Shift in the equilibrium between

Another way of decreasing the retention volume of functional macromolecules can be found in raising the column temperature. In this case, it should be borne in mind that a change in temperature causes a number of changes in the chromatographic system, mainly associated with the shift in the equilibrium between the components of the adsorbed mobile phase and water, which results in a change in the eab of the mixture and the adsorbent activity. 

Photochromic tautomerism refers to a photochemically induced shift in the equilibrium between tautomers which have significantly different absorption spectra. 

Arrhenius interpreted the equation by suggesting that there exists an equilibrium between normal molecules and what he called active molecules, and that only the active molecules undergo chemical change. The active molecules were supposed to be formed endothermically from the normal molecules. The rapid increase in the rate of chemical change with rising temperature is therefore caused by the shift in the equilibrium between the two kinds of molecules, and, since k is proportional to the number of active molecules, the equation d log k/dT = A/RT2 represents this shift in the ordinary thermodynamic way. A is the heat absorbed in the formation of an active molecule from a normal one and is therefore called the heat of activation. 

Figure 8 Schematic representation of the shift in the equilibrium between left- and right-handed helices (A) and the transition from left- to right-handed helix of poly isocyanates (B) by a photochemical trans-cis isomerization of the azobenzene unit.

Even if 5-NS and 5-NS (Me) did not have a small pressure region where a negative temperature expansion dependence was observed, the zero temperature dependence over most of the isotherm is still remarkable and calls for an explanation. For this region, a decrease in area/ molecule with temperature caused by a shift in the equilibrium between the two primary conformations (erect and bent) apparently balances the normal expansion of the films with increasing temperature. Moreover, since 5-NS and 5-NS (Me) can apparently attain close-packed conformations (as indicated by the inflection), a close-packed bent conformation is more stable then with 8-NS(Me). Nevertheless, 8-NP(Me) reveals a more extensive negative area-temperature dependence, presumably because the decrease in area/molecule is greater for 8-NP(Me) when the... 

Dwyer (2) and Kirschner (3) have attributed this change in rotation to a shift in the equilibrium between the dextro- and /ev -enantiomers of the complex, a shift that occurs because of the presence of an optically active "environment" (the /evo-malic acid) around the complex. Such an equilibrium shift is not possible in the case of optically stable complexes because no equilibrium exists between such enantiomers, as evidenced by the tendency of enantiomers of such complexes to resist racemization in solution for very long periods of time. Other proposals have also been put forth for the mechanism of this Effect, which are described in excellent reviews by Gillard and Williams (4) and Schipper (5), but space does not permit discussing these here. 

In the case of solvatochromism a shift in the equilibrium between two types of molecules can occur (for example, keto enol equilibrium, associated nonassociated form, or the stabilization of polarized or the unpolarized form of the dye can take place under the influence of the polarity of the solvent. For example, the behavior of 4-phenylazo-l-naphthol (I) (12, 14) represents a clear example of the shift in the tautomeric equilibrium under the influence of the solvent. When dissolved in pyridine this compound gives a yellow solution and contains exclusively azonaphthol form (la). 

---