Membrane hydration number of Li⁺ ion in highly concentrated aqueous LiCl solutions

The hydration numbers of Li+ ions in various concentrated aqueous LiCl electrolytes have been determined using a novel physico-chemical environment of conducting polypyrrole polymer in order to investigate the size of the primary hydration shell of Li+ ions. Simultaneous cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) techniques were used.

The primary (membrane) hydration numbers obtained are 2 M LiCl, 5.4–5.5; 2.5 M LiCl, 4.9–5.0; 3 M LiCl, 3.9–4.0; 4 M LiCl, 3.5–3.6; 5 M LiCl, 2.8–3.1; 6 M LiCl, 2.6–2.7; 7 M LiCl, 1.7–1.8. The values obtained are found to remain constant and independent of the concentrations of the electrolytes below ~ 2 M but to decrease considerably with increasing concentrations.

The estimation of free bulk water implies that the structure of highly concentrated aqueous electrolytes is different than that in the standard concentrations and that electrolytes may contain two types of water molecules, namely tightly bound water molecules and loosely bound water molecules but hardly contain any free bulk water molecules.