Determination of Thermodiffusion Parameters from Thermal Field-Flow Fractionation Retention Data

Abstract

Field-flow fractionation (FFF) encompasses a broad family of methods of separation and characterization of supramolecular species. They are all based on the application of a field force perpendicularly to the direction of the flow of a carrier liquid in a thin parallel plate channel. The principle of the method is described and the general relationships between retention time and analyte properties are presented. Then, the specific characteristics (such as range of applications, relaxation times, flow stability) of thermal field-flow fractionation (thermal FFF), which is the FFF method that is based on the application of a temperature gradient, are discussed. Thermal FFF basically relies on the Soret effect and the thermal FFF retention time is related to the Soret coefficient, S-T, of the polymer sample in the carrier liquid. However, the extraction of S-T from retention time is rendered complicated by the temperature dependence of all relevant physico-chemical parameters (thermal conductivity, viscosity, density, and Soret coefficient itself). The various methods which have been developed to take into account some or all of these dependences are described. One of the most complete method, the so-called linear lambda method, is applied to the determination of the Soret coefficients of a series of polystyrene standards in ethylbenzene, and to their temperature dependence. The resulting data are further analyzed to obtain the molar mass dependence of ST. This dependence can be expressed as a power law. It is found that the prefactor and the exponent of this power law both depend on temperature and have an optimum in the investigated temperature range. Furthermore, this exponent appears to be somewhat larger than the exponent of the power law relating the ordinary mass diffusion coefficient to the molar mass. Hence, depending on temperature, the thermodiffusion coefficient of polystyrene in ethylbenzene might slightly depend on molar mass.