Impact of Surface Energy on Capillary Condensation in Multiscale Synthetically Altered Porous Material

Capillary condensation is a phenomenon that occurs in porous media where a gas condenses into a liquid phase outside of the typical thermodynamic phase boundary. The size of the confining pore is attributed as the controlling factor in causing capillary condensation. A condensed phase in a porous medium could be detrimental to transport of the gaseous phase. Experimental methods to determine the existence of a condensed liquid phase are limited in sample size and flexibility in testing conditions. Recent findings indicate that capillary condensation can be identified via high-field nuclear magnetic resonance (NMR). Changes in chemical shift and shape of select peaks in 1D proton spectra can be correlated with fluid density changes during phase transition. The ethane phase diagram is probed using finely controlled temperature and pressure in the NMR instrument equipped with a high-pressure NMR cell. For this study, NMR, atomic force microscopy and adsorption isotherms were utilized to systematically examine the impact of surface energy changes on capillary condensation in synthetically altered silicate porous media. Additionally, comparison studies with Kelvin's equation were performed to validate findings. The porous media samples were modified to have different wetting properties using a silane deposition method to create uniform surface conditions. Several surface conditions were created by selecting silane based on contact angle measurements between ethane gas and water at ambient conditions. Our results thus far show that materials with relatively large pore sizes (of the order of one micron) with highly wetting surfaces can produce capillary condensation. The formation of this liquid phase is believed to occur in the contact points between spheres and in surface roughness and defects. A comparison with Kelvins equation indicated that the phase change would occur at the bulk saturation pressure. Adsorption isotherms show a change in the rate of adsorbed volume at a lower relative pressure when the surface was strongly wetting.