BAND BROADENING IN MICELLAR LIQUID-CHROMATOGRAPHY

The effect of temperature on efficiency in micellar liquid chromatogra phy (MLC) has been investigated using an SDS micellar mobile phase and a C-18 stationary phase. Application of the Knox equation to plate co unt data yielded crucial information about band broadening in MLC. The improvement in chromatographic efficiency with temperature is due to a decrease in both the A (flow anisotropy) and C (stationary phase mas s transfer) terms of the Knox equation. The decrease in the A term can be attributed to a shift in the position of the equilibrium of the so lute away from the micelle and towards the bulk solvent; the decrease in the C term with temperature can be explained in terms of surfactant adsorption which tends to increase both the thickness and viscosity o f the stationary phase. By increasing the operating temperature of the column, less surfactant is adsorbed on the stationary phase. Due to c oncerns about the validity of the Knox equation to describe band broad ening in MLC, it was necessary to consider other studies of micellar m obile phases in relation to their role in MLC, in order to obtain phys ically meaningful values for the A, B, and C terms of the Knox equatio n. When the different equilibria involving the micelle and surfactant were taken into account, the low rate of surfactant desorption could n ot be ignored as the underlying cause of band broadening in MLC. If th e desorption rate of surfactant molecules on an alkyl bonded phase is too low, the result is a disturbance in the various equilibria involvi ng the micelle and surfactant monomer. This disturbance would explain the continued adsorption of SDS on the stationary phase at concentrati ons in excess of the critical micelle concentration of the surfactant. The low rate of surfactant desorption would also affect the dynamics of micellization which play a crucial role in MLC mobile phase mass tr ansfer.