Thermodynamics of Transport Phenomena in Field-Flow Fractionation: Organized Structures Formation

The classical theoretical model of the field-flow fractionation, assuming the exponential concentration distribution of the species affected by the external fields in polarization mechanism, is just an approximation neglecting the possible formation of the organized structures. Original use of Riemann’s integral to calculate the dimensionless exponential concentration distribution enables to study the thermodynamic aspects and the related transport phenomena for model systems where the field forces are weak and to obtain precise and predictive results on the equilibrium positions of the species forming the focused zones by two different mechanisms. The samples injected into the separation units of the field-flow fractionation are rather diluted but the applied field forces transport the macromolecular or particulate species and produce the zones of much higher concentration. Consequently, the organized structures, that constitute the spacially oriented concentration gradients in the direction of the effective field forces, appropriate to each particular separation mechanism, may appear whenever the repulsive interactions among the dissolved or dispersed species restrict their free Brownian motion. It is difficult to detect the existence of such structures directly in thin field-flow fractionation channels but it was unquestionably proven by the small angle X-ray scattering of larger volume of the negatively charged nanometer sized particles in aqueous suspension of silica. The experimental equilibrium positions of the particles of different densities but almost the same size, focused by the action of lift forces, were identical, but their trajectories from the starting takeoff from the accumulation wall to the equilibrium positions were different. This finding inspired the idea to exploit this effect for rapid focusing micro-TFFF.