Thermodynamic and Rheological Properties of Polyelectrolyte Systems

The chapter provides a treatment of the interaction between acidic or basic polyelctrolytes (PE) and ionizable organic molecules (selected model drugs) in aqueous environments, in terms of acid-base reactions. The electrostatic attraction between the ionized pending groups of the PE and the organic ions yields a high proportion of counterionic condensation with affinity constants in the range of 103 to 105. The high proportion of counterionic condensation in PE-drug aqueous dispersions determines many of the particular properties of these systems such as the effects of addition of electrolytes and non-electrolytes, the kinetic of drug release under different conditions, the raise of compatibility of low solubility drugs, the increase of chemical stability and the rheological behavior. The aqueous systems of acidic PE are characterized by their building viscosity capacity. Flow curves of PE-drug systems reflex the behavior of model PE-Na systems. However, complexes of a set of model drugs under similar conditions exhibit a wide range of viscosities. The determination of the kinetic of water sorption of PE-drug complexes in solid state provides valuable complementary information related to their swelling capacity. Rheology of PE-drug aqueous dispersions as well as their swelling capacity are relevant properties in the fields of mucoadhesivity and drug release.