Interfacial shear rheology of protein-surfactant layers

The shear rheology of adsorbed or spread layers at air/liquid and liquid/liquid phase boundaries is relevant in a wide range of technical applications such as mass transfer, monolayers, foaming, emulsification, oil recovery, or high speed coating. Interfacial shear rheological properties can provide important information about interactions and molecular structure in the interfacial layer. A variety of measuring techniques have been proposed in the literature to measure interfacial shear rheological properties and have been applied to pure protein or mixed protein adsorption layers at air/water or oil/water interfaces. Such systems play for example an important role as stabilizers in foams and emulsions. The aim of this contribution is to give a literature overview of interfacial shear rheological studies of pure protein and protein/surfactant mixtures at liquid interfaces measured with different techniques. Techniques which utilize the damping of waves, spectroscopic or AFM techniques and all micro-rheological techniques will not discuss here.     

Interfacial rheology of food emulsifiers and proteins

The scientific literature from 1997 (inclusive) to the present on the interfacial rheology of emulsifiers and proteins of relevance to food has been reviewed. Both shear and dilatational rheology of oil–water and air–water interfaces have been covered and the main factors affecting interfacial rheology have been tabulated. Special attention is paid to: the sensitivity of interfacial rheology to film composition and structure; the growing viewpoint of treating proteins films as a two-dimensional gel state; recent theoretical modelling of interfacial rheological effects; those few publications that attempt to relate interfacial rheology to bulk stability. It is concluded that there have been few major advances in the last 4 or 5 years, but the heterogeneity of such adsorbed films seems to be better recognised, both spatially and rheologically, with the challenge remaining to connect this picture to the stability of the corresponding bulk systems.     

Effect of interfacial rheology on model emulsion coalescence I. Interfacial rheology

Interfacial elasticity and "dynamic" surface pressure isotherms were measured for interfaces between a dispersed water phase and a continuous phase of asphaltenes, toluene, and heptane. The interfacial modulus is a function of asphaltene concentration and in all cases reached a maximum at an asphaltene concentration of approximately 1 kg/m(3). The modulus increased significantly as the interface aged and slightly as the heptane content increased to a practical limit of 50 vol%. The modulus was approximately the same at 23 and 60 degrees C. The modulus correlated with the inverse of the initial compressibility determined from surface pressure isotherms. The surface pressure isotherms also indicated that a phase transition occurred as the interface was compressed leading to the formation of low compressibility films. Crumpling was observed upon further compression. The phase transition shifted to a higher film ratio with an increase in heptane content and interface age. Asphaltene concentration and temperature (23 and 60 degrees C) has little effect on the surface pressure isotherms. The surface pressure and elasticity measurements are consistent with the gradual formation of a cross-linked asphaltene network on the interface.     

Surface shear rheology of saponin adsorption layers

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. Langmuir2011, 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.     

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.     

Interfacial rheology of stable and weakly aggregated two-dimensional suspensions

The interfacial rheological properties of stable and weakly aggregated two-dimensional suspensions are studied experimentally using a magnetic rod interfacial rheometer. Particle monolayers with well controlled structures were prepared. Charged polystyrene particles create two-dimensional colloidal crystals at the water-decane interface over a wide range of concentrations. Under similar conditions a predominantly liquid structure is obtained at the water-air interface for the same particles. The addition of appropriate combinations of the anionic surfactant sodiumdodecylsulfate (SDS) and sodium chloride (NaCl) to the aqueous subphase leads to a destabilization of these monolayers with the formation of fractal aggregates at low concentrations and a heterogeneous gel forming as the surface coverage is increased. After the structures have been built up a reproducible structure can be obtained, of which the interfacial rheological properties can be investigated using a magnetic rod stress rheometer. In all cases, numerical calculations were used to assess the importance of instrumental artifacts and the effect of the coupling between surface and subphase flows. The rheology of aggregated suspensions was compared to the reference case of a colloidal crystal. The two-dimensional aggregated suspensions display rheological features which are similar to their three-dimensional counterparts. These include an elastic response with small linearity limits, a power law dependence on surface coverage and a dependence on the strength of attraction. The results shed some light on the possible role of interfacial rheology on the stability of particle laden high interface systems. Additionally, the 2D suspensions could present fundamental insights in the rheological properties of dense colloidal suspensions.     

Interfacial rheology of petroleum asphaltenes at the oil-water interface

A biconical bob interfacial shear rheometer was used to study the mechanical properties of asphaltenic films adsorbed at the oil-water interface. Solutions of asphaltenes isolated from four crude oils were dissolved in a model oil of heptane and toluene and allowed to adsorb and age in contact with water. Film elasticity (G') values were measured over a period of several days, and yield stresses and film masses were determined at the end of testing. The degree of film consolidation was determined from ratios of G'/film mass and yield stress/G'. Asphaltenes with higher concentrations of heavy metals (Ni, 330-360 ppm; V, 950-1000 ppm), lower aromaticity (H/C, 1.24-1.29), and higher polarity (N, 1.87-1.99) formed films of high elasticity, yield stress, and consolidation. Rapid adsorption kinetics and G' increases were seen when asphaltenes were near their solubility limit in heptane-toluene mixtures (approximately 50% (v/v) toluene). In solvents of greater aromaticity, adsorption kinetics and film masses were reduced at comparable aging times. Poor film forming asphaltenes had yield stress/G' values ((1.01-1.21) x 10(-2)) more than 4-fold lower than those of good film forming asphaltenes. n-heptane asphaltenes fractionated by filtering solutions prepared at low aromaticity (approximately 40% toluene in mixtures of heptane and toluene) possessed higher concentrations of heavy metals and nitrogen and higher aromaticity. The less soluble fractions of good film forming asphaltenes exhibited enhanced adsorption kinetics and higher G' and yield stress values in pure toluene. Replacing the asphaltene solutions with neat heptane-toluene highlighted the ability of films to consolidate and become more elastic over several hours. Adding resins in solution to a partially consolidated film caused a rapid reduction in elasticity followed by gradual but modest consolidation. This study is among the first to directly relate asphaltene chemistry to adsorption kinetics, adsorbed film mechanical properties, and consolidation kinetics.     

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.     

Interfacial rheology and structure of straight-chain and branched fatty alcohol mixtures

Langmuir monolayers of mixtures of straight-chain and branched molecules of hexadecanol and eicosanol were studied using surface pressure-area isotherms, Brewster angle microscopy, and interfacial rheology measurements. For hexadecanol mixtures below 30% branched molecules, the isotherms show a lateral shift to a decreasing area proportional to the fraction of straight chains. Above a 30% branched fraction, the isotherms are no longer identical in shape. The surface viscosities of both straight and mixed monolayers exhibit a maximum in the condensed untilted LS phase at pi = 20 mN/m. Adding branched chains results in a nonmonotonic increase in surface viscosity, with the maximum near 12% branched hexadecanol. A visualization of these immiscible monolayers using Brewster angle microscopy in the liquid condensed phase shows the formation of discrete domains that initially increase in number density and then decrease with increasing surface pressure. Eicosanol mixtures exhibit different rheological and structural behavior from hexadecanol mixtures. The addition of branched chains results in a lateral shift to increasing area, proportional to the fraction and projected area of both straight and branched chains. A phase transition is seen for all mixtures, including pure straight chains, at pi = 15 mN/m up to 50% branched chains. A second transition is seen at pi = 25 mN/m when the isotherms cross over. Above this transition, the isotherms shift in the reverse direction with increasing branched fraction. The surface viscosities of both straight and mixed monolayers show a maximum in the L2' phase near pi = 5 mN/m. The surface viscosity is constant for low branched fractions and decays beyond 15% branched chains.     

Crystal-melt coexistence under shear: Interpreting the nonlinear rheology

We propose a phenomenological model for shear-induced melting aimed at assisting the design of experimental studies of this phenomenon. For increasing strain rates, the model predicts the changes in liquid fraction and shear stress as a function of interfacial supercooling. We discuss the experimental conditions under which shear-induced melting could be observed in a range of materials.     

Simulation of nonlinear shear rheology of dilute salt-free polyelectrolyte solutions

Brownian dynamics simulations are used to conduct a systematic analysis of the nonlinear shear rheology of dilute polyelectrolyte solutions, exploring its relationship to shear rate, Bjerrum length, and concentration. A simple coarse-grained bead-spring chain model that incorporates explicit counterions is used. It is found that the polyelectrolyte chains exhibit a shear thinning behavior at high shear rate (as characterized by bead Peclet number Pe) that is independent of the electrostatic strength due to the stripping of ions from close proximity to the chain caused by the flow. In contrast, at low values of Pe, the viscosity increases monotonically with increasing Bjerrum length over the range studied here, in contrast to the nonmonotonic trend displayed by the chain size. Furthermore, at fixed Bjerrum length, the reduced viscosity increases monotonically with concentration. The mechanism underlying these observations is essentially the primary electroviscous effect; the ion cloud surrounding a polyelectrolyte chain deforms in flow, causing a significant increase in viscosity as concentration increases. Finally, the authors have also considered the role of hydrodynamic interactions in these simulations, finding that for low concentration studies in shear flow, these do not qualitatively affect the results.     

Interfacial dynamics and rheology of polymer-grafted nanoparticles at air-water and xylene-water interfaces

Particle-stabilized emulsions and foams offer a number of advantages over traditional surfactant-stabilized systems, most notably a greater stability against coalescence and coarsening. Nanoparticles are often less effective than micrometer-scale colloidal particles as stabilizers, but nanoparticles grafted with polymers can be particularly effective emulsifiers, stabilizing emulsions for long times at very low concentrations. In this work, we characterize the long-time and dynamic interfacial tension reduction by polymer-grafted nanoparticles adsorbing from suspension and the corresponding dilatational moduli for both xylene-water and air-water interfaces. The dilatational moduli at both types of interfaces are measured by a forced sinusoidal oscillation of the interface. Surface tension measurements at the air-water interface are interpreted with the aid of independent ellipsometry measurements of surface excess concentrations. The results suggest that the ability of polymer-grafted nanoparticles to produce significant surface and interfacial tension reductions and dilatational moduli at very low surface coverage is a key factor underlying their ability to stabilize Pickering emulsions at extremely low concentrations.     

Interfacial rheology and conformations of triblock copolymers adsorbed onto the water-oil interface

The conformation and the dilatational properties of three non-ionic triblock PEO-PPO-PEO (where PEO is polyethyleneoxide and PPO is polypropyleneoxide) copolymers of different hydrophobicity and molecular weight were investigated at the water-hexane interface. The interfacial behavior of the copolymers was studied by combining dilatational rheology using the oscillating drop method and ellipsometry. From the dilatational rheology measurements the limiting elasticity values, E(0), of the Pluronics as function of surface pressure, Π, and adsorption time were obtained, i.e. E(0)(t) and E(0)(Π). Here, it is shown that E(0)(t) depends on the number of PEO units and on the bulk concentration, showing maximum and minimum surface elasticity values which indicate conformational changes in the interfacial layer. Furthermore, in the framework of the polymer scaling law theory, conformational transitions were discussed in E(0) vs. Π plots. In a dilute regime (Π<14 mN m(-1)) at the water-hexane interface, E(0)=2Π fits well all the data, which indicates a two-dimensional "stretched chain" conformation. Increasing Π, two other interfacial transitions could take place. The different behavior of Pluronic copolymers could be also described by the local minima of E(0), which depends on the hydrophobicity of the copolymers. Conformational transitions observed by interfacial rheology were compared to ellipsometric data. Experimental results were discussed and explained on the basis of two- and three-dimensional copolymer structure taking into account that PPO chains could be partially immersed in hexane and water.     

Structure, rheology and shear alignment of Pluronic block copolymer mixtures

The structure and flow behaviour of binary mixtures of Pluronic block copolymers P85 and P123 is investigated by small-angle scattering, rheometry and mobility tests. Micelle dimensions are probed by dynamic light scattering. The micelle hydrodynamic radius for the 50/50 mixture is larger than that for either P85 or P123 alone, due to the formation of mixed micelles with a higher association number. The phase diagram for 50/50 mixtures contains regions of cubic and hexagonal phases similar to those for the parent homopolymers, however the region of stability of the cubic phase is enhanced at low temperature and concentrations above 40 wt%. This is ascribed to favourable packing of the mixed micelles containing core blocks with two different chain lengths, but similar corona chain lengths. The shear flow alignment of face-centred cubic and hexagonal phases is probed by in situ small-angle X-ray or neutron scattering with simultaneous rheology. The hexagonal phase can be aligned using steady shear in a Couette geometry, however the high modulus cubic phase cannot be aligned well in this way. This requires the application of oscillatory shear or compression.     

Importance of physical vs. chemical interactions in surface shear rheology

The stability of adsorbed protein layers against deformation has in literature been attributed to the formation of a continuous gel-like network. This hypothesis is mostly based on measurements of the increase of the surface shear elasticity with time. For several proteins this increase has been attributed to the formation of intermolecular disulfide bridges between adsorbed proteins. However, according to an alternative model the shear elasticity results from the low mobility of the densely packed proteins. To contribute to this discussion, the actual role of disulfide bridges in interfacial layers is studied. Ovalbumin was thiolated with S-acetylmercaptosuccinic anhydride (S-AMSA), followed by removal of the acetylblock on the sulphur atom, resulting in respectively blocked (SX) and deblocked (SH) ovalbumin variants. This allows comparison of proteins with identical amino acid sequence and similar globular packing and charge distribution, but different chemical reactivity. The presence and reactivity of the introduced, deblocked sulfhydryl groups were confirmed using the sulfhydryl-disulfide exchange index (SEI). Despite the reactivity of the introduced sulfhydryl groups measured in solution, no increase in the surface shear elasticity could be detected with increasing reactivity. This indicates that physical rather than chemical interactions determine the surface shear behaviour. Further experiments were performed in bulk solution to study the conditions needed to induce covalent aggregate formation. From these studies it was found that mere concentration of proteins (to 200 mg/mL, equivalent to a surface concentration of around 2 mg/m(2)) is not sufficient to induce significant aggregation to form a continuous network. In view of these results, it was concluded that the adsorbed layer should not be considered a gelled network of aggregated material (in analogy with three-dimensional gels formed from heating protein solutions). Rather, it would appear that the adsorbed proteins form a highly packed system of proteins with net-repulsive interactions.     

Interfacial nucleic acid chemistry studied by acoustic shear wave propagation

Acoustic wave devices have continued to gain attention as biosensor structures because of their relative ease of operation and sensitivity to interfacial biochemical events. In the present paper, we review the use of the thickness-shear mode device for the label-free detection of processes involving nucleic acid moieties that are imposed at the sensor-liquid interface. Following a concise discussion of the theory and technology connected to the operation of the sensor in liquids, we outline a number of protocols that have been adopted for the attachment of oligonucleotides to sensor surfaces, many of which have been employed in ultrasonic biosensing. The various categories of applications are then surveyed in some detail. By far, the largest group is the study of duplex formation at the sensor surface, involving a compendium of experiments involving complementary and mismatched sequences. Considerably less attention has been paid to the detection of interaction of surface-bound nucleic acids with small molecules such as specific-binding peptides and drugs.A comprehensive appraisal of the literature in this field strongly suggests that acoustic coupling phenomena are particularly sensitive to interfacial physical chemistry. Accordingly, acoustic shear wave technology offers unique advantages over other sensor configurations because of its ability to produce multidimensional information through the recording of various parameters obtained from acoustic network analysis.     

Interfacial rheology of an ultrathin nanocrystalline film formed at the liquid/liquid interface

We report the interfacial properties of monolayers of Ag nanoparticles 10-50 nm in diameter formed at the toluene-water interface under steady as well as oscillatory shear. Strain amplitude sweep measurements carried out on the film reveal a shear thickening peak in the loss moduli (G") at large amplitudes followed by a power law decay of the storage (G') and loss moduli with exponents in the ratio 2:1. In the frequency sweep measurements at low frequencies, the storage modulus remains nearly independent of the angular frequency, whereas G" reveals a power law dependence with a negative slope, a behavior reminiscent of soft glassy systems. Under steady shear, a finite yield stress is observed in the limit of shear rate .gamma going to zero. However, for .gamma > 1 s-1, the shear stress increases gradually. In addition, a significant deviation from the Cox-Merz rule confirms that the monolayer of Ag nanoparticles at the toluene-water interface forms a soft two-dimensional colloidal glass.     

Surface shear rheology of WPI-monoglyceride mixed films spread at the air-water interface

Surface shear viscosity of food emulsifiers may contribute appreciably to the long-term stability of food dispersions (emulsions and foams). In this work we have analyzed the structural, topographical, and shear characteristics of a whey protein isolate (WPI) and monoglyceride (monopalmitin and monoolein) mixed films spread on the air-water interface at pH 7 and at 20 degrees C. The surface shear viscosity (etas) depend on the surface pressure and on the composition of the mixed film. The surface shear viscosity varies greatly with the surface pressure. In general, the greater the surface pressure, the greater are the values of etas. The values of etas for the mixed WPI-monoolein monolayer were more than one order of magnitude lower than those for a WPI-monopalmitin mixed film, especially at the higher surface pressures. At higher surface pressures, collapsed WPI residues may be displaced from the interface by monoglyceride molecules with important repercussions on the shear characteristics of the mixed films. A shear-induced change in the topography and a segregation between domains of the film forming components were also observed. The displacement of the WPI by the monoglycerides is facilitates under shear conditions, especially for WPI-monoolein mixed films.