Former post-doc of Prof Paunov
Dr Hartmut Wege worked as a Post-Doctoral Fellow in Paunov’s research group at the University of Hull 2004-2006. After that he held post-doc positions in NCSU (USA) and CNRS (France).
Supervisor: Prof Vesselin N. Paunov (PI) and Prof Orlin Velev (Co-PI, NCSU)
Research project 1:
Foams and Emulsions Stabilised by In-Situ Formed Microparticles from Hydrophobic Cellulose
(in collaboration with Velev’s Research Group, North Carolina State University)
We report a simple method to produce foams and emulsions of extraordinary stability by using hydrophobic cellulose microparticles, which are formed in situ by a liquid-liquid dispersion technique . The hydrophobic cellulose derivative, hypromellose phthalate (HP), was initially dissolved in water-miscible solvents such as acetone and ethanol/water mixtures. As these HP stock solutions were sheared in aqueous media, micron sized cellulose particles formed by the solvent attrition (Fig. 1). We also designed and investigated an effective and simple process for making HP particles without any organic solvents, where both the solvent and antisolvent were aqueous buffer solutions at different pH. Consequently, the HP particles adsorbed onto the water/air or water/oil interfaces created during shear blending, resulting in highly stable foams or foam/emulsions (Fig. 3). The formation of HP particles and their ability for short-term and long-term stabilization of interfaces strongly depended on the HP concentration in stock solutions, as well as the solvent chemistry of both stock solutions and continuous phase media. Some foams and emulsion samples formed in the presence of ca. 1 wt% HP were stable for months. This new class of nontoxic inexpensive cellulose-based particle stabilizers has the potential to substitute conventional synthetic surfactants, especially in edible, pharmaceutical and biodegradable products.
Fig. 1. Schematics of the mechanism of forming foams and emulsions stabilized by in situ formed HP particles.
Fig. 2 Optical micrographs of the morphology of typical hydrophobic cellulose (HP) particles formed from 2 wt% (A), 5 wt% (B), 15 wt% (C), and 30% (D) of HP stock solutions in acetone, respectively. Scale bars are 50 μm.
Fig. 3 (A) Appearance of the foams immediately after formation with 15% HP in stock solutions (from left to right). Optical micrograph of foam system prepared from the stock solution of 15% HP in the first hour after formation (B). (C). Air bubble stabilised by HP particles.
- Wege, H.A., Kim, S., Paunov, V.N., Zhong, Q., Velev, O.D., “Long-term stabilization of foams and emulsions with in-situ formed microparticles from hydrophobic cellulose”, Langmuir, 24 (2008) 9245-9253.
Research project 2:
Anisotropic magnetically-functionalized lens- and donut-shaped microparticles
We developed a simple method for fabricating anisotropic non-spherical polymeric microparticles of various morphologies, including microlenses, doughnuts and hexagons . Magnetically functionalized lens-like and donut-shaped polymeric microparticles were prepared by spreading a magnetite-doped paraffin–polymer solution at the air/water interface in the presence of an external magnetic field. The method is based on spreading a paraffin–polymer solution on an air/water surface followed by solidification of the paraffin–polymer melt due to cooling and solvent attrition (Fig. 1). We examine the parameters that affect the particle morphology and interfacial aggregation behaviour. The particle size and shape can be changed by adjusting the paraffin–polymer concentration in the spreading solution and the temperature of the aqueous sub-phase. By embedding colloidal magnetite in the polymer melt, we produced magnetic microparticles with a permanent magnetic dipole moment of controllable strength and direction, as deduced from their interfacial aggregation behaviour (Fig. 2).
Fig. 1 Optical microscope images of donut-shaped microparticles produced by spreading an isopropanol solution of paraffin–ceresin doped with a fluorescent dye onto an aqueous solution of (A) 5 mM SBDS and (B) 2 mM hithenol BC-20 at T = Tm = 55 °C. The second row shows the corresponding fluorescence images with filter set for perylene. Scale bars are 20 μm .
Fig. 2 Optical microscope images of magnetite-doped (10% w/w) microlenses of various morphologies produced by spreading an isopropanol solution of paraffin–ceresin at T = 55 °C in the presence of external magnetic field which is (A) perpendicular to the air/water interface and (B) parallel to the air/water interface. The aqueous sub-phase is 10 mM SBDS .
- Wege, H.A., Dyab, A.K.F., Velev, O.D., Paunov, V.N., “Fabrication of magnetically-functionalized lens- and donut-shaped microparticles by a surface-formation technique”, Phys. Chem. Chem. Phys., 9 (2007) 6300-6303.