Subject electrostatic energy

With the valence bond structures of the exercise, we can try to estimate the effect of the enzyme just in terms of the change in the activation-free energy, correlating A A g with the change in the electrostatic energy of if/2 and i/r3 upon transfer from water to the enzyme-active site. To do this we must first analyze the energetics of the reaction in solution and this is the subject of the next exercise. 

For more complex molecules a reasonable estimate of the heat of sorption can sometimes be made by considering group contributions. Such an approach works best for nonpolar sorbates on nonpolar surfaces but is subject to considerable error for polar systems in which the electrostatic energies of adsorption are large. 

Such a situation has been thoroughly reported in solid state physics books on dielectrics subjected to external electric fields [3] and may be applied directly here to aqueous solutions. Consider, for example, a pair consisting of a cation and an anion (each of charge - - q and — q) that are freshly formed in vacuum. Suppose their centers are separated by a distance r. Then, Coulomb s law of electrostatics gives the change in electrostatic energy of attraction as... 

Figure 1-5 If polar molecules, such as HF, are subjected to an electric field, they tend to line up very slightly in a direction opposite to that of the field. This minimizes the electrostatic energy of the molecules. Nonpolar molecules are not oriented by an electric field. The effect is greatly exaggerated in this drawing.

The minimum ignition energy of Hquid acetylene under its vapor, when subjected to electrostatic sparks, has been found to depend on the temperature as indicated in Table 3 (86). Ignition appears to start in gas bubbles within the Hquid. 

Best approach toward a general solution of all problems of induced stability appears to be a two-pronged surface treatment involving electrostatic and steric protection. In order to increase repulsion energy, zeta should be increased and to enable the particles to resist compression to a distance of separation less than that at E, a bulky molecule should be attached firmly to the surface. Some systems do not accept both steric and ionic protection but for those that do, the combination shows most promise. Er should not be increased without some assurance that the particles will not be subjected to drastic compressive forces. 

The ions in solution are subject to two types of forces those of interaction with the solvent (solvation) and those of electrostatic interaction with other ions. The interionic forces decrease as the solution is made more dilute and the mean distance between the ions increases in highly dilute solutions their contribution is small. However, solvation occurs even in highly dilute solutions, since each ion is always surrounded by solvent molecules. This implies that the solvation energy, which to a first approximation is independent of concentration, is included in the standard chemical potential and has no influence on the activity. 

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