- Peter A. Kralchevsky (1)
- Katerina Alba (1)
- Krassimir D. Danov (1)
- Nikolai D. Denkov (1)
- Simeon D. Stoyanov (2)
- Krastanka G. Marinova (1)
- Edward G. Pelan (1)
- Konstantin Golemanov (1)
- Vassilis Kontogiorgos (1)
- Lydia M. Dimitrova (1)
- Fatmegul Mustan (1)
- Mariana P. Boneva (1)
- Nevena Pagureva (1)
- Elka S. Basheva (1)
- L.M.C. Sagis (1)
- Jordan T. Petkov (1)
- Slavka Tcholakova (1)
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.
Limited coalescence and Ostwald ripening in emulsions stabilized by hydrophobin HFBII and milk proteins
Dimitrova, Lydia M. ; Boneva, Mariana P. ; Danov, Krassimir D. ; Kralchevsky, Peter A. ; Basheva, Elka S. ; Marinova, Krastanka G. ; Petkov, Jordan T. ; Stoyanov, Simeon D. - \ 2016
Colloids and Surfaces. A: Physicochemical and Engineering Aspects 509 (2016). - ISSN 0927-7757 - p. 521 - 538.
Drop size distribution - Emulsification - Emulsion stability - HFBII hydrophobin - Ostwald ripening
Hydrophobins are proteins isolated from filamentous fungi, which are excellent foam stabilizers, unlike most of the proteins. In the present study, we demonstrate that hydrophobin HFBII can also serve as excellent emulsion stabilizer. The HFBII adsorption layers at the oil/water interface solidify similarly to those at the air/water interface. The thinning of aqueous films sandwiched between two oil phases ends with the formation of a 6 nm thick protein bilayer, just as in the case of foam films, which results in strong adhesive interactions between the emulsion drops. The drop-size distribution in hydrophobin stabilized oil-in-water emulsions is investigated at various protein concentrations and oil volume fractions. The data analysis indicates that the emulsification occurs in the Kolmogorov regime or in the regime of limited coalescence, depending on the experimental conditions. The emulsions with HFBII are very stable – no changes in the drop-size distributions are observed after storage for 50 days. However, these emulsions are unstable upon stirring, when they are subjected to the action of shear stresses. This instability can be removed by covering the drops with a second adsorption layer from a conventional protein, like β-lactoglobulin. The HFBII surface layer is able to suppress the Ostwald ripening in the case when the disperse phase is oil that exhibits a pronounced solubility in water. Hence, the hydrophobin can be used to stabilize microcapsules of fragrances, flavors, colors or preservatives due to its dense adsorption layers that block the transfer of oil molecules.
Engineering of acidic O/W emulsions with pectin
Alba, Katerina ; Sagis, L.M.C. ; Kontogiorgos, Vassilis - \ 2016
Colloids and Surfaces. B: Biointerfaces 145 (2016). - ISSN 0927-7765 - p. 301 - 308.
Pectin - Emulsions - Ostwald ripening - Lissajous plots - biopolymer - Fluorescence
Pectins with distinct molecular design were isolated by aqueous extraction at pH 2.0 or 6.0 and were examined in terms of their formation and stabilisation capacity of model n-alkane–in–water emulsions at acidic pH (pH 2.0). The properties and stability of the resulting emulsions were examined by means of droplet size distribution analysis, Lifshitz-Slyozov-Wagner modelling, bulk rheology, interfacial composition analysis, large-amplitude oscillatory surface dilatational rheology, electrokinetic analysis and fluorescence microscopy. Both pectin preparations were able to emulsify alkanes in water but exhibited distinct ageing characteristics. Emulsions prepared using pectin isolated at pH 6.0 were remarkably stable with respect to droplet growth after thirty days of ageing, while those prepared with pectin isolated at pH 2.0 destabilised rapidly. Examination of chemical composition of interfacial layers indicated multi-layered adsorption of pectins at the oil-water interface. The higher long-term stability of emulsions prepared with pectin isolated at high pH is attributed to mechanically stronger interfaces, the highly branched nature and the low hydrodynamic volume of the chains that result in effective steric stabilisation whereas acetyl and methyl contents do not contribute to the long-term stability. The present work shows that it is possible by tailoring the fine structure of pectin to engineer emulsions that operate in acidic environments.