(719ab) Molecular Simulation Study of Spontaneous Desorption In Cage-Like Mesoporous Adsorbents

Cavitation of metastable adsorbate in ink-bottle, or blocked mesopores causes a sharp step on the desorption isotherm near the lower closure point of the hysteresis loop [1]. For a long time, the position of this step was considered independent of the pore size. However, recent experimental studies showed that the pressure of cavitation could depend on the pore size and shape [2]. We hypothesize that the position of cavitation depends on the free energy barrier of formation of the critical nucleus. We show that the nucleation barrier can be estimated by comparing simulated and experimental isotherms. In order to establish a relationship between the cavitation pressure and the pore size, a series of argon and nitrogen adsorption-desorption isotherms were calculated in spherical pores ranging from 5.7 to 9.7 nm, which corresponded to the samples of SBA16 materials explored experimentally. The gauge cell Monte Carlo method was employed to calculate the critical nucleus size and the nucleation barrier [3,4]. The grand canonical Monte Carlo simulation was employed to mimic the spontaneous cavitation process. The simulated isotherms were matched to the experimental data. We found that the nucleation barrier of cavitation increases with the pore size. The nucleation barriers vary from 30 kT to 50 kT in the pores of 5.7nm to 9.7nm.

1. P.I. Ravikovitch and A.V. Neimark, Langmuir, 2002, 18, 9830-9837.

2. M. Thommes, B. Smarsly, P.I. Ravikovitch, A.V. Neimark, Langmuir, 2006, 22, 756-764.

3. A. V. Neimark and A. Vishnyakov, Journal of Chemical Physics, 2005, 122, 054707.

4. A. V. Neimark and A. Vishnyakov, Journal of Physical Chemistry, 2006, 110, 9403-9412.