Implementation of an interfacial solubility calculation to properly account for concentration gradients.
For example,
For a liquid-gas system there is the liquid-gas interface where the concentration at that interface is different than the bulk concentrations based upon the solubility of the species as determined by a law such as Henry's law which states that the concentration can be related to the partial pressure.
C_i/K_H = P_i
So the concentration changes across the interface for use in mass balance equations depends therefore on the stated type of relation on either side.