Mechanical properties of protein adsorption layers at the air/water and oil/water interface: A comparison in light of the thermodynamical stability of proteins

Over the last decades numerous studies on the interfacial rheological response of protein adsorption layers have been published. The comparison of these studies and the retrieval of a common parameter to compare protein interfacial activity are hampered by the fact that different boundary conditions (e.g. physico-chemical, instrumental, interfacial) were used. In the present work we review previous studies and attempt a unifying approach for the comparison between bulk protein properties and their adsorption films. Among many common food grade proteins we chose bovine serum albumin, β-lactoglobulin and lysozyme for their difference in thermodynamic stability and studied their adsorption at the air/water and limonene/water interface. In order to achieve this we have i) systematically analyzed protein adsorption kinetics in terms of surface pressure rise using a drop profile analysis tensiometer and ii) we addressed the interfacial layer properties under shear stress using an interfacial shear rheometer under the same experimental conditions. We could show that thermodynamically less stable proteins adsorb generally faster and yield films with higher shear rheological properties at air/water interface. The same proteins showed an analog behavior when adsorbing at the limonene/water interface but at slower rates.     

RHEOLOGICAL PROPERTIES AND APPLICATIONS OF MESQUITE TREE (PROSOPIS JULIFLORA) GUM. 5. THE RHEOLOGICAL PROPERTIES AND STABILITY OF CORN OIL -IN-WATER EMULSIONS STABILISED BY BLENDS OF MESQUITE GUM AND TWEEN 60.

Corn oil-in-water emulsions were prepared using mesquite gum and Tween 60 in conjunction as emulsifier-stabilisers. The mesquite gum/ Tween 60 ratio influenced the viscoelasticity and stability of the emulsions. The viscoelasticity parameters decreased in magnitude, and the rate of drop coalescence increased, as the mesquite gum/Tween 60 ratio decreased. In addition, this ratio influenced the pattern of change in the viscoelasticity parameters when the emulsions were aged. Mixed films of mesquite gum and Tween 60 adsorbed at a flat corn oil-water interface exhibited viscoelasticity, the parameter magnitudes decreasing as the mesquite gum/ Tween 60 ratio decreased. The influence of the mesquite gum/Tween 60 ratio on both the emulsions and model interfacial films is attributed to the relative proportions of the two emulsifier-stabilisers adsorbed at the oil-water interface, their configuration in the adsorbed state and their ability to interlink to form a network.     

The adsorption of hyperbranched polymers on silicon oxide surfaces

The effects of a control blocking of free cystein by N-ethylmaleimide on the interfacial behavior (kinetics of adsorption at the air/water interface, rheology of the interfacial layer) as well as on the foaming properties (density, stability) of beta-lactoglobulin were investigated. Compared to native beta-lactoglobulin (unmodified beta-lactoglobulin), sulfydryl-modified beta-lactoglobulin exhibited higher surface hydrophobicity, adsorbed faster at the air/water interface, had the capability to develop rapidly an interfacial layer with high shear elastic constant but exhibited a considerably lower shear elastic constant plateau value. Moreover, sulfydryl-modified beta-lactoglobulin exhibited better foaming properties especially regarding the short-term foam stability suggesting that the initial rheology of the interfacial film is at least as much important for the general mechanism of foam stabilization as the potential viscoelasticity the interfacial film could reach on aging.     

Salt effects on the dilational viscoelasticity of surfactant adsorption layers

Interfacial rheology of adsorbed layers of surfactants, demonstrating the response of the interface to interfacial deformations, plays a key role in formation and stability of foams and emulsions. It also provides insights into complex surfactant systems in different applications, in particular, medical treatments and diagnostics. The response of the interface is mainly determined by the composition of a surfactant system, the equilibrium and kinetic adsorption properties of the included surface-active compounds and their interaction within the adsorption layer. The subject of ongoing investigations is interfacial rheology of surfactant layers in the presence of inorganic ions. Although these ions have no surface activity, they can strongly influence the interfacial rheological properties owing to their interaction with the surface-active molecules.This work aims to present recent developments in the interfacial rheology of surfactant adsorbed layers at liquid–fluid interfaces in the presence and absence of salts, highlighting the state of the art of experimental and theoretical works in this area. We highlight drawbacks of recently developed techniques for measuring dilational interfacial properties of surfactant layers, compared with previous techniques. Moreover, this review shows the dearth of research on the ion-specific effect on the interfacial rheology of surfactant layers. This demonstrates the necessity of further investigation of the effect of ion specificity on interfacial viscoelasticity.     

Effect of Coconut Protein and Xanthan Gum,Soybean Polysaccharide and Gelatin Interactions in Oil-Water Interface

We report on our study of the interactions between coconut protein extracted from coconut meat and three hydrocolloids (gelatin, xanthan gum, and soybean polysaccharide) and their interfacial adsorption and emulsification properties. We used Zeta potential, fluorescence spectroscopy scanning and ITC to investigate the interactions between a fixed concentration (1%) of coconut protein and varying concentrations of hydrocolloid. Through the interfacial tension and interfacial viscoelasticity, the interfacial properties of the hydrocolloid and coconut protein composite solution were explored. The physical stability of the corresponding emulsion is predicted through microstructure and stability analysis. Xanthan gum forms a flocculent complex with coconut protein under acidic conditions. Soy polysaccharides specifically bind to coconut protein. Under acidic conditions, this complex is stabilized through the steric hindrance of soy polysaccharides. Due to gelatin-coconut protein interactions, the isoelectric point of this complex changes. The interfacial tension results show that as time increases, the interfacial tensions of the three composite solutions decrease. The increase in the concentration of xanthan gum makes the interfacial tension decrease first and then increase. The addition of soybean polysaccharides reduces the interfacial tension of coconut protein. The addition of xanthan gum forms a stronger elastic interface film. Emulsion characterization showed that the gelatin-added system showed better stability. However, the addition of xanthan gum caused stratification quickly, and the addition of soybean polysaccharides also led to instability because the addition of polysaccharides led to a decrease in thermodynamic compatibility. This research lays the foundation for future research into coconut milk production technology.     

Correlation between mechanical behavior of protein films at the air/water interface and intrinsic stability of protein molecules

The relation between mechanical film properties of various adsorbed protein layers at the air/water interface and intrinsic stability of the corresponding proteins is discussed. Mechanical film properties were determined by surface deformation in shear and dilation. In shear, fracture stress, sigma(f), and fracture strain, gamma(f), were determined, as well as the relaxation behavior after macroscopic fracture. The dilatational measurements were performed in a Langmuir trough equipped with an infra-red reflection absorption spectroscopy (IRRAS) accessory. During compression and relaxation of the surface, the surface pressure, Pi, and adsorbed amount, Gamma (determined from the IRRAS spectra), were determined simultaneously. In addition, IRRAS spectra revealed information on conformational changes in terms of secondary structure. Possible correlations between macroscopic film properties and intrinsic stability of the proteins were determined and discussed in terms of molecular dimensions of single proteins and interfacial protein films. Molecular properties involved the area per protein molecule at Pi approximately 0 mN/m (A(0)), A(0)/M (M = molecular weight) and the maximum slope of the Pi-Gamma curves (dPi/dGamma). The differences observed in mechanical properties and relaxation behavior indicate that the behavior of a protein film subjected to large deformation may vary widely from predominantly viscous (yielding) to more elastic (fracture). This transition is also observed in gradual changes in A(0)/M. It appeared that in general protein layers with high A(0)/M have a high gamma(f) and behave more fluidlike, whereas solidlike behavior is characterized by low A(0)/M and low gamma(f). Additionally, proteins with a low A(0)/M value have a low adaptability in changing their conformation upon adsorption at the air/water interface. Both results support the conclusion that the hardness (internal cohesion) of protein molecules determines predominantly the mechanical behavior of adsorbed protein layers.     

Nematic-isotropic interfaces under shear: a molecular-dynamics simulation

We present a large-scale molecular-dynamics study of nematic-paranematic interfaces under shear. We use a model of soft repulsive ellipsoidal particles with well-known equilibrium properties, and consider interfaces which are oriented normal to the direction of the shear gradient (common stress case). The director at the interface is oriented parallel to the interface (planar). A fixed average shear rate is imposed with moving periodic boundary conditions, and the heat is dissipated with a profile-unbiased thermostat. First, we study the properties of the interface at one particular shear rate in detail. The local interfacial profiles and the capillary wave fluctuations of the interfaces are calculated and compared with those of the corresponding equilibrium interface. Under shear, the interfacial width broadens and the capillary wave amplitudes at large wavelengths increase. The strain is distributed inhomogeneously in the system (shear banding), the local shear rate in the nematic region being distinctly higher than in the paranematic region. Surprisingly, we also observe (symmetry-breaking) flow in the vorticity direction, with opposite direction in the nematic and the paranematic state. Finally, we investigate the stability of the interface for other shear rates and construct a nonequilibrium phase diagram.     

Shear and dilatational relaxation mechanisms of globular and flexible proteins at the hexadecane/water interface

Proteins adsorbed at fluid/fluid interfaces influence many phenomena: food emulsion and foam stability (Murray et al. Langmuir 2002, 18, 9476 and Borbas et al. Colloids Surf., A 2003, 213, 93), two-phase enzyme catalysis (Cascao-Pereira et al. Biotechnol. Bioeng. 2003, 83, 498; 2002, 78, 595), human lung function (Lunkenheimer et al. Colloids Surf., A 1996, 114, 199; Wustneck et al.; and Banerjee et al. 2000, 15, 14), and cell membrane mechanical properties (Mohandas et al. 1994, 23, 787). Time scales important to these phenomena are broad, necessitating an understanding of the dynamics of biological macromolecules at interfaces. We utilize interfacial shear and dilatational deformations to study the rheology of a globular protein, lysozyme, and a disordered protein, beta-casein, at the hexadecane/water interface. Linear viscoelastic properties are measured using small amplitude oscillatory flow, stress relaxation after a sudden dilatational displacement, and shear creep response to probe the rheological response over broad experimental time scales. Our studies of lysozyme and beta-casein reveal that the interfacial dissipation mechanisms are strongly coupled to changes in the protein structure upon and after adsorption. For beta-casein, the interfacial response is fluidlike in shear deformation and is dominated by interfacial viscous dissipation, particularly at low frequencies. Conversely, the dilatational response of beta-casein is dominated by diffusion dissipation at low frequencies and viscous dissipation at higher frequencies (i.e., when the experimental time scale is faster than the characteristic time for diffusion). For lysozyme in shear deformation, the adsorbed protein layer is primarily elastic with only a weak frequency dependence. Similarly, the interfacial dilatational moduli change very little with frequency. In comparison to beta-casein, the frequency response of lysozyme does not change substantially after washing the protein from the bulk solution. Apparently, it is the irreversibly adsorbed fraction that dominates the dynamic rheological response for lysozyme. Using stress relaxation after a sudden dilatational displacement and shear creep response, the characteristic time of relaxation was found to be 1000 s in both modes of deformation. The very long relaxation time for lysozyme likely results from the formation of a glassy interfacial network. This network develops at high interfacial concentrations where the molecules are highly constrained because of conformation changes that prevent desorption.     

Biosourced Poly(lactic acid)/polyamide-11 Blends: Effect of an Elastomer on the Morphology and Mechanical Properties

Fully biobased polylactide (PLA)/polyamide-11 (PA11) blends were prepared by melt mixing with an elastomer intermediate phase to address the low elasticity and brittleness of PLA blends. The incorporation of a biobased elastomer made of poly(butylene adipate-co-terephthalate) (PBAT) and polyethylene oxide (PEO) copolymers was found to change the rigid interface between PLA and PA11 into a much more elastic/deformable one as well as promote interfacial compatibility. The interfacial tension of the polymer pairs and spreading coefficients revealed a high tendency of PEO to spread at the PLA/PA11 interface, resulting in a complete wetting regime (interfacial tension of 0.56 mN/m). A fully percolated rubbery phase (PEO) layer at the PLA/PA11 interface with enhanced interfacial interactions and PLA chain mobility contributed to a better distribution of the stress around the dispersed phase, leading to shear yielding of the matrix. The results also show that both the morphological modification and improved compatibility upon PEO addition (up to 20 wt %) contributed to the improved elongation at break (up to 104%) and impact strength (up to 292%) of the ternary PLA/PA11/PEO blends to obtain a super-tough multiphase system.     

Surfactant-Influenced Gas-Liquid Interfaces: Nonlinear Equation of State and Finite Surface Viscosities

A canonical flow geometry was utilized for a fundamental study of the coupling between bulk flow and a Newtonian gas-liquid interface in the presence of an insoluble surfactant. We develop a Navier-Stokes numerical model of the flow in the deep-channel surface viscometer geometry, which consists of stationary inner and outer cylinders, a floor rotating at a constant angular velocity, and an interface covered initially by a uniformly distributed surfactant. Here, the floor of the annular channel is rotated fast enough so the flow is nonlinear and drives the film toward the inner cylinder. The boundary conditions at the interface are functions of the surface tension, surface shear viscosity, and surface dilatational viscosity, as described by the Boussinesq-Scriven surface model. A physical surfactant system, namely hemicyanine, an insoluble monolayer on an air-water interface, with measured values of surface tension and surface shear viscosity versus concentration, was used in this study. We find that a surfactant front can form, depending on the Reynolds number and the initial surfactant concentration. The stress balance in the radial direction was found to be dominated by the Marangoni stress, but the azimuthal stress was only due to the surface shear viscosity. Numerical studies are presented comparing results of surfactant-influenced interface cases implementing the derived viscoelastic interfacial stress balance with those using a number of idealized stress balances, as well as a rigid no-slip surface, providing added insight into the altered dynamics that result from the presence of a surfactant monolayer. Copyright 2000 Academic Press.     

Methods of measuring rheological properties of interfacial layers (Experimental methods of 2D rheology)

Current methods of studying the rheological properties of interfacial layers at the interfaces of fluids are reviewed. This area of research includes two-dimensional 2D rheology. Regardless of the similarities between the parameters of rheological properties of two-dimensional and bulk (three-dimensional) systems, when measuring surface properties, it is necessary to reformulate the main experimental methods to allow for the different dimensions of surface and bulk characteristics of material. Parameters of shear and dilational (measured upon expansion-compression) properties of interfacial layers are distinguished, and the latter are considered to be independent parameters of a system. The most attention was given to the rotational methods of measuring shear viscosity and the components of the complex 2D elastic modulus, as well as to measuring surface tension upon harmonic changes of the bubble (droplet) surface area, which allows characteristics of the dilational behavior of thin liquid films to be determined. Both groups of methods are widely used in laboratory practice and realized in the form of a number of original and commercial instruments. Dilational measurements of interfacial layers can also be performed with oscillations of a movable barrier on a Langmuir trough. In addition, methods based on the propagation of capillary waves across the surface of a liquid, as well as rarer methods of capillary flow in thin channels forced by either a surface tension gradient or the motion of the interface, are considered.     

Interfacial rheology of mixed layers of food proteins and surfactants

Mixed protein–surfactant adsorption layers at liquid interfaces are described including the thermodynamic basis, the adsorption kinetics and the shear and dilational interfacial rheology. It is shown that due to the protrusion of hydrophobic protein parts into the oil phase the adsorption layers at the water–hexane interface are stronger anchored as compared to the water-air surface. Based on the different adsorption protocols, a sequential and a simultaneous scheme, the peculiarities of complexes between proteins and added surfactants are shown when formed in the solution bulk or at a liquid interface. The picture drawn from adsorption studies is supported by the findings of interfacial rheology.     

The effect of polymeric surfactants on the rheological properties of nanoemulsions

We have investigated the rheological properties of submicron emulsions and how they are affected by the structure of polymeric surfactants. We have prepared oil-in-water emulsions stabilized with five steric surfactants, two of them belonging to the Myrj family and three belonging to the Pluronic family, with key differences on their structures. Droplet size and volume fraction have been kept constant to analyze only the influence of the surfactant. The viscoelasticity has been characterized by dynamic oscillatory shear experiments, while the shear viscosity was measured during steady shear flow tests. The results show a qualitatively similar gel-like behavior for all the emulsions, but with remarkable quantitative differences. Surfactants with longer hydrophilic tails produced emulsions with higher viscoelasticity. Pluronics, having a central hydrophobic part between two hydrophilic tails, produced emulsions with notably higher viscoelasticity and yield stress than Myrjs with comparable hydrophilic tails. The reason for this seems to be a more efficient steric barrier at the interface, induced by this central hydrophobic part.     

Entropic wetting and the free isotropic-nematic interface of hard colloidal platelets

We study bulk and interfacial properties of a model suspension of hard colloidal platelets with continuous orientations and vanishing thickness using both density functional theory, based on either a second virial approach or fundamental measure theory (FMT), and Monte Carlo (MC) simulations. We calculate the bulk equation of state, bulk isotropic-nematic (IN) coexistence, and properties of the (planar) free IN interface and of adsorption at a planar hard wall, where we find complete wetting of the nematic phase at the isotropic-wall interface upon approaching bulk IN coexistence. We investigate in detail the asymptotic decay of correlations at large distances. In all cases, the results from FMT and MC agree quantitatively. Our findings are of direct relevance to understanding interfacial properties of dispersions of colloidal platelets.     

Interfacial rheology and structure of tiled graphene oxide sheets

The hydrophilic nature of graphene oxide sheets can be tailored by varying the carbon to oxygen ratio. Depending on this ratio, the particles can be deposited at either a water-air or a water-oil interface. Upon compression of thus-created Langmuir monolayers, the sheets cover the entire interface, assembling into a strong, compact layer of tiled graphene oxide sheets. With further compression, the particle layer forms wrinkles that are reversible upon expansion, resembling the behavior of an elastic membrane. In the present work, we investigate under which conditions the structure and properties of the interfacial layer are such that free-standing films can be obtained. The interfacial rheological properties of these films are investigated using both compressional experiments and shear rheometry. The role of surface rheology in potential applications of such tiled films is explored. The rheological properties are shown to be responsible for the efficiency of such layers in stabilizing water-oil emulsions. Moreover, because of the mechanical integrity, large-area monolayers can be deposited by, for example, Langmuir-Blodgett techniques using aqueous subphases. These films can be turned into transparent conductive films upon subsequent chemical reduction.     

Effect of pH on the adsorption kinetics of egg yolk at the triacylglycerol–water interface and viscoelastic properties of interfacial egg yolk films: a dynamic drop tensiometry study

Interfacial properties of normal egg yolk (EY), as well as stabilized, i.e. enzymatically modified with phospholipase A2, egg yolk (SEY) at the triacylglyceride (TAG) oil–water interface have been investigated with the use of the dynamic drop tensiometry (DDT) technique in the wide interval of pH values of aqueous EY solutions. We found that for both EY and SEY pH values of their aqueous solutions affect absolute values of interfacial tension at the TAG oil–water interface. In the presence of EY this effect was more pronounced, with minimum of interfacial tension values at pH nearly equal to the isoelectric point of EY proteins. For SEY solutions no clear trend was noticed, although a reduction of interfacial tension near pH 6 was also observed. Moreover, the pH-dependence of nearly steady values of interfacial tension in the presence of EY was substantially less pronounced than it has been reported previously. It was also found that there is a difference in the interfacial viscoelasticity of SEY and EY films formed at the TAG oil–water interface. Although the dependence of dilational modulus, ∣ε∣, versus surface pressure for SEY solutions goes through a maximum, absolute values of ∣ε∣ increase for EY solutions in a wide pH range. At the same time, no visible effect of pH on the viscoelasticity of EY and SEY interfacial films was noticed. It became clear from the dilational modulus versus surface pressure curves for both EY and SEY that adsorption of their surface-active components at the TAG oil–water interface occurs in a step-wise manner. We found also that the phase angle values for the adsorbed EY layers were lower that those observed in the presence of SEY, indicating an increasing viscous contribution to the dilational modulus in the SEY-containing system.     

Structure and mechanical properties of a polyglycerol ester at the air-water surface

We studied the structure and mechanical properties of surface films resulting from the adsorption of a dispersed L beta phase at the air-water interface. This L beta phase corresponds to multilamellar vesicles and is formed by a commercial polyglycerol fatty acid ester (PGE) in aqueous solution at temperatures below the main chain-melting temperature (Tm=58 degrees C). We measured the adsorption kinetics using the pendant drop technique and mechanical properties of PGE films using oscillatory surface shear and dilatational rheometric methods. Though the adsorption kinetics are very slow, we show that the L beta phase of PGE is surface-active and forms viscoelastic films at the air-water surface after sufficiently long adsorption times. The rheological response functions to shear and dilatational deformation are reminiscent of those of temporary networks, indicating an intermolecular connectivity at the surface. This temporary network is probably created by hydrophobic interactions of alkyl chains. We obtained more detailed information about the properties of this network by comparing the rheological signature of an adsorbed PGE film (unknown structure) with a solvent-spread monolayer (known structure). We characterized the structural features of spread PGE films by recording the Langmuir isotherm and Brewster angle micrographs (BAM).We show that the rheological responses of the adsorbed film and the solvent-spread monolayer are very close to each other, indicating a structural similarity. From this study, we conclude that a dispersed L beta phase of PGE is able to adsorb at the air-water surface at T相似文献   

Chemical oscillation with periodic adsorption and desorption of surfactant ions at a water/nitrobenzene interface.

Chemical oscillations with periodic adsorption and desorption of surfactant ions, alkyl sulfate ions, at a water/nitrobenzene interface have been investigated. The interfacial tension was measured with a quasi elastic laser scattering (QELS) method and the interfacial electrical potential was obtained. We found that this oscillation consists of a series of abrupt adsorptions of ions, followed by a gradual desorption. In addition, we observed that each abrupt adsorption was always accompanied by a small waving motion of the liquid interface. From the analysis of the video images of the liquid interface or bulk phase, we could conclude that each abrupt adsorption is caused by nonlinear amplification of mass transfer of ions from the bulk phase to the liquid interface by a Marangoni convection, which was generated due to local adsorption of the surfactant ions at the liquid interface that resulted in the heterogeneity of the interfacial tension. In the present paper, we describe the mechanism of the chemical oscillation in terms of the hydrodynamic effect on the ion adsorption processes, and we also show the interfacial chemical reaction with ion exchange during the ion desorption process.