Self-assembly of cyclodextrin–oil inclusion complexes at the oil–water interface: a route to surfactant-free emulsions

Abstract

We investigated the self-assembly of cyclodextrin (CD) molecules at the tetradecane–aqueous solution interface through formation of inclusion complexes (ICs). We studied the surface activity of CDs at both the air–water and the oil–water interface. Although α-CD and β-CD are not surface active at the air–water interface, they form pseudo-surfactants as inclusion complexes with linear oil molecules at the oil–water interface. We discussed the factors affecting the formation of these ICs and their assembly into microcrystals at the oil–water interface. We discovered that the morphology and the size of the aggregates formed by these ICs are dependent on the type of CD and oil used. Lamella sheets and long microrods were obtained from α-CD molecules and tetradecane. In contrast, β-CD–tetradecane gave short microrods which assembled in microcrystals. We characterised the CD–tetradecane ICs using optical microscopy, SEM, TEM and FT-IR. The crystallinity of the ICs was assessed using cross-polarised light microscopy. We demonstrated the spontaneous formation of a dense layer of adsorbed CD–tetradecane IC microcrystals at the tetradecane–water interface. At large oil volume fractions, this phenomenon led to the formation of a Pickering type of oil-in-water emulsion stabilised by adsorbed CD–oil microcrystals while at low oil volume fractions it completely solubilises the oil in the form of IC microcrystals. This emulsion stabilisation mechanism with sustainable materials like CDs may find applications in surfactant free pharmaceutical and cosmetic formulations reducing the release of surfactants in the environment.