  We experimentally explore pressure-driven flow of water and n-hexane across
nanoporous 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 by
applying hydrostatic pressures via inert gases (argon and helium, pressurized
up to 70 bar) on the upstream side in a capacitor-based membrane permeability
setup. For the native, hydrophilic silica walls, the measured hydraulic
permeabilities can be quantitatively accounted for by bulk fluidity provided we
assume a sticking boundary layer, i.e. a negative velocity slip length of
molecular dimensions. The thickness of this boundary layer is discussed with
regard to previous capillarity-driven flow experiments (spontaneous imbibition)
and with regard to velocity slippage at the pore walls resulting from dissolved
gas. Water flow across the silanized, hydrophobic nanopores is blocked up to a
hydrostatic pressure of at least 70 bar. The absence of a sticking boundary
layer quantitatively accounts for an enhanced n-hexane permeability in the
hydrophobic compared to the hydrophilic nanopores.
