Micro-thermal field-flow fractionation of colloidal particles: Effect of temperature on retention and relaxation processes

An important decrease in the heat flux across the micro-thermal field-flow fractionation (micro-TFFF) channel compared with the standard size channel allowed to control independently the temperatures of the cold and hot walls and to study the behaviour of colloidal particles within an extended temperature range. The only limitation was imposed by the freezing and boiling points of the carrier liquid. The retention and the relaxation processes were found to be influenced by temperature. A decrease in the viscosity of the carrier liquid with increasing temperature results in an increase in the diffusion coefficient of the retained species. Consequently, the relaxation processes associated with the establishment of the steady-state concentration distribution are accelerated, the time to reach the steady state decreases with increasing temperature, and the retention ratio of moderately retained particles decreases to reach a minimum value. The fractograms of colloidal samples of narrow particle size distribution (PSD) obtained at lowest temperatures can exhibit two peaks; the first corresponding to the unrelaxed particles eluted even at higher than the average velocity of the carrier liquid and the second one corresponding to the particles retained less compared with the zone eluted at a steady state from the very beginning. The further increase in temperature above that at which the retention ratio reaches the mentioned minimum produces inversion of the retention ratio, i.e. its increase. The peak corresponding to the unrelaxed part of the particles progressively disappears with increasing temperature. If the flow of the carrier liquid is stopped immediately after the injection of the sample for a time to reach a steady-state concentration distribution of the particles across the channel and restarted after that period, the retention ratio does not exhibit a pronounced minimum. On the other hand, the width of the zone generally decreases with increasing temperature but less if the stop-flow procedure is applied. These findings confirm that the retention ratio is influenced by temperature in agreement with the theoretical prediction. A practical conclusion is that higher resolution and reduced time of the separation can be achieved at higher temperatures of the accumulation wall. The micro-TFFF thus becomes a highly competitive method of separation and determination of the PSD of colloidal particles.