Micropore Filling and Multilayer Formation in Stöber Spheres upon Water Adsorption

Abstract

The presence of porosity critically affects the performance of solid systems. The pore accessibility to adsorbate molecules and the corresponding adsorption/desorption behavior are crucial aspects to understand the properties of porous materials but are difficult to address, principally when dealing with narrow micropores. A prominent example is colloidal silica (Stöber) spheres whose microporosity, inaccessible for some adsorbates, can be readily filled by water molecules to a large extent but exhibiting a complex adsorption behavior with unexpected hystereses. Here, we perform water adsorption isotherms on Stöber spheres at different temperatures using an original analysis of the Dubinin-Radushkevich representation to examine both the accessibility to the microporosity and the formation of water multilayers on the outer sphere surface. The micropore filling (and emptying) is found to be limited by the kinetic energy of the water molecules, causing low-pressure hysteresis. We further discover that the (temperature-dependent) completion of the micropore filling delays the onset of multilayer adsorption, leading to hysteresis at a high relative pressure. The number of adsorbed water layers is determined, and the adsorption-induced swelling of the spheres is discussed.