Time lag for transmembrane transport without interface equilibrium

Equilibrium at an interface is a key assumption in the traditional theory, describing a relationship of a time lag and diffusion coefficients during nonsteady-state transport through a membrane. Recent experiments demonstrated that this assumption is not always valid. Interfacial resistance has been identified in several separation processes, such as gas, vapor permeation, solvent extraction, liquid membrane, drug control release, etc. Role of non-equilibrium interface in the overall mass transfer is addressed in this paper and expanded equations for the time lag are derived. Analytical expressions allow the utilization of transient state information for the evaluation of interfacial resistance. It is demonstrated theoretically that ignorance of the interfacial resistance could decrease the estimated diffusion coefficient by a factor of 3. A concept of interfacial resistance index (IR index) is proposed, which allows the evaluation of interfacial resistance in the overall mass transfer process. For a given system the possible role of interfacial resistance in the overall mass transfer can be evaluated even before the actual interfacial transfer rate is known.