Solvation process, energy

The specific processes discussed above are all special cases of the general process (9.2.1). In all of these cases we have seen the explicit modification of the equilibrium constant of the corresponding process. As indicated in Eq. (9.2.3), the general modification requires knowledge of the solvation Gibbs energies of all the components involved in the process. For macromolecules such as proteins or nucleic acid, none of these is known, however. Nevertheless, some specific solvation effects are examined in Sections 9.4 and 9.5. 

In the following model example, we assume that each species involved in the binding process has a spherical shape and that the FGs on its surface are distributed in such a way that each pair of FGs on the surface (i.e., exposed to the solvent) is independently solvated. In other words, the conditional solvation Gibbs energy of the ith FG (given the hard core H) is independent of the presence or absence of any other FGs. Formally, this is equivalent to taking only the first sum over i in the expansion on the rhs of Eq. (9.4.2). 

For this particular solvent we can now combine the solvation Gibbs energies for the process (9.6.1) to obtain... 

When a solute particle is introduced into a liquid, it interacts with the solvent particles in its environment. The totality of these interactions is called the solvation of the solute in the particular solvent. When the solvent happens to be water, the term used is hydration. The solvation process has certain consequences pertaining to the energy, the volume, the fluidity, the electrical conductivity, and the spectroscopic properties of the solute-solvent system. The apparent molar properties of the solute ascribe to the solute itself the entire change in the properties of the system that occur when 1 mol of solute is added to an infinite amount of solution of specified composition. The solvent is treated in the calculation of the apparent molar quantities of the solute as if it had the properties of the pure solvent, present at its nominal amount in the solution. The magnirndes of quantities, such as the apparent molar volume or heat content, do convey some information on the system. However, it must be realized that both the solute and the solvent are affected by the solvation process, and more useful information is gained when the changes occurring in both are taken into account. 

One can consider two facets of the solvation process, the energetics and the kinetics. Clearly, the kinetics will not matter if reactions take place on a time scale that is much faster or much slower than the solvation process. However, if reaction and solvation occur on the same time scale, the considerable energy changes that the solvation process can engender will affect the reaction. In fact, exactly what solvent motions take place during the solvation process may well be important. Thus, it is of interest to understand the kinetics of the solvation process. 

One of the models that has had considerable success for predicting solvation processes of dipoles in non-hydrogen-bonded solvents is the dielectric continuum model [5,14]. In this model, the amount of solvation will depend on the dipole density— that is, the molar concentration and strength of dipoles. While the position of the absorption maximum is not directly related to the energy of solvation that a molecule experiences, one would expect the two to be very strongly correlated. However, for the three different... 

Another physical effect associated with solvation is cavitation. It is again helpful to visualize the solvation process as a stepwise procedure. Here, we imagine the first step as being creation of a cavity of vacuum within the solvent into which the solute will be inserted as a second step. The energy cost of the cavity creation is the cavitation energy. Note that energy is always required to create the cavity - if it were favorable to create bubbles of vacuum in the liquid, the solvent would not remain in the liquid phase. 

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