Ion-specific repulsive interactions in colloidal silica dispersions evidenced through osmotic compression measurements and implication in frontal ultrafiltration experiments.

Comparative osmotic compression experiments were performed on colloidal silica dispersions in the presence of various chloride salts at the same 0.01 M concentration with different counterions and highlighted the influence of ionic specificity on the resistance to water removal. These results were complemented with frontal ultrafiltration measurements that demonstrate modulation of the permeate flux according to the salt used.

Purification of biological molecules by continuous flow electrophoresis in the Second International Microgravity Laboratory.

The RAMSES instrument is a continuous-flow electrophoresis device that performs separation and purification of biological materials on a preparative scale. It was flown on board the shuttle Columbia during the Second International Microgravity Laboratory mission that took place from 8 to 23 July 1994. This project had a technological aim to qualify the design and the technologies used, and a more scientific one to improve understanding of the phenomena involved in this process. To reach these goals, experiments were performed in which different samples and a variety of operating conditions were used, some of which cannot be applied on earth. Useful purifications were obtained, allowing samples to be collected for further analysis on the ground. These results confirm the predictions of a numerical model that could be used for future process optimization and the samples were found to have retained their biological activity.

Influence of ionic specificity on the microstructure and the strength of gelled colloidal silica suspensions.

The dynamic rheological behavior of gelled Ludox suspensions in the presence of the structure-breaker ammonium counterion and the structure-maker sodium counterion has been investigated. Depending on the nature of the electrolyte and its concentration, the results highlight the crucial effect of the network microstructure on the rheological results. Within the same microstructure, the strength of gelled networks has been found to be greater in the presence of the poorly hydrated ammonium counterions at any Ludox volume fraction. The obtained results were discussed with respect to recent contradictory trends reported in the literature concerning the behavior of silica suspensions.

Influence of buoyancy-driven convection on protein separation by free-flow electrophoresis.

In free-flow electrophoresis, the stability and reproducibility of the flow field are prerequisites for a satisfactory separation. Due to the presence of the species to be separated and due to mass- and heat-transfer phenomena, non-uniformities in density inevitably appear, both within the carrier buffer and between the carrier buffer and the sample stream. On earth they give rise to buoyancy-driven convection which interferes with the separation. These effects have been quantified by numerical modelling. Agreement has been found between the numerical results and experimental observations.

Numerical simulation of protein separation by continuous-flow electrophoresis.

Continuous-flow electrophoresis is a process for separating protein mixtures on a preparative scale. Its resolution is determined by the migration distance at the collection plane and by the fineness of the filament occupied by each protein species. Filaments undergo spreading due to a number of different phenomena, among which electrokinetics and electrohydrodynamics are known to be important. In the first of these, differences in migration velocity between the ionic species give rise to local variations in pH and electrical conductivity near the protein filament. In the second, the local change in electrical conductivity distorts the electric field, thus inducing shear stress in the liquid and creating a local flow pattern. A numerical model has been developed to describe these phenomena when two proteins are being separated.

Optimization of protein separation by continuous-flow electrophoresis: influence of the operating conditions and the chamber thickness.

The separation of protein by continuous-flow electrophoresis is disturbed by a number of effects which cause spreading of the protein streams with a loss of resolution. Two numerical models have been used to describe the coupling between the various spreading phenomena. A simple model takes into account molecular diffusion, electroosmosis and residence time gradients; the main model differs from the simple one by taking account of electrohydrodynamics. The influence of the separation chamber thickness, the radius of the sample filament, the field strength, and the residence time is explored. The simulations show that each dispersive effect has its own range of predominance, so that an optimum is found for the thickness of the separation chamber.

The relative importance of transport phenomena in recycling isoelectric focusing.

The various transport phenomena involved in recycling isoelectric focusing are analyzed for their contributions to band spreading so as to find ways of improving the resolution of this liquid-phase protein purification method. A numerical model is proposed that takes into account diffusion, electroosmosis and electrophoretic migration as a function of pH. The electrohydrodynamic effects have so far been neglected in these calculations. The results of these calculations are compared with experimental measurements performed in different chamber geometries, with a variety of proteins and under different operating conditions, always chosen to avoid flow instabilities. This comparison shows that the resolution of this process is greatly impaired if the electroosmotic slip velocity at the wall is not suppressed.

Protein separation by continuous-flow electrophoresis in microgravity.

During the IML-2 space shuttle mission, the RAMSES instrument was operated in the Spacelab module. This continuous-flow electrophoresis device performs separation and purification of protein solutions on a preparative scale. Samples containing artificial mixtures of pure proteins were used to test the capabilities of the device, and useful separations were obtained for proteins having a mobility difference of only 3 x 10(-9) m2 V-1 s-1. Operating conditions that cannot be applied on earth were explored for two different sample concentrations, one of which is too high to allow treatment on earth. It agrees well with a previously published numerical model in that the main cause of loss in resolution in this process is the electrohydrodynamic spreading of the protein filaments.

Sensitivity models in simulation of capillary zone electrophoresis: the effect of buffer power and initial buffer concentration.

Separation of proteins by capillary zone electrophoresis in an amphoteric medium is investigated by numerical simulation. We explain how sensitivity models can be used to gain further information about this process. The sensitivity models that are derived are used to study the sensitivity of the separation to the specific buffer capacity of the amphoteric molecule that serves as a buffer. The effect of changing the overall concentration of this ampholyte is also examined and it is shown that a change in its concentration is equivalent to a change in its buffer capacity.

Protein separation by electrophoresis in a nonsieving amphoteric medium.

A numerical model has been used to study the separation of protein mixtures by zone electrophoresis in a nonsieving amphoteric medium. An amphoteric buffer fixes the pH of the solution close to its isoelectric point, where the buffer molecules are unchanged: they thus contribute very little to the conductivity of the solution. This means that high field strengths can be used for rapid separation without sacrificing resolution. The numerical study shows that in this process the band spreading that can reduce resolution is essentially due to differences in migration rate between protein-rich and low-protein zones: both differences in solution conductivity and in protein mobility are involved. Rules for judging the buffer capacity of amphoteric molecules are presented and it is shown how, for a given protein, the effectiveness of this technique varies with the range of pH in which it is applied.

Conditions for purification of proteins by free-flow zone electrophoresis.

The separation of proteins by free-flow zone electrophoresis is generally impaired by a number of secondary effects which cause spreading of the protein streams with resultant loss in resolution. A strategy is outlined, based on experimental observations and numerical modeling, which allows the operating conditions and separation chamber dimensions to be chosen so as to obtain complete separation of two proteins of known mobility. This approach takes into account some dispersive phenomena such as molecular diffusion, electroosmosis and residence time gradients. In some cases, the right operating conditions cannot be achieved on earth and electrophoresis separations in microgravity may then be justified.