Theory of electron paramagnetic relaxation in liquid solutions

Theoretical and Experimental Chemistry -     

Isotope effects in the relaxation of6Li and7Li nuclei in paramagnetic aqueous solutions

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, p. 2429, October, 1988.     

Lanthanide-ion-induced paramagnetic shifts in NMR spectra in aqueous solutions

Theoretical and Experimental Chemistry -     

Enthalpic relaxation in frozen aqueous trehalose solutions

Our object was to investigate the effect of annealing on the glass transition temperatures and enthalpic recovery of frozen aqueous solutions of trehalose. Trehalose solutions were subjected to differential scanning calorimetry wherein they were first cooled from room temperature to −60 °C, and heated to the annealing temperature, which ranged between −34 and −48 °C. Following isothermal annealing for the desired time period, the glass transition temperatures and the enthalpic recovery were determined in the final heating scan. Frozen unannealed trehalose solutions were characterized by two glass transition events. At a heating rate of 2 °C/min, the observed Tg₁′ and Tg₂′ were ∼−45 and −31 °C, respectively. Annealing resulted in an increase in the lower transition temperature, Tg₁′, while the higher transition temperature, Tg₂′, was unaffected. Enthalpic recovery due to annealing was associated only with Tg₂′. Annealing at −36 °C resulted in the highest value of Tg₁′ and the maximum enthalpic recovery. Irrespective of the heating rates, the magnitude of enthalpic recovery and Tg₁′ showed a similar trend (first an increase, followed by a decrease) as a function of annealing temperature. This suggests that annealing led to crystallization of ice and subsequently the system became maximally freeze-concentrated. Annealing, at temperatures higher than −36 °C, led to a reduction in enthalpic recovery associated with Tg₂′ and a lowering of Tg₁′. These observations are consistent with the hypothesis that the higher transition temperature coincides with the onset of ice melting. We have attempted to bridge two conflicting schools of thought regarding the origin of multiple glass transitions in frozen aqueous sugar solutions. The two glass transitions are attributed to the formation of two “populations” in the freeze-concentrated phase during “non-equilibrium” or rapid cooling—one, that is maximally freeze-concentrated and the other that contains a higher amount of water. The higher transition temperature also overlaps with the onset of ice melting.     

Microwave dielectric permittivity and relaxation for aqueous potassium trifluoroacetate solutions

The results of microwave dielectric measurements in aqueous potassium trifluoroacetate solutions at seven frequencies (ranging within 7.5–25 GHz) at 288, 298, and 308 K are presented. Static dielectric constants, dielectric relaxation times and activation parameters are calculated. The H-bond network in potassium trifluoroacetate solutions is shown to experience molecular-kinetic stabilization and an increase in connectivity and structuring, which are similar to those experienced by water in potassium acetate solutions. These changes are associated with the hydrophobic hydration of trifluoroacetate ion, which was first determined by microwave dielectric spectroscopy and arises from the effect of the low-polarity CF₃ group of trifluoroacetate ion.     

Nuclear magnetic relaxation and structure of aqueous solutions

By the interpretation of the intermolecular nuclear magnetic relaxation rate of¹⁹F the orientation of the water molecules in the hydration sphere of F^– can be determined. Similarly, the orientation of the water molecules around the methyl group of propionic acid in aqueous solution has been studied. Experiments are described which give information about the nature of association of solute in aqueous solution of a number of carboxylic acids and of ethanol. The local dynamic details of the I^– ion have been investigated. Some new results are briefly descussed regarding the nuclear magnetic relaxation by quadrupole interaction in electrolyte solutions.This paper was presented at the symposium, The Physical Chemistry of Aqueous Systems, held at the University of Pittsburgh, Pittsburgh, Pennsylvania, June 12–14, 1972, in honor of the 70th birthday of Professor H. S. Frank.