engleski

Are ions reflected in water IR signature?

Hydrogen bond (HB) network formed between water (H2O) molecules in liquid phase displays exceptional structural and dynamic features resulting in unique physicochemical properties. Owing to two HB donating (OH) and two HB accepting centers (free electron pairs on O- atom), one H2O molecule can be simultaneously engaged in four HBs. Despite being scrutinized for decades, the average occupancy of HB centers remains ambiguous and the employment of different experimental methods participates in the discrepancy enhancement [1, 2]. Properties of water HB network become additionally complicated with the presence of ions in water solution. The traditional division of ions on kosmotropes and chaotropes, making HB network between water molecules stronger or weaker, respectively, is questioned by MD simulations [3], whereas experimental study based on steady-state IR spectra of aqueous solutions of various salts implies opposite conclusions [4]. In addition to the finding that the presence of monoatomic ions in aqueous solutions can be detected from signals of (anti-)symmetric stretching of water molecules ((a)sOH) [5], water signature originated from the combination of water bending (HOH) and libration of water molecules (L) revealed high sensitivity on water surroundings as well [4]. In this regard, we have examined aqueous solutions of sodium (Na+) and potassium (K+) salts of chloride (Cl), bromide (Br), iodide (I), nitrate (NO3), nitrite (NO2) and acetate (CH3COO) of equal ionic strengths (I = 100 mM) by IR spectroscopy in transmission regime, in the temperature range 25 – 70 °C. In order to examine the impact of pH on the signals of water, IR spectra of NaOH and KOH (I = 100 mM) were acquired as well. The signatures of different ions entangled in the combination band are unraveled by multivariate analysis of IR spectra (Fig. 1) [6]. 1. Y. Maréchal, The Hydrogen Bond and the Water Molecule, First Ed., Elsevier Science, Amsterdam, 2007, pp. 215–248. 2. F. Cipcigan, V. Sokhan, G. Martyna, J. Crain, Sci Rep. 8 (2018) 1718. 3. H. I. Okur, J. Hladílkova, ́ K. B. Rembert, Y. Cho, J. Heyda, J. Dzubiella, P. S. Cremer, P. Jungwirth, J. Phys. Chem. B 121 (2017) 1997–2014. 4. P. K. Verma, A. Kundu, M. S. Puretz, C. Dhoonmoon, O. S. Chegwidden, C. H. Londergan, M. Cho, J. Phys. Chem. B 122 (2018) 2587–2599. 5. F. Rauh and B. Mizaikof, Appl. Spectrosc. 70 (2016) 1214–1227. 6. P. Maleš, Z. Brkljača, I. Crnolatac, D. Bakarić, Colloids Surf. B 201 (2021) 111645.

Aqueous solutions ; Salts ; FT-IR spectroscopy ; MCR-ALS with EFA ; Fluctuations

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