Effects of Critical Fluctuations on Adsorption-Induced Deformation of Microporous Carbons

We study adsorption-induced deformation of microporous carbons in the vicinity of the critical temperature of the adsorbed fluid for a range of subcritical pressures. The thermodynamic model (Kowalczyk, P.; Ciach, A.; Neimark A. Langmuir <bold>2008</bold>, 24, 6603) coupled with molecular simulations and experimental dilatometric measurements at T/T-c = 1.623 (T and Tc denote experimental and critical temperature, respectively) is used for constructing dilatometric deformation curve at T/T-c = 1.001. We find that the initial contraction of a microporous carbon sample upon near-critical argon adsorption is similar to 1.5 vol % (similar to 2 orders of magnitude higher than at T/T-c = 1.623). Large initial contraction of microporous carbon is a result of significantly higher solvation pressures in carbon micropores generated by the near-critical argon adsorption. In supermicropores and narrow mesopores (pore size similar to 0.75.0 nm) we observe a crossover between the oscillatory solvation force in thin pores, and the attractive thermodynamic Casimir force in pores of very large thickness. In the crossover regime, the oscillatory decay is superimposed on an attractive background, and the repulsion between the confining surfaces appears only when the pressure exceeds a certain value depending on the pore width. The attractive force in all pores exceeding a certain width can lead to a significant increase of the contraction of the sample containing supermicropores and narrow mesopores when the critical temperature is approached at sufficiently low subcritical pressure (similar to 0.4 MPa in the case of argon).