Modeling of adsorption and desorption in pores of simple geometry using molecular dynamics

We present results from molecular dynamics simulations of adsorption and de sorption by diffusive mass transfer into model pores of well-defined geomet ry. The pore geometries considered were chosen to investigate issues relate d to classical theories of hysteresis in adsorption. We consider the slit p ore with or without a closed end, a wedge or triangular pore, and finally a model with an ink bottle geometry. In each case we show that the molecular dynamics simulations yield the same results for adsorption and desorption as grand canonical Monte Carlo simulations of the same systems. Using compu ter graphics visualization, we show how hysteresis for the slit and wedge g eometries is related to the evolution of vapor-liquid interfaces in a manne r largely consistent with the classical picture of these systems. We find t hat for the ink bottle geometry the large pore can empty during desorption even while the small pore remains filled with fluid. This behavior is diffe rent from the classical picture of adsorption and desorption in this geomet ry based on the concept of pore blocking.