Hydration and coordination at ionised nitrogen centres.

Abstract

The apparent molar volumes, Ov, of a series of tetra-n-alkylammonium bromides have been measured down to high dilutions by means of a modified Stokes and Hepler dilution dilatometer and at higher concentrations with a differential buoyancy balance. Despite conflicting indications in earlier literature, the present measurements have shown that the limiting law slope for the apparent molar volume is approached at sufficiently high dilutions. Measurements of the apparent molar volume and the apparent molar adiabatic compressibility have also been carried out on a series of substituted pyridinium and piperidinium salts. The effects associated with indirect and direct blocking at the N+ centre were observed with these two series of salts and allowed comparisons to be made with the results of measurements carried out on a series of alkylammonium salts where direct blocking of the charge centre arises. The partial molar volume and adiabatic compressibility of aqueous solutions of neutral pyridine and piperidine bases were also investigated. The apparent molar volume of these solutes showed pronounced maxima and minima when evaluated as a function of concentration. Some measurements of Ov in D2O solutions for the same solutes also showed a similar type of concentration dependence.