Ionic liquids as tools to improve gel formulations containing sparingly soluble phenolic acids

Hydroxycinnamic acids, namely caffeic and p-coumaric acids, have several pharmaceutical and cosmetic applications, but due to their low aqueous solubility, their applicability can be limited. Ionic liquids (ILs) have been shown to be a valuable tool to improve the solubility and assist in the incorporation of various phenolic compounds into delivery systems. Thus, this work aims to evaluate the impact of incorporating three choline-based ILs, namely (2-hydroxyethyl)-trimethylammonium-L-phenylalanine [Cho][Phe], (2-hydroxyethyl)-trimethylammonium-L-glutamate [Cho][Glu] and (2-hydroxyethyl)-trimethylammonium-glycinate [Cho][Gly], into an aqueous gel formulation containing the poorly soluble caffeic and p-coumaric acids. The results obtained confirm that the ILs not only increase the drug solubility, but also allow higher amounts of both studied drugs to be incorporated into the gels, without interfering with the stability of the aqueous gels. Moreover, the ILs altered the fluidity of the gels, both in the absence and presence of both phenolic acids, as they increase the viscosity of the formulations, contributing to higher flow resistance of the gel, which may be better accepted by the consumer. Keywords: Ionic liquids; Hydroxycinnamic acids; Poorly soluble drugs; Gels; Increased Viscosity