Since the molar flux density 𝐽 𝑖 e in itself depends on the derivatives of the concentration 𝑐 and the composition 𝑥 𝑖 , the overall result is a system of coupled ordinary differential equations of 2nd order with respect to 𝑐 and 𝑥 𝑖 . 𝐽 𝑖 e is calculated with the binary friction model according to equation ( ) .