how to solve an equation at an interface

sreenivas

i am solving a foam flow problem. it is a two phase problem containing liquid and air as the two phases. the liquid contains another component (surfactant) to stabilise the foam. the air phase doesnt contain any surfactant. the surfactant particles will be present at the interface.there will be an equillibrium between the surfactant present in the bulk and the surfactant particles present at the interface. i have to solve another equation for the concentration of the surfactant at the interface(boundary between air and liquid). this is not the same as the equation for the concentration of the surfactant in the bulk. i am not understanding how to incorporate this equation which has to be solved only at the interface.

i will be very thankful if i could get any suggestions in this regard.