Solute-induced changes in the water H-bond network of different alcohol-aqueous systems

Abstract In this work, alcohol-water solutions for eight monohydric and two polyhydric alcohols at different concentrations and temperatures were analyzed by infrared spectroscopy to investigate the perturbation of the structure of water surrounding the amphiphilic alcohol molecule. The solute-induced structural changes of water were studied based on the solute-correlated spectral component, which reflects the vibrational properties of the solute and the water molecules perturbed by the solute. This component was obtained from the spectral decomposition using the vibrational multivariate curve resolution method. We used the HOH bending mode as the main probe to investigate the water hydrogen bonding network and estimate the number of water molecules perturbed by the solute molecule. In addition, the CH and CO stretching modes were analyzed to gain a more comprehensive insight into the hydration of these amphiphilic molecules and to provide a reasonable explanation for the observed phenomena. The blue shifts we identified in the HOH bending frequency from the solute-correlated spectra can be attributed to the water molecules with strengthened hydrogen bonds (i.e., the water molecules perturbed by the solute). The perturbation of the water H-bond network is found to depend on the size and shape of the hydrophobic moiety of alcohol, although the differences are small, i.e., the number of perturbed water molecules per alcohol molecule varies depending on the type of alcohol, but is generally between 1 and 3. It seems that this small perturbation of the solvent-water structure does not occur at the immediate water contacts, but rather at the distances beyond the immediate neighbor contact distance. A strong indication of the latter is the relatively large fraction of the bulk-like water (compared to the fraction of perturbed water) still present even in the highly concentrated alcohol solutions. In summary, the number of perturbed water molecules testifies to the fact that it is unlikely that hydration of alcohols at atmospheric pressure and at room temperature and above will result in larger distorted structural units of water different from those found in bulk water.