SEPARATIONS INVOLVING PHASE CHANGES 1 Volatilization

Like growth reactions, corrosion at surfaces involves a change in surface composition however, it is typically associated with a deleterious change. Corrosion reactions can be separated into two classes those involving the removal of material from the solid and those involving the formation of a surface reaction layer. In the first type of reaction, gas-or solution-phase species react with the surface to produce volatile or soluble products as in the etching of silicon by hydrofluoric acid. The second type of reaction involves the reaction of surface atoms to form a new compound which remains on the surface. The oxidation of many metals is considered a corrosion reaction of this type. 

The design of a plate tower for gas-absorption or gas-stripping operations involves many of the same principles employed in distillation calculations, such as the determination of the number of theoretical plates needed to achieve a specified composition change (see Sec. 13). Distillation differs from gas absorption in that it involves the separation of components based on the distribution of the various substances between a gas phase and a hquid phase when all the components are present in Doth phases. In distillation, the new phase is generated From the original feed mixture by vaporization or condensation of the volatile components, and the separation is achieved by introducing reflux to the top of the tower. 

Other methods to determine the interactions between aroma compounds and wine matrix components do not involve gas phase measurements. For example, the equilibrium dialysis method has been applied for determining interactions between yeast macromolecules and some wine aroma compounds (Lubbers et al. 1994a) and more recently to study the interaction of aroma compounds and catequins in aqueous solution (Jung and Ebeler 2003). While this method can be set up in different ways, a simple approach is to fill a dialysis cell (two chambers separated by a semiper-meable membrane) with an aromatized liquid. A non-volatile component of wine can be added to one chamber of the cell and then the system allowed to come to equilibrium. If the added non-volatile component binds the aroma compound, the other chamber will be depleted by this binding. Quantification of this change in concentration permits calculating the quantity that is bound to the added substrate. 

The same type of molecular forces are involved as those in GC, except that, as the solutes no longer need to be volatile, ionic interactions can now be used to control retention, in addition to dispersive and polar interactions, as in GC. It will be seen that temperature can also be used to control retention in LC, in a somewhat similar manner to GC. The distribution coefficient of a solute between the two phases in LC will always result from both standard free entropy and enthalpy changes during distribution, as in GC. In addition, the separation of enantiomers will also depend primarily on a difference in the standard free entropy between the two isomers, that results from spatial variations and which are then augmented by standard free enthalpy differences. 

---