Solute-solvent Effects Research Articles

Effect of solvent-water mixtures on the prototropic equilibria of fluorescein and on the spectral properties of the monoanion

A spectral resolution procedure was used to resolve the absorption, excitation and emission spectra of the fluorescein monoanion in a number of solvent-water mixtures. This permitted an analysis of the effect of the solvent environment on the spectral properties of the monoanion and on the lactone/monoanion/dianion transitions of fluorescein. The monoanion excitation and emission spectra show relatively small changes with changing environment, a behavior that is related to the hydrogen-bonding environment of the solvent-water mixtures. There is also a general increase in the quantum yield of the monoanion from 0.36 in water to values up to 0.49 in the solvent-water mixtures. The presence of solvent also results in a general increase in the lactone content and in the monoanion:dianion and lactone:monoanion ratios. General polarity effects alone cannot account for the observed effects on the prototropic transitions indicating that specific solute-solvent effects involving hydrogen bonding perturb the prototropic equilibria of fluorescein.

Ionization thermodynamics of methylbenzoic acids in water-DMF mixtures

The ionization enthalpies of p-, m- and o-methylbenzoic acid were measured calorimetrically at 298.15 K in water-DMF mixtures ranging from pure water to 70% DMF by weight. The ionization entropies were calculated based on the free energy values obtained from experiments. The results were interpreted in terms of solute-solvent interactions and different substituent effects.

Quantitative Studies of the Internal Referencing Method with the Bulbed Capillary External Referencing Method for Proton Nuclear Magnetic Resonance Spectroscopy

Serious methodological defects of the internal referencing method are revealed by quantitative studies using the bulbed capillary external referencing method. Entirely new quantitative data on solute-solvent effects fatally affecting the internal referencing method are presented and discussed. Possibilities of new solute-solvent effect studies are also suggested.

Solute–solvent effects in the dissociation of thymolsulfonephthalein (an uncharged acid) in aqueous mixtures of acetonitrile (ACN), urea, and dimethyl sulfoxide (DMSO)

The thermodynamic first dissociation constants, [Formula: see text] of thymolsulfonephthalein (H2A), an uncharged acid, have been determined at 25 °C in aqueous mixtures of 10, 30, 50, 70, and 80 wt% acetonitrile (ACN), 11.52, 20.31, 29.64, and 36.83 wt% urea, 20, 40, 60, and 80 wt% dimethyl sulfoxide (DMSO) by spectrophotometric measurements. The solvent effect represented by ∂(ΔG0) = 2.303RT[p(sK)N − p(wK)N] is found to increase in ACN + H2O system as mol% ACN increases in the solvent. In contrast, the corresponding values in urea + H2O as well as DMSO + H2O solvent systems decrease with increase in proportion of organic component in the solvent, the decrease being sharp in urea + H2O. The results have been discussed in terms of the standard Gibbs energies of transfer of H+ from water to the mixed solvent, [Formula: see text] and the relative values of the standard Gibbs energies of transfer of HA−, [Formula: see text] and of [Formula: see text] in all the solvent systems. The overall dissociation behaviour of the acid (H2A) is found to be dictated by the specific solute-solvent interactions of the species participating in the dissociation equilibria.

Solute–solvent effects in the dissociation of thymolsulphonephthalein (an uncharged acid) in aqueous mixtures of protic (methanol) and dipolar aprotic (dimethylformamide) solvents

The thermodynamic first dissociation constants, (sK)H2A of thymolsulphonephthalein, an uncharged acid, have been determined spectrophotometrically at 25 °C in aqueous solutions containing 10, 30, 50, 70, and 80 wt% protic solvent (methanol) and in aqueous solutions containing 20, 40, 60, 70, and 80 wt% dipolar aprotic solvent (NN-dimethylformamide)(DMF). p(sK)H2AN Increases gradually with increase in proportion of methanol in the solvent. In contrast, as DMF is progressively added to water, p(sK)H2AN decreases sharply at the beginning followed by a very small decrease before showing a weak tendency to rise after ca. 40 mol% DMF. The solvent effect on the dissociation of the acid has been discussed in terms of the standard Gibbs energy of transfer of H+ from water to the mixed solvent, ΔGto(H+), and the relative values of the standard Gibbs energies of transfer of HA–, ΔGto(HA–), and of H2A, ΔGto(H2A), in the two solvent systems. The contrasting behaviour of the indicator acid is found to be dictated by the specific solute–solvent interactions of the species involved in the ionization equilibria.

Separation of Calcium Isotopes by Liquid Phase Thermal Diffusion

Significant separation of the isotopes of calcium was obtained by thermogravitational thermal diffusion of an aqueous calcium nitrate solution. A flow of solvent was used to partially offset the large solute-solvent separation effect in the experimental column. Further development of this technique may lead to separation of the isotopes of calcium and of other elements on a practical scale.

Effects on hyperpolarizabilities of molecular interactions in associating liquid mixtures

We have measured the second and third order hyperpolarizabilities β and γ using the technique of electric field induced second harmonic generation in a variety of weakly associating (e.g., nitrobenzene–benzene) and strongly associating (e.g., water–methanol) liquid mixtures. These experiments are shown to yield interesting information on dipolar forces, hydrogen bonding, charge transfer interactions, and other solute–solute, solute–solvent effects.

Transfer energies and solute-solvent effects in the dissociation of protonated Tris in N-methylpropionamide-methanol solvents

Transfer energies and solute-solvent effects in the dissociation of protonated Tris in N-methylpropionamide-methanol solvents

Solute-solvent effects in the acidic dissociation of the ampholyte N-tris(hydroxymethyl)methylglycine (?tricine?) in 50 mass % methanol-water solvent

The pK values for the two acidic dissociation steps of the ampholyte N-tris-(hydroxymethyl)methylglycine (“tricine”) in 50 mass % methanol-water solvent have been determined by emf measurements of cells of the type Pt|H2(g, 1 atm), tricine buffer, Br−, AgBr|Ag over the range 5 to 50°C (pK 1)and 5 to 60°C (pK 2).The standard thermodynamic quantities ΔHo, ΔSo, and ΔC p o for the two dissociation processes have been derived and are compared with the corresponding values for tricine and the parent glycine in water and with those for other acids in 50 mass % methanol-water solvent. Both tricine and protonated tricine become weaker acids when methanol is added to the aqueous solvent. It appears that a strong stabilization of the zwitterion in water is responsible for this behavior. This conclusion is supported by comparing the changes in entropy and heat capacity for the dissociation of tricine with the values of these quantities for the dissociation of “model” acids of simple structure, such as ammonium ion and acetic acid.

Transfer energies and solute-solvent effects in the dissociation of protonated tris in N-methylpropionamide-water solvents

Transfer energies and solute-solvent effects in the dissociation of protonated tris in N-methylpropionamide-water solvents

Solute-solvent effects in the ionization of tris(hydroxymethyl)acetic acid and related acids in water and aqueous methanol

Solute-solvent effects in the ionization of tris(hydroxymethyl)acetic acid and related acids in water and aqueous methanol