THz spectroscopy was used to probe changes that occur in the dynamics of the hydrogen bond network upon solvation of alcohol chains. The THz spectra can be decomposed into the spectrum of bulk water, tetrahedral hydration water, and more disordered (or interstitial) hydration water. The tetrahedrally ordered hydration water exhibits a band at 195 cm and is localized around the hydrophobic moiety of the alcohol. The interstitial component yields a band at 164 cm which is associated with hydration water in the first hydration shell. These temperature-dependent changes in the low-frequency spectrum of solvated alcohol chains can be correlated with changes of heat capacity, entropy, and free energy upon solvation. Surprisingly, not the tetrahedrally ordered component but the interstitial hydration water is found to be mainly responsible for the temperature-dependent change in ΔC and ΔG. The solute-specific offset in free energy is attributed to void formation and scales linearly with the chain length.

We have investigated the low frequency (30-350 cm(-1)) spectra of solvated MnCl2, MnBr2, NiCl2, and NiBr2. Using a chemical equilibrium model in combination with principal component analysis, we were able to dissect the spectra into molar extinction coefficients due to the solvated ions and - for MnCl2, MnBr2, and NiCl2- to extract information on the ion pair spectra. The deduced anion spectra (calculated as MnCl2-MnBr2 and NiCl2-NiBr2) are very similar and nearly identical to the anion spectra observed for LaCl3-LaBr3. The differences between the cationic contributions MnCl2-NiCl2 and MnBr2-NiBr2 indicate that the solvated cation spectra can be understood in terms of distinct resonances of the octahedrally solvated cation complex that are red-shifted for Mn(2+) compared to Ni(2+). The description of the full extinction spectra requires the introduction of an additional resonance at a center frequency of around 130 cm(-1) that we tentatively assign to hydration water. Cooperative effects are small and are reflected in a change in the band intensity. However, the center frequencies of the observed modes remain unchanged when exchanging the counter ion. Analysis of the ion pair extinction spectra supports contact ion pair formation for MnBr2 and NiCl2 and MnCl2.

Ion radius and charge density are important parameters that determine the solvation behavior in aqueous electrolyte solutions. Here, we report on high precision THz absorption measurements of solvated LaCl3 and LaBr3 using narrow-band (75-90 cm(-1)) p-Ge laser and wideband (30-350 cm(-1)) Fourier transform spectroscopy. The concentration dependent absorption up to 3.3 M shows a prominent nonlinearity indicating ion pair formation with increasing electrolyte concentration. A more detailed analysis in terms of a chemical equilibrium model allowed us to separate the ion and ion pair contributions from bulk and solvation water. Thus we were able to characterize anion and cation solvation independently. The center frequencies of the Cl(-) and Br(-) rattling modes are in agreement with those found in aqueous alkali and earth alkali halide solutions. The coupling between anion and cation hydration is found to be small. Based upon our detailed analysis we propose increasing formation of solvent shared ion pairs with increasing solute concentration. The well defined ion resonances imply that in spite of its high charge density La(3+) acts locally on the water structure. Terahertz absorption spectroscopy is found here to be an experimental tool which allows us to directly observe solute hydration shells as well as ion pair formation.

We have investigated the hydration dynamics of solvated iron(ii) and iron(iii) chloride. For this, THz/FIR absorption spectra of acidified aqueous FeCl2 and FeCl3 solutions have been measured in a frequency range of 30-350 cm(-1) (≈1-10 THz). We observe a nonlinear concentration dependence of the absorption, which is attributed to the progressive formation of chloro-complexes of Fe(ii) and Fe(iii), respectively. By principal component analysis of the concentration dependent absorption spectra, we deduced the molar extinction spectra of the solvated species Fe(2+) + 2Cl(-) and FeCl(+) + Cl(-), as well as FeCl(2+) + 2Cl(-) and FeCl2(+) + Cl(-). In addition, we obtain ion association constants log KFeCl2 = -0.88(5) and log KFeCl3 = -0.32(16) for the association of Fe(2+) and Cl(-) to FeCl(+) and the association of FeCl(2+) and Cl(-) to FeCl2(+), respectively. We performed a simultaneous fit of all the effective extinction spectra and their differences, including our previous results of solvated manganese(ii) and nickel(ii) chlorides and bromides. Thereby we were able to assign absorption peaks to vibrational modes of ion-water complexes. Furthermore, we were able to estimate a minimum number of affected water molecules, ranging from ca. 7 in the case of FeCl(+) + Cl(-) to ca. 21 in the case of FeCl(2+) + Cl(-).

We have studied ion pairing of ytterbium halide solutions. THz spectra (30-400 cm) of aqueous YbCl and YbBr solutions reveal fundamental differences in the hydration structures of YbCl and YbBr at high salt concentrations: While for YbBr no indications for a changing local hydration environment of the ions were experimentally observed within the measured concentration range, the spectra of YbCl pointed towards formation of weak contact ion pairs. The proposed anion specificity for ion pairing was confirmed by supplementary Raman measurements.