Adsorption of fluids in nanoporous media causes mechanical stresses and results in deformation [1]. This phenomenon is ubiquitous, and its magnitude depends on the pore size and geometry. Adsorption and adsorption-induced deformation is typically modeled in convex pores (cylindrical or spherical pores) or planar geometry (slit pores). However, many porous materials are composed of spherical grains, so that the pores are formed by the intergranular spaces between the convex surfaces, thus becoming non-convex. Here we present a first theoretical study of adsorption-induced deformation of materials with non-convex pores. We take the templated mesoporous carbons (CMK-3) which have an ordered hexagonal structure of parallel cylindrical rods [2], and model adsorption of nitrogen in this system using the classical density functional theory (DFT) within the local density approximation. Adsorbed fluid density profile in CMK-3 geometry can be reduced to two dimensions, but unlike slit or cylindrical pore model it cannot be made one dimensional. The fluid-fluid interaction parameters are fit to experimental nitrogen binodal, and reproduce other thermodynamic properties (such as surface tension and vapor pressure). The solid-fluid interaction parameters are fit to reproduce the nitrogen adsorption at 77 K on a reference carbon surface. Using the resulting parameters we predict the adsorption isotherms and strain isotherms for nitrogen adsorption in CMK-3 carbons. The shape of adsorption isotherm matches the shape of experimental isotherm. The predicted strain isotherms are qualitatively different from the strain isotherms in cylindrical pores [3]. We attribute this difference to formation of liquid bridges between the adjacent rods [4]. Our results suggest that adsorption-induced deformation in materials with non-convex pore walls cannot be predicted within the existing models. These results may shed some light on understanding adsorption-induced deformation of consolidated granular media.
References:
1. Gor, G. Y.; Huber, P.; Bernstein, N., 2017. Adsorption-induced deformation of nanoporous materials - a review. Applied Physics Reviews, 4(1), p.011303.
2. Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O., 2000. Synthesis of new, nanoporous carbon with hexagonally ordered mesostructure. Journal of the American Chemical Society, 122(43), pp.10712-10713.
3. Gor, G. Y.; Neimark, A. V., 2011. Adsorption-induced deformation of mesoporous solids: Macroscopic approach and density functional theory. Langmuir, 27(11), pp.6926-6931.
4. Ludescher, L.; Braxmeier, S.; Balzer, C.; Reichenauer, G.; Putz, F.; Hüsing, N.; Gor, G. Y.; Paris, O., 2020. Capillary Bridge Formation between Hexagonally Ordered Carbon Nanorods, Adsorption, DOI: 10.1007/s10450-020-00215-6
