Effect of Metal Ions on the Thermodynamics of Water: 1-Butanol Partitioning of Ciprofloxacin

Henry Akpobor Okeri, Chukwuenweniwe Jonathan Eboka


The purpose of this study is to determine the effect of metal cations on the partitioning of ciprofloxacin. The thermodynamics of partitioning of ciprofloxacin between aqueous buffer (pH 7.4) and 1-butanol was studied alone, and in the presence of compounds containing metal cations which included Fe 2+ , Al 3+ , Mg 2+ , Fe 3+ , K + , Ca 2+ and Na + . The
determinations were carried out at 20°C, 25°C, 30°C, 37°C and 45°C and the absorbance values of the solutions were determined using UV-visible spectrophotometry at appropriate wavelengths of maximum absorption. The results showed that partition coefficient of ciprofloxacin between aqueous buffer:1-butanol medium was 0.176 at 25°C, demonstrating that ciprofloxacin is not predominantly lipophilic. The van’t Hoff thermodynamic analyses of the partitioning revealed that the transfer of ciprofloxacin from the aqueous to the organic phase was exothermic. The presence of metal cations caused a decrease in the partition coefficient of ciprofloxacin and increased the standard change in Gibbs free energy (  G ) because of complexation leading to the formation of a more polar product with concomitant increase in aqueous solubility and hence a reduction in the transfer of ciprofloxacin from the aqueous phase. When the amount of metal cation was doubled, there was a further though insignificant increase in  G values showing that the transfer of ciprofloxacin from the aqueous phase was less favourable and will therefore require more energy before partitioning can occur. The presence of metal cations decreased the partitioning of ciprofloxacin from the aqueous phase to 1-butanol. This may explain the reported reduction in absorption, and consequently reduced bioavailability and antibacterial activity observed when ciprofloxacin was co-administered with metal cations.

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