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 terms of a self-aggregation process of 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.

Transmittance Fourier transform infrared (FT-IR) spectra of liquid water in the 4-80 degrees C temperature range are reported in the whole mid-infrared (MIR) region (4000-360 cm (-1)). The spectra were recorded by using a newly developed, home-made transmittance cell, working in light vacuum conditions (pressures of the order of 3-4 millibar). This permits the elimination of the aqueous vapor bands from the liquid spectra, particularly in the bending region, and the rapid collection of data without fluxing large amounts of nitrogen through the interferometer sample chamber. The temperature evolution of the OH stretching and HOH deformation bands is discussed in terms of Gaussian components analysis and a two-state model describing the equilibrium between different H-bond structures of liquid water. From this picture, structural and thermodynamic information about the hydrogen-bonding network of water is obtained.

We report the study of the effect of tert-butyl alcohol (TBA) and trimethylamine-N-oxide (TMAO) on a typical process governed by the hydrophobic interaction, such as the micellization of a surfactant in aqueous solution. The behaviour of the critical micelle concentration (c.m.c.) of the sodium dodecyl sulfate (SDS) was investigated as a function of the added amounts of TBA or TMAO by conductivity measurements. The hydration properties of the two solutes was studied by IR spectroscopy, in the region of the frequency of the O-H stretching band for H 2 O and of the methylic band for TBA and TMAO. The overall behaviour of the TBA/H 2 O and TMAO/H 2 O solutions is also studied by compressibility measurements. The data were discussed in terms of the hydration properties of TBA and TMAO and of the thermodynamical properties of the mixtures. Significant correlations between these properties and the effect of TBA and TMAO on the micellization process are shown.

The hydration properties of two biologically relevant molecules, trimethylamine-N-oxide (TMAO) and tertbutyl alcohol (TBA), were investigated by monitoring the effects of these two solutes on the near-infrared (NIR) spectra of water. In particular, the 1450-nm ν 1 + ν 3 water combination band (ν 1 is the symmetric stretching and ν 3 is the asymmetric stretching) and the 1928-nm ν 2 + ν 3 band (ν 2 is the bending) were recorded at 25°C in aqueous solutions of TBA and TMAO over the 0-0.1 and 0-0.05 solute mole fraction intervals for TBA and TMAO, respectively. NIR data show, in agreement with molecular dynamics simulations and other suggestions found in the literature, that on the whole water molecules are more tightly coordinated by TMAO than by TBA. Furthermore, nonadditive perturbations of the water's H-bond network are observed for TBA and are absent in the TMAO case. These results are discussed in connection to the significantly different action exerted by these two solutes on typical processes governed by hydrophobic interactions, such as protein folding and micellization of a surfactant. In these processes, the data support the assumption that the presence of TMAO or TBA modifies the extent of the free-energy contribution associated with structural reorganization of water.

In this paper, we report on IR and dielectric spectroscopy of Ca(AOT) 2 reverse micelles containing water pools of different sizes. We investigate the effect of changing the nature of the counterion (Na + f Ca 2+ ) on the hydration mechanisms and dynamics of AOT microaggregates. The hydration has been studied as a function of the molar ratio W ) [H 2 O]/[AOT] by means of IR absorption of O-H stretching modes in the 3800-3000 cm -1 frequency range. The dynamical properties of the system have been investigated by dielectric permittivity measurements performed in the range 0.02-3 GHz, as a function of W. A relaxation phenomenon has been observed in this region, and a close connection between the behavior of the relaxation time vs W and the progressive hydration of AOT polar headgroups has been found, similar to the case of the NaAOT micellar system. These results could be interpreted in terms of two coexisting diffusion mechanisms: the reorientation of the whole micellar aggregate and the free rotational diffusion of the completely hydrated AOT headgroups.