Evaporation from an Ink-Bottle Pore: Mechanisms of Adsorption and Desorption

We present a molecular simulation study of argon adsorption and desorption in ink-bottle pores in order to investigate the effects of pore width, length, and temperature on the form of the hysteresis loop, with particular emphasis on the pressure at which evaporation occurs. We show that <i>intrinsic</i> cavitation occurs only in an ink-bottle pore with a sufficiently long and narrow neck. The tension in the condensed fluid is characterized by the average value of the shortest distance between molecules, and when this reaches a critical value, cavitation occurs. As the neck width is increased or the neck length is decreased, the evaporation mechanism in the cavity switches from cavitation to evaporation at higher pressure than the cavitation pressure (which we call cavitation-like pore opening). For cavitation-like pore opening, the evaporation pressure is greater than the intrinsic cavitation pressure and the tensile strength just prior to evaporation is less than the critical tension. For a given temperature, the reduced pressure of cavitation, (<i>P</i>/<i>P</i>₀)_cav, is always lower than the cavitation-like pore opening reduced pressure, and (<i>P</i>/<i>P</i>₀)_cav follows a linear dependence on the temperature. The intersection between the cavitation curve and the cavitation-like pore opening curve for a given neck size is the transition temperature, <i>T</i>_trans: for <i>T</i> < <i>T</i>_trans, cavitation-like pore opening is the operating mechanism, while cavitation occurs for <i>T</i> > <i>T</i><sub>trans