Abstract

The pure polycrystaline ice is formed during freezing of aqueous solutions. Most of the solutes are expelled to the grain boundaries of ice crystals. The UV/vis absorption spectroscopy of the various organic compounds frozen in the aqueous solutions was used to elucidate the changes in absorption spectra and to characterize the amount of aggregation of organic compounds, H+ and OH ions availabilities, as well as solvatochromic properties. Molecular aggregations in ice change the absorption characteristics of the organic molecules due to changed interactions with the host water molecules of the cavity as well as intermolecular interactions among the solute molecules themselves. The extent of aggregation of organic compounds was studied on a model compound (methylene blue). The absorption spectra of this compound are known to change upon aggregation. The re-evaluated equilibrium constants of dimerization and trimerization in liquid water solution allowed us to estimate local concentrations in the frozen samples. The freezing rate, not the temperature of the sample, plays the central role in the degree of aggregation. While the local concentration of methylene blue at the grain boundaries of polycrystalline ice increased by approx. 3 orders of magnitude upon fast freezing at 77 K compared to the liquid phase, the concentration raised at least by 6 orders of magnitude upon slow freezing at 243 K. The protonation degree of cresol red in frozen aqueous solutions at 253 or 77 K, containing various acids (HF, HCl, HNO3, H2SO4 and p-toluenesulfonic acid), sodium hydroxide, NaCl, or NH4Cl, was examined. The results showed that the extent of cresol red protonation, enhanced in the solid state by 2 - 4 orders of magnitude in contrast to the liquid solution, is principally connected to an increase in the local concentration of acids. It was found that this enhancement was not very sensitive to either the freezing rate or the type of acid used, and that cresol red apparently established an acid-base equilibrium prior to solidification. In addition, the presence of inorganic salts, such as NaCl or NH4Cl, is reported to cause a more efficient deprotonation of CR in the former case and an enhanced protonation in the latter case, being well explained by the theory of Bronshteyn and Chernov. In addition, absorption spectra of nine solvatochromic compounds were examined in the frozen aqueous solutions to obtain quantitative information about the solvation types on the grain boundaries of ice. According to our measurements, electron-pair donating and/or hydrogen-bond accepting interactions are the most prevailing at the grain boundaries of ice but the hydrogen-bond donating effect was significant too. In contrast, the polarity/dipolarity interactions were unimportant. From the results of the UV/vis absorption measurements in the ice, we can envisage the grain boundary containing the probes as a complex concentrated mixture of more or less organized organic molecules that specifically interact with water molecules in the vicinity. Since changes caused by the freezing of aqueous solutions plays an important role in (photo)chemical transformations, this work helps to understand how the initial conditions control the course of the process. The results of our research are relevent in other interdisciplinary fields, such as environmental chemistry, cosmochemistry, or geochemistry. …víceméně