Non-Equilibrium Theory of Gas-Liquid Chromatography

Abstract

IN a gas-liquid chromatographic column the assumption of equilibrium demands that the rate constants for solute transfer between the gas and the liquid phases must be infinitely large and the flow-rates extremely small. In practical columns neither of these conditions is ever fulfilled; the kinetic rate constants are finite, and the use of extremely slow flow-rates is out of the question because of undesirably long retention times and pronounced longitudinal diffusion in the gas phase which enhances band broadening. This has led various workers to advance theories which take account of the effect of non-equilibrium conditions on band structure and column efficiencies^1–4. Of particular importance is the van Deemter theory⁴, which uses the driving force concept to describe the departure from equilibrium in terms of resistances to mass transfer in the gas and the liquid phases. The hypothesis of interfacial equilibrium involved in the theory is one the validity of which cannot be demonstrated directly by experiment. The existence of equilibrium at the interface would mean that if the concentration in the liquid phase at the interface is specified, the concentration in the gas phase at the interface is the same as if the two phases had been in contact for an indefinite length of time. Owing to continuous flow of carrier gas it is scarcely likely that the molecules once evaporated from the surface will get to the same spot again, and this, obviously, introduces conceptual difficulties regarding the nature of dynamic equilibrium at any point within the column. A natural consequence of the assumption of interfacial equilibrium is that once the diffusing molecules strike the interface, they pass into the other phase without experiencing any resistance at the interface itself. On the contrary, discrepancies between the calculated and observed rates of absorption suggest that there should be resistance to mass transfer at the interface^5–7. From the point of view of the theory of absolute reaction-rates, there are sound theoretical grounds for postulating the existence of interfacial resistance. One can imagine an energy barrier which may involve the formation of an ‘intermediate-activated state’; of all the molecules which strike the boundary surface only those which have the required energy of activation for diffusion surmount the energy barrier while the rest rebound into the gas phase, and furthermore the orientation of molecules may play a part.