Density Depletion at Solid−Liquid Interfaces:  a Neutron Reflectivity Study

Abstract

Neutron reflectivity experiments conducted on self-assembled monolayers (SAMs) against polar (water) and nonpolar (organic) liquid phases reveal further evidence for a density reduction at hydrophobic-hydrophilic interfaces. The density depletion is found at the interface between hydrophobic dodecanethiol (C12) and hexadecanethiol (C16) SAMs and water and also between hydrophilic SAMs (C12/C11OH) and nonpolar fluids. The results show that the density deficit of a fluid in the boundary layer is not unique to aqueous solid-liquid interfaces but is more general and correlated with the affinity of the liquid to the solid surface. In water the variation of pH has only minor influence, while different electrolytes taken from the Hofmeister series seem to increase the depletion. On hydrophobic SAMs an increase in density depletion with temperature was observed, in agreement with Monte Carlo simulations performed on corresponding model systems. The increase in the water density depletion layer is governed by two effects: the surface energy difference between water and the substrate and the chemical potential of the aqueous phase.

Depletion at solid/liquid interfaces: Flowing hexadecane on functionalized surfaces

“…As a result, we obtained a surface which was completely wetted by hexadecane, corresponding to an interfacial energy of γ sl = 34 mJ/m 2 at room temperature. 6,13 For the lower interfacial energy substrate another identically cleaned silicon wafer was subsequently chemically coated with an OTS monolayer by immersing it in a solution of 50 g bicyclohexane, 2.8 g carbon tetrachloride, and 0.8 g OTS for 15 min and flushing afterward with chloroform to remove any cross-linked OTS chains. This procedure was repeated three times to ensure a fully covered substrate, and the resulting advancing contact angle of hexadecane was 40 • ± 2 • at 22 • C, 39 • ± 2 • at 40 • C, and 37.3 • ± 2 • at 60 • C. This corresponds to an interfacial energy of γ sl = 7.2 mJ/m 2 at room temperature.…”

“…6 Note that in the present study the depletion layer can be modeled separately from the SAM due to the high resolution of the instrument, and, therefore, no reference system is needed anymore, which could introduce a systematic error. A further indication of the intrinsic nature of the observed depletion at the OTS/d-hexadecane interface is the enhancement of the depletion distance with temperature.…”

“…The reduction of the depletion layer with increasing interfacial energy is confirmed by the other studies of static alkanes on solid surfaces. 6 The main result is that the density depletion of liquids at solid boundaries cannot be generally attributed to surface slip.…”

What x rays can tell us about the interfacial profile of water near hydrophobic surfaces

“…In reality, although the water-hydrophobic interface has been studied by a number of groups, [30][31][32][33][34][35][36][37] the results are inconsistent and have not led to a consensus. Early neutron reflectivity studies [31][32][33] of hydrocarbon-water interfaces reported gap widths larger than 10Å, while later x-ray reflectivity measurements on similar systems reported gap widths of less than 5Å.…”

“…Thus silicon-supported SAMs provide excellent, well-defined hydrophobic surfaces that have been used in many experimental studies. [31][32][33][35][36][37] However, there is a high price: the system now contains at least three interfaces, namely silicon-oxide, oxide-SAM, and SAM-water. Only the last of these is of interest here, but all interfaces reflect X-rays or neutrons and must be accounted for during data analysis.…”