Studying orthogonal self-assembled systems: phase behaviour and rheology of gelled microemulsions

Abstract

Microemulsions in a gelled form are desirable for applications like topical and transdermal drug delivery as they enable local application and enhanced residence times of the drug. One method for gelling microemulsions consists of adding an appropriate low molecular weight gelator. Although our group has reported this method before, a systematic study on the nature and properties of such gelled microemulsions has been missing to date. In this paper we present phase studies and rheology results which demonstrate that microemulsions gelled by a low molecular weight gelator are orthogonal self-assembled systems. We studied the gelled microemulsion H₂O–n-decane/12-hydroxyoctadecanoic acid (12-HOA)–tetraethylene glycol monodecyl ether (C₁₀E₄) with 1.5 wt%, 2.5 wt% and 5.0 wt% of the low molecular weight organogelator 12-HOA. We found that the phase boundaries of the gelled microemulsion are about 6 K below those of the non-gelled “base” microemulsion H₂O–n-decane–C₁₀E₄, irrespective of the gelator concentration. Moreover, we detected by differential scanning calorimetry and rheological measurements a sol–gel boundary about 20 K below that of the respective binary gel n-decane/12-HOA. Both temperature shifts are not surprising considering that (a) 12-HOA is surface active, thus influencing the microemulsion's phase behaviour and (b) that the microemulsion can be treated as a solvent in a 12-HOA gel which influences the gel properties. The general phase behaviour of both base systems, however, is indeed maintained in the gelled microemulsion. For the rheological properties we found, accordingly, perfect agreement with the respective binary gel. Thus, our data clearly substantiate that gelled microemulsions are orthogonal self-assembled systems.