## 2 Materials and Methods
### Terahertz Spectra Analysis
In other cases, different models are used to describe the dielectric function of water in the terahertz region [51], including up to 3 relaxation and 3 vibrational bands. In this regard, the question arises: how many bands should be taken into account in this case? If we take into account a large number of bands, this will allow us to more accurately describe the experimental dielectric function. However, it would require taking into account a larger number of model parameters which, in turn, leads to a greater uncertainty in the values of these parameters in the calculation. If we take a small number of bands, then the model parameters are determined with a smaller spread. In this case, we will lose the ability to analyze other parameters. Thus, the information content of such a model will decrease. It is also important to mention that the interpretation of some bands from the terahertz region is still a subject of discussion [52; 53; 54]. Thus, even after obtaining the exact values of the parameters of the additional bands, we encounter uncertainty in the interpretation. In this study, an attempt was made to analyze aqueous solutions with very small differences in the spectra in the terahertz region. Therefore, the stability of the results obtained and the small scatter are extremely important for us. As we found out, this was achievable by taking into account three spectral bands. At the same time, the model we selected (3) is quite informative and allows to extensively characterize molecular structure and dynamics of the water phase, since each of these bands is associated with certain types of water dynamics at the intermolecular level. The model chosen (3) enables describing the properties of the solutions using defined physical parameters, such as the time of rotational relaxation of water molecules, the resonant frequency of intermolecular vibrations, and the contribution of particular molecular dynamic processes to the total dielectric response. It should be noted that two of the three bands chosen for the model have the peaks outside the measuring range: Debye relaxation band (at the low-frequency end of the range) and intermolecular stretching vibrational band (at the high-frequency end). However, these bands are very intense, and they quite strongly influence the features of spectra in the measured range [18]; therefore, it would be incorrect to ignore their impact. Model (4) initially contains 9 parameters: \(\tau\), \(\Delta\varepsilon\), \(\text{A}_{1}\), \(\omega_{1}\), \(\gamma_{1}\), \(\text{A}_{2}\), \(\omega_{2}\), \(\gamma_{2}\), \(\sigma_{0}\), the optimal values of which are to be determined. However, it is plausible to reduce the number of variable parameters in order to increase the reliability of fitting. First, the conductivity \(\sigma_{0}\) was determined by direct measurements. Second, the two intermolecular bands (stretching and bending) have a similar molecular dynamic meaning--intermolecular vibrations of hydrogen-bonded water molecules. Therefore, one can omit some parameters of one of these two bands and focus on the analysis of the equivalent parameters of the other band. In this study, we used an approximation of consistency of the resonant frequency of the intermolecular stretching band \(\omega_{2}\), setting it equal to 200 cm\({}^{-1}\), which is a characteristic value for this type of vibration. The maximum of this band is located far from the analyzed spectral region, and its position varies slightly between aqueous solutions at the same temperature. Therefore, we did not include \(\omega_{2}\) among the varied parameters. Third, when the binding of water molecules in the solution changes, the mean relaxation time \(\tau\) may alter, which is related to a shift in the band maximum.