Observation of Surface Layering in a Nonmetallic Liquid

Oscillatory density profiles (layers) have previously been observed at the free surfaces of liquid metals but not in other isotropic liquids. We have used x-ray reflectivity to study a molecular liquid, tetrakis(2-ethylhexoxy)silane. When cooled to $T/T_c\approx 0.25$ (well above the freezing point for this liquid), density oscillations appear at the surface. Lateral order within the layers is liquidlike. Our results confirm theoretical predictions that a surface-layered state will appear even in dielectric liquids at sufficiently low temperatures, if not preempted by freezing.

Figure 1

Normalized specular reflectivity for $\sim 5000\AA$ TEHOS films on rough silicon wafers at different temperatures. At 227 K and below, the reflectivity data develop structure, indicating a change in the surface density profile. Lines are best fits using the electron density profiles shown in Fig. 2.

Figure 2

The dashed lines show best-fit electron densities as functions of distance from the surface, for $\sim 5000\AA$ TEHOS films on rough silicon wafers at different temperatures. The solid lines show the density profiles with capillary broadening removed (see text).

Figure 3

Liquid surface width vs temperature. The surface width is defined as the distance from $\rho /{\rho }_{\mathrm{bulk}}=0.9$ to $\rho /{\rho }_{\mathrm{bulk}}=0.1$ in each best-fit electron density function (dashed lines in Fig. 2); thus, the absolute values will differ from widths calculated using other definitions.

Figure 4

Grazing-incidence (in-plane) scattering from a TEHOS surface (○), compared to scattering in transmission from a bulk TEHOS sample (●). The two curves show the same features, indicating that there is only liquidlike lateral order at the surface.