Mecánica estadística de fluidos y equilibrio químicoaplicación de la teoría MSA al estudio del efecto de la fuerza iónica sobre equilibrios ácido-base y de complejación de moléculas orgánicas

Resumen

Some properties of aminoacids are of a high interest on account of their biological significance as structural units for proteins; good proof is the fact that IUPAC recently revised acid–base and complex stability constants. IUPAC’s work in this direction revealed the absence of systematic studies on the influence of ionic strength on acid–base equilibrium constants, particularly at moderate to high ionic strength values. Our group recently undertook the systematic use of various semiempirical models for activity coefficients (viz. those of Pitzer, Scatchard and Guggenheim) to interpret pK*–I data at moderate ionic strength values in both pure and mixed electrolytes. The ensuing treatments rely on the equation , which relates the thermodynamic constant to the stoichiometric constant and the ratio of the activity coefficients for the species involved in the equilibrium considered. The different expressions used for the activity coefficients are usually based on modificated versions of the Debye–Hückel limiting law obtained by including different ionic strength terms that lead to various pKi vs. ionic strength analytical functions for different ionic strength ranges. One plausible alternative for modelling the Q(
i) function is the use of current statistical mechanics treatments, particularly approximations based on integral equations, the most simple of which is the mean spherical approximation (MSA), a straightforward analytical theory for electrolytes that is useful for representing thermodynamic properties of ionic solutions over a wide concentration range. As in the Debye–Hückel theory, solution properties are represented by means of a simple parameter, 2, in addition to the charges, diameters and concentrations of the ions. Because MSA is the essentially correct way of including the effects of ion sizes in the D–H theory, it is more accurate than these. In this work, equations for equilibria typical of many processes (e.g. those involving carboxylic acids and amines), were derived by using the mean spherical approximation. MSA results were used to interpret the effect of ionic strength on the stability and acid–base equilibrium constants for several amino acids on the basis of previously obtained both potenciometric and polarographic experimental results. The curves thus obtained fitted closely experimental data within the experimental error range. The data were also analysed by using some semi-empirical treatments (viz. those of Pitzer, Scatchard and Guggenheim). Based on the obtained results, the mean spherical approximation leads to equations that fit tightly experimental pK–I data, just like various equations based on different ionic strength terms; the most substantial advantage of MSA over semi-empirical models that lead to ionic strength terms with coefficients that defy theoretical interpretation is that MSA procedure relies on ionic diameters, charges and molar concentrations. Although the Debye-Hückel limiting law (DHLL) is the theoretical relation for log
± assumed to hold at high dilution, use of a log
± expression based on a quasi-lattice model (a relation between log
± and the cubic root of the ionic strength) allows one to derive pKi expressions that reproduce highly faithfully experimental data over the ionic strength range studied. Use of such expressions in regions away from the high dilution limit is theoretically justified as an alternative to modifications involving the inclusion of empirical terms for the ionic strength. While the extrapolation at I 0 was not carried out on the basis of the Debye–Hückel equation, the results obtained using both approximations were quite consistent in most cases.