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

Baghali G Mathapa,Vesselin N Paunov

doi:10.1039/c3ta12108a

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.

Highlights

The design of functional nanomaterials is a subject of great interest because of their unique properties and potential applications in a wide range of elds such as medical formulations, agrochemicals, skin care/cosmetics and pharmaceuticals
Oil droplet images of varying light intensities were captured at different time intervals to show the formation of inclusion complexes (ICs) microcrystals due to the CD threading and assembly at the o/w interface
We discovered that threading of CD along the tetradecane molecule and the microcrystal formation is dependent on the CD used and the size of its hydrophobic cavity

Summary

Introduction

The design of functional nanomaterials is a subject of great interest because of their unique properties and potential applications in a wide range of elds such as medical formulations, agrochemicals, skin care/cosmetics and pharmaceuticals. Click chemistry and spontaneous self-assembly are one of the promising bottom-up approaches to form supramolecular nanomaterials from molecular building blocks by non-covalent interactions such as van der Waals and hydrogen bonding.[1,2] Such self-assembly processes are reversible and simpler compared to the systems involving covalent bonds.[3] Cyclodextrins (CDs) are widely used as host molecules which can form supramolecular structures. CDs are cyclic sugars derived from hydrolysis of starch by Cyclodextrin Glycosyl Transferase bacterial enzyme (CG-Tase) consisting of six (a), seven (b) and eight (g) glucose units bridged through 1,4-glycosidic bonds.[4,5] They have a cone shaped chemical structure with a hydrophobic inner cavity which allows CDs to incorporate other molecules

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