The Adiabatic Compressibility of Nonelectrolyte Aqueous Solutions in Relation to the Structures of Water and Solutions

The sound velocity (V) and density of aqueous solutions of urea, dimethylurea, thiourea, acetamide, acetone, and dimethylformamide have been measured, and the adiabatic compressibility (β) has been calculated at various concentrations (μ: mole fraction). The β-μ curves and V-μ curves at various temperatures have a common intersection at the fixed concentrations of μcβ and μcV respectively. At these concentrations, the compressibility (βc) and the sound velocity (Vc) of a solution are independent of the temperature over a certain range. When βc is plotted against the mole ratio, rcβ=(water)/(solute) at μcβ, the plots for acetyl-derivatives fall on one straight line, and those for urea-derivatives, on another line. In the cases of acetone and ethanol, the rcβ values of these solutes coincide with the number of water molecules constituting the framework of the solid clathrate hydrate. Also, for the other aqueous solutions, it may be reasonable to conclude that the rcβ value corresponds to the number of water molecules constituting some other type of liquid-clathrate hydrate characteristic of each solute.