Effect of Caffeine in the Diffusion Behaviour of Analgesic Drugs in Aqueous Solutions

The aim of the study was to examine the effect of caffeine on the diffusion of two pain-relieving drugs, namely paracetamol and sodium salicylate, in aqueous solutions at a temperature of 25.00ºC, using the Taylor dispersion method to determine the mutual diffusion coefficients for these ternary aqueous solutions. Ternary mutual diffusion coefficients are reported for aqueous solutions of paracetamol + caffeine, and sodium salicylate + caffeine, at 25.00ºC, and concentrations up to 0.050mol·dm-3. The data indicate that the presence of caffeine affects the diffusion of analgesic drugs. For example, a coupled diffusion of these drugs with caffeine was observed through the non-zero values of the cross-diffusion coefficients. Support for this came from the good agreement between our data and the predicted by a model of the diffusion that includes 1:1 complex, being obtained the association constants for systems [(caffeine and paracetamol) and (caffeine and salicylate sodium)], K=70mol-1·dm3 and K=80, respectively. These data provide us with a better understanding of the mechanisms of mass transport through diffusion in biological systems, as well as the structure of these systems.