Gas sorption.
Example 1, following the sorption kintics in a linear scale of time :

The absorbed mass increases very rapidly at short times and the kinetic curve M(t) has got a vertical tangent straight at t=0. When the time increases, the absorbed mass increases less and less rapidly. At long times the equilibrium of sorption is reached step by step and the mass curve tends to an horizontal plateau. The concentration profile in the membrane becomes to be flat.