Fick's law assumes that thickness of the sheet is constant. However, in case of gel it gets swelled as sorption proceeds. I just wonder how to calculate the diffusion coefficient in such case.
Hello, I know that you asked a long time ago, but still.. Ficks law can be used to describe diffusion in swelling materials, you just have to account for this phenomenon (e.g., by having a concentration dependent density). We did this for diffusion in PS (http://pubs.acs.org/doi/abs/10.1021/je300916f).
However, you could encounter concentration dependent diffusivities or changing properties of the gel during sorption / desorption, but I do not have experience with this.