We introduce the difference of chemical potential between the real fluid and perfect gas in both sides, leading to the following expression e -β(µ gas (P water )-µ water (P water )) P water = e -β(µ CnAm gas (P CnAm water )-µ CnAm water (P CnAm water )) P CnAm water (10) The chemical potential of gas phase for both water and water in the presence of electrolyte are written as: