Molecular simulation and experiments of water adsorption in a high surface area activated carbon: Hysteresis, scanning curves and spatial organization of water clusters

Despite decades of research, quantitatively accurate molecular models of water sorption on activated carbons remain elusive, while many phenomena are not properly understood even qualitatively. Here we focus specifically on scanning phenomena. Scanning isotherms, obtained by reversing either adsorption or desorption process before the closure of the hysteresis loop, have been recently explored as a sensitive experimental probe for structural characterization of activated carbons and at the same time have been subject of several competing theories of adsorption. We employ adsorption experiments and molecular simulations to understand the nature of the states along the adsorption and scanning curves for water at 308 K in a high surface area activated carbon, Maxsorb. The molecular model considered here is based on a random packing of fullerene-like fragments, functionalized with carboxylate groups. The model is able to reproduce reasonably well the shape of the adsorption and scanning desorption isotherms. We investigate spatial organization, structure, size and interaction between the clusters on the adsorption branch. Furthermore, we show that the scanning desorption isotherm consists of a series of states of the system where a single cluster of water shrinks in size as water evaporates from its surface, before it finally disintegrates into separate smaller clusters.