A Comparison between the Diffusion Properties of Theophylline/β-Cyclodextrin and Theophylline/2-Hydroxypropyl-β-Cyclodextrin in Aqueous Systems

Ternary mutual diffusion coefficients (D11, D22, D12, and D21) measured by the Taylor dispersion method at T = 298.15 K in two different laboratories (University of Naples, Federico II, Italy, and University of Coimbra, Portugal) are reported for aqueous systems, involving as components 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) or β-cyclodextrin (β-CD) and theophylline. The aim of the paper was to determine the transport properties of these systems as well as the presence of an inclusion mechanism. On the basis of the diffusion coefficient data it can be argued that theophylline is included neither into β-cyclodextrin nor into 2-hydroxypropyl β-cyclodextrin. The variations of the main diffusion coefficients with composition are ascribable to nonspecific solute–solute interactions such as the viscosity increase induced by the presence of the drug or to an obstruction effect of which the bulky cyclodextrin is responsible.