Pawel Plucinski

Interfacial transport processes in the reversed micellar extraction of proteins

Abstract Interfacial mass transfer coefficients for the transfer of the proteins α-chymotrypsin and cytochrome c between a bulk aqueous and a reversed micellar phase were measured using a stirred diffusion cell. The strong dependence of interfacial forward transport kinetics on pH and salt concentration indicates the dominant role that charge interactions play in this system. This role is elucidated theoretically by determining the electrostatic interactions between a protein particle and the bulk interface, and the degree to which that interface will deform in response to these interactions. Back transfer rates were found to be three orders of magnitude slower than those for forward transfer. The dependence of these rates on aqueous pH suggests that coalescence of the protein-filled micelle with the bulk interface dominates the desolubilization kinetics.

Novel techniques for iol/water separation

Abstract In this work two novel methods of separation of oil/water dispersions are proposed. In both methods wetting plays an essential role. It has been found that during the flow of a dispersion through thin microporous hydrophobic membranes with pore size similar to drop size, a spontaneous droplet enlargement takes place. This was manifested by very fast gravitational phase separation after one passage of the dispersion through the membrane. The fraction of separated oil was independent of initial oil concentration and also independent in a broad range from the flow rates. As an alternative to the gravitational separators, the enlarged droplets can be removed using hollow fiber modules. The dispersion is led through the lumen of the hollow fibers, the oil phase permeates through the porous membrane. A simplified mechanism is proposed to explain the influence of residence time and flow rate on the efficiency of the hollow fiber separation.

Two-phase kinetics of the solubilization in reverse micelles

Abstract The rates of the solubilization of methylene blue, water, and different electrolytes were measured in reversed AOT micelles in a liquid/liquid system using a two-phase stirred cell. Variation in the forced convection in the cell allowed the diffusional and chemical steps of the process to be differentiated. Applying the usual concept of mass transfer resistances it is possible to prove unequivocally that the rates of solubilization of methylene blue and water are controlled by the convective transport in the aqueous and organic phase, respectively. The transfer of metal cations, however, is a very slow process that is independent of the stirring rate and dependent on the interfacial area, which indicates that an interfacial step is rate controlling. With these results, particularly related to the kinetic behavior of mixed electrolytes, a mechanism for the interfacial solubilization is proposed, and some conclusions concerning the kind of aggregation are drawn. The values of the interfacial tension and tension gradients caused by convection suggest that appearance of a locally negative interfacial tension could be the reason for the encapsulation process.

Kinetics of the reextraction of hydrophilic solutes out of AOT-reversed micelles

Abstract Reversed micellar solutions, made of AOT in various nonpolar hydrocarbons and containing water, salt, amino acids, and/or chymotrypsin in their aqueous core were contacted with aqueous buffers with high ionic strength, thus evoking reextraction of the hydrophilic solute. The kinetics of these reextraction processes were measured by means of a two-phase stirred cell. The reextraction of all solutes was shown to be controlled by an interfacial process, and the reextraction rate was nearly independent of the solute. Water itself, however, was reextracted one to two orders of magnitude faster than the other solutes. These results are consistent with an interfacial-coalescence-controlling mechanism for solutes, the central step of which is the merging of the reversed micellar surfactant shell with the macroscopic interface. In contrast to this, an osmotic driven permeation mechanism is proposed for the fast reextraction of water, being characterized by diffusion of water through the AOT layers of micelle and macroscopic interface without exchange of salt.

The interactions between polyelectrolytes and AOT in an oil/water system

Abstract The interactions between sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and polyelectrolytes were studied by measuring the mass transfer of polycations from an aqueous to a reverse micellar phase. The quarternized poly(vinylpyridine) derivatives reacted with AOT and formed an insoluble complex (in isooctane or water) whenever surfactant/cationic monomer concentration ratio was less than 1.4. Surplus AOT in the system led to quantitative transfer of the complex from the aqueous to the micellar phase, which indicates a strong role of electrostatic interactions and probably hydrophobic ones. The phase behaviour and ion balance suggest a mode of “solubilization” of the polyelectrolytes in the micellar solution that differs from the previously observed incorporation of solute molecules into the reverse micelle. In the case investigated, the reverse micelles and surfactant-polyelectrolyte complex co-exist. The results of sedimentation (ultracentrifuge) measurements indicated strong interactions between the polyelectrolyte-AOT complex and the reverse micelles. The mass balance of AOT and water also suggests a certain kind of aggregation between the reverse micelles and the polyelectrolyte-surfactant complex. When barium was used instead of sodium as the counterion to AOT, the stoichiometry of the interphase-micellar phase transition changed from an AOT/monomer ratio of 1.4:1 to 1.1:1. The yield of the mass transfer of the protonated polyvinylpyridine was much lower than that of the quaternized product in the same system. A saturation-like isotherm of extraction was obtained if the AOT surplus was at least 20-fold. At AOT/monomer concentration ratios below 20, a third phase was formed. The addition of co-surfactants (n-alkanols) strongly increased the efficiency of mass transfer, but the critical AOT/monomer concentration ratio at the transition point (the appearance of a third phase or a clear oil/water system) remained the same.

The influence of cosurfactants on the solubilization of phenylalanine in water-in-oil microemulsion

Abstract The influence was studied of cosurfactants (a series of n-alkanols) on the equilibria of phenylalanine (Phe) solubilization in a water-in-oil (w/o) microemulsion (Winsor II system) as well as on the kinetics of phenylalanine mass transfer from an aqueous phase to a micellar phase. The distribution coefficient of phenylalanine was independent of the alcohol type and concentration. The resulting values of the water ratio and the interfacial tension as well as alcohol partitioning between the microemulsion and solvent phases were measured, also. The kinetics results (obtained from stirred cell measurements) are discussed from the view point of the three-step bud mechanism previously proposed for interfacial solubilization in w/o microemulsions. The observed “catalytic” effect of the added alcohol is discussed with respect to its influencing the incorporation of Phe molecules into the Aerosol OT (sodium bis(2-ethylhexyl) sulfosuccinate) layer as well as influencing the dynamics of the fusion of monolayers in the neck of the buds. Generally, the short-chain alkanols (pentanol, hexanol) accelerate the mass transfer and enable the easy reorganization of Aerosol OT molecules during the Phe incorporation, while the long-chain alkanols (n >/ 9) retard the mass transfer from the aqueous to the micellar phase. These effects seem to be connected with the mechanical state of the surfactant/cosurfactant interfacial layer: rigid or fluid.

Kinetics of the interfacial ion exchange in winsor II microemulsion systems

Abstract The kinetics of the ion exchange between an aqueous and a reverse micellar phase (Winsor II system) was studied in detail using a two-phase stirred cell. The observation that the solubilization rate was independent of convection in all experiments leads to the conclusion that the formation of reverse micelles with new counterions (i.e., the ion solubilization) is controlled by an interfacial process. Furthermore, it was observed that main group cations differ strikingly from cations of the transition groups of the periodic system of elements in solubilization rate, in interfacial tension, and in the influence of electrolyte concentration. This different behavior was explained by specific interactions, probably by some kind of chelate structures being possible between transition metals and the sulfonate group of the surfactant molecule (AOT) at the interface. A further series of experiments showed that the solubilization rate pronouncedly increases with the number of micelles. From this the conclusion can be drawn that the AOT coverage at the macroscopic interface increases as well. These facts point to a specific mechanism of adsorption being controlled by coalescence and new formation of micelles. The observed influence of solvents on the formation of reverse micelles is interpreted by the different ability of solvent molecules to penetrate into the palisade layer of AOT molecules at the interface. This penetration ability is higher for short-chain or cycloalkanes which therefore can stabilize the interfacial layer more effectively. Thus, the rate of micelle formation is significantly reduced.