Protein-stabilized emulsions and nanoemulsions

This presentation will summarize briefly our current understanding of the scientific and technological basis for the formation of emulsions containing micrometer and nanometer sized droplets, with applications in food, cosmetic and pharma industries [1-3]. The focus will be on protein-stabilized emulsions. Their specific features will be summarized in comparison to the surfactant- and particle-stabilized emulsions [3]. The subtle relations between the material characteristics of the emulsions (oil type, emulsifier, pH, etc.) and the optimal hydrodynamic conditions for emulsification will be discussed [3] in the context of obtaining emulsions with desired properties. Examples of appropriate mixtures of proteins with other emulsifiers (lipids, lysolipids, natural gums) will be given. The differences between oil-in-water and water-in-oil emulsions will be briefly discussed [4,5]. Special focus will be given on the recent advance in production of nanoemulsions using high pressure homogenizers, high viscosity of the continuous phase, and/or high oil volume fraction during emulsification [6,7]. Some new methods for self-emulsification will be briefly reviewed [8,9]. The basic physicochemical and hydrodynamic concepts will be illustrated by multiple examples with real systems. References: David Julian McClements, Food Emulsions: Principles, Practices, and Techniques, 3rd Edition, CRC Press, 2015. Andreas Håkansson, Emulsion Formation by Homogenization: Current Understanding and Future Perspectives, Annu. Rev. Food Sci. Technol. 10 (2019) 239–258 (review article). S. Tcholakova, N. D. Denkov, and A. Lips, Comparison of Solid Particles, Globular Proteins and Surfactants as Emulsifiers. Phys. Chem. Chem. Phys. 10 (2008) 1608-1627 (review article). N. Politova, S. Tcholakova, N. D. Denkov, Factors Affecting the Stability of Water-oil-water Emulsion Films. Colloids Surfaces A 522 (2017) 608–620. N. Politova, S. Tcholakova, S. Tsibranska, N. D. Denkov, K. Muelheims, Coalescence Stability of Water-in-Oil drops: Effects of Drop Size and Surfactant Concentration. Colloids Surfaces A 531 (2017) 32–39. S. Tcholakova, I. Lesov, K. Golemanov, N. Denkov, S. Judat, R. Engel, T. Daner, Efficient Emulsification of Viscous Oils at High Drop Volume Fraction. Langmuir 27 (2011) 14783-14796. D. Gazolu-Rusanova, I. Lesov, S. Tcholakova, N. Denkov, B. Ahtchi, Food grade nanoemulsion preparation by rotor-stator homogenization. Food Hydrocolloids (2019) under review. S. Tcholakova, Z. Valkova, D. Cholakova, Z. Vinarov, I. Lesov, N. D. Denkov, K. Smoukov, Efficient Self-Emulsification via Cooling-Heating Cycles. Nature Comm. 8 (2017) 15012. Zh. Valkova, D. Cholakova, S. Tcholakova, N. Denkov, S. K. Smoukov, Mechanisms and Control of Self-Emulsification upon Freezing and Melting of Dispersed Alkane Drops. Langmuir 33 (2017) 12155−12170

Control of drop shape transformations in cooled emulsions.

The general mechanisms of structure and form generation are the keys to understanding the fundamental processes of morphogenesis in living and non-living systems. In our recent study (Denkov et al., Nature 528 (2015) 392) we showed that micrometer sized n-alkane drops, dispersed in aqueous surfactant solutions, can break symmetry upon cooling and "self-shape" into a series of geometric shapes with complex internal structure. This phenomenon is important in two contexts, as it provides: (a) new, highly efficient bottom-up approach for producing particles with complex shapes, and (b) remarkably simple system, from the viewpoint of its chemical composition, which exhibits the basic processes of structure and shape transformations, reminiscent of morphogenesis events in living organisms. In the current study, we show for the first time that drops of other chemical substances, such as long-chain alcohols, triglycerides, alkyl cyclohexanes, and linear alkenes, can also evolve spontaneously into similar non-spherical shapes. We demonstrate that the main factors which control the drop "self-shaping", are the surfactant type and chain length, cooling rate, and initial drop size. The studied surfactants are classified into four distinct groups, with respect to their effect on the "self-shaping" phenomenon. Coherent explanations of the main experimental trends are proposed. The obtained results open new prospects for fundamental and applied research in several fields, as they demonstrate that: (1) very simple chemical systems may show complex structure and shape shifts, similar to those observed in living organisms; (2) the molecular self-assembly in frustrated confinement may result in complex events, governed by the laws of elasto-capillarity and tensegrity; (3) the surfactant type and cooling rate could be used to obtain micro-particles with desired shapes and aspect ratios; and (4) the systems studied serve as a powerful toolbox to investigate systematically these phenomena.This work was funded by the European Research Council (ERC) grant to Stoyan Smoukov, EMATTER (# 280078). The study falls under the umbrella of European networks COST MP 1106 and 1305.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.cis.2016.06.00

Shape-Shifting Polyhedral Droplets

Cooled oil emulsion droplets in aqueous surfactant solution have been observed to flatten into a remarkable host of polygonal shapes with straight edges and sharp corners, but different driving mechanisms - (i) a partial phase transition of the liquid bulk oil into a plastic rotator phase near the droplet interface and (ii) buckling of the interfacially frozen surfactant monolayer enabled by drastic lowering of surface tension - have been proposed. Here, combining experiment and theory, we analyse the hitherto unexplored initial stages of the evolution of these 'shape-shifting' droplets, during which a polyhedral droplet flattens into a polygonal platelet under cooling and gravity. Using reflected-light microscopy, we reveal how icosahedral droplets evolve through an intermediate octahedral stage to flatten into hexagonal platelets. This behaviour is reproduced by a theoretical model of the phase transition mechanism, but the buckling mechanism can only reproduce the flattening if surface tension decreases by several orders of magnitude during cooling so that the flattening is driven by buoyancy. The analysis thus provides further evidence that the first mechanism underlies the 'shape-shifting' phenomena.Comment: 11 pages, 12 figure

Self-shaping of oil droplets via the formation of intermediate rotator phases upon cooling.

Revealing the chemical and physical mechanisms underlying symmetry breaking and shape transformations is key to understanding morphogenesis. If we are to synthesize artificial structures with similar control and complexity to biological systems, we need energy- and material-efficient bottom-up processes to create building blocks of various shapes that can further assemble into hierarchical structures. Lithographic top-down processing allows a high level of structural control in microparticle production but at the expense of limited productivity. Conversely, bottom-up particle syntheses have higher material and energy efficiency, but are more limited in the shapes achievable. Linear hydrocarbons are known to pass through a series of metastable plastic rotator phases before freezing. Here we show that by using appropriate cooling protocols, we can harness these phase transitions to control the deformation of liquid hydrocarbon droplets and then freeze them into solid particles, permanently preserving their shape. Upon cooling, the droplets spontaneously break their shape symmetry several times, morphing through a series of complex regular shapes owing to the internal phase-transition processes. In this way we produce particles including micrometre-sized octahedra, various polygonal platelets, O-shapes, and fibres of submicrometre diameter, which can be selectively frozen into the corresponding solid particles. This mechanism offers insights into achieving complex morphogenesis from a system with a minimal number of molecular components.European Research Council (Grant ID: EMATTER 280078), European networks COST MP 1106 and 1305 and the capacity building project BeyondEverest of the European Commission (Grant ID: 286205)This is the author accepted manuscript. The final version is available from Nature Publishing Group via http://dx.doi.org/10.1038/nature1618