The absorption spectra of liquid water and various aqueous solutions were analyzed in a terahertz frequency domain (from 6 to 200 cm(-1)) which characterize the collective dynamics of water molecules. Particular attention was paid to the relaxation process iii the tange of, similar to 6-80 cm(-1). The physical essence of this process on the molecular level is still unclear. We found that the amplitude of this relaxation process correlates with the degree of destruction of water structure. The obtained data allowed us to interpret this process as a thonomolecular relaxation of free water molecules. On the basis of a consideration of the water polarization in the electric field, we proposed a method of calculation of the amount of free water molecules in solution.