How Water Meets a Very Hydrophobic Surface

Is there a low-density region (“gap”) between water and a hydrophobic surface? Previous x-ray and neutron reflectivity results have been inconsistent because the effect (if any) is subresolution for the surfaces studied. We have used x-ray reflectivity to probe the interface between water and more hydrophobic smooth surfaces. The depleted region width increases with contact angle and becomes larger than the resolution, allowing definitive measurements. Large fluctuations are predicted at this interface; however, we find that their contribution to the interface roughness is too small to measure.

Figure 1

Upper panel: Unnormalized reflectivity data and best fits (lines through data) for the dry FAS25 SAM surface (stars) and for the FAS25-water interface (circles). We show $R$ rather than $R/R_F$ for consistency with other papers in this area [15, 16, 17, 23]. Inset: schematic diagram of the sample cell, shown in cross section. X rays penetrate a Kapton window and are reflected from the water-substrate interface. Lower panel: fitted density profile of the dry FAS25 SAM (dashed line) and wet FAS25 SAM (solid line). Slab thickness variations much smaller than the resolution width ($\sim 3\AA$) are not significant. The thin dashed lines show the unrounded slabs for the wet SAM.

Figure 2

Unnormalized reflectivity data, and best fits (lines through data), for dry FAS13 SAM surface (stars) and FAS13-water interface (circles). Inset: Density profile of dry FAS13 SAM (dashed line) and FAS13-water interface (solid line). Thin dashed lines show unrounded slabs for the wet SAM.

Figure 3

Dependence of the $D_{\mathrm{eq}}$ on advancing contact angle. (a): from Ref. 17, for partially deuterated alkanethiol SAMs, $p\mathrm{H}=5.5$, after subtracting the gap with nonpolar liquid; (b): from Ref. 16, for OTS, after subtracting the “hydrogen gap” (error bar not reported; contact angle of sample studied not reported, value shown is estimated); (c): from Ref. 15, for OTE, also with the hydrogen gap subtracted; (d): from Ref. 23, for FAS17 (error bar not reported; contact angle of sample studied not reported, value shown is average of three contact angles reported in Refs. 20, 29). The unlabeled data are results from the present Letter, for FAS13 and FAS25. The dashed line is a guide to the eye.