Effects of polymer addition on the rheology of o/w microemulsions

Microemulsions are profitably employed in the pharmaceutical field to prepare drug delivery systems release for guest drugs sparingly soluble in water. In particular, they can be used for topical and transdermal administration in place of ointments or creams, on condition that their rheological properties are properly modulated. The present work concerns the analysis of the rheological effects produced by the addition of different amounts of Carbopol 940, a polymer widely used for topical applications, to an O/W microemulsion containing a lipophilic phase (Labrafac Hydro, 21 wt%) and stabilized by the surfactant (Cremophor RH40)/co-surfactant (Transcutol) couple. The contribution of the disperse phase is evaluated by comparing the linear and nonlinear properties of the Carbopol/microemulsion system (CM) with those of the corresponding aqueous Carbopol systems with co-surfactant (CWT) and without co-surfactant (CW). Four polymer concentrations (0.25%, 0.5%, 1%, and 2%) are taken into consideration. The linear viscoelastic properties of the microemulsions essentially mirror those of the corresponding aqueous Carbopol systems when the polymer concentration is sufficiently high (1-2%). In these conditions the oil phase is reasonably hosted within the meshes of the three-dimensional polymeric gel network and gives only a slight contribution to the connectivity of the whole system. Similar considerations can be drawn from the flow behavior of the systems examined in the low shear region.