Exploring the influence of urea on the proton-transfer reaction in aqueous amine solutions with Raman and ultrasonic relaxation spectroscopy

Abstract

A combined ultrasonic and vibrational spectroscopic approach is performed, which allows measuring all the kinetic and thermodynamic parameters associated with the proton-transfer mechanisms taking place in the ternary urea- 1,1,3,3-tetramethylguanidine (TMG) -water system. The addition of urea affects extensively the already existing proton-transfer reaction that occurs when guanidine is dissolved in water. An almost perfect agreement is found between measured sound absorption dispersion and that predicted by a single Debye-type relaxation process, which can be attributed to the protonation reaction of either urea or guanidine. The experimental and theoretically predicted Raman spectra confirm the protonation of urea when added in the guanidine-water binary system. By comparing the relaxation spectra obtained for urea and amine, we observe a difference in the amplitude by several orders of magnitude, which means that since urea is the one with the higher amplitude, its protonation is the dominant contribution in the acoustic spectra of the ternary system. From the isothermal standard volume change in the ternary system with increasing urea concentration, the concentration dependence of the sound velocity, density, kinematic viscosity and classical absorption coefficient, it is evident that the addition of urea makes it difficult for the water molecules to accommodate the molecules of TMG. Urea restrains more water molecules away from the amine and breaks down the associated network of the binary system driving it to a less rigid one.