Incompatible Liquids in Confined Conditions

TL;DR

This study uses small angle neutron scattering to investigate how water and toluene adsorb on oxidized microporous carbon under various humidity conditions, revealing unexpected co-adsorption behavior and high water concentrations in the adsorbed phase.

Contribution

It demonstrates the co-adsorption of water and toluene on carbon surfaces at high humidity, challenging previous assumptions about their incompatibility in confined conditions.

Findings

Water forms discontinuous clusters at high humidity.

Toluene and water adsorb as a single phase at low RH.

Water concentration in the adsorbed phase exceeds bulk solubility.

Abstract

In applications involving organic vapour, the performance of high surface area carbons is often challenged by water vapour in the atmosphere. Small angle neutron scattering (SANS), through its ability to distinguish between different components by means of contrast variation, is ideally suited to investigating the behaviour of adsorbed layers in simultaneous contact with a mixed vapour phase. Even at high relative humidity (RH), water alone forms a discontinuous film composed of clusters on the surface of the oxidized microporous carbon used for these studies. When toluene is also present, all the available carbon surface is wetted. Toluene and water adsorb as a single phase already at RH 11.5%, and the concentration of water present in the adsorbed phase is as high as 2.9 wt.%, far above its solubility in bulk toluene (0.033 wt.% at 25 deg C). At RH 87% the concentration of water in the adsorbed phase is four times higher, approximately 12 wt.%. The recently proposed mechanism of anchoring of the water by the aromatic molecules may provide an explanation for this phenomenon.