Urea and urea-water solutions-an infrared study

Infrared studies of urea both in crystal and in water solution are presented. On the basis of temperature and concentration measurements of urea-water solutions, as well as from the spectra of crystalline urea films of hydrogenated and partially deuterated molecules, we have assigned the main bands in urea spectrum. It is shown that the series of spectra recorded at various concentrations in water can be decomposed into four basic spectra: the first one is the same as the spectrum of liquid water, the second one has features only due H2O but is slightly disturbed as compared with the first one and the other two display in addition features due to vibrations in urea. The relative intensities of these components show a strong concentration dependence, particularly for what concerns the ratio of the last two components. We ascribe one of these two components to a structure where urea-urea interaction is predominant in the solution while the other one is ascribed to a structure where spectral features are mainly due to urea-water interactions. The first component with urea-urea features can be observed only at urea concentrations higher than I M. Concentration dependence of the population of these two components shows that at higher urea concentrations the interaction between urea molecules can be observed presumably as a consequence of aggregation of urea molecules. Such aggregation of urea molecules in dimers and/or oligomers may explain some peculiarities connected with the unusual behaviour of urea solutions in water as predicted by the Shellman, Kreschek, Scheraga and Stokes model. Concentration measurements also show that urea molecules only slightly alter the structure of bulk water, and hence cannot be described as 'structure breakers'. (C) 2002 Elsevier Science B.V. All rights reserved.