We have described hydration of ions of different kinds on the basis of infrared spectroscopy and HDO as a probe. Electrolytes with amphiphilic anion have been studied in aqueous solutions of Mg(C2H5SO4)2, NaC2H5SO4, KC2H5SO4, NaC2H5SO3, KC2H5SO3, NaCH3SO4, NaCH3SO3, LiCF3COO, NaCF3COO, KCF3COO, and NaCF3SO3 by means of FTIR spectra of HDO isotopically diluted with H2O. A procedure has been applied that allowed us to remove the contribution of bulk water and thus to separate the spectra of ion-affected HDO. The results and the known literature data treated in the described way were discussed. Ions of different character have been taken into consideration:  mono-, di-, and trivalent, including hydrophobic and amphiphilic ones. Anions and cations appear to have a very different effect on water structure when their polarization power is considered. The energy of affected water H-bonds changes proportionally to the polarization power of anions. On the contrary, cations reveal the zone-type influence. The origin of such hydration behavior and the consequences have been discussed.

Mixtures of water and acetonitrile in the full concentration range have been studied by infrared spectroscopy. OD and CN stretching vibrations of HDO and CD3CN molecules have been used as probes of the structural environments. Acetonitrile molecules which are unaffected by water molecules are found for a broad concentration range (0.1 I XHP I 0.8), showing that a preferential solvation occurs. The strong tendency for self-association of water molecules is evident from the occurrence of a broad OD stretching band. Chains of water molecules linked by hydrogen bonds are suggested to be formed rather than spherical clusters.

The hydration of tetrabutylammonium cation (Bu4N+) has been studied in aqueous solutions of Bu4NBr, Bu4NCl, and Bu4NF salts by means of the FTIR method. Isotopically diluted HDO in H2O has been applied by using the stretching vibration OD band as a probe of the solutes' hydration. The spectra have been analyzed in a way that led to removal of the contribution of the bulk water and thus to the separation of the spectra for salt-affected HDO. In several steps of a curve-fitting procedure the Bu4N+- and the respective anion-affected OD bands have been refined from the salt-affected HDO spectra. The Bu4N+-affected OD band profile and the bulk spectrum have been converted to a probability distribution function of interatomic O···O distances for influenced water, by the use of a correlation curve. Comparison of the band for HDO affected by cation with that of the bulk water, as well as the derived respective distributions of O···O distances, leads to the conclusion that ”iceberg formation” is a wrong expression for the effect of hydrophobic groups in water. The results have been discussed with respect to enthalpy and entropy effects.

The influence of urea and trimethylamine-N-oxide (TMAO) on the structure of water and secondary structure of hen egg white lysozyme (HEWL) has been investigated. The hydration of these osmolytes was studied in aqueous solutions by means of FTIR spectra of HDO isotopically diluted in H(2)O. The difference spectra procedure was applied to remove the contribution of bulk water and thus to separate the spectra of solute-affected HDO. The structural-energetic characteristic of these solute-affected water molecules shows that, on average, water affected by TMAO forms stronger H-bonds and is more ordered than pure water. In the case of urea, the H-bonds are very similar to those in pure water. To facilitate the interpretation of the obtained spectral results, calorimetric measurements, DFT calculations, and molecular dynamics (MD) simulations of aqueous osmolyte clusters were performed. All of these results confirmed that the interactions of TMAO with water molecules are much stronger than those of urea with water. Additional ATR FTIR measurements were performed to characterize the influence of the examined osmolytes on the secondary structure of HEW lysozyme. The type of interactions (direct or indirect) was determined, based on the second derivatives of ATR protein spectra record during an increase in the osmolyte concentration. The changes in the amide I band shape caused by urea or TMAO were found to correlate quite well with changes in the water structure around these osmolytes.