Adsorption and permeation events in molecular diffusion

How many times a diffusing molecule can permeate across a membrane or be adsorbed on a substrate? We employ the encounter-based approach to find the statistics of adsorption or permeation events for molecular diffusion in a general confining medium. Various features of these statistics are illustrated for two practically relevant cases of a flat boundary and a spherical confinement. Some applications of these fundamental results are discussed.