Ciystal solvent, effects

Although there are examples of enzymes which maintain their catalytic activity even when ciystallized, they normally work in their natural (i.e., aqueous) environment. This is the reason why the majority of the simulations are carried out applying a technique that accounts for solvent effects. But what is the effect of a solvent ... 

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or ciystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binaiy mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirec tly cooled, and/or solvent is evaporated to effect ciystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt ciystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining sec tion of the apparatus. 

Interactions between solute and solvent molecules can have a significant effect on the shape of a crystal. This can be accounted for by specific adsorption of the solvent molecule on ciystal faces. Current oystal growth theories indicate that when interactions between solute and solvent are strong the solute molecules are solvated and a solvation layer dsts at the oystal-liquid interface which likely can vary as a function of ciystal face. Crystal growth requires desolvation of die solute molecule and desolvation of the surface site on the crystal. The molecule then surface diffuses imtil it reaches an incorporation (kink) site. 

A few years ago, a largely empirical approach was used to quantify the effect of admixtures and solvents. A theoretical description of the effect of admixtures has been developed only rather recently. Nevertheless, a consistent theory of the effect of admixtures on individual aspects of the process of crystallization is still missing. Various admixtures probably operate with different mechanisms. Some of them are selectively adsorbed on ciystal faces and deactivate individual growth centers, others can change the structural properties of the solution or of the interface they may be incorporated into the crystal lattice or pushed away by the growing crystal and sometimes there exists a chemical Interaction between the micro- and macrocomponents. It is obvious that this situation enables us to give subsequent explanations of individual effects but the prediction Is... 

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