Solvation simplified

Abstract

Recebido em 24/5/10; aceito em 28/9/10; publicado na web em 16/11/10The effects of solvents on chemical phenomena is complex because there are various solute-solvent interaction mechanisms. Solvatochromism refers to the effects of solvents on the spectra of probes. The study of this phenomenon sheds light on the relative importance of the solvation mechanisms. Solvation in pure solvents is quantitatively analyzed in terms of a multi-parameter equation. In binary solvent mixtures, solvation is analyzed by considering the organic solvent, S, water, W, and a 1:1 hydrogen bonded species (S-W). The applications of solvatochromism to understand distinct chemical phenomena, reactivity and swelling of cellulose, is briefly discussed.Keywords: solvation ; solvatochromism; solute-solvent interactions.The need for understanding solvation is clear: most reactions are carried out in the liquid phase; the solvent is not a "spectator". It acts as a heat-and mass-transfer agent; it participates in proton transfers (for acid/base catalyzed reactions) and in the solvation of ions, dipolar species, etc. Consider the decomposition of 6-nitro-3-carboxybenzisoxazole, whose reaction scheme is depicted in Figure 1. This example is particularly illustrative of the effects of solvents on reactivity because it is a simple, spontaneous decomposition reaction. Therefore, effects of changing the solvent on the observed rate constant, k obs , can be unequivocally attributed to differences in solvation between the reactant state-where the negative charge is concentrated on the carboxylate anion-and the transition state, where the charge is dispersed over several atoms. The half-lives of this reaction in hexamethylphosphotriamide, acetonitrile, and water, are 0.001 s, 11.6 min, and one day, respectively! 1Can these large differences in k obs be correlated with solvent properties? The results of such attempt is shown in Table 1, where the subscript (S) refers to solvent, e r , E T (30), a S , b S refer to the solvent relative permittivity, its empirical polarity, hydrogen-bond donation capacity or "acidity", and hydrogen-bond acceptance capacity or "basicity", respectively (vide infra for more discussion of the last three solvent descriptors). Table 1 reveals that there is no correlation between log k obs and any single solvent property. Inclusion of a second descriptor leads to a noticeable improvement in the regression analysis; a four-descriptor equation gives the best correlation coefficient. The conclusion from Table 1 is obvious: solvent effects on chemical reactivity, and presumably other phenomena, e.g., chemical equilibria and spectroscopic data, are complex; any successful correlation with a single solvent descriptor is, most certainly, fortuitous.The following questions now arise: (i) What is the reason for this complex dependence of chemical phenomena on solvent properties?(ii) Can the relative importance of each solute-solvent interaction be identified and quantified? (iii) How can we treat solvation in solvent mixtures? The object...

Understanding Solvation: Comparison of Reichardt’s Solvatochromic Probe and Related Molecular “Core” Structures

“…(ii) As discussed for probes of distinct chemical classes in binary mixtures of water with organic solvents and with ILs, this nonideal behavior is due to PS of the probe by one component of the mixture. The extent of this PS depends on the components of the binary mixture and the hydrophobicity of the probe; more hydrophobic probes show more deviation from linearity. − This agrees with the larger deviation of the more hydrophobic p -RB.…”

“…(iii) We treated this PS according to a model discussed elsewhere, in which the medium is composed of a mixture of water (W), the organic solvent (S), and a “complex” one (S–W) formed by hydrogen bonding between the two pure solvents. This model was discussed in detail elsewhere; − we give here the essentials, as summarized by eqs –: Here, the concentrations are given on the mole fraction scale, ( m ) is the number of solvent molecules that perturb the intramolecular charge transfer within the probe, hence affecting the value of E T (probe). Note that m is not the solvation number of the probe.…”

Enhanced CO2 Adsorption and Selectivity of CO2/N2 on Amino-MIL-53(Al) Synthesized by Polar Co-solvents

“…19,20 It was reported that the effects of solvents on MOF synthesis and structure are complicated as a result of various solution− solvent interaction mechanisms. 21 The reaction medium in a synthesis procedure can be greatly affected by the polar properties; therefore, the selection of solvents may profoundly impact the derived product. Moreover, the polarity of solvents can be enriched by adding less polar constituents.…”