Former PhD student of Prof Paunov Baghali did his PhD in Paunov Research Group. He graduated in 2013 and is currently working as a Lecturer at Botswana Agricultural College.   PhD Supervisor: Prof. Vesselin N. Paunov (PI)

Research project 1:

Self-assembly of cyclodextrins at fluid interfaces: surfactant-free emulsions, cyclodextrinosomes and cyclodextrin-functionalised living cells

 We explored the self-assembly of cyclodextrins (CDs) at the tetradecane/water interface through formation of inclusion complexes (ICs) [1]. 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 an inclusion complex with linear oil molecules at the oil-water interface which further assemble into microcrystals at the oil-water interface. We discovered that the morphology and the size of the microcrystals formed by these ICs are dependent on the type of CD and oil used. We demonstrated the spontaneous formation of a dense layer of adsorbed CD-tetradecane IC microcrystals at the tetradecane-water interface [1,2]. 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 application in surfactant free pharmaceutical and cosmetic formulations reducing the release of surfactants in the environment. We also report the preparation of o/w emulsions stabilised by microcrystals of cyclodextrin-oil inclusion complexes [2]. The inclusion complexes are formed by threading cyclodextrins from the aqueous phase on n-tetradecane or silicone oil molecules from the emulsion drop surface which grow further into microrods and micro-platelets depending on the type of cyclodextrin (CD) used. These microcrystals remain attached at the surface of the emulsion drops and form densely packed layers. The novelty in this emulsion stabilisation mechanism is that molecularly dissolved cyclodextrin from the continuous aqueous phase is assembled into colloid particles directly onto the emulsion drop surface, i.e. molecular adsorption leads to effective Pickering stabilisation [2]. The β-CD stabilised tetradecane-in-water emulsions were so stable that we used this system as a template for preparation of cyclodextrinosomes. These structures were produced solely through formation of cyclodextrin-oil inclusion complexes and their assembly into a crystalline phase on the drop surface retained its stability after the removal of the core oil. We also reported the preparation of CD-stabilized emulsions with a range of other oils and studied the effect of the salt concentration in the aqueous phase, the type of CD and the oil volume fraction on the type of emulsion formed.

Fig.1. Influence of α-CD and β-CD concentrations on air–water surface tension and tetradecane–water interfacial tension. (d) Molecular structures of tetradecane, α-CD, β-CD and γ-CD.

Fig.2. Schematics for preparation of cyclodextrinosome microcapsules by templating oil-in-water emulsions with in-situ formed CD-oil inclusion complex micro-crystals. Optical and SEM images of the actual cyclodextrinosomes (in the RHS image the capsule is cracked open)

In addition, by cross-linking CDs at the emulsion drops interface with epichlorohydrin and PAH we produced microcapsules with CD-membrane functionality [3]. Similar crosslinking process with CDs and PAH in a homogeneous aqueous solution with epichlorohydrin gave nanoporous microparticles of CD-functionalised polymer [4]. In a separate study, we also achieved surface functionalization of living cells with CDs without significant loss of viability [5. The CD-stabilized emulsions can find applications in a range of surfactant-free formulations in cosmetics, home and personal care. Cyclodextrinosomes and CD-functionalized microcapsules could find applications in pharmaceutical formulations as drug delivery vehicles.

References

1. Mathapa, B.G., Paunov, V.N., Self-assembly of cyclodextrin-oil inclusion complexes at the oil-water interface: a route to surfactant-free emulsions, J. Mater. Chem. A, 1 (2013) 10836 – 10846.
2. Mathapa, B.G., Paunov, V.N., Cyclodextrin stabilised emulsions and cyclodextrinosomes, Physical Chemistry Chemical Physics, 15 (2013) 17903 – 17914.
3. Mathapa, B.G., Paunov, V.N., “Fabrication of novel cyclodextrin-polyallylamine hydrochloride co-polymeric microcapsules by templating oil-in-water emulsions”, Soft Matter, 9 (2013) 4780-4788.
4. Mathapa, B.G., Paunov, V.N., Nanoporous cyclodextrin based co-polymer microspheres for encapsulation of active components, J. Mater. Chem. B, 1 (2013) 3588 – 3598.
5. Mathapa, B.G., Paunov, V.N., Fabrication of viable cyborg cells with cyclodextrin functionality, Biomaterials Science, 2 (2013) 212-219.