Ultrasonic Study of Dilute Aqueous Solutions of Urea, Guanidine Hydrochloride and Dioxane

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An ultrasonic study on dilute aqueous solutions of urea, guanidine hydrochloride and dioxane was made by means of a pulse method, with the intention of investigating the influence of these solutes upon the structure of water. Ultrasonic absorption coefficient was measured in the range of frequency from 15 to 45 Mc/sec, and ultrasonic velocity was measured at 5 Mc/sec. Temperature was varied from 10 to 40°C. In urea solutions, the magnitude of α⁄f2 (α=absorption coefficient, f= frequency) decreases first very slowly with concentration, then rapidly in the range from about 0.5 to 3M and at about 4–6M reaches a nearly constant value. In guanidine hydrochloride solutions a similar change in the relation α⁄f2 vs. concentration takes place with a smaller amount than in urea solution. This shows that guanidine hydrochloride has greater ability as a structure breaker in liquid water than urea. In the dioxane-water system, absorption increases with the concentration of dioxane. A nearly flat region in the absorption vs. concentration curve is found for a small concentration of dioxane. This feature in the α⁄f2vs. concentration curve, which is found in the small concentration region, is ascribed to the "cooperative breaking effect" upon the "open-packed structure" in water.
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