|Nonlinear surface rheology and interfacial microstructure imaging of WPI particles and their constituents
Yang, Jack - \ 2019
protein pickering stabilizer - air/water interface - microstructure - surface rheology - Lissajous plots - atomic microscopy
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.
Quantitative description of the parameters affecting the adsorption behaviour of globular proteins
Delahaije, R.J.B.M. ; Gruppen, H. ; Giuseppin, M.L.F. ; Wierenga, P.A. - \ 2014
Colloids and Surfaces. B: Biointerfaces 123 (2014). - ISSN 0927-7765 - p. 199 - 206.
air-water-interface - bovine serum-albumin - beta-lactoglobulin - rheological properties - air/water interface - surface rheology - kinetics - ovalbumin - charge - denaturation
The adsorption behaviour of proteins depends significantly on their molecular properties and system conditions. To study this relation, the effect of relative exposed hydrophobicity, protein concentration and ionic strength on the adsorption rate and adsorbed amount is studied using ß-lactoglobulin, ovalbumin and lysozyme. The curves of surface elastic modulus versus surface pressure of all three proteins, under different conditions (i.e. concentration and ionic strength) superimposed. This showed that the interactions between the adsorbed proteins are similar and that the adsorbed proteins retain their native state. In addition, the adsorption rate (kadsorb) was shown to scale with the relative hydrophobicity and ionic strength. Moreover, the adsorbed amount was shown to be dependent on the protein charge and the ionic strength. Based on these results, a model is proposed to predict the maximum adsorbed amount (Gmax). The model approximates the adsorbed amount as a close-packed monolayer using a hard-sphere approximation with an effective protein radius which depends on the electrostatic repulsion. The theoretical adsorbed amount was in agreement with experimental Gmax (±10%).
Molecular assembly, interfacial rheology and foaming properties of oligofructose fatty acid esters
Kempen, S.E.H.J. van - \ 2013
Wageningen University. Promotor(en): Erik van der Linden, co-promotor(en): Leonard Sagis; Henk Schols. - S.l. : s.n. - ISBN 9789461737328 - 238
vetzure esters - oppervlaktespanningsverlagende stoffen - estervorming - oppervlaktereologie - schuimen - fatty acid esters - surfactants - esterification - surface rheology - foaming
Aerated food products consist of air bubbles that are surrounded by a matrix that can be either liquid or solid. Due to the large number of air bubbles that are generally present in aerated products, these systems contain a large interfacial area. Therefore, the properties of the interfaces are considered to contribute significantly to the macroscopic properties of the system. The properties of these interfaces are largely determined by the type of surfactant that adsorbs. Two major types of surfactants that are used within the food industry are proteins and low molecular weight (LMW) surfactants. Proteins are macromolecules consisting of hydrophilic and hydrophobic patches that adsorb at the interface, where they lower the surface tension and can unfold to create a two-dimensional network that can provide a high modulus. In contrast, LMW surfactants are molecules with a well-defined hydrophilic and hydrophobic part. They can form more compact surface layers than proteins, leading to lower surface tensions. They generally do not provide the interface with a high modulus, instead they stabilize the interface through the Gibbs-Marangoni mechanism that relies on rapid diffusion of surfactants after deformations of the interface. A molecule that can lower the surface tension considerably, like a LMW surfactant, and at the same time provide a high modulus, like a protein, has the potential to be an excellent foam stabilizer. In this thesis we focus on a series of molecules that obey these criteria: oligofructose fatty acid esters. We address the influence of changes in chemical fine structure (fatty acid chain length and degree of saturation, degree of esterification and size of the hydrophilic group) on the functional properties.
These esters are synthesized by esterification of fatty acids to oligofructose, which is a mixture of oligomers with different degrees of polymerization. As we show in chapter 2, reasonable yields are obtained when using lipase as the catalyst in a mixture of DMSO and ButOH. The conversion into mono-esters increased with increasing fatty acid chain length and is consistent with the preference of the enzyme for more hydrophobic substrates. The crude reaction product consisted of a mixture of unreacted oligofructose and fatty acids, the main reaction products mono-esters and small amounts of di-esters. The crude product was fractionated using RP-SPE. MALDI-TOF MS and (2D) NMR were used to confirm the structure and purity of the esters; >90% for mono-esters and >80% for di-esters.
Similar to typical LMW surfactants, the oligofructose esters formed spherical micelles in the bulk after a certain critical concentration. As we show in chapter 3, the CAC depended on the hydrophobicity of the molecules. The efficiency also increased with increasing hydrophobicity and the effectiveness was similar. The area occupied by a single molecule at the interface was determined by fitting the CAC curves with the Gibbs adsorption model and measured directly using ellipsometry. The area occupied at the interface was larger for oligofructose mono-esters compared to sucrose esters. Furthermore, oligofructose di-esters occupied slightly more area than sucrose esters. All esters occupied significantly more area than a single fatty acid chain. This shows that the oligofructose group dominates the area occupied at the interface.
The rheological properties, as studied in chapter 4, were determined using a traditional approach, where the dependency of the surface dilatational modulus on surface pressure and frequency was determined, and using a novel approach, where we show how the surface dilatational modulus is dependent on deformation amplitude and temperature. Furthermore, we show how Lissajous plots of surface pressure versus deformation may be used to gain information about the correlation between surface rheological properties and interfacial microstructure. Sucrose esters behaved like typical LMW surfactants, with low surface dilatational moduli, scaling exponents in the frequency dependency close to 0.5, and fairly viscous Lissajous plots without significant asymmetries. In contrast, oligofructose mono-esters formed interfaces with high surface dilatational moduli, low scaling exponents in the frequency dependency and asymmetric Lissajous plot with strain hardening during compression and strain softening during expansion. We conclude that the oligofructose mono-esters form a two-dimensional soft glass. The oligofructose di-esters behaved like typical LMW surfactants at high surface pressures, showing that the presence of the second fatty acid chain prevent the formation of the glass by the oligofructose part.
In chapter 5 we focus on the difference in functionality between the crude reaction product, the individual components that are present in the crude product and mixes of these products. Unreacted fatty acids migrated to the interface only in very small amount, due to the low solubility in the bulk. The addition of mono-esters slightly improved the amount of fatty acid that could migrate to the interface. Oligofructose was not surface active and its addition to the mono-ester only diluted the mono-ester which did not lead to significant changes in functional properties because the concentration of mono-ester was still close to the CMC. When mono-esters and di-esters were mixed, the rheological results showed that the ratio between mono-ester and di-ester was very important for the rheological profile. In both cases the results suggest the presence of islands of glass phase formed by the mono-esters surrounded by a viscous phase formed by the di-esters. When the surface concentration of mono-esters was high, the glassy patches dominated the interface, leading to a high modulus, low frequency dependency and Lissajous plots with a high degree of asymmetry. When the surface concentration of mono-esters decreased, the lower connectivity between the glassy patches lead to a low modulus, intermediate frequency dependency, and Lissajous plots with moderate asymmetry.
To study the potential of oligofructose esters as food grade surfactants it is important to consider that many food products contain ingredients with the potential to be surface active. Therefore, in chapter 6 we have studied the functional properties of an oligofructose mono-ester in the presence of whey protein isolate, a commonly used food protein. Except for at the highest protein concentration, the surface was dominated by the oligofructose ester. The stabilization mechanisms of oligofructose ester and WPI were mutually exclusive, leading to interfaces with a low surface dilatational modulus. Since the foaming properties were not negatively affected, we conclude that the Gibbs-Marangoni mechanism occurred. Only at the highest protein concentration, the surface concentration of WPI was sufficiently high to interfere with this mechanism, leading to a significant decrease in foam stability. Oligofructose esters were also able to displace a fully developed WPI network.
In chapter 7 we discuss the foaming properties of the esters. We show that only esters of intermediate hydrophobicity are able to form foams with small bubbles and a uniform bubble size distribution that lead to high foam stability. The affinity of esters with shorter fatty acid chains, up to 8 carbon atoms, for the interface was quite low as a result of the relatively hydrophilic nature of the molecules. Therefore, they were not effective foam stabilizers. The most hydrophobic components (mono-ester with a chain length of 18 carbon atoms and di-ester with a chain length of 12 carbon atoms) were too slow to migrate to the interface. Therefore, also these components were poor foam stabilizers. We show that the surface tension at short time scales is the most accurate predictor of foam stability. However, despite similar initial surface tension values, oligofructose esters lead to higher foam stability. This could be attributed to the oligofructose part that forms a two-dimensional glass phase and provides mechanical stability to the foam films.
In the general discussion that is presented in chapter 8 we integrate the results from the different chapters. One of the factors that is persistent throughout the different chapters is the rheological profile of the interfaces. We have shown that by using amplitude sweeps and Lissajous plots, a lot more information on the interfacial microstructure can be extracted from rheological data than by using more conventional methods. In the last part of the general discussion improvements to the synthesis are discussed, as the optimization of the synthesis was not considered in this thesis. Furthermore, improvements for the functional experiments and additional applications were identified.
Modeling interfacial dynamics using nonequilibrium thermodynamics frameworks
Sagis, L.M.C. - \ 2013
The European Physical Journal. Special Topics 222 (2013)1. - ISSN 1951-6355 - p. 105 - 127.
extended irreversible thermodynamics - scanning angle reflectometry - in-water emulsions - adsorption layers - surface rheology - 2-dimensional suspensions - superficial viscosity - reciprocal relations - general formalism - polymer-solutions
In recent years several nonequilibrium thermodynamic frameworks have been developed capable of describing the dynamics of multiphase systems with complex microstructured interfaces. In this paper we present an overview of these frameworks. We will discuss interfacial dynamics in the context of the classical irreversible thermodynamics, extended irreversible thermodynamics, extended rational thermodynamics, and GENERIC framework, and compare the advantages and disadvantages of these frameworks.
Rheology of complex fluid-fluid interfaces: a unified approach based on nonequilibrium thermodynamics
Sagis, L.M.C. - \ 2010
Applied Rheology 20 (2010)2. - ISSN 1430-6395 - p. 24380 - 24380.
extended irreversible thermodynamics - general formalism - surface rheology - dynamics - microbubbles - deformation - equilibrium - equation - systems - layers
Surface rheological properties affect the dynamics of vesicles, nanoparticles, emulsion droplets, foam bubbles, polymer microcapsules, liquid jets, living cells, lung avioli, thin liquid films, and many other multiphase systems. Surface rheology is therefore relevant for a wide range of disciplines in the areas of physics, chemistry, engineering, biology, and medicine. Currently used descriptions of surface rheology have a number of limitations, and in particular are hard to generalize to the large deformation regime. Data are often analyzed with constitutive equations based on straightforward generalizations of models developed for describing bulk phase rheology. Since the latter are in general designed to describe incompressible materials, they are not guaranteed to describe highly compressible interfaces correctly. Here we discuss a unified approach to surface rheology based on nonequilibrium thermodynamics (NET) that provides a consistent set of balance and constitutive equations for the unambiguous determination of surface rheological parameters, both near and far beyond equilibrium. A closer integration of experimental surface rheology and multiphase nonequilibrium thermodynamics would clearly be beneficial for both disciplines
Binding of ß-lactolobulin to pectins varying in their overall and local charge density
Sperber, B.L.H.M. ; Cohen Stuart, M.A. ; Schols, H.A. ; Voragen, A.G.J. ; Norde, W. - \ 2009
Biomacromolecules 10 (2009)12. - ISSN 1525-7797 - p. 3246 - 3252.
continuous capillary-electrophoresis - galacturonic acid distribution - long amphiphilic polymers - frontal analysis - fluorescence anisotropy - protein particles - surface rheology - lactoglobulin - complexes - dna
The formation of complexes between proteins and polysaccharides is of great importance for many food systems like foams, emulsions, acidified milk drinks, and so on. The complex formation between ß-lactoglobulin (ß-lg) and pectins with a well-defined physicochemical fine structure has been studied to elucidate the influence of overall charge and local charge density of pectin on the complex formation. Binding isotherms of ß-lg to pectin are constructed using fluorescence anisotropy, which is shown to be an excellent technique for this purpose, as it is fast and requires low sample volumes. From the binding isotherms the maximal adsorbed amount, binding constant (kobs) and the cooperativity of binding are obtained at different ionic strengths. The Hill model is used to fit the binding isotherms and is shown to be preferable over a Langmuir fit. At pH 4.25, kobs shows a maximum at an ionic strength of 10 mM when using a low methyl esterified pectin (LMP) due to the balance of attractive and repulsive electrostatic forces between ß-lg and pectin and ß-lg neighbors. For two high methyl esterified pectins, one with a blockwise distribution of methyl esters (HMPB) and one with a random distribution (HMPR), this ionic strength maximum is absent and kobs decreases with increasing ionic strength. kobs is found to be largest for LMP and HMPB and considerably lower for HMPR. A positive cooperativity is observed for both LMP (above an ionic strength of 45 mM) and HMPR (above an ionic strength of 15 mM) but not for HMPB. Positive cooperativity is thought to be caused by a rearrangement of the pectin helix structure caused by binding of ß-lg, thus creating new or binding sites with a higher affinity. To attain strong binding of ß-lg to pectin it is preferable to use a pectin with a blockwise distribution of methyl esters. When complex formation takes place in high ionic strength media an LMP gives the best results, while at low ionic strength a high methyl esterified pectin with blockwise distribution may give better results, due to reduced electrostatic repulsion between both pectin and ß-lg and ß-lg neighbors