Fig. 6.3: Interface between two phases
If is total volume then
Also
where is number of moles of species assigned to the interface, is total number of moles of and and are the number of moles of assigned to bulk phases of liquid 1 and liquid 2 respectively. Therefore
is total volume then
is number of moles of species 
assigned to the interface, 
is total number of moles of 
and 
are the number of moles of 
