Limiting single-ion molar volumes. Intrinsic volume as a function of the solvent parameters

Abstract

Examination of the individual ion limiting molar volumes for series of ions of similar size but different charge indicates that electrostriction is indeed a linear function of Z as proposed by Couture and Laidler. This conclusion is supported by calculations using expressions developed by Conway and co-workers. The intrinsic volumes of ions of all Z are calculated easily using a linear function and it is found that irrespective of charge or hydration, similar sized ions have similar intrinsic volumes. It is shown that in series of large molecular ions, RNH⁺₃, R₄N⁺, O₃XOX(O)₂O^n–…, that an increment in size of one carbon atom or one oxygen atom results in a volume increment equivalent to the displacement of one water molecule. It follows that the intrinsic volume of an ion, or of any solute, is determined primarily by the structure of the solvent and the way this forms cavities to accept a solute and only secondarily by the parameters of the ion. The monatomic ions do have intrinsic volumes which vary widely with radius but it is possible to define a radius below which the majority of ions displace one water molecule and have very similar volumes irrespective of ionic radius. Above this radius the ionic volume is determined by the way the second hydration sphere has to expand to accommodate the hydrated ion in the solvent structure.