## 1 Introduction  
Terahertz spectroscopy is a modern and rapidly developing method capable of analyzing the structure of matter. The terahertz region corresponds to intermolecular vibrations of molecules in terms of energy (meV) and frequency (THz) values which allows investigating the intermolecular structure and dynamics. The method has been successfully tested in various studies and has been proven to be applicable for addressing tasks in the fields of molecular biology [1; 2; 3; 4; 5; 6; 7], medicine [8; 9], pharmacy [10; 11; 12], etc. However, in most of these works the studied objects were substances with low quantity of water, whereas this method has been rarely used to study aqueous solutions, for example in [13; 14]. This is due to a strong absorption of the water itself which masks the absorption of dissolved substances. Nevertheless, of particular interest are some areas of research exploiting terahertz spectroscopy which have made it possible to obtain previously unknown and fundamentally new data on aqueoussolutions. In a number of studies aimed at analyzing aqueous solutions of various substances, in particular sugars and proteins, the molecules of these substances have been shown to have hydration shells 10 to 50 A thick [15; 16; 17] which corresponds to dozens of layers of water molecules. None of the previously known experimental approaches have made it possible to register such extended hydration shells. In another series of papers [18; 19; 20; 21; 22], terahertz spectroscopy was used for studying the structure and dynamics of molecules in various aqueous solutions which resulted in establishing quantitative relationships between the spectral parameters in the terahertz region and the fraction of free water molecules. Based on the obtained information, a novel method for the analysis of water structuredness has been proposed. These studies describe the features of spectra measurement which allow for the exclusion of a number of unwanted optical and spectral artifacts. As a result, it becomes possible to determine the dielectric function of aqueous solutions with high accuracy. In the article [23] a study of the effect of protein molecules in various conformations on the water environment has been described. The abovementioned results indicate a very high potential of the terahertz spectroscopy technique which generally makes it possible to study a wide range of objects with high water content. Such objects include highly diluted solutions obtained by serial dilution technology with vigorous shaking at each stage. To date, more and more data on their effectiveness appear. In particular, it has been shown that high dilutions of antibodies can be used to treat various diseases [24; 25; 26; 27; 28]. High dilutions exert a modifying effect on the properties of the starting material [29; 30; 31; 32; 33], despite the colossal difference in the quantities of mixed substances (of the original substance and its high dilution). All this indicates that in the process of dilution the organization of hydration shells of molecules can change which determines the effects of high dilutions [34]. Unfortunately, the sensitivity limit of modern analytical methods does not allow for the direct detection of the active component in highly diluted solutions of substances. However, a THz technique can solve the problem as it allows to investigate the peculiarities of intermolecular bonds in solutions sensitive to changes in the composition of the solution. It is capable of providing information on the intermolecular structure and interactions in solutions in order to elucidate the physicochemical nature of the effects of the above-mentioned drugs. One of the most up-to-date modifications of the method is the terahertz time-domain spectroscopy (THz-TDS) which offers wide opportunities for conducting reliable measurements of spectra in the 5-110 cm\({}^{-1}\) region (0.15-3.3 THz) [35; 36; 37; 38; 39; 40]. THz-TDS allows us to simultaneously obtain both the absorption and refractive index spectra of the substance.