Effect of Morphological Defects on Gas Adsorption in Nanoporous Silicas

Abstract

This paper reports a molecular simulation study on the adsorption of a simple fluid in nanoporous silicas with or without morphological defects (constrictions). All of the pores considered in this work are of a finite length and are connected to a bulk reservoir so that they mimic real materials for which the confined fluid is always in contact with the external gas phase. The adsorption isotherms for the regular cylindrical pores conform to the typical experimental behavior for MCM-41 as the adsorbed amount increases continuously in the multilayer adsorption regime until a jump occurs due to capillary condensation of the fluid within the pore. The evaporation pressures are lower than the condensation pressures, so that hysteresis loops are observed. The condensation and evaporation mechanisms for the pores with constrictions depart significantly from what is observed for regular nondefective nanopores. Depending on the size, length, and number of constrictions in the pores, the filling and emptying processes are found to be of different nature. In every case, these mechanisms involve the coexistence between the confined liquid and some gas nanobubbles that are trapped within the main cavities of the pore. It is also found that the finite length of the pore introduces some heterogeneity in the adsorption and desorption processes; the filling and emptying of the regions (constrictions and cavities) near the pore surface differs from those of the regions in the pore center. The desorption process can occur through different mechanisms such as cavitation, pore blocking effects with at equilibrium evaporation, or several combined pore blocking effects. Adsorption isotherms can be used to assess and characterize morphological defects in nanopores. In contrast, our results suggest that microcalorimetry experiments such as measurements of the isosteric heat of adsorption cannot be used to gain information regarding such defects as all of the data for the regular and constricted pores fall on the same curve.