The thermodynamic characteristics of structural changes in water connected with the hydration of polyatomic and complex ions

Abstract

On the basis of previously developed ideas [1] we have considered the thermodynamic characteristics of structural changes in water connected with the hydration of polyatomic and complex ions. We have shown that during the hydration of these ions, due to their rotation in the solution, there is an additional change in the structure of water compared with the case of monatornic ions of the same size and charge. It follows from the obtained data that for a large number of the ions considered there is negative hydration, ideas on which were developed by O. Ya. Samoilov. Previously [1] we studied the thermodynamic characteristics of strnctural changes in water connected with the hydration of rnonatomic ions. We showed that these changes could be characterized quantitatively by the quantity S AShydr is the change in entropy, during the hydration of monatomic ions; AS I is the change in entropy of an ion on transition from the gaseous state to solution. ASII characterizes the structural chan~es in water durinE the hydration of monatomic ions, due to their potential in solution. From this point of view it is of interest to consider the thermodynamic characteristics of structural changes in water connected with the hydration of polyatornic and complex ions. Polyatomic and complex ions exhibit other motions besides translational motion: rotational, vibrational, intraionie, etc. The entropy of the studied gaseous ions is made up of three components: translational (St), rotational (Sr) and vibrational (Sv). The electronic component of entropy can be taken equal to zero, since for most polyatomie gases the multiplet electron levels are absent. Providing there are sufficient data, each of these components can be calculated by methods of statistical thermodynamics [2-4]. Consequently, to solve the problem it is essential to find the peculiarities in the behavior of polyatomic and complex ions compared with monatomic ions in solution and their effect on the structure of water. In a previous communication [1], as the "true" entropy of a monatomic ion in an aqueous solution we took the entropy of an inert gas which had the same atomic weight and which was isoelectronic with it. This fact makes it possible to determine the quantity &SII, characterizing the structural changes in water during the hydration of monatomic ions, which is connected with the presence of an ion potential. The appearance of a charge on a particle in solution does not change its "true" entropy, since the electron component of entropy does not change. For polyatomie and complex ions in solution,the solution of the problem is somewhat more difficult. As for monatomie ions, the entropy of a polyatomic or complex ion in solution is conveniently taken to be the entropy of a neutral particle in solution, isoelectronic and isostructural to the considered ion,with the same molecular weight. This choice also makes it possible to separate the change in entropy connected with the structural changes in water during the hydration of polyatomic or complex ions from thatconneeted with the potential of the ions under consideration. Some of the features compared with monatomic ions, which are available here, will be considered later. We can therefore use equation (1) to calculate the change in entropy connected with structural changes in water during the hydration of polyatomic and complex ions,and that caused by their potential. The values of AShydr