Solubility Coefficient (S)

The solubility coefficient, S _i , of a gas i in a membrane is the ratio between its solubility C _i in the membrane and its fugacity f _i or partial pressure p _i in the gas phase at constant temperature:

$$ {S}_i=\frac{C_i}{f_i} $$

(1)

For gas sorption in polymer membranes, a common unit is cm³(STP)/(cm³(pol) atm). The solubility coefficient of a low-pressure gas in a liquid is the reciprocal of Henry’s law coefficient H.

In the case of gas sorption in polymer membranes, of particular interest is the infinite dilution solubility coefficient, S ₀ , that is evaluated in the limit of infinite dilution or zero pressure of penetrant in the gaseous phase.

$$ {S}_{i0}={\displaystyle \underset{p\to 0}{ \lim }}\frac{C_i}{f_i}={\displaystyle \underset{p\to 0}{ \lim }}\frac{C_i}{p_i} $$

(2)

For vapor solubility in polymer membranes, the activity-based solubility coefficient is sometimes also used, S _i p _SAT , that is the ratio between solubility and corresponding penetrant activity, if the vapor...