An Ultrasonic Absorption Study of the DMF–Water System

Abstract

Abstract Using the pulse method, the ultrasonic absorption was measured for binary mixtures of water and N,N-dimethylformamide (DMF) at 5.5 and 57 MHz in the temperature range of 5–45 °C. At lower temperatures α⁄f2 as a function of the concentration produces a plateau up to x (x is the mole percent of DMF in solution)=1.5, a minimum point at x=6, and a maximum point at x=25. An increase in the temperature causes these characteristic features of α⁄f2 to disappear. At 45 °C the values of α⁄f2 increase monotonically. No relaxation process is observed in the frequency range of this work. The general features of the concentration dependence of α⁄f2 resemble those of the urea–water system in a water-rich region and those of the dioxane–water system in a DMF-rich region. The mechanism of the plateau can be explained by the cooperative nature of the formation and breaking of water clusters. The maximum of α⁄f2 is ascribed to the complex formation between DMF and water molecules, while the minimum of α⁄f2 arises from the competition between the breaking-down of the water structure and the complex formation in solution. The maximum behavior in the ultrasonic absorption can be interpreted in terms of a kinetic model, DMF+mH2O\ightleftharpoonsDMF·(H2O)m. m is determined to be 3, and the following values have been obtained for the model mentioned above: ΔVm\simeq1 cm3/mol, Km=10±2.