A novel application of inverse gas chromatography for estimating contact angles in porous media

Hypothesis: Surface wettability is a critical factor in multi-phase flow within porous media, a processes essential in various applications e.g. in the energy sector. Traditional methods for assessing wettability of porous media by contact angle measurements, such as sessile droplet and micro-CT techniques, are limited by interface pinning, sample size or resolution impacting precision and accuracy. We hypothesized that using smaller and unconstrained probes, specifically gas molecules, to retrieve interactions along a representative sample size via inverse gas chromatography (IGC) could provide a more accurate determination of contact angles. Method: We propose a procedure to relate IGC results with macro-scale wettability descriptions, such as the Young equation. To test the effectiveness of IGC method, glass bead samples with varying wettability, modified through a silanization process, were prepared. Contact angles for a distilled water-air-sample system were measured using the sessile droplet method and micro-CT for comparative analysis. IGC was employed to determine the surface energy components of these samples, which were then used in the extended Young-Dupré equation to calculate the contact angles. Findings: The contact angle ranges determined by IGC and micro-CT for untreated glass beads, the most hydrophilic samples, showed great alignment. This consistency is attributed to the chemical amorphous nature of the untreated beads reflected in the assumption that dispersive and specific energetic components of surface sites are uncorrelated, on which the proposed analysis is based. For treated samples, where the silanization process creates correlations between surface energetic components, the alignment between IGC and micro-CT results was less precise. This study successfully demonstrated that IGC, a molecular-scale probe-based technique, can effectively determine the contact angle range, a macroscopic property, for amorphous samples. Future work should incorporate correlations between energetic components of surface detected by IGC to extend this method's applicability to a wider material range.