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.
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 paper attempts to explain the mutual influence of nonpolar and electron-donor groups on solute hydration, the problem of big importance for biological aqueous systems. Aprotic organic solvents have been used as model solutes, differing in electron-donating power. Hydration of acetonitrile, acetone, 2-butanone, and triethylamine has been studied by HDO and (partially) H2O spectra. The quantitative version of difference spectra method has been applied to determine solute-affected water spectra. Analysis of the data suggests that solvent-water interaction via the donor center of the solute is averaged between water-water interactions around the solute. Such behavior can be simply explained by the model of solute rotating in a cavity of water structure, which is formed by clathratelike hydrogen-bonded water network. On the basis of the band shape of solute-affected HDO spectra and the corresponding distribution of intermolecular distances, the criterion for hydrophobic type hydration has been proposed. From that point of view, all the studied solutes could be treated as hydrophobic ones. The limiting band position and the corresponding intermolecular distance of affected water, gained with increasing electron-donating power of solutes, has been inferred from the data obtained. These observations are important for interpretation of vibrational spectra of water as well as for volumetric measurements of solutions. The simple model of hydration, proposed to better justify the results, connects the values obtained from the methods providing microscopic and macroscopic characteristics of the system studied.
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