Much stronger binding is seen in aqueous solutions between the anionic polyelectrolyte potassium poly(vinyl sulfate) and the substitution labile aluminium(III) than with the kinetically inert chromium(III). This strongly supports the idea that entropy driven water loss from the hydration sphere of the metal ion plays a major role in driving binding of the trivalent metal ion to the polyelectrolyte.

Simultaneous administration of antacids containing magnesium or aluminium and ciprofloxacin or other quinolones decreases the gastrointestinal absorption of those antibacterial agents. Current speculation about the mechanism of this interaction has focused on drug-cation chelation. The present study was designed to detect the protonation in solutions and the formation of the complex species at the pH levels typical of the gastrointestinal tract. It involves the study of ciprofloxacin in aqueous solutions containing Al3+ and (or) Mg2+ by combining the results of potentiometric and spectroscopic (1H nuclear magnetic resonance) techniques. Calculations were only performed for data in the range 4.5 < pH < 5.5 (pH levels typical of gastrointestinal tract) and the results of both methods are made self-consistent, assuming an equilibrium model including complex species MHL, MLOH (where H2L denotes ciprofloxacin and M is Al3+ or Mg2+); their formation constants are given.

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The structure of mono- and diprotonated cyclohexyldiamine isomers in aqueous solution is investigated theoretically by the application of the CPCM continuum solvation model combined with the MP2/aug-cc-pVDZ model chemistry. The calculated Gibbs energy of hydration (ΔGhyd) is expressed in different terms with physical meaning: cavity formation, solute conformational variation, and solute-solvent interaction. Significant differences of the ΔGhyd values are found among isomers, which are interpreted based on the analysis of the factors accounting for the stability of the conformers/isomers in the gas and solution phases. Particular attention is given to the role played by the formation of an intramolecular hydrogen bond in the monoprotonated forms and by the Coulombic repulsion between the NH3(+) groups in the diprotonated ones. From the Gibbs energies of the acid/base pairs in the gas phase and respective hydration Gibbs energies, the acidity constants (pKa) are calculated and interpreted. For some isomers, the constants are also determined experimentally by potentiometric titration. A good agreement was found between the calculated and experimental values.

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