Prof. Krassimir D. Danov, Ph.D., D.Sc.

Corresponding Member of Bulgarian Acad. Sci.

Head of the Laboratory of Complex Fluids

kd@lcpe.uni-sofia.bg
+359 2 8161 396
Interests
  • Hydrodynamics of Liquid Films, Drops and Bubbles
  • Interfacial Rheology and Bulk Rheology of Dispersions
  • Thermodynamics and Kinetics of Surfactant Adsorption
  • Kinetics of Coagulation and Flocculation
  • Instabilities and Critical Thickness of Liquid Films
Bio

M.Sc. Mechanics and Mathematics (1979); Ph.D. Mathematics (1985); Associate Professor (1997); D.Sc. (2001), Full Professor of Mathematical Modeling and Applied Mathematics (2005) – Department of Chemical Engineering (DCE), Faculty of Chemistry, Sofia University, Bulgaria. Five times, he has been visiting professor in the Institute of Fluid Mechanics, University of Erlangen – Nuremberg, Germany. He was also senior researcher in the CRPP, Bordeaux, France, 1999, and in the Laboratory of Ultrastructure Research, NIPS, Okazaki, Japan, 2001. His research interests are in the area of the hydrodynamics of liquid films, drops and bubbles; thermodynamics and kinetics of surfactant adsorption, incl. micellar surfactant solutions; interfacial rheology and bulk rheology of suspensions; evaporation, hydrodynamic instabilities and critical thickness of liquid films; dynamics of flocculation and coalescence in emulsions; electrostatic and hydrodynamic interactions of colloidal particles at fluid phase boundaries and in thin liquid films. So far, he has published 217 research and review papers (cited 5471 times in the scientific literature, h-index 44, Scopus), and has given 296 presentations at international conferences, including 76 invited lectures and 115 lectures. He was awarded the F. W. Bessel Prize of the Humboldt Foundation (Germany) in 2002 and the Blue Ribbon Medal of the Sofia University “St. Kliment Ohridski” for significant achievements in science (2016). He is recipient of the highest national award for scientific achievements “Pythagoras” by the Bulgarian ministry of education and science (2019).

Publications
Most recent publications
R. D. Stanimirova, K. D. Danov, M. T. Georgiev, J. T. Petkov
J. Colloid Interface Sci. 2025
677
250-263

Hypothesis
Polyglycerol esters of fatty acids are generated via the esterification of a polydisperse mixture of polyglycerol with naturally derived fatty acids. The polymerization process of polyglycerol results in the production of various oligomers, ranging from di-, tri-, and higher-order forms, which contribute to the complexity of final products. The combination of complementary experimental techniques and adequate theoretical interpretations can reveal the wide variety of their physicochemical properties.

Experiments
The colloid and interface properties of polyglyceryl mono-laurate, mono-stearate, mono-oleate, and a mixture of mono-caprylate and mono-caprate esters solutions were characterized by measurements of the electrolytic conductivity, static and dynamic surface tension, aggregate and micelle sizes and distributions, thin liquid film stability and stratification, and solubility in aqueous and in oil phases. The formation, stability, and bubble size distribution of foams generated from polyglycerol esters aqueous solutions were systematically investigated.

Findings
The low concentrations of double-tail molecules and fatty acids in polyglycerol esters affect considerably their micellar, aggregation, and vesicle formations in aqueous solutions. The theoretical data interpretation of polyglycerol esters isotherms and thin liquid films data provide information on the adsorption energies, excluded areas per molecule, interaction parameters of molecules at interfaces, surface electrostatic potential, and the size of micelles. Polyglyceryl mono-oleate exhibits spontaneous emulsification properties. Short chain length polyglycerol esters have excellent foaming ability but relatively low foam stability. The optimal weight fractions of the short-chain polyglyceryl esters and polyglyceryl mono-stearate mixtures with respect to good foaminess and foam stability upon Ostwald ripening are obtained. The reported physicochemical characterization of the water-soluble polyglycerol esters could be of interest to increase the range of their applicability in practice.

V. Yavrukova, K. Danov, T. Slavova, R. Stanimirova, Y. Ung, A. Suan, H. Xu, J. Petkov
J. Colloid Interface Sci. 2024
660
896 - 906

Hypothesis: Methyl ester sulfonates (MES) show limited water solubility at lower temperatures (Krafft point). One way to increase their solubility below their Krafft points is to incorporate them in anionic surfactant micelles. The electrostatic interactions between the ionic surfactant molecules and charged micelles play an important role for the degree of MES solubility. Experiments: The solubility and electrolytic conductivity for binary and ternary surfactant mixtures of MES with anionic sodium alpha olefin sulfonate (AOS) and sodium lauryl ether sulfate with two ethylene oxide groups (SLES-2EO) at 5 °C during long-term storage were measured. Phase diagrams were established; a general phase separation theoretical model for their explanation was developed and checked experimentally. Findings: The binary and ternary phase diagrams for studied surfactant mixtures include phase domains: mixed micelles; micelles + crystallites; crystallites, and molecular solution. The proposed general phase separation model for ionic surfactant mixtures is convenient for construction of such complex phase diagrams and provides information on the concentrations of all components of the complex solution and on the micellar electrostatic potential. The obtained maximal MES mole fraction of transparent micellar solutions could be of interest to increase the range of applicability of MES–surfactants.

T. Slavova, G. Radulova, K. Danov
Colloids Int. 2024
8
11

Different oils can be homogeneously dispersed in the network junctions of the separated bicontinuous micellar phases. Upon dilution, these dispersions spontaneously form nanoemulsions. The possibility of a micellar sponge phase formation in the case of mixtures with three anionic and two zwitterionic surfactants in the presence of divalent and monovalent salts is studied. The best results are obtained using sodium lauryl ether sulfate with 1 ethylene oxide group (SLES-1EO) and both cocamidopropyl betaine (CAPB) or N,N-dimethyldodecylamine N-oxide (DDAO) in the presence of an appropriate small amount of MgCl2 and CaCl2. Bicontinuous micellar phases can be produced also in high-salinity NaCl solutions. The bulk properties of these phases are independent of the concentration of the initial solutions from which they are separated, and their Newtonian viscosities are in the range from 0.3 Pa·s to 0.8 Pa·s. Both 8 wt% CAPB- and DDAO-containing sponge phases engulf up to 10 wt% limonene and spontaneously form nanoemulsion upon dilution with droplet sizes of 110–120 nm. Vitamin E can be homogeneously dispersed only in CAPB-containing saturated micellar network, and upon dilution, these dispersions spontaneously form nanoemulsions with smaller droplet sizes of 66 nm for both 8 diastereomers and 2 diastereomers mixtures of vitamin E.

K. Danov, G. Radulova, M. Georgiev, P. Kralchevsky
J. Technical Univ. Gabrovо 2023
66
24 - 28

The concentrated surfactant solutions have a wide application in industry, oil recovery, drug delivery, turbulent drag reduction, etc. The competition between the companies-producers has led to use of new kind of formulations to improve: washing action; skin and eye irritation; stability and durability; biodegradability; tolerance to hard water. Here, we present a review on the state of the art and our contributions to the molecular thermodynamic theory and experiment on the growth of giant micellar aggregates. Despite the considerable advances in theory and computer simulations, agreement with experimental data has been achieved only in isolated cases. Our predictive molecular thermodynamic approach accounts for the different contributions to the micellar scission energy in the case of nonionic, zwitterionic and ionic surfactant solutions and their mixtures. Excellent agreement was achieved between the theoretical model and experimental data for wormlike surfactant micelles at various concentrations of salt and temperatures. At high salt concentrations, the model also predicts loss of chemical equilibrium, which implies a transition to self-assemblies of other morphology or the onset of crystallization and phase separation. The results have applications for the design of new products and nanostructured materials.

M. Georgiev, B. Konstantinov, K. Marinova, J. Petkov, K. Danov
J. Technical Univ. Gabrovо 2023
66
38 - 42

The research paper explores the adsorption properties of cationic surfactants on silicon wafers through imaging ellipsometry. The objective of this research is to shed light on the layer structures formed by cationic surfactants, specifically those based on dimethyl ammonium chloride, on silicon wafers. The study involved the deposition of three distinct cationic surfactants on the wafer’s surface, followed by the measurement of the adsorption layers formed. The findings reveal the creation of thin, smooth, and irregular adsorption layers. Interestingly, no correlation was found between the thickness of the adsorption layer and the surfactant tail’s chain length. The research underlines the significant role which the imaging ellipsometry can have for studying surfactants’ adsorption properties on surfaces, contributing to their optimal usage in various fields.

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