Phase behaviour of the lidocaine-monoolein-water system

Abstract

The phase behaviour of the four-component system, lidocaine base-lidocaine HCl-monoolein-water, was investigated. It was found that the cubic phase formed in the monoolein-water system was transformed into a lamellar liquid crystalline phase on addition of lidocaine HC1, and to a reversed hexagonal liquid crystalline phase or a reversed micellar phase when the base form was added. With roughly equal amounts of the base and salt forms of lidocaine, the cubic phase persists. These findings strongly indicate not only that lidocaine in both forms participates in the lipid aggregation, but also that the interfacial curvature changes in the opposite direction in relation to the curvature of the cubic phase. These changes were rationalized by making use of the so-called packing concept of amphiphilic molecules. From a pharmaceutical point of view, the phase behaviour of the system shows a number of interesting properties. Some phases may be in equilibrium with excess water solutions, and temperature-induced transitions occur between phases of very different rheology.