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We experimentally explore pressure-driven flow of water and n-hexane acrossnanoporous silica (Vycor glass monoliths with 7 or 10 nm pore diameters,respectively) as a function of temperature and surface functionalization(native and silanized glass surfaces). Hydraulic flow rates are measured byapplying hydrostatic pressures via inert gases (argon and helium, pressurizedup to 70 bar) on the upstream side in a capacitor-based membrane permeabilitysetup. For the native, hydrophilic silica walls, the measured hydraulicpermeabilities can be quantitatively accounted for by bulk fluidity provided weassume a sticking boundary layer, i.e. a negative velocity slip length ofmolecular dimensions. The thickness of this boundary layer is discussed withregard to previous capillarity-driven flow experiments (spontaneous imbibition)and with regard to velocity slippage at the pore walls resulting from dissolvedgas. Water flow across the silanized, hydrophobic nanopores is blocked up to ahydrostatic pressure of at least 70 bar. The absence of a sticking boundarylayer quantitatively accounts for an enhanced n-hexane permeability in thehydrophobic compared to the hydrophilic nanopores.

Hydraulic Transport Across Hydrophilic and Hydrophobic Nanopores: Flow Experiments with Water and n-Hexane