Peculiarities of structure in aqueous electrolyte solutions and specificity of hydration effects

Quantitative detection of potassium chloride solutions at different concentrations using terahertz waves

Terahertz detection, utilizing electromagnetic waves within the terahertz spectral band, offers a sophisticated approach for the analysis and identification of materials. This technique capitalizes on the distinctive resonant interactions between terahertz waves and the vibrational modes of various non-polar substances and biochemical entities. A notable challenge arises in the terahertz time-domain spectroscopy (THz-TDS) when applied to water-rich samples, owing to the significant absorption characteristics of water molecules at these frequencies. Although various methodologies exist for the detection of potassium chloride, a common substance with extensive applications, its analysis using THz-TDS, particularly in aqueous solutions, remains unexplored in literature. This investigation delineates the terahertz spectral behavior of potassium chloride in powdered form and extends to evaluate aqueous solutions with concentrations ranging from 3 to 60 mmol/L. The findings indicate a pronounced absorption peak within the terahertz range for potassium chloride solutions, which progressively lowers in intensity as the concentration increases. Consequently, THz-TDS emerges as a swift, precise, non-invasive, and safe modality for the quantification of potassium chloride in solutions.

Manifestation of hydration of Na+ and Cl- ions in the IR spectra of NaCl aqueous solutions in the range of 2750-4000 cm-1

This work is aimed at the study at studying the influence of the interaction of solvate shells on the profiles of the IR spectra of sodium chloride solutions in the 2750-4000 cm^-1 range. The IR spectra of distilled water and sodium chloride solutions were obtained with the limit (0.356 g per 100 g of water) and 50 % of the limit (0.178 g per 100 g of water) concentrations at a temperature of 21˚. Theoretical methods based on the use of the DFT approach with the XLYP exchange-correlation potential are used to calculate the profiles of the IR spectra of clusters containing 9 water molecules per one NaCl molecule at the limit concentrations of the solution. In the case when the cluster contained a NaCl molecule, the spectra were calculated for interacting and non-interacting solvate shells in which the number of H₂O molecules varied from 3 to 6. The expansion of the experimental band profile on a basis containing the profiles of the theoretical bands made it possible to study the features of NaCl hydration with a change in the concentration of solutions. It was found that the IR spectrum band is formed mainly by interacting Na⁺ and Cl^- solvation shells, each containing 4 H₂O molecules, while the ninth H₂O molecule provides the bond between the solvated ions. As the salt concentration increases, the contribution of the solvation shells to the band profile increases too. The agreement reached in the positions and profiles of experimental and theoretical water bands at different solution concentrations substantiates the adequacy of the theoretical description of NaCl hydration. Theoretical studies explained the effect of a decrease in the band width, an increase in the peak intensity, and a shift of its maximum toward higher wavenumbers with increasing solution concentration.