INFRARED STUDY OF LOW CONCENTRATION AQUEOUS IONIC SALT SOLUTIONS: PURE AND MIX SALTS JEAN-JOSEPH MAX – ITF Labs, Montréal CAMILLE CHAPADOS – UQTR, Trois-Rivières Canada

2 Introduction IR study of 10 salts: Li, Na, K, Cs, Mg with Cl, Br, I Aqueous solution are made of 2 species Pure water and pure salt solvated water Ions are close bond in pairs (Na + Cl – ) Solvated with a fixed number of water molecules from saturation to ~0.2 M Is the situation the same at lower concentrations?

3 Factor Analysis (1) (2) simple use of scalar products of spectra no special algorithm factor orthogonalization is needed

4 Experiment FTIR ATR (3.3 internal reflections, ZnSe) 500 scans successive addition of mother solution volumes to the measured mixture circulating liquid in the IR cell ( not moved)

5 IR spectra of NaI from 0 to 1 M

6 FA residues with 2 principal factors

7 Spectra of orthogonalized factors

8 IR retrieved vs. [NaI] and hydration number

9 Conclusion on the pure salts NaI hydration is stable from saturation (8 M) down to 2 mM NaCl, BaCl 2 and CaCl 2 gave similar results Down to a few mM, ions are solvated in pairs with a fixed number of water molecules to form a neutral species Down to a few mM these water molecules are strongly perturbed by ion pairs

10 IR spectra of NaI-CsCl mixtures

11 Successive difference spectra

12 3 principal factor spectra and their MFs

13 FA residues with 3 principal factors

14 Salts chemical reaction in water Use of NaI, CsCl solvated waters, and pure water produces abstract spectra (showing negative bands) In water NaI and CsCl react to form solvated NaCl + CsI

15 Spectra of orthogonalized factors

16 IR retrieved concentrations

17 Conclusions In water, ions stay in neutral pairs down to a few mM. A fixed number of water molecules are strongly perturbed in the vicinity of the ion pairs. The dielectric medium (water) varies in the ion vicinities. Ion pairs are solvated in neutral clusters stabilized by the highest number of water molecules. Ions pairs in water reorganize following this rule. These results were obtained using a simple experimental setup but with rigorous data treatment. These are important findings on electrolyte solutions.

18 Acknowledgements NSERC (National Science and Engineering Research Council of Canada) UQTR (Université du Québec à Trois-Rivières) ITF Lab, Montréal, QC, Canada