Infrared study of the hydrophobic hydration and hydrophobic interactions in aqueous solutions of tert-butyl alcohol and trimethylamine-n-oxide

This work concerns a comparison of the hydration properties and self-association behavior in aqueous solution of two biologically relevant simple molecules: the trimethyl-amine-N-oxide (TMAO) and the tert-butyl alcohol (TBA). These molecules are geometrically very similar, having the same hydrophobic moiety and different polar groups. Both molecules were used as a model to study hydrophobic behavior in water solution. In particular, water perturbation induced by TBA and TMAO molecules, Was studied as a function of the solute molar fraction X-2 (0 < X-2 < 0.05) by using the IR absorption bands due to the vibrational modes of water in the 4000-2600 cm(-1) frequency region. Furthermore, possible clustering effects in aqueous solution of the TBA and TMAO hydrophobic groups were investigated by studying the behavior of the alkyl CH stretching band in the 3200-2800 cm(-1) frequency region as a function of X-2. The OH stretching absorption data show, in agreement with molecular dynamics simulation results and other suggestion found in the literature, that the interaction of the TBA and the TMAO with water are remarkably different, In fact, water molecules are more coordinated by TMAO than by TBA. Significant differences are also evident in the CH stretching data for the two molecules. For TBA, the data can be interpreted in term,; of a self-aggregation process or the alcohol molecules occurring beyond a threshold value of the alcohol molar fraction (X-2* = 0.025). This phenomenon seems to be absent in the TMAO samples.