A study of the behaviour of ampicillin in aqueous solution and thermodynamic characterization of its aggregation

Abstract

The aggregation of ampicillin in water has been examined by conductivity measurements over the temperature range 288.15–313.15 K and light scattering. These measurements indicate the formation of two critical concentrations over the range 0–0.35 mol kg⁻¹. Aggregation number and effective aggregate charge were calculated from the static light scattering data according to the Anacker and Westwell treatment. Thermodynamic parameters of aggregate formation were obtained from a form of the mass action model applicable to systems of low aggregation number. This method was applied at both critical concentrations. A valence of one was used for the monomers present in the first equilibrium. The second equilibrium was between aggregates of two different sizes, in this case, the valence of the aggregates being the effective charge calculated from the Anacker and Westwell treatment. Experimental results show that free energies of micellization for ampicillin are higher for the first critical concentration and in the same range, but lower than for other penicillins. The enthalpies of micellization become negative when the temperature is increased, but the variation is three times greater for the first critical concentration than for the second.