The equilibrium at the interface between the liquid droplet and the surrounding gas is based on the assumption that the chemical potential for the liquid phase, l, and the vapor phase, v, are equal for each species, i. Assuming an ideal solution, the surface mass fraction of fuel vapor can be determined using Raoult’s law. For a mixture of discrete components, Raoult’s law is

(6–81)

where is the partial pressure of species i in the vapor phase at the droplet surface, is the vapor pressure of species i at temperature T, x is the mole fraction and the subscripts v and l denote the vapor phase and liquid phases, respectively. The species vapor pressure is given by the Clausius-Clapeyron equation.