Non-disruptive mixing of cyclodextrins and wormlike micelles in the non-dilute regime

Abstract

The encapsulation of hydrophobic molecules for different applications is of great importance, especially for pharmaceutical science, as such molecules can be used as carriers of other molecules. Among other possible molecules than can be used for encapsulation, cyclodextrins (CDs) are of particular interest as they have the ability of hosting hydrophobic molecules due to their structure which consist of a hydrophobic inner cavity and a hydrophilic outside surface. This property allows them to form inclusion complexes with amphiphilic molecules like surfactants. In this work we investigated the interaction of Cetyltrimethylammonium bromide and sodium salicylate mixtures (CTAB-NaSal) in aqueous solution with α-cyclodextrin (αCD), β-cyclodextrin (βCD) or 2-hydroxypropyl-α-cyclodextrin (HPαCD), in a regime of concentration where the CTAB-NaSal system self-assembles in wormlike micelles. Depending on CD concentration, the new system features a color change from clear to white. The rheological behavior of the new systems was measured, obtaining viscoelastic spectra and flow curves in function of CD concentration. Structural micellar changes produced by the CD incorporation were obtained and analyzed via MD simulations. The addition of CD slightly changes the typical viscoelastic behavior of the CTAB-NaSal system and maintains the Maxwellian behavior typical of wormlike micelles, although the viscosity of the solution decreases. These experimental and numerical findings show that the addition of CDs instead of disrupting the micellar structure, roughly preserve it, and produce important changes in the optical properties above a critical CD concentration due to the formation of small aggregates on the surface of the micelle. Our results are evidence of a non-disruptive interaction between self-assembled structures and CDs, giving a new type of material not only with possible encapsulation properties but also with a non-trivial rheology.