Gelation of steroid-bearing dextrans by surfactants

Abstract

Gelation of hydropho-bically modified dextrans, prepared by covalent attachment of deoxy-cholic acid (DCA) to dextran (Dex), with several surfactant categories was investigated with viscosity and rheological techniques. Freshly prepared Dex—DCA solutions do not gel over a large range of polymer and surfactant concentration for any surfactant categories, anionic, cationic, nonionic, and zwitterionic, as indicated by only minor change in the solution’s apparent viscosity (capillary viscometers). For 1% (w/w) Dex—DCA solutions at intermediate sodium deoxycholate (DCA) concentrations (0.008–0.03 M) a stiff macroscopic transparent gel is formed when the samples mature for more than 10 days at room temperature (independently of temperature and sonication during preparation). The time required for gel formation varies with the surfactant concentration and shows a minimum of 10 days around about 10 mM DCA and increases up to about 30 days near the gel phase boundaries. Continuous-flow measurements were carried out for the Dex-DCA/DCA gel mixture. The rheological measurements indicate a high yield stress but an easy breakdown of the gel into small particles when stressed. The extrapolated zero-shear viscosity shows a transition from a gel to a fluid solution at about 33 °C (heat scan) and an opposite gelation around 20 °C (cool scan). In these gelation—fluidization cycles the gel recovers practically its initial mechanic properties.