Phase transitions in porous materials: influence of physico-chemical heterogeneities

Essentially all surfaces adsorb fluids due to favourable interactions [1, 2]. For strong enough interactions, chemical bonds may be broken in the adsorbed molecules and/or established between the adsorbent and the adsorbate (chemisorption). For weak interactions, the adsorption process occurs reversibly without breaking any molecule (physisorption). Despite weak, the fluid/substrate interactions induce observable modifications of the fluid properties in the vicinity of the surface. Moreover, collective effects may be induced when confining fluids in porous materials, because the local curvature associated to porosity enhances fluid/substrate interactions. The measured adsorption isotherms (giving the amount of fluid adsorbed versus fluid pressure) thus provide a powerful experimental tool for porous materials characterization [3, 4]. An important feature of such isotherms is their possible irreversibility between adsorption and desorption (existence of a hysteresis, to be discussed later). Focusing on heterogeneous surfaces, we show how surface chemistry and geometry influence the adsorption properties of a condensable fluid and hence its liquid/vapor coexistence diagram. In most cases, molecular simulation, taking into account the fluid/substrate interactions at atomistic scale, has proven to be a powerful tool to understand adsorbed fluid properties in the vicinity of a surface. In particular for studying the effect of surface chemistry or corrugation, local curvature, pore morphology, or any other nanometer-scale property of the surface We show that, increasing the surface heterogeneities has drastic effects on coexistence diagram. Not only a shift of the critical point (i.e. quantitative changes), but also the appearance of many metastable states that introduce qualitative changes [5, 6]. A multi-scale approach has been used to explore this issue, showing the importance of the interdependence between the various heterogeneities for the adsorption and desorption mechanisms, the observed hysteresis, and the distribution of the underlying metastable states. Such approach, including both an atomistic and a large-scale description (from interatomic forces to large scale domain network) is expected to offer a powerful tool for experimental data analysis.