Multicomponent diffusion in solutions where crystals grow

The correct study of diffusive time evolution of concentration boundaries i n n-component systems requires the use of all the (n-1)(2) diffusion coeffi cients defined by Fick's law. However, to simplify the analysis, the so-cal led pseudo-binary approximation is very often used. This can lead to very m isleading results. On the other hand, the possibility to predict the diffus ional behaviour of rt-component systems from the properties of correspondin g binaries and from the knowledge of the solute-solute cross-interactions s hould be a very important goal. If no "chemical" solute-solute interactions are present in solution, the diffusion coefficients depend only on the "hy drodynamic" or volumetric solute-solute interactions. This contribution, wh ich is mostly reflected in the cross-diffusion coefficient values, is alway s present and assumes an important role in solutions containing macromolecu lar solutes. It is then very important in modelling the diffusion phenomena in systems where a protein can crystallise in the presence of polymeric so lutes as precipitating agents. The present paper is devoted to the study of the hydrodynamic effects on the diffusion coefficients of poly(ethylenegly col) samples, which is one of the widely precipitating agents used in the p rotein precipitation. A predictive model to evaluate the diffusion coeffici ents in the presence of the only hydrodynamic effect was applied with good success to the systems presented and to a literature system NaCl-lysozyme-w ater. (C) 2000 Published by Elsevier Science S.A.