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

TL;DR

This study experimentally investigates how water and n-hexane flow through hydrophilic and hydrophobic nanopores, revealing boundary layer effects and flow blockage at high pressures, with implications for nanofluidics and surface chemistry.

Contribution

It provides quantitative analysis of flow behavior in nanopores with different surface chemistries, highlighting the role of boundary layers and slip conditions in nanofluidic transport.

Findings

Hydrophilic silica exhibits flow consistent with bulk fluidity plus a boundary layer.

Hydrophobic nanopores block water flow up to 70 bar hydrostatic pressure.

n-Hexane permeability is higher in hydrophobic pores due to reduced boundary effects.

Abstract

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.