Tetraalkylammonium salt hydrates

Other tetraalkylammonium salts are also used in electrochemical measurements they are tetraalkylammonium nitrates, picrates, carboxylates, sulfonates, etc. They can be prepared in the laboratory, by neutralizing the corresponding acid in water with ftjNOH just to the equivalence point, removing water, and then drying. If necessary, the products are recrystallized. Some tetraalkylammonium salts form hydrates and are difficult completely to dehydrate. For practical information, see, for example, Ref. [19]. 

Tetraalkylammonium salts, first synthesized by Hofmann in 1851 by the reaction of a tertiary amine with an alkyl halide, are soluble in various polar solvents. These salts form crystalline hydrates that contain large numbers of water of hydration molecules. The unusual physical properties of tetraalkylammonium salts and their effects on the structure of bulk water have been reported.2 3,4,5 currently, several theories exist as to the effects of the tetraalkylammonium cations on the structure or entropy of bulk water. Yet to be understood are the effects of the anions associated with the tetraalkylammonium ions on the overall structure of water. Not only Is the theory of interactions of such salts with water of interest to those engaged in basic chemistry, but the salts also are used in various applications. 

It is well-established that tetraalkylammonium salts and other nonpolar solutes exert a strong structure-stabilizing effect on the water lattice [Z09], documented for example in the slowing down of water rotational [Z08] and translational motion [ZIO]. Another important aspect on the interaction between tetraalkylammonium halides and water is that they form crystalline hydrates of high stability. In these the hydrogen-bonded water molecules form large clathrates in which the hydrophobic cations are enclosed [211]. One remarkable observation is that the halide ions may replace water molecules in the water clathrate lattice [312]. 

Let it be assumed that the value of the interaction energy of an ion with a solvent is an inverse function of the ion-first water shell distance, r. Then, if one has a series of salts (R,A, R2A,...) where R is, say, a tetraalkylammonium ion, and the anion is constant, the electrolyte property (e.g., the heat of hydration) can be plotted for the series of RAs, against l/ f (where r represents the cation radius), and the extrapolated value for l/rj" = 0 is then the individual heat of hydration for the common anion. A". 

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