobic/aqueous interfaces. One of the oils, perfluorohexane, is a fluorocarbon whose superhydrophobic interface with water provides a stringent test for the presence of a depletion layer. The other oil, heptane, is a hydrocarbon and, therefore, is more relevant to the study of biomolecular hydrophobicity. These results are consistent with the subangstrom proximity of water to soft hydrophobic materials.

Figure 1

Water drop placed at the perfluorohexane/air interface.

Figure 2

X-ray reflectivity from the perfluorohexane/water interface at $23.5°\mathrm{C}$ as a function of wave vector transfer normal to the interface. Solid line: fit to capillary wave theory. Dashed line: Fresnel reflectivity $R_F$. Error bars are smaller than the symbols. The point at $Q_z=0$ is the direct (not reflected) beam that is used to normalize the reflectivity. Inset: X-ray reflectivity normalized to $R_F$. Dashed (red) line: Fit to capillary wave theory. Solid line: Fit to perfluorohexane multilayering.

Figure 3

Depletion layer analysis for the perfluorohexane/water interface. Data from Fig. 2. Solid line: Fit to Eq. (1). Dashed lines: Calculated from Eq. (2) with $D_{\mathrm{dep}}=0.5$ to 3 Å in steps of 0.5 Å (bottom to top) with $\sigma =3.4\AA$. Inset: Electron density profile normalized to bulk water, $\rho (z)/{\rho }_w$, that is used to calculate the corresponding x-ray reflectivity. Solid lines: Water and perfluorohexane densities for $D_{\mathrm{dep}}=3\AA$. The 3 Å separation between the midpoints of the profiles is shown by vertical lines. Dashed lines: Total profiles for $D_{\mathrm{dep}}=0$ to 3 Å in steps of 0.5 Å (top to bottom), where $D_{\mathrm{dep}}=0\AA$ corresponds to the solid line in the main figure.

Figure 4

Depletion layer analysis for the heptane/water interface. Symbols: X-ray reflectivity data at $25°\mathrm{C}$. Solid line: Fit to Eq. (1). Long dashed line: Capillary wave fit with ${\sigma }_{\mathrm{cap}}=3.44\AA$. Short dashed lines: Calculated from Eq. (2) with $D_{\mathrm{dep}}=0.5$ to 3 Å in steps of 0.5 Å (bottom to top) with ${\sigma }_{\mathrm{cap}}=3.44\AA$. Data point at $Q_z=0$ is the direct (not reflected) beam.