Specific Features of the Temperature and Frequency Dependences of the Hypersound Velocity in an Aqueous Solution of 4-Methylpyridine with a Singular Point

The frequency dependence of sound velocity in the frequency range from 5.4 MHz to 6.1 GHz in an aqueous solution of 4-methylpyridine with a concentration corresponding to the singular point has been experimentally studied. The specific features of the positive sound dispersion and the found negative dispersion in a narrow range of hypersonic frequencies are discussed from the viewpoint of different structural organization of the solution components in the temperature ranges below and above the singular point temperature. An experimental indication of the existence of a structural phase transition in the solution at a temperature below the singular point, described within the mean-field approximation of Landau theory, has been obtained.