Sorption isotherm hysteresis and turbidity phenomena in mesoporous media

Abstract

Based on equilibrium sorption isotherm studies of porous Vycor discs whose circular faces were coated with a clear epoxy resin, it was possible to study the variable sorbed vapor disposition coincident with sorption isotherm hysteresis. Along the adsorption isotherm, condensate lenses were found to be more or less randomly distributed uniformly throughout the Vycor, but upon subsequent desorption, the absence of cavitation allowed the porous Vycor disc to remain condensate saturated. The desorption isotherm was, therefore, nearly horizontal until, at a sufficiently low vapor pressure, the outermost liquid-vapor interfaces became unstable. The resulting instability caused rheon jump desorption, which proceeded radially inward over an unusually small relative pressure range. This desorption process left behind residual isolated condensate ganglia in the outer portion of the porous medium in metastable equilibrium with a condensate saturated central core. After descending approximately two-thirds of the near vertical desorption isotherm, the condensate saturated core had evaporated and the remainder of the desorption hysteresis isotherm was primarily ganglia evaporation. Nonuniform desorption behavior was sensibly absent along descending scanning curves because each curve began at a partially saturated adsorption state containing randomly distributed vapor regimes from which internal rheon jump desorption could begin. This nonuniform desorption, observed with porous Vycor, may apply to a variety of porous media, thereby negating any simple application of the Kelvin equation in computing pore size distributions for these media. Such a calculation, which assumes that all pores communicate with the vapor, would lead to a considerable error in that the number of larger pores is greatly underestimated.