Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Origin of the extremely high elasticity of bulk emulsions, stabilized by Yucca Schidigera saponins
    Tsibranska, Sonya ; Tcholakova, Slavka ; Golemanov, Konstantin ; Denkov, Nikolai ; Arnaudov, Luben ; Pelan, Eddie ; Stoyanov, Simeon D. - \ 2020
    Food Chemistry 316 (2020). - ISSN 0308-8146
    Drop aggregation - Drop-drop adhesion - Emulsion - Emulsion elasticity - Interfacial elasticity - Non-purified oil - Phytosterols - Saponin

    We found experimentally that the elasticity of sunflower oil-in-water emulsions (SFO-in-W) stabilized by Yucca Schidigera Roezl saponin extract, is by >50 times higher as compared to the elasticity of common emulsions. We revealed that strong specific interactions between the phytosterols from the non-purified oil and the saponins from the Yucca extract lead to the formation of nanostructured adsorption layers which are responsible for the very high elasticity of the oil-water interface and of the respective bulk emulsions. Remarkably, this extra high emulsion elasticity inhibits the emulsion syneresis even at 65 vol% of the oil drops – these emulsions remain homogeneous and stable even after 30 days of shelf-storage. These results demonstrate that the combination of saponin and phytosterols is a powerful new approach to structure oil-in-water emulsions with potential applications for formulating healthier functional food.

    Role of interfacial elasticity for the rheological properties of saponin-stabilized emulsions
    Tsibranska, Sonya ; Tcholakova, Slavka ; Golemanov, Konstantin ; Denkov, Nikolai ; Pelan, Eddie ; Stoyanov, Simeon D. - \ 2020
    Journal of Colloid and Interface Science 564 (2020). - ISSN 0021-9797 - p. 264 - 275.
    Emulsion - Emulsion rheology - Interfacial rheology - Saponin - Surface elasticity - Surface rheology

    Hypothesis: Saponins are natural surfactants which can provide highly viscoelastic interfaces. This property can be used to quantify precisely the effect of interfacial dilatational elasticity on the various rheological properties of bulk emulsions. Experiments: We measured the interfacial dilatational elasticity of adsorption layers from four saponins (Quillaja, Escin, Berry, Tea) adsorbed on hexadecane-water and sunflower oil-water interfaces. In parallel, the rheological properties under steady and oscillatory shear deformations were measured for bulk emulsions, stabilized by the same saponins (oil volume fraction between 75 and 85%). Findings: Quillaja saponin and Berry saponin formed solid adsorption layers (shells) on the SFO-water interface. As a consequence, the respective emulsions contained non-spherical drops. For the other systems, the interfacial elasticities varied between 2 mN/m and 500 mN/m. We found that this interfacial elasticity has very significant impact on the emulsion shear elasticity, moderate effect on the dynamic yield stress, and no effect on the viscous stress of the respective steadily sheared emulsions. The last conclusion is not trivial, because the dilatational surface viscoelasticity is known to have strong impact on the viscous stress of steadily sheared foams. Mechanistic explanations of all observed effects are described.

    Growth of wormlike micelles in nonionic surfactant solutions : Quantitative theory vs. experiment
    Danov, Krassimir D. ; Kralchevsky, Peter A. ; Stoyanov, Simeon D. ; Cook, Joanne L. ; Stott, Ian P. ; Pelan, Eddie G. - \ 2018
    Advances in Colloid and Interface Science 256 (2018). - ISSN 0001-8686 - p. 1 - 22.
    Micelle free energy - Micelle growth - Nonionic surfactants - Polyoxyethylene alkyl ethers - Wormlike micelles
    Despite the considerable advances of molecular-thermodynamic theory of micelle growth, agreement between theory and experiment has been achieved only in isolated cases. A general theory that can provide self-consistent quantitative description of the growth of wormlike micelles in mixed surfactant solutions, including the experimentally observed high peaks in viscosity and aggregation number, is still missing. As a step toward the creation of such theory, here we consider the simplest system – nonionic wormlike surfactant micelles from polyoxyethylene alkyl ethers, CiEj. Our goal is to construct a molecular-thermodynamic model that is in agreement with the available experimental data. For this goal, we systematized data for the micelle mean mass aggregation number, from which the micelle growth parameter was determined at various temperatures. None of the available models can give a quantitative description of these data. We constructed a new model, which is based on theoretical expressions for the interfacial-tension, headgroup-steric and chain-conformation components of micelle free energy, along with appropriate expressions for the parameters of the model, including their temperature and curvature dependencies. Special attention was paid to the surfactant chain-conformation free energy, for which a new more general formula was derived. As a result, relatively simple theoretical expressions are obtained. All parameters that enter these expressions are known, which facilitates the theoretical modeling of micelle growth for various nonionic surfactants in excellent agreement with the experiment. The constructed model can serve as a basis that can be further upgraded to obtain quantitative description of micelle growth in more complicated systems, including binary and ternary mixtures of nonionic, ionic and zwitterionic surfactants, which determines the viscosity and stability of various formulations in personal-care and house-hold detergency.
    Hardening of particle/oil/water suspensions due to capillary bridges : Experimental yield stress and theoretical interpretation
    Danov, Krassimir D. ; Georgiev, Mihail T. ; Kralchevsky, Peter A. ; Radulova, Gergana M. ; Gurkov, Theodor D. ; Stoyanov, Simeon D. ; Pelan, Eddie G. - \ 2018
    Advances in Colloid and Interface Science 251 (2018). - ISSN 0001-8686 - p. 80 - 96.
    Capillary bridges - Capillary suspensions - Pendular state - Rheology of suspensions - Wet granular materials - Yield stress
    Suspensions of colloid particles possess the remarkable property to solidify upon the addition of minimal amount of a second liquid that preferentially wets the particles. The hardening is due to the formation of capillary bridges (pendular rings), which connect the particles. Here, we review works on the mechanical properties of such suspensions and related works on the capillary-bridge force, and present new rheological data for the weakly studied concentration range 30-55 vol% particles. The mechanical strength of the solidified capillary suspensions, characterized by the yield stress Y, is measured at the elastic limit for various volume fractions of the particles and the preferentially wetting liquid. A quantitative theoretical model is developed, which relates Y with the maximum of the capillary-bridge force, projected on the shear plane. A semi-empirical expression for the mean number of capillary bridges per particle is proposed. The model agrees very well with the experimental data and gives a quantitative description of the yield stress, which increases with the rise of interfacial tension and with the volume fractions of particles and capillary bridges, but decreases with the rise of particle radius and contact angle. The quantitative description of capillary force is based on the exact theory and numerical calculation of the capillary bridge profile at various bridge volumes and contact angles. An analytical formula for Y is also derived. The comparison of the theoretical and experimental strain at the elastic limit reveals that the fluidization of the capillary suspension takes place only in a deformation zone of thickness up to several hundred particle diameters, which is adjacent to the rheometer's mobile plate. The reported experimental results refer to water-continuous suspension with hydrophobic particles and oily capillary bridges. The comparison of data for bridges from soybean oil and hexadecane surprisingly indicate that the yield strength is greater for the suspension with soybean oil despite its lower interfacial tension against water. The result can be explained with the different contact angles of the two oils in agreement with the theoretical predictions. The results could contribute for a better understanding, quantitative prediction and control of the mechanical properties of three-phase capillary suspensions solid/liquid/liquid.
    Rheology of particle/water/oil three-phase dispersions : Electrostatic vs. capillary bridge forces
    Georgiev, Mihail T. ; Danov, Krassimir D. ; Kralchevsky, Peter A. ; Gurkov, Theodor D. ; Krusteva, Denitsa P. ; Arnaudov, Luben N. ; Stoyanov, Simeon D. ; Pelan, Eddie G. - \ 2018
    Journal of Colloid and Interface Science 513 (2018). - ISSN 0021-9797 - p. 515 - 526.
    Capillary bridges - Capillary suspensions - Pendular state - Silica particles - Suspension rheology - Wet granular materials - Yield stress
    Hypothesis Particle/water/oil three-phase capillary suspensions possess the remarkable property to solidify upon the addition of minimal amount of the second (dispersed) liquid. The hardening of these suspensions is due to capillary bridges, which interconnect the particles (pendular state). Electrostatic repulsion across the oily phase, where Debye screening by electrolyte is missing, could also influence the hardness of these suspensions. Experiments We present data for oil-continuous suspensions with aqueous capillary bridges between hydrophilic SiO2 particles at particle volume fractions 35–45%. The hardness is characterized by the yield stress Y for two different oils: mineral (hexadecane) and vegetable (soybean oil). Findings and modelling The comparison of data for the “mirror” systems of water- and oil-continuous capillary suspensions shows that Y is lower for the oil-continuous ones. The theoretical model of yield stress is upgraded by including a contribution from electrostatic repulsion, which partially counterbalances the capillary-bridge attraction and renders the suspensions softer. The particle charge density determined from data fits is close to that obtained in experiments with monolayers from charged colloid particles at oil/water interfaces. The results could contribute for better understanding, quantitative prediction and control of the mechanical properties of solid/liquid/liquid capillary suspensions.
    Role of surface properties for the kinetics of bubble Ostwald ripening in saponin-stabilized foams
    Tcholakova, Slavka ; Mustan, Fatmegul ; Pagureva, Nevena ; Golemanov, Konstantin ; Denkov, Nikolai D. ; Pelan, Edward G. ; Stoyanov, Simeon D. - \ 2017
    Colloids and Surfaces. A: Physicochemical and Engineering Aspects 534 (2017). - ISSN 0927-7757 - p. 16 - 25.
    Adsorption layer - Foam coarsening - Ostwald ripening - Saponin - Surface rheology
    Bubble Ostwald ripening (OR) leads to a gradual increase of the mean bubble size in foams with time. The rate of OR can be reduced significantly or even arrested completely using appropriate solid particles and/or surfactants as foam stabilizers. In the current paper, we show that saponins, a widespread class of natural surfactants, can reduce significantly the rate of OR in foams. To reveal the reasons for the reduced rate of OR in saponin-stabilized foams, we performed measurements of the rate of bubble diminishing, for single air bubbles placed below a solution surface, with a series of saponin bio-surfactants. These saponin surfactants form adsorption layers with surface elasticity, spanning a very wide range - from almost zero up to several thousand mN/m. The measured rate of bubble OR showed no correlation with the surface elastic modulus (dilatational or shear), as measured at 0.1. Hz frequency of surface oscillations. A reasonable correlation was observed only with the surface stress (deviation from the equilibrium surface tension), measured at very slow rate of surface deformation, which mimics much better the actual processes of bubble OR in foams - higher surface stress corresponds to lower OR rate. New theoretical expression, accounting for the out-of- equilibrium surface tension during bubble shrinkage and for the gas flux across the meniscus regions surrounding the foam films, was derived and used to calculate theoretically the rate of bubble diminishing. The comparison of the theoretical predictions with the experimental data shows clearly that the main reason for the reduced rate of OR in the studied systems is the high resistance to gas transfer of the saponin adsorption layers. The deviations from the equilibrium surface tension, although noticeable, have smaller effect. The complementary experiments with actual foams showed that the rate of OR is even lower (compared to the rate measured with single bubbles) which is explained with the thicker non-equilibrium foam films, formed between the neighboring bubbles in saponin-stabilized foams.
    Production and characterization of stable foams with fine bubbles from solutions of hydrophobin HFBII and its mixtures with other proteins
    Dimitrova, Lydia M. ; Petkov, Plamen V. ; Kralchevsky, Peter A. ; Stoyanov, Simeon D. ; Pelan, Eddie G. - \ 2017
    Colloids and Surfaces. A: Physicochemical and Engineering Aspects 521 (2017). - ISSN 0927-7757 - p. 92 - 104.
    Hydrophobins are proteins that are excellent foam stabilizers. We investigated the effects of pH and addition of other proteins on the foaminess, bubble size, and stability of foams from aqueous solutions of the protein HFBII hydrophobin. The produced stable foams have bubbles of radii smaller than 40 μm that obey the lognormal distribution. The overrun of most foams is in the range from 5 to 8, which indicates a good foaminess. The foam longevity is characterized by the time dependences of the foam volume and weight. A combined quantitative criterion for stability, the degree of foam conservation, is proposed. The produced foams are stable for at least 12–17 days. The high foam stability can be explained with the formation of dense hydrophobin adsorption layers, which are impermeable to gas transfer and block the Ostwald ripening (foam disproportionation). In addition, the population of small bubbles formed in the HFBII solutions blocks the drainage of water through the Plateau borders in the foam. The variation of pH does not essentially affect the foaminess and foam stability. The addition of “regular” proteins, such as beta-lactoglobulin, ovalbumin and bovine serum albumin, to the HFBII solutions does not deteriorate the quality and stability of the produced foams up to 94% weight fraction of the added protein. The results and conclusions from the present study could be useful for the applications of hydrophobins as foam stabilizers.
    Adhesion of bubbles and drops to solid surfaces, and anisotropic surface tensions studied by capillary meniscus dynamometry
    Danov, Krassimir D. ; Stanimirova, Rumyana D. ; Kralchevsky, Peter A. ; Marinova, Krastanka G. ; Stoyanov, Simeon D. ; Blijdenstein, Theodorus B.J. ; Cox, Andrew R. ; Pelan, Eddie G. - \ 2016
    Advances in Colloid and Interface Science 233 (2016). - ISSN 0001-8686 - p. 223 - 239.
    Bubble and drop adhesion to walls - Capillary meniscus dynamometry - Disjoining pressure vs. transversal tension - Foams and emulsions - Isotropic and anisotropic interfaces - Protein and egg yolk solutions

    Here, we review the principle and applications of two recently developed methods: the capillary meniscus dynamometry (CMD) for measuring the surface tension of bubbles/drops, and the capillary bridge dynamometry (CBD) for quantifying the bubble/drop adhesion to solid surfaces. Both methods are based on a new data analysis protocol, which allows one to decouple the two components of non-isotropic surface tension. For an axisymmetric non-fluid interface (e.g. bubble or drop covered by a protein adsorption layer with shear elasticity), the CMD determines the two different components of the anisotropic surface tension, σs and σϕ , which are acting along the "meridians" and "parallels", and vary throughout the interface. The method uses data for the instantaneous bubble (drop) profile and capillary pressure, but the procedure for data processing is essentially different from that of the conventional drop shape analysis (DSA) method. In the case of bubble or drop pressed against a substrate, which forms a capillary bridge, the CBD method allows one to determine also the capillary-bridge force for both isotropic (fluid) and anisotropic (solidified) adsorption layers. The experiments on bubble (drop) detachment from the substrate show the existence of a maximal pulling force, F max, that can be resisted by an adherent fluid particle. F max can be used to quantify the strength of adhesion of bubbles and drops to solid surfaces. Its value is determined by a competition of attractive transversal tension and repulsive disjoining pressure forces. The greatest F max values have been measured for bubbles adherent to glass substrates in pea-protein solutions. The bubble/wall adhesion is lower in solutions containing the protein HFBII hydrophobin, which could be explained with the effect of sandwiched protein aggregates. The applicability of the CBD method to emulsion systems is illustrated by experiments with soybean-oil drops adherent to hydrophilic and hydrophobic substrates in egg yolk solutions. The results reveal how the interfacial rigidity, as well as the bubble/wall and drop/wall adhesion forces, can be quantified and controlled in relation to optimizing the properties of foams and emulsions.

    Toward Scalable Fabrication of Hierarchical Silica Capsules with Integrated Micro-, Meso-, and Macropores
    Zhou, Weizheng ; Tong, Gangsheng ; Wang, Dali ; Zhu, Bangshang ; Ren, Yu ; Butler, Michael ; Pelan, Eddie ; Yan, Deyue ; Zhu, Xinyuan ; Stoyanov, Simeon D. - \ 2016
    Small 12 (2016)13. - ISSN 1613-6810 - p. 1797 - 1805.
    Hierarchical materials - Multi-templating - Pickering emulsions - Porous structures - Silica capsules

    Hierarchical porous structures are ubiquitous in biological organisms and inorganic systems. Although such structures have been replicated, designed, and fabricated, they are often inferior to naturally occurring analogues. Apart from the complexity and multiple functionalities developed by the biological systems, the controllable and scalable production of hierarchically porous structures and building blocks remains a technological challenge. Herein, a facile and scalable approach is developed to fabricate hierarchical hollow spheres with integrated micro-, meso-, and macropores ranging from 1 nm to 100 μm (spanning five orders of magnitude). (Macro)molecules, micro-rods (which play a key role for the creation of robust capsules), and emulsion droplets have been successfully employed as multiple length scale templates, allowing the creation of hierarchical porous macrospheres. Thanks to their specific mechanical strength, these hierarchical porous spheres could be incorporated and assembled as higher level building blocks in various novel materials.

    Surface properties of adsorption layers formed from triterpenoid and steroid saponins
    Pagureva, N. ; Tcholakova, S. ; Golemanov, K. ; Denkov, N. ; Pelan, E. ; Stoyanov, S.D. - \ 2016
    Colloids and Surfaces. A: Physicochemical and Engineering Aspects 491 (2016). - ISSN 0927-7757 - p. 18 - 28.
    Air-water interface - Dilatation - Saponins - Shear - Surface rheology

    Saponins are natural surfactants with non-trivial surface and aggregation properties which find numerous important applications in several areas (food, pharma, cosmetic and others). In the current paper we study the surface properties of ten saponin extracts, having different molecular structure with respect to the type of their hydrophobic fragment (triterpenoid or steroid aglycone) and the number of sugar chains (1 to 3). We found that the triterpenoid saponins Escin, Tea Saponin and Ginsenosides have area per molecule in the range between 0.5 and 0.7nm2, and the adsorbed molecules are orientated perpendicularly to the interface. The comparison of the experimentally measured surface elasticities with theoretically estimated ones shows that the saponins with very high dilatational and shear elasticities (up to 2000mN/m) have molecular interaction parameter in the adsorption layers which is above the threshold value for two-dimensional phase transition. In other words, the highly elastic layers are in surface condensed state, due to strong attraction between the adsorbed molecules. Furthermore, these adsorption layers have non-linear rheological response upon expansion and contraction, even at relatively small deformation. Layers from the other studied saponins (steroids and crude mixtures of triterpenoid saponins), which are unable to form strong intermolecular bonds within the adsorption layer, have zero shear elasticity and viscosity and low dilatational elasticity and viscosity, comparable in magnitude to those reported in literature for protein adsorption layers. The comparison of the results, obtained by several independent experimental methods, allowed us to formulate the conditions under which the results from different interfacial rheology tests could be compared, despite the complex non-linear response of the saponin adsorption layers.

    Hydrophobic modification of chitin whisker and its potential application in structuring oil
    Huang, Yao ; He, Meng ; Lu, Ang ; Zhou, Weizheng ; Stoyanov, S.D. ; Pelan, E.G. ; Zhang, Lina - \ 2015
    Langmuir 31 (2015)5. - ISSN 0743-7463 - p. 1641 - 1648.

    A facile approach was developed to modify chitin whiskers by reacting them with bromohexadecane, and the potential application of modified whiskers in structuring oil was evaluated. The results of Fourier transform infrared spectra (FT-IR), wide-angle X-ray diffraction (XRD), elemental analysis, solid 13C NMR, and differential scanning calorimeter (DSC) confirmed that the long alkyl chains were successfully introduced to the chitin whiskers and endowed them with improved hydrophobicity and thermal transition. By hot pressing the modified whiskers, the highly hydrophobic whisker sheets were constructed, showing high contact angles close to 150°. The hydrophobic interaction between the long alkyl chains and chitin backbone induced the crystal alignment with micro-nano structure, leading to the surface roughness and high hydrophobicity of the sheets. Furthermore, the modified whiskers could form a stable dispersion in sunflower oil, displaying a remarkable thickening effect. The viscosity of the oily suspension exhibited temperature dependence and shear-thinning behavior, suggesting great potentials to fabricate oleogel without adding any saturated fat. Furthermore, the intrinsic biocompatibility of α-chitin structure benefits its application in foodstuff, cosmetics, and medical fields.

    Capillary meniscus dynamometry - Method for determining the surface tension of drops and bubbles with isotropic and anisotropic surface stress distributions
    Danov, K.D. ; Stanimirova, R.D. ; Kralchevsky, P.A. ; Marinova, K.G. ; Alexandrov, N.A. ; Stoyanov, S.D. ; Blijdenstein, T.B.J. ; Pelan, E.G. - \ 2015
    Journal of Colloid and Interface Science 440 (2015). - ISSN 0021-9797 - p. 168 - 178.
    Anisotropic interfacial layers - Drop shape analysis - Non-uniform surface tension - Pendant drops and buoyant bubbles - Protein adsorption layers - Surface stress balances

    The stresses acting in interfacial adsorption layers with surface shear elasticity are, in general, anisotropic and non-uniform. If a pendant drop or buoyant bubble is covered with such elastic layer, the components of surface tension acting along the "meridians" and "parallels", σs and σϕ, can be different and, then, the conventional drop shape analysis (DSA) is inapplicable. Here, a method for determining σs and σϕ is developed for axisymmetric menisci. This method, called 'capillary meniscus dynamometry' (CMD), is based on processing data for the digitized drop/bubble profile and capillary pressure. The principle of the CMD procedure for data processing is essentially different from that of DSA. Applying the tangential and normal surface stress balance equations, σs and σϕ are determined in each interfacial point without using any rheological model. The computational procedure is fast and could be used in real time, during a given process. The method is applied to determine σs and σϕ for bubbles and drops formed on the tip of a capillary immersed in solutions of the protein HFBII hydrophobin. Upon a surface compression, meridional wrinkles appear on the bubble surface below the bubble "equator", where the azimuthal tension σϕ takes negative values. The CMD method allows one to determine the local tensions acting in anisotropic interfacial layers (films, membranes), like those formed from proteins, polymers, asphaltenes and phospholipids. The CMD is applicable also to fluid interfaces (e.g. surfactant solutions), for which it gives the same surface tension as the conventional methods.

    Shear rheology of mixed protein adsorption layers vs their structure studied by surface force measurements
    Danov, K.D. ; Kralchevsky, P.A. ; Radulova, G.M. ; Basheva, E.S. ; Stoyanov, S.D. ; Pelan, E.G. - \ 2015
    Advances in Colloid and Interface Science 222 (2015). - ISSN 0001-8686 - p. 148 - 161.
    nonlinear viscoelastic model - dependent relaxation-times - class-ii hydrophobin - low ionic-strength - beta-casein - interfacial rheology - air/water interface - liquid interfaces - bubble stability - hfbii
    The hydrophobins are proteins that form the most rigid adsorption layers at liquid interfaces in comparison with all other investigated proteins. The mixing of hydrophobin HFBII with other conventional proteins is expected to reduce the surface shear elasticity and viscosity, Esh and ¿sh, proportional to the fraction of the conventional protein. However, the experiments show that the effect of mixing can be rather different depending on the nature of the additive. If the additive is a globular protein, like ß-lactoglobulin and ovalbumin, the surface rigidity is preserved, and even enhanced. The experiments with separate foam films indicate that this is due to the formation of a bilayer structure at the air/water interface. The more hydrophobic HFBII forms the upper layer adjacent to the air phase, whereas the conventional globular protein forms the lower layer that faces the water phase. Thus, the elastic network formed by the adsorbed hydrophobin remains intact, and even reinforced by the adjacent layer of globular protein. In contrast, the addition of the disordered protein ß-casein leads to softening of the HFBII adsorption layer. Similar (an even stronger) effect is produced by the nonionic surfactant Tween 20. This can be explained with the penetration of the hydrophobic tails of ß-casein and Tween 20 between the HFBII molecules at the interface, which breaks the integrity of the hydrophobin interfacial elastic network. The analyzed experimental data for the surface shear rheology of various protein adsorption layers comply with a viscoelastic thixotropic model, which allows one to determine Esh and ¿sh from the measured storage and loss moduli, G' and G¿. The results could contribute for quantitative characterization and deeper understanding of the factors that control the surface rigidity of protein adsorption layers with potential application for the creation of stable foams and emulsions with fine bubbles or droplets.
    Role of the hydrophobic phase for the unique rheologica properties of saponin adsorption layers
    Golemanov, K. ; Tcholakova, S. ; Denkov, N. ; Pelan, E.G. ; Stoyanov, S.D. - \ 2014
    Soft Matter 10 (2014)36. - ISSN 1744-683X - p. 7034 - 7044.
    oil/water interfaces - aqueous foams - surface rheology - water-interface - quillaja bark - air/water - shear - drainage - monolayers - emulsions
    Saponins are a diverse class of natural, plant derived surfactants, with peculiar molecular structure consisting of a hydrophobic scaffold and one or several hydrophilic oligosaccharide chains. Saponins have strong surface activity and are used as natural emulsifiers and foaming agents in food and beverage, pharmaceutical, ore processing, and other industries. Many saponins form adsorption layers at the air–water interface with extremely high surface elasticity and viscosity. The molecular origin of the observed unique interfacial visco-elasticity of saponin adsorption layers is of great interest from both scientific and application viewpoints. In the current study we demonstrate that the hydrophobic phase in contact with water has a very strong effect on the interfacial properties of saponins and that the interfacial elasticity and viscosity of the saponin adsorption layers decrease in the order: air > hexadecane » tricaprylin. The molecular mechanisms behind these trends are analyzed and discussed in the context of the general structure of the surfactant adsorption layers at various nonpolar phase–water interfaces.
    Sonication–Microfluidics for Fabrication of Nanoparticle-Stabilized Microbubbles
    Chen, H. ; Li, J. ; Zhou, W. ; Pelan, E.G. ; Stoyanov, S.D. ; Arnaudov, L.N. ; Stone, H.A. - \ 2014
    Langmuir 30 (2014)15. - ISSN 0743-7463 - p. 4262 - 4266.
    flow-focusing device - contrast agents - foams - emulsions - delivery - bubbles
    An approach based upon sonication–microfluidics is presented to fabricate nanoparticle-coated microbubbles. The gas-in-liquid slug flow formed in a microchannel is subjected to ultrasound, leading to cavitation at the gas–liquid interface. Therefore, microbubbles are formed and then stabilized by the nanoparticles contained in the liquid. Compared to the conventional sonication method, this sonication–microfluidics continuous flow approach has unlimited gas nuclei for cavitation that yields continuous production of foam with shorter residence time. By controlling the flow rate ratios of the gas to the liquid, this method also achieves a higher production volume, smaller bubble size, and less waste of the nanoparticles needed to stabilize the microbubbles.
    Growth of bubbles on a solid surface in response to a pressure reduction
    Li, J. ; Chen, H. ; Zhou, W. ; Wu, B. ; Stoyanov, S.D. ; Pelan, E.G. - \ 2014
    Langmuir 30 (2014)15. - ISSN 0743-7463 - p. 4223 - 4228.
    contact angles - drop size - dynamics - water - nanobubbles - cavitation - interface - line
    A diffusion-controlled method is presented to study the growth of bubbles on a solid surface. The bubbles are nucleated spontaneously on a hydrophobic smooth surface in response to a sudden pressure reduction and then grow with an expanding contact line. The evolution of the bubbles in the early stage is found to grow with a constant bubble radius and a decreasing contact angle, while the bubbles continue their growth with a constant contact angle and an increasing bubble radius after the contact angle reaches its equilibrium value. A total variation of about 60° of the contact angle is observed during the growth of the bubbles with the size scale of 10–100 µm in radius. The growing process is described by the diffusion theory with the validation of the growth constant.
    Competitive adsorption of the protein hydrophobin and an ionic surfactant: Parallel vs sequential adsorption and dilatational rheology
    Stanimirova, R. ; Marinova, K.G. ; Danov, K.D. ; Kralchevsky, P.A. ; Basheva, E.S. ; Stoyanov, S.D. ; Pelan, E.G. - \ 2014
    Colloids and Surfaces. A: Physicochemical and Engineering Aspects 457 (2014). - ISSN 0927-7757 - p. 307 - 317.
    air-water-interface - sodium dodecyl-sulfate - beta-casein - air/water interface - fluid interfaces - layers - hfbii - elasticity - stability - mixtures
    The competitive adsorption of the protein HFBII hydrophobin and the anionic surfactant sodium dodecyl sulfate (SDS) is investigated in experiments on parallel and sequential adsorption of the two components. The dynamic surface tension and the surface storage and loss dilatational moduli are determined by the oscillating bubble method. A new procedure for data processing is proposed, which allows one to collect data from many different runs on a single master curve and to determine more accurately the dependence of the dilatational elasticity on the surface pressure. Experiments on sequential adsorption are performed by exchanging the HFBII solution around the bubble with an SDS solution. Experiments with separate thin foam films bring additional information on the effect of added SDS. The results indicate that if HFBII has first adsorbed at the air/water interface, it cannot be displaced by SDS at any concentration, both below and above the critical micellization concentration (CMC). In the case of parallel adsorption, there is a considerable difference between the cases below and above the CMC. In the former case, SDS cannot prevent the adsorption of HFBII at the interface, whereas in the latter case adsorption of HFBII is absent, which can be explained with hydrophilization of the hydrophobin aggregates by the SDS in the bulk. The surface dilatational elasticity of the HFBII adsorption layers markedly decreases in the presence of SDS, but it recovers after washing out the SDS. With respect to their dilatational rheology, the investigated HFBII layers exhibit purely elastic behavior, the effect of dilatational viscosity being negligible. As a function of surface tension, the elasticity of the investigated interfacial layers exhibits a high maximum, which could be explained with the occurrence of a phase transition in the protein adsorption layer.
    Remarkably high surface visco-elasticity of adsorption layers of triterpenoid saponins
    Golemanov, K. ; Tcholakova, S. ; Denkov, N. ; Pelan, E. ; Stoyanov, S.D. - \ 2013
    Soft Matter 9 (2013)24. - ISSN 1744-683X - p. 5738 - 5752.
    amplitude oscillatory shear - acacia-concinna saponins - steroidal saponins - beta-lactoglobulin - alkaline hydrolysate - sorbitan tristearate - interfacial rheology - tribulus-terrestris - plasma-cholesterol - panax-ginseng
    Saponins are natural surfactants, with molecules composed of a hydrophobic steroid or triterpenoid group, and one or several hydrophilic oligosaccharide chains attached to this group. Saponins are used in cosmetic, food and pharmaceutical products, due to their excellent ability to stabilize emulsions and foams, and to solubilize bulky hydrophobic molecules. The foam and emulsion applications call for a better understanding of the surface properties of saponin adsorption layers, including their rheological properties. Of particular interest is the relation between the molecular structure of the various saponins and their surface properties. Here, we study a series of eight triterpenoid and three steroid saponins, with different numbers of oligosaccharide chains. The surface rheological properties of adsorption layers at the air-water interface, subjected to creep-recovery and oscillatory shear deformations, are investigated. The experiments showed that all steroid saponins exhibited no shear elasticity and had negligible surface viscosity. In contrast, most of the triterpenoid saponins showed complex visco-elastic behavior with extremely high elastic modulus (up to 1100 mN m(-1)) and viscosity (130 N s m(-1)). Although the magnitude of the surface modulus differed significantly for the various saponins, they all shared qualitatively similar rheological properties: (1) the elastic modulus was much higher than the viscous one. (2) Up to a certain critical value of surface stress, sC, the single master curve described the dependence of the creep compliance versus time. This rheological response was described well by the compound Voigt model. (3) On increasing the surface stress above sC, the compliance decreased with the applied stress, and eventually, all layers became purely viscous, indicating a loss in the layer structure, responsible for the elastic properties. The saponin extracts, showing the highest elastic moduli, were those of Escin, Tea saponins and Berry saponins, all containing predominantly monodesmosidic triterpenoid saponins. Similarly, a high surface modulus was measured for Ginsenosides extracts, containing bidesmosidic triterpenoid saponins with short sugar chains.
    Surface Pressure and Elasticity of Hydrophobin HFBII Layers on the Air-Water Interface: Rheology Versus Structure Detected by AFM Imaging
    Stanimirova, R.D. ; Gurkov, T.D. ; Kralchevsky, P.A. ; Balashev, K.T. ; Stoyanov, S.D. ; Pelan, E.G. - \ 2013
    Langmuir 29 (2013)20. - ISSN 0743-7463 - p. 6053 - 6067.
    class-ii hydrophobins - air/water interface - langmuir monolayers - trichoderma-reesei - proteins - films - adsorption - stability - emulsions - mechanisms
    Here, we combine experiments with Langmuir trough and atomic force microscopy (AFM) to investigate the reasons for the special properties of layers from the protein HFBII hydrophobin spread on the airwater interface. The hydrophobin interfacial layers possess the highest surface dilatational and shear elastic moduli among all investigated proteins. The AFM images show that the spread HFBII layers are rather inhomogeneous, (i.e., they contain voids, monolayer and multilayer domains). A continuous compression of the layer leads to filling the voids and transformation of a part of the monolayer into a trilayer. The trilayer appears in the form of large surface domains, which can be formed by folding and subduction of parts from the initial monolayer. The trilayer appears also in the form of numerous submicrometer spots, which can be obtained by forcing protein molecules out of the monolayer and their self-assembly into adjacent pimples. Such structures are formed because not only the hydrophobic parts, but also the hydrophilic parts of the HFBII molecules can adhere to each other in the water medium. If a hydrophobin layer is subjected to oscillations, its elasticity considerably increases, up to 500 mN/m, which can be explained with compaction. The relaxation of the layers tension after expansion or compression follows the same relatively simple law, which refers to two-dimensional diffusion of protein aggregates within the layer. The characteristic diffusion time after compression is longer than after expansion, which can be explained with the impedence of diffusion in the more compact interfacial layer. The results shed light on the relation between the mesoscopic structure of hydrophobin interfacial layers and their unique mechanical properties that find applications for the production of foams and emulsions of extraordinary stability; for the immobilization of functional molecules at surfaces, and as coating agents for surface modification.
    Surface Shear Rheology of Saponin Adsorption Layers
    Golemanov, K. ; Tcholakova, S. ; Denkov, N. ; Pelan, E. ; Stoyanov, S.D. - \ 2012
    Langmuir 28 (2012)33. - ISSN 0743-7463 - p. 12071 - 12084.
    physico-chemical properties - quillaja bark saponin - air-water-interface - thin liquid-films - plant saponins - chromatographic determination - phospholipid monolayers - yucca-schidigera - acid saponins - cholesterol
    Saponins are a wide class of natural surfactants, with molecules containing a rigid hydrophobic group (triterpenoid or steroid), connected via glycoside bonds to hydrophilic oligosaccharide chains. These surfactants are very good foam stabiliziers and emulsifiers, and show a range of nontrivial biological activities. The molecular mechanisms behind these unusual properties are unknown, and, therefore, the saponins have attracted significant research interest in recent years. In our previous study (Stanimirova et al. Langmuir 2011, 27, 12486-12498), we showed that the triterpenoid saponins extracted from Quillaja saponaria plant (Quillaja saponins) formed adsorption layers with unusually high surface dilatational elasticity, 280 +/- 30 mN/m. In this Article, we study the shear rheological properties of the adsorption layers of Quillaja saponins. In addition, we study the surface shear rheological properties of Yucca saponins, which are of steroid type. The experimental results show that the adsorption layers of Yucca saponins exhibit purely viscous rheological response, even at the lowest shear stress applied, whereas the adsorption layers of Quillaja saponins behave like a viscoelastic two-dimensional body. For Quillaja saponins, a single master curve describes the data for the viscoelastic creep compliance versus deformation time, up to a certain critical value of the applied shear stress. Above this value, the layer compliance increases, and the adsorption layers eventually transform into viscous ones. The experimental creep recovery curves for the viscoelastic layers are fitted very well by compound Voigt rheological model. The obtained results are discussed from the viewpoint of the layer structure and the possible molecular mechanisms, governing the rheological response of the saponin adsorption layers.
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