A conductance study of guanidinium chloride, thiocyanate, sulfate, and carbonate in dilute aqueous solutions: ion-association and carbonate hydrolysis effects.

We study the conductance of dilute aqueous solutions for a series of guandinium salts at 298.15 K. The experimental molar conductivities were analyzed within the framework of the Quint-Viallard theory in combination with Debye-Hückel activity coefficients. From this analysis, we find no evidence for significant ion association in aqueous solutions of guanidinium chloride (GdmCl) and guanidinium thiocyanate (GdmSCN), and the molar conductivity of these electrolytes can be modeled assuming a complete dissociation. The limiting ionic conductivity of the guanidinium ion (Gdm(+)) is accurately determined to λ(Gdm(+)) = 51.45 ± 0.10 S cm(2) mol(-1). For the bivalent salts guanidinium sulfate (Gdm(2)SO(4)) and guanidinium carbonate (Gdm(2)CO(3)), the molar conductivities show small deviations from ideal (fully dissociated electrolyte) behavior, which are related to weak ion association in solution. Furthermore, for solutions of Gdm(2)CO(3), the hydrolysis of the carbonate anion leads to distinctively increased molar conductivities at high dilutions. The observed ion association is rather weak for all studied electrolytes and cannot explain the different protein denaturing activities of the studied guanidinium salts, as has been proposed previously.

Amino acid solvation in aqueous kosmotrope solutions: temperature dependence of the L-histidine-glycerol interaction.

We have studied thermodynamics of interaction between the aromatic amino acid L-histidine and glycerol, which is one of the most important stabilizing agents for proteins in water. The pair and triplet interaction parameters have been extracted from enthalpy and solubility data using standard thermodynamic manipulations in a wide temperature range. Our results indicate for the first time that the L-histidine-glycerol pair and triplet interactions are characterized by rather small enthalpy and entropy changes, which do not depend on temperature in either cold or hot water. These temperature-independent enthalpies and entropies of interaction lead to zero heat capacity changes during the amino acid transfer from water to both dilute and rather concentrated aqueous glycerol solutions. We attribute this behavior to a delicate balance between contributions from hydrophobic and hydrophilic fragments in the solute molecules. This unique feature appears to be the major reason that thermodynamics of pair and triplet interactions are nearly identical at standard and physiological temperatures.

An analysis of electrical conductances of aqueous solutions of polybasic organic acids. Benzenehexacarboxylic (mellitic) acid and its neutral and acidic salts.

A general approach is proposed to analyze electrical conductivities in aqueous solutions of polybasic organic acids. Experimental conductivities are examined in the context of dissociation and hydrolysis reactions by applying the Quint-Viallard conductivity equations and the Debye-Hückel equations for activity coefficients. The proposed numerical procedure is illustrated by the case of benzenehexacarboxylic (mellitic) acid and its neutral and acidic salts. From conductivity measurements of mellitic acid and its salts, performed in dilute aqueous solutions in the 278.15-308.15 K temperature range, the limiting conductances of mellitic anions, lambda(0)(1/jH(6-j)Mel(-j), T), j = 1, 2, 3, 4, 5, 6 are determined.