How to determine a boundary condition at a thin membrane for diffusion from experimental data

We present a new method of deriving a boundary condition at a thin membrane for diffusion from experimental data. Based on experimental results obtained for normal diffusion of ethanol in water, we show that the derived boundary condition at a membrane contains a term with the Riemann– Liouville fractional time derivative of the 1/2 order. Such a form of the boundary condition shows that a transfer of particles through a thin membrane is a ‘long memory process’. Presented method is an example that an important part of mathematical model of physical process may be derived directly from experimental data.