IR spectroscopy of aqueous alkali halide solutions: Pure salt-solvated water spectra and hydration numbers

Abstract

Extrapolation techniques were used to obtain pure salt-solvated water spectra from the attenuated total reflection infrared spectra (ATR-IR) of aqueous solutions of the nine alkali halide salts LiCl, NaCl, KCl, CsCl, NaBr, KBr, NaI, KI, and CsI and the alkaline–earth chloride salt MgCl2. These salts ionize completely in water. The ions by themselves do not absorb in the IR, but their interactions with water can be observed and analyzed. A pure salt-solvated water spectrum is easier to analyze than that of a combined solution of pure water and salt-solvated water. Although the salt-solvated water spectra examined have distinctive signatures, they can be classified in three categories: those similar to NaCl; those not similar to NaCl; and MgCl2, in a class by itself. Each of the pure salt-solvated water spectra differs from that of liquid water, though the number of bands is the same. From the Gaussian band fitting, we found that the positions of the bands were fairly constant, whereas their intensities differed. The salt hydration numbers were determined: for NaCl, KCl, NaBr, KBr, and CsI solutions it is 5; for KI and MgCL2 it is 4; for NaI it is 3.5; for CsCl it is 3; and for LiCl it is 2. From these results we found that each pair of ions (monoatomic ions) of the ten salt solutions studied are close bound and form a complex in a cluster organization with a fixed number of water molecules.

Defective Continuous Hydrogen‐Bond Networks: An Alternative Interpretation of IR Spectroscopy

“…[27] Although dispersion in the refractive index (anomalous dispersion) is of potential concern, it is often neglected in transmission experiments [25] and has been previously shown to have little impact on ATR studies on ionic solutions. [28,31] Indeed, concerns regarding the significance of anomalous dispersion are also debated for IR spectra of H 2 O and D 2 O solutions, where some authors have accounted for it [34,35] and others have not. [36] We do not consider the effect of anomalous dispersion to deter from the broader conclusions about the IR analysis of low-concentration aqueous solutions.…”

Label-free sensing of the binding state of MUC1 peptide and anti-MUC1 aptamer solution in fluidic chip by terahertz spectroscopy

“…Namely, the binding between MUC1 and anti-MUC1 can take place in D-PBS solution but cannot do so in PBS solution. However, ions can modify the polarization properties of water molecules and alter the surrounding water shell, resulting in aberrant absorption compared to double deionized water [33]. Hence, we need to evaluate this interference effect of the divalent metal ions by measuring the THz absorption spectroscopy of the PBS and D-PBS solutions separately.…”

Conductivity of Cellulose Acetate Membranes from Pandan Duri Leaves (Pandanus tectorius) for Li-ion Battery

“…It can be seen that the absorbance of hydroxyl functional group at 3397.34 cm -1 of 65/35 CA/LiCl composition is higher than pure cellulose acetate and peak at 1639.58 cm -1 (Figure 2b) as indicated HOH hydration due to the presence of lithium chloride in cellulose acetate membrane. In addition, the absorbance peaks at 541.31 and 592.71 cm -1 suggest the existence of lithium in 65/35 CA/LiCl composition that enhance the conductivity of its membrane [18]. The XRD pattern of 65/35 CA/LiCl shows in Figure 3, was obtained 2θ value around 17.25° and 23.02°.…”