Prof. DSc. Nikolai Denkov and Assoc. Prof. Slavka Tcholakova

Within the frame of the project a team from the Department of Engineering Chemistry (Prof. N. Denkov, Assoc. Prof. S. Tcholakova) will work on synthesis of nanoparticles in emulsions. There are two major approaches for mass production of nanoparticles – top-down approach by grinding of large particles into smaller ones, and bottom-up approach through nucleation. Various production approaches, however, have different technical problems. An alternative root is to use batch water-in-oil emulsions, in which the water droplets are used as microreactors for particle synthesis. The group of Prof. Denkov has worked on several aspects of the emulsification process which can be combined together to develop a new approach for particle synthesis in batch emulsions. These aspects include [[1],[2]]: (1) Detailed study of the effects of hydrodynamic conditions on the drop-size distribution during emulsification; (2) Choice of appropriate surfactants for selective control of emulsion stability; (3) Analysis of the drop-drop coalescence in emulsions with a special emphasis on the effect of surfactant type on the coalescence process. Most of these studies were made in collaboration with the emulsification team of BASF company, Ludwigshafen, Germany, which is the major producer of nano- and microparticles in Europe (with wide range of applications).

Based on previous studies and preliminary experiments, the following approach will be developed. Two emulsions, containing the different reagents will be stabilized by nonionic surfactant with well defined concentration, which ensures stabilization of the drops in these emulsions by forming a dense adsorption layer on the drop surfaces, see Figure 1A. Upon mixing these emulsions in the homogenizer, smaller droplets will be formed, whose surfaces are not protected by a dense surfactant monolayer due to the deficiency of surfactant in the system, Figure 1B. As a result, the droplets in the obtained emulsion will spontaneously coalesce until they become sufficiently large, so that their surfaces become again covered with dense adsorption layer preventing the further drop-drop coalescence, Figure 1C. In this coalescence process, the interior of the various droplets will be mixed so that the reaction will be initiated. At the end of the cycle, the emulsion will be destroyed (by increase of temperature above the surfactant cloud point) and the particles will be collected in a medium with appropriate pH, electrolytes, and surfactants.

Figure 1. Controlled limited drop-drop coalescence. Two emulsions stabilized by nonionic surfactant are prepared so that a complete adsorption layer of surfactant is formed, and afterwards mixed under intensive stirring. The hydrodynamic agitation leads to breakage of the initial drops into smaller ones, which are not well protected by the dilute adsorption layer of surfactant. As a result, the small droplets coalesce until their size becomes equal to that of the initial emulsions and the reactants are mixed inside the drops.



[1] N. Vankova, S. Tcholakova, N. D. Denkov, I. B. Ivanov, V. D. Vulchev, T. Danner, “Emulsification in turbulent flow: 1. Mean and maximum drop diameters in inertial and viscous regimes”, J. Colloid Interface Sci. 312 (2007) 363.

[2] S. Tcholakova, N. D. Denkov, A. Lips, “Comparison of solid particles, globular proteins, and surfactants as emulsifiers”, Phys. Chem. Chem. Phys. 12 (2008) 1608 (invited review).