Probing the microstructure of liquid crystalline surfactant systems; a physicochemical study using rheometry, DTA and 17O NMR

Abstract

Mixtures of Brij 96 (decaethylene oxide oleyl ether) and water form liquid crystalline systems (creams) which have a potential as dosage forms for controlled dermal and transdermal drug delivery. By changing the water-surfactant mixing ratio in these systems, lamellar, viscous isotropic or hexagonal liquid crystalline phases can be obtained. First, a number of liquid crystalline creams were characterized using rheometry. Going from a lamellar cream (low water content) to a hexagonal (high water content) one, the macroscopic viscosity strongly increased. In Brij 96-water creams, the polyfethylene oxide) (PEO) chains of the surfactant molecules and the water form a continuous hydrophilic domain, the microstructure of which win be important for the diffusion of drugs through the creams. The hydrophilic domains in the liquid crystalline creams were investigated using 17O nuclear magnetic relaxation and differential thermal analysis and compared with those of PEO-water solutions. From these studies the conclusions were drawn that (i) the hydrophilic domains may be regarded as PEO solutions in water, and that (ii) the cream structure has no detectable influence on the properties of the microstructure of the hydrophilic domains. In the most concentrated creams and PEO solutions studied, having a water-EO ratio of 1.5, an 8-fold retardation of the rotational mobility of the water molecules was observed using 17O nuclear magnetic relaxation.