Effect of temperature perturbation on hydrogen bonding in aqueous solutions of different urea concentrations

Water absorption peaks in near-infrared (NIR) and attenuated total reflectance (ATR)-Fourier transform infrared (FTIR) spectra at different temperatures both in the absence and presence of urea have been analyzed to investigate hydrogen bonding in aqueous solution by perturbing the temperature and concentration of urea. Concentration-dependent basic spectra of aqueous urea solutions represent different clusters in the system originating from the different extent of self-aggregation of urea molecules and water-urea interactions. Derivative and deconvoluted spectra confirm the presence of different structural components or clusters in pure water, but in the presence of urea a new strong water cluster could be identified for the first time above a certain concentration of urea. The degree of perturbation has been evaluated by 2D correlation and difference spectroscopy. The apparent molar volume, free energy change of activation (Delta G), change in enthalpy of activation (Delta H) and change in entropy of activation (Delta S) for the viscous flow of water in the presence and absence of urea have been analyzed. The comprehensive analyses help to infer the different extent of aggregation of the urea molecules and formation of clusters by water-urea interactions in aqueous urea solutions.