How to set user defined boundary condition

I'm now trying to calculate heterojunction. As an example of model library， I use the "Transport of Diluted species" model to calculate. But I want to set a condition that " log(n1)+c1=log(n2)+c2" at the boundary. n1, n2 represent the concentration, c1,c2 are different const depending on material. There are no boundary condition same as this need，so how can I set that boundary condition？