Thermodynamics and kinetics of solute transfer in reversed-phase liquid chromatography

Abstract

In this study, the thermodynamic and kinetic behavior of a homologous series of fatty acids is examined using a polymeric octadecylsilica stationary phase and a methanol mobile phase. The zone profiles are evaluated as the temperature is varied from 20 to 60 °C and the average pressure from 400 to 4570 p.s.i. (1 p.s.i.=6894.76 Pa). The rate constant for solute transfer from mobile to stationary phase ( k ms) appears to be relatively constant with carbon number. In contrast, the rate constant from stationary to mobile phase ( k sm) decreases logarithmically with increasing carbon number. This suggests that the mass transport processes become progressively slower, owing to the smaller diffusion coefficients of the larger solutes in the stationary phase. The activation energy decreases slightly in the mobile phase and increases slightly in the stationary phase with increasing carbon number. The activation energy in the stationary phase ranges from 41.6 to 55.9 kcal/mol, while the thermodynamic change in internal energy ranges from −9.8 to −29.0 kcal/mol for C 10 to C 22, respectively (1 cal=4.184 J). The activation volume increases with increasing carbon number in both the mobile and stationary phase. The activation volume in the stationary phase ranges from 31.7 to 211 cm 3/mol, while the thermodynamic change in molar volume ranges from −27.1 to −104 cm 3/mol for C 10 to C 22, respectively. These large changes in activation energy and volume suggest that the solutes do not enter and leave the stationary phase in a single step, but in a stepwise or progressive manner.