Interactions competing ions, role

The hydration of halide anions is of intrinsic interest to the process of solvation. The most important aspect of water, its hydrogen bond network, is directly perturbed by the anion in a simple and direct way. It competes for the protons with its own ionic hydrogen bond. The gas phase studies of the smallest hydrated ions show extremely large shifts in the 0-H stretch in the 0-H-X bond. This strong interaction must play a role in the bulk solvation process. Interesting implications will be discussed in the final section of this chapter. 

The classical picture of the role of ions in aqueous polymer solutions is related to the notion that hydrophilic colloids are heavily hydrated in solution. The ions are considered to exert their influence by competing successfully with the polymer for the available molecules of water. Because of the charge on the ions, it might be expected that the ion-dipole interactions would be stronger than the dipole-dipole interactions between the water and the polymer. The classical viewpoint would therefore claim that the ions dehydrate the polymer and so induce flocculation. 

Nitrogen-containing compounds form an important class of N-bases due to their role in life processes. This is one of the reasons for our interest in reactions of free carbenium ions with amines [1-3]. In the course of these studies we found that, in contrast to the other N-bases, the interaction of carbocations with amines occurs via two competing channels, i.e. the formation of the condensation complex as a result of the overlap of the vacant p-orbital of the cation with the lone pair orbital of nitrogen and the proton transfer from the carbenium cation to the amine. The latter channel is very effective, due to the high proton affinities of amines. 

Enzymes have also been incorporated into the PAMAM dendrimer to produce silica nanoparticles. At physiological pH, the interaction between an enzyme and the cationic surface of the PAMAM dendrimer plays an important role in forming supramolecular complexes. As a result, the cationic dendrimer (pXa 9.5) interacts with low-pf enzymes, for example, glucose oxidase (GO, pf -4.2) was encapsulated in water with a quantitahve yield. As phosphate ions compete with the enzyme, GO was poorly encapsulated as phosphate buffer concentrations increased because the phosphate ions significantly reduced the electrostatic interactions between the enzyme and dendrimer (Figure 1.25). 

Polar solvents may interact strongly with a mineral oxide surface. In principle, the adsorption of die solvent must be considered. Claesson [13] studied the adsorption of fatty acids by sihca from solvents of various polarities. The results show that polar solvents compete with the solute for available sites on the surface, while nonpolar solvents show little competition. The polarity of the solvent is often determined from the measured dielectric properties. Krishnakumar and Somasundaran [13] studied surfactant adsorption on to silica and alumina from solvents with various dielectric properties. The aim of the study was to look at the effect of adsorbent and smfactant acidities and solvent polarity on the adsorption properties of the surfactant molecules. They used anionic and cationic surfactants as adsorption probes. The results show that polar interactions control the adsorption from solvents of low dielectric properties while hydrocarbon chain interactions with the surface play an important role in determining adsorption from solvents of higher dielectric properties. It was also found that an acidic surfactant interacts strongly with a basic adsorbent, and vice versa. One should be aware that the polarity of a molecule as measmed from the dielechic properties is not always eorrelated with the ability of the molecules to form ion pairs. For example, dimethylformamide and nihomethane have almost equal dielechic constants. However, the extent of ion pairing in nihomethane is much greater than that in dimethylformamide. Thus, the solvent acidity and basicity are the physical properties which can best characterize the ability of the solvent to compete with the solute for available sites on the mineral surface. 

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