Interactions of organic molecules at grain boundaries in ice: A solvatochromic analysis

Empirical solvent polarity parameters were used to evaluate the nature and magnitude of the intermolecular interactions of eight dipolar organic solvatochromic indicators in aqueous solutions frozen at 253 or 77 K, using the concept that is generally employed to study the polarity of liquid solvents or solid surfaces. ET(30), ET(33), and ETN as well as α, acceptor number (AN) (hydrogen-bond donation ability), β (hydrogen-bond acceptor ability), and π* (polarity/polarizability) parameters were obtained by measuring the differences in the shifts of the absorption spectra of the probes. It was found that hydrogen-bond and electron-pair donating interactions were significant contributors to the polarity of a probe environment in ice and, at the same time, they were found to be substantially larger than those measured in liquid aqueous solutions and relatively insensitive to the sample temperature. While the former interaction type is attributed rather to the presence of water in a close vicinity of the probe molecules, the latter is evidently connected with the inter-probe interactions within the self-assembled molecular aggregations in conjunction with the water–probe interactions. The solvatochromic analysis revealed very weak dipole–dipole interactions (π*) but the results are inconclusive. The data are consistent with a model according to which, upon freezing the aqueous solutions, the organic solute molecules are ejected to the grain boundaries to form highly concentrated liquid or frozen mixtures of organic and water molecules, having a high degree of complexity and exhibiting specific intermolecular interactions. Evaluation of the intermolecular polar interactions at the grain boundaries in ice should be of a great value in advancing our understanding of physical and chemical processes occurring in natural ice and snow.