Multicomponent diffusion in solutions where crystals grow

The correct study of diffusive time evolution of concentration boundaries in n-component systems requires the use of all the (n−1)2 diffusion coefficients defined by Fick’s law. However, to simplify the analysis, the so-called pseudo-binary approximation is very often used. This can lead to very misleading results. On the other hand, the possibility to predict the diffusional behaviour of n-component systems from the properties of corresponding binaries and from the knowledge of the solute–solute cross-interactions should be a very important goal. If no “chemical” solute–solute interactions are present in solution, the diffusion coefficients depend only on the “hydrodynamic” or volumetric solute–solute interactions. This contribution, which is mostly reflected in the cross-diffusion coefficient values, is always present and assumes an important role in solutions containing macromolecular solutes. It is then very important in modelling the diffusion phenomena in systems where a protein can crystallise in the presence of polymeric solutes as precipitating agents. The present paper is devoted to the study of the hydrodynamic effects on the diffusion coefficients of poly(ethyleneglycol) samples, which is one of the widely precipitating agents used in the protein precipitation. A predictive model to evaluate the diffusion coefficients in the presence of the only hydrodynamic effect was applied with good success to the systems presented and to a literature system NaCl–lysozyme–water.