Abstract
The objective of this study is to determine the Lifshitz–van der Waals/Acid–Base (LW/AB) surface energy components of urea–water-solutions (UWS) for different urea mass fractions. The surface energy parameters are evaluated by ring tensiometry and contact angle measurements of sessile drops placed onto pre-determined solid substrates. Therefore, the energetic characteristics of UWS are evaluated in relation to probe liquids. The results indicate that aqueous solutions of urea become less polar with increasing urea mass fraction while their overall surface tension is also increased. This is attributed to a significant grow of the Lifshitz–van der Waals surface energy component that compensates the reduction of the polar part. In addition, aqueous solutions of urea are characterised by a significant electron donor (γl−) capacity compared to pure water while their electron acceptor parameter (γl+) is reduced. Subsequently, γl+/γl− is continuously reduced with increasing urea concentration. The enhancement of electron-donicity is also reflected to the pH of the solutions while the overall trends are independent from the selection of acid-to-base ratio for pure water. The above findings are related to physicochemical aspects based on molecular and intermolecular interactions.
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