A simple and efficient method to characterize bonded water molecules in aqueous solutions of electrolytes: Application to sodium sulphate decahydrate

During a study on crystallization in viscous media, an unexpected relevant theoretical and experimental result about the organization of water molecules in salt solutions has been obtained. Experimental results obtained during measurement of viscosity of saccharose and water solutions demonstrate that the salt dissolved in saccharose solution has the capacity to catch the solvent, namely water, from the solvated substance, here saccharose, which induces a quite large viscosity gap. There is a certain mutual electrolyte-water attraction; it may be pushed on by stronger water-salt than water-saccharose interactions. Determination of the binding of water with the salt in the solution can be done directly from measured viscosity. The number of water molecules withdrawn by Na2SO4 in solution is computable from viscosity values. Significance of the results obtained is a relevant demonstration that the 10 water molecules of Na2SO4 center dot 10H(2)O are already organised around the solute before the crystallization. The phenomenon observed here questions theoretical predictions that rely on solvent organization and salt effect in solution. Novelty of this study may thus encourage further experimental work in order to elucidate and test the validity of such investigational method to determine the number of water molecules withdrawn by salt in solution. (C) 2007 Elsevier B.V. All rights reserved.