Highlights in Solute-Solvent Interactions

The history of studies on ionic solvation is briefly reviewed, and structural and dynamic properties of solvated ions in aqueous and nonaqueous solutions are discussed. An emphasis is placed on ionic solvation in nonaqueous mixed solvents in which preferential solvation of ions takes place. A new parameter for expressing the degree of preferential solvation of an ion is proposed.

The newer literature on spin equilibria of Fe(II) complexes in solution is reviewed. The thermodynamics as well as the kinetics in different solvents are discussed.

The thermochromic behaviour of various coordination compounds in solution is discussed, with a special focus on cyclic diamine chelates. Thermally induced spin-crossover phenomena of iron(II) complexes are also considered. The solvatochromic behaviour of mixed-ligand complexes is presented in detail.

Following a critical survey of the vast recent literature, the state of the art may be summarized as follows:

Solubility measurements as a function of temperature have been shown to be a powerful tool for the determination of thermodynamic properties of sparingly-soluble transition metal carbonates. In contrast to calorimetric methods, such as solution calorimetry or drop calorimetry, the evaluation of solubility data avoids many systematic errors, yielding the enthalpy of solution at 298.15 K with an estimated uncertainty of ±3kJ·mol ^-1. A comprehensive set of thermodynamic data for otavite (CdCO₃), smithsonite (ZnCO₃), hydrozincite (Zn₅(OH)₆(CO₃)₂), malachite (Cu₂(OH)₂CO₃), azurite (Cu₃(OH)₂(CO₃)₂), and siderite (FeCO₃) was derived. Literature values for the standard enthalpy of formation of malachite and azurite were disproved by these solubility experiments, and revised values are recommended. In the case of siderite, data for the standard enthalpy of formation given by various data bases deviate from each other by more than 10 kJ • mor¹ which can be attributed to a discrepancy in the auxiliary data for the Fe^e+ ion, A critical evaluation of solubility data from various literature sources results in an optimized value for the standard enthalpy of formation for siderite. The Davies approximation, the specific ion-interaction theory, and the Pitzer concept are used for the extrapolation of the solubility constants to zero ionic strength in order to obtain standard thermodynamic properties valid at infinite dilution, T = 298.15 K, and p =10⁵ Pa. The application of these electrolyte models to both homogeneous and heterogeneous (solid-solute) equilibria in aqueous solution is reviewed. A comprehensive set of thermodynamic data of otavite (CdCO₃), smithsonite (ZnCO₃), hydrozincite (Zn₅(OH)₆(CO₃)₂), malachite (Cu₂(OH)₂(CO₃) azurite (Cu₃(OH)₂(CO₃)₂) and siderite (FeCO₃) was derived. Literature values for the standard enthalpy of formation of malachite and azurite were disproved by the solubility experiments, and reviced values are recommended. In the case of siderite, data for the standard enthalpy of formation given various data bases deviate from each other by more than 10kJ · mol^-1 which can be attributed to discrepancy in the auxiliary data for the Fe²⁺ ion. A critical evaluation of solubility data from various literature sources results in an optimized value for the standard enthalphy of formation for siderite. The Davies approximation, the specific ion-interaction theory, and the Pitzer concept are used for the extrapolation of the solubility constants to zero ionic strength in order to obtain standard thermodynamic properties valid at infinite dilution, T = 298.15K, and p = 10⁵ Pa. The application of these electrolyte models to both homogeneous and heterogeneous (solid-solute)equilibria in aqueous solution is reviewed.

Kamlet-Taft’s a (hydrogen bond donor acidity) and π*(dipolarizability) values of various silica batches measured in various solvents are presented. The α and π*parameters for the various solid acids are analyzed by means of Fe(phen)₂(CN)₂(cis-dicyano-bis-(1,10)-phenanthrolineiron(II), 1), Michler’s ketone (4,4’-bis-(dimethylamino)-benzophenone, 2), and two hydrophilic derivatives of 2, (4-(dimethylamino)-4’-(di-2-hydroxyethyl)-amino-benzophenone(3a) and 4,4’-bis(di-(2-hydroxyethyl)-amino)-benzophenone (3b) as well as coumarin 153 (4) as solvatochromic surface polarity indicators. Apparent β (hydrogen bond acceptor basicity) parameters for bare silica have been evaluated by means of an aminobenzodifuranone dye (5) as solvatochromic probe.

Theoretical E_T (30) values of the solid/solvent interfaces are calculated by applying linear solvation energy (LSE) relationships using the independently measured a and π* values of the solid acids according to E_T (30) = (E_T (30))₀+ aα +sπ*.

A method is proposed that permits the proper extrapolation of thermodynamic quantities represented by the Pitzer equations from 25°C to other temperatures. The new method, which assumes temperature independent heat capacities, was tested for the NaCI—H₂O system and found very satisfactory. A new evaluation of the Na₂CO₃—H₂O system according to the CALPHAD method is presented, and solubilities for the quaternary Na₂CO₃—NaC1—NaOH—H₂O and its ternary subsystems are predicted. Limitations of (i) these predictions and of (ii) the Pitzer model regarding extrapolations to high molalities are discussed.

Compared to the simple one-component case, the phase behaviour of binary liquid mixtures shows an incredibly rich variety of phenomena. In this contribution we restrict ourselves to so-called binary symmetric mixtures, i.e. where like-particle interactions are equal (Φ ₁₁(r) = Φ₂₂(r)), whereas the interactions between unlike fluid particles differ from those of likes ones (Φ ₁₁(r)≠Φ ₁₂(r)). Using both the simple mean spherical approximation and the more sophisticated self-consistent Ornstein-Zernike approximation, we have calculated the structural and thermodynamic properties of such a system and determine phase diagrams, paying particular attention to the critical behaviour (critical and tricritical points, critical end points). We then study the thermodynamic properties of the same binary mixture when it is in thermal equilibrium with a disordered porous matrix which we have realized by a frozen configuration of equally sized particles. We observe — in qualitative agreement with experiment — that already a minute matrix density is able to lead to drastic changes in the phase behaviour of the fluid. We systematically investigate the influence of the external system parameters (due to the matrix properties and the fluid—matrix interactions) and of the internal system parameters (due to the fluid properties) on the phase diagram.

Six mixed-ligand Nickel(II) and Copper(II) chelates with square-planar geometry of the formula [Ni/Cu(O-0)(S-tmpn)]B(C6H₅)₄ were prepared, where O-O represents acetylacetonate, tropolonate, or hinokitiolate and S-tmpn is (S)-tetramethyl-1,2-propanediamine. The compounds were investigated with respect to their function as receptor for unprotected amino acids, taking advantage of their high solubilities in non-polar organic solvents. In liquid-liquid extraction experiments between a 1,2-dichloroethane phase containing the metal chelates and an aqueous phase containing amino acids (rac-phenylglycine, rac-phenylalanine, or rac-tryptophan), the nickel(II) chelates effectively extracted amino acids from the aqueous phases under neutral conditions, forming octahedral ternary chelates.

The distribution of tetraalkylammonium ions (C _n H_2n+1)₊N⁺(R ⁺,TAAn ⁺,n=4−7) with picrate ion (pic ⁻) and inorganic anions X ⁻(Cl⁻, Br⁻, Cl0⁻ ₄), into various inert organic solvents was studied at 25.0°C. The distribution data were analyzed by taking into consideration the distribution of ion pairs, R ⁺ • X⁻, and the dimerization of the ion pairs, (R ⁺ •X⁻)₂, in the organic phase. The ion-pair, distribution constant, K _dist, increases with increasing chain length of the tetraalkylammonium ion and with increasing ionic radius of the anion. The values of K _dist show a good correlation with the E _T value of solvent, i.e. the solvation ability with respect to the anion, and smoothly increase with increasing E _T. The effect of the solvent on the dimerization constants, K _dim, is markedly different between the ion pairs of picrate ion and inorganic anions. In the case of picrate, K _dim significantly decreases with decreasing length of the alkyl chain of the tetraalkylammonium ion, but hardly changes by changing the solvent. On the other hand, in the case of ion pairs of inorganic anions the value of K _dim decreases with decreasing E _T and is almost constant for all anions. These results were reasonably explained by the difference of the solvation of the anion moieties of the monomeric and dimeric ion pairs.