Experimental Confirmation of Different Mechanisms of Evaporation from Ink-Bottle Type Pores:  Equilibrium, Pore Blocking, and Cavitation

We present a new classification of physical mechanisms of adsorption hysteresis in ink-bottle type pores confirmed by experimental studies of capillary condensation of N2, Ar, and Kr at 77.4 and 87.4 K in ordered 3D cage-like structures of FDU-1 and SBA-16 nanoporous templated silicas with narrow pore size distribution. An analysis of the hysteresis loops and scanning desorption isotherms on FDU-1 with ∼15 nm spherical pores reveals three mechanisms of evaporation: (1) evaporation from the blocked cavities controlled by the size of connecting pores (classical ink-bottle or pore blocking effect); (2) spontaneous evaporation caused by cavitation of the stretched metastable liquid; and, for the first time, (3) near-equilibrium evaporation in the region of hysteresis from unblocked cavities that have access to the vapor phase. Studies of the temperature dependence of the hysteresis loop showed a transition between the cavitation and the pore blocking regimes of evaporation, which opens up the possibility to tune the experimental conditions in order to characterize the pore neck size distribution. A similar transition was also observed for Kr in SBA-16 silica with ∼8.5 nm pores. The pressure of cavitation was found to depend on the pore geometry. This conclusion questions the conventional assumption that the pressure of cavitation, which determines the lower closure point of the hysteresis loop, is a unique function of the adsorbate and the temperature. The experimental results agree with the nonlocal density functional theory (NLDFT) of capillary condensation hysteresis in spherical cavities reported earlier (Ravikovitch, P. I.; Neimark, A. V. Langmuir 2002, 18, 1550).