The concentration of the transferring species A at a boundary surface is specified.
Surface concentration can assume a variety of units, for example, molar concentration cAs,
mass concentration rAs, gas mole fraction yAs, liquid mole fraction xAs, etc. When the
boundary surface is defined by a pure component in one phase and a mixture in the second
phase, then the concentration of transferring species A in the mixture at the interface is
usually at thermodynamic saturation conditions. Specifically, for a gas mixture in contact
with a pure volatile liquid A or pure volatile solid A, the partial pressure of species A in the
gas at the surface is saturation vapor pressure, PA, so that pAs ¼ PA. For a liquid mixture in
contact with a pure solid A, the concentration of species A in the liquid at the surface is the
solubility limit of A in the liquid, c
A so that cAs ¼ c
A.
For a contacting gas and liquid where transferring species A is present in both phases,
there are two ways to specify the concentration at the gas–liquid interface. First, if both of
the species in the liquid phase are volatile, then the boundary condition at the gas–liquid
surface is defined for an ideal liquid mixture by Raoult’s law