Are N-methyl groups of Tetramethylurea (TMU) Hydrophobic? A composition and temperature-dependent fluorescence spectroscopic investigation of TMU/water binary mixtures

Abstract

This paper reports results from temperature and composition dependent steady state UV-Visible absorption and fluorescence measurements and time-resolved fluorescence experiments, using a dipolar probe coumarin 153 (C153), in TMU/water binary mixtures. Both steady state and time-resolved spectroscopic data indicate, much like in water/alcohol mixtures, TMU-induced structural stiffening and transition of the tetrahedral H-bond network of water. The structural transition referred to above means cosolvent-induced transition of three dimensional tetrahedral H-bond network to two dimensional zig-zag chain-like structure often found in alcohols. A comparison to the results obtained for aqueous alcohol solutions suggests that the cosolvent concentration at which the structural transition occurs depends both on the polarity of the cosolvent and the size of the cosolvent molecules. UV-Visible absorption measurements reveal aggregation among TMU molecules at lower TMU concentration which shows a temperature maximum. In addition, red edge excitation effects have been observed at very dilute TMU concentration suggesting distribution of C153 among heterogeneous environments. All these results indicate hydrophobic interaction-induced aggregation of TMU in dilute aqueous solutions which corroborate well with the existing simulation observation. Temperature dependence of absorption spectral red-shift of dipolar probe coumarin 153 in TMU/Water binary mixtures has been shown here. The redshift grows up with increase of temperature from 278K and it reaches maximum at 293K.