Rheological transition of concentrated emulsions during successive shearing cycles

Rheological investigations have identified a shear viscosity transition from shear thinning to Newtonian at low to moderate shear rates for concentrated polydimethylsiloxane emulsions during successive shearing cycles. The viscosity "flattening" behavior is dependent on the maximum shear rate applied and on droplet deformation. Atomic force microscopy measurements indicate attraction between the "repulsive" emulsion droplets under deformation. The results suggest formation of stable droplet layers due to deformation under high shear hydrodynamic compression. Based on these findings, unique methods to control the post-shear rheology of concentrated emulsions can be envisaged.     

Formation of interfacial milk protein complexation to stabilize oil-in-water emulsions against calcium

The interactions between cationic liposomes doped with the anionic nucleolipid 1,2-dipalmitoyl-sn-glycero-3-cytidine diphosphate (DP-Cyt) and deoxyribonucleic acid (DNA) were investigated. Toward this goal, new liposomal and lipoplex formulations characterized by the presence of the anionic amphiphile DP-Cyt were proposed. The effects of incorporation of the cytosine functionalized lipid DP-Cyt into the cationic bilayers were analyzed by means of electrophoretic mobility, dynamic light scattering (DLS) and fluorescence spectroscopy techniques. These approaches allowed us to follow the DNA condensation process and to identify specific electrokinetic characteristics of liposome and DNA-liposome complexes formation. Specifically, DP-Cyt liposomes and DNA were shown to form electrically stable or unstable complexes depending on the charge ratio between the phosphate group of DNA and the cationic lipid. Remarkably, a prominent role for DP-Cyt in enhancing the DNA binding capacity on liposomes was demonstrated. Zeta potential experiments performed on systems with different liposomes/DNA ratio showed that the value of the charge neutralization point is a function of the content of the incorporated DP-Cyt. As a whole, our data demonstrate that the association of cationic DP-Cyt doped liposomes with DNA is driven by both electrostatic interaction and additional specific interactions at the polar head level based on the cytidine nucleobase.     

Milk protein interfacial layers and the relationship to emulsion stability and rheology

The properties of milk protein-stabilised, oil-in-water emulsions are determined by the structure and surface rheology of the adsorbed layer at the oil-water interface. Analysis of the segment density profiles normal to the surface show differences in the structure between adsorbed layers of disordered casein and globular whey protein. Systematic studies of stability and rheology of model oil-in-water emulsion systems made with milk proteins as sole emulsifiers give insight into the relation between adsorbed layer properties and bulk emulsion stability. Of particular importance are effects of pH, temperature, calcium ions and protein content. Colloidal interactions between adsorbed layers on different surfaces can be inferred from an analysis of dynamic collisions of protein-coated emulsion droplets in shear flow using the colloidal particle scattering technique. The role of competitive adsorption on emulsion properties can be derived from experiments on systems containing mixtures of milk proteins and small-molecule surfactants. Shear-induced destabilisation is especially influenced by the presence of fat crystals in the emulsion droplets. Aggregated gel network properties are dependent on the balance of weak and strong interparticle interactions. In heat-set whey protein emulsion gels, the rheological behaviour is especially sensitive to surfactant type and concentration. Rearrangements of transient caseinate-based emulsion gels can have a profound influence on the quiesent stability behaviour. Computer simulation provides a general link between particle interactions, microstructure and rheological properties.     

A monolayer model of the interfacial region of microemulsions

Mixed monolayers of tetradecanol and oleic acid at the water-air interface were studied to provide a static related structure featuring the interface between water and oil of water-dodecane microemulsions.The films of pure components as a function of temperature show a strong area contraction between 25 ° and 30 °C, caused by a change in the head groups hydration. This agrees with similar discontinuities found for some properties of the microemulsion in the same temperature range. At the water-air interface, the composition range of tetradecanol/oleic acid mixtures with the highest thermodynamic stability corresponds to the same stability range of the water-in -dodecane-potassium oleate microemulsions.Adsorption isotherms of tetradecanol and hexanol at the dodecane-water interface were studied to compare the surface behaviour of the two alcohols; the results indicate that the two alcohols have very similar two-dimensional surface phases and adsorption energies.     

Rheological investigations on the creaming of depletion-flocculated emulsions

Preventing creaming or sedimentation by the addition of thickeners is an important industrial challenge. We study the effect of the addition of a "free" nonadsorbing polymer (xanthan gum) on the stability against creaming of sterically stabilized O/W emulsions. Therefore, we analyze our samples using microscopy and rheological measurements. At low xanthan concentrations, the emulsions cream. However, above a certain concentration a three-dimensional network of droplets is formed, which can prevent creaming. We attribute the formation of this structure to depletion attraction. The rheological behavior of an emulsion that is macroscopically stable should be elastic, while it should be viscous for a creaming emulsion. In order to distinguish between stable and unstable samples, we measure their relaxation time by mechanical rheology and find a good correlation to the visual observation. However, the measured relaxation times are much shorter than the time-scales, on which we observe creaming. We hypothesize that the measured relaxation time is related to the droplet-droplet interaction. This determines the frequency at which microscopic rearrangements occur, which weaken the network structure prior to creaming. Based on this interpretation, the relaxation time gives direct access to the microstructural processes involved in creaming. We therefore suggest using it as a predictive parameter of creaming stability.     

Current knowledge on the interfacial behaviour limits our understanding of plant protein functionality in emulsions

For different reasons, there is an increasing interest in plant-based foods as well as vegetarian and vegan dairy and meat alternatives. Frequently, those foods represent dispersed systems and more specifically food emulsions with proteins as emulsifying food ingredients. Owing to a very heterogeneous composition of plant proteins and a wide range of structural varieties in the proteins, it is worth discussing if our current understanding of interfacial and emulsifying properties of proteins is sufficient to meet the challenges associated with the utilisation of plant proteins for the stabilisation of food emulsions. To this purpose, we review the current understanding of the interfacial behaviour of proteins, summarise analytical techniques for their characterisation and critically discuss the available literature on oilseed and legume proteins to identify future research needs and opportunities for customised emulsion design.     

Preparation of a stable ultradispersed AgBr colloid in water-oil microemulsions

Conclusions AgBr microparticles, containing a calculated number of Ag⁺ ions in the range from 6 to 20, were synthesized in the dispersed phase of water-oil microemulsions.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 479–480, February, 1989.     

Effect of surfactant sucrose ester on physical properties of dairy whipped emulsions in relation to those of O/W interfacial layers

Dairy foams were manufactured on a pilot plant with various sucrose ester contents. Oil-in-water emulsions were produced by high-pressure homogenisation of anhydrous milk fat (20 wt%) with an aqueous phase containing skim milk powder (6.5 wt%), sucrose (15 wt%), hydrocolloids (2 wt%), and sucrose esters. Sucrose ester content was varied from 0 to 0.35 wt%. Firmness and stability of dairy foams were determined. The fraction of protein associated with emulsion fat droplets and the compression isotherms of those droplets were determined as a function of sucrose ester content. With less than 0.1 wt% sucrose ester, no foam could be produced. The most firm and stable foams were obtained with ca. 0.1 wt% sucrose ester. The fraction of protein associated with emulsion droplets suddenly falls from 60% at a sucrose ester content lower than 0.1125% down to ca. 10-20% for higher surfactant content. Compression isotherms of emulsion droplets at the air-water interface show that, in the presence of surfactant, emulsion droplets disrupt and spread at the interface whilst without surfactant they become dispersed. This means that the presence of sucrose ester causes some destabilisation of fat droplet interfacial layers. There is hence an optimal sucrose ester content that allows some destabilisation of the oil-water interface without concomitant protein displacement from that interface. Consequently, with the recipe and manufacturing process used to produce dairy foams, there exists a compromise in sucrose ester content with regards to manufacture and shelf-life of dairy foams.     

Influence of interfacial rheology on the viscosity of concentrated emulsions

New models are developed for the viscosity of concentrated emulsions taking into consideration the effects of interfacial rheology and Marangoni phenomenon. The interface is assumed to be viscous with non-zero surface-shear and surface-dilational viscosities. The Marangoni effect is accounted for through non-zero Gibbs elasticity of the interface. The experimental viscosity data for a number of emulsion systems are interpreted in terms of the proposed models.     

Geometric parameters as a criterion for assessment of the bioactive conformations of opiate receptor ligands

The mutual positions of the phenyl fragment and the protonated amino group in the molecules of opiate receptor ligands of various structural classes were studied. It was concluded that two bioactive ligand conformations exist and their implementation does not depend on the structural class of the ligand, selectivity of its action on receptors, or relationship between the agonistic and antagonistic properties. A set of geometric parameters describing the three-dimensional arrangement of the phenyl fragment and the protonated amino group in bioactive conformations was proposed; this can be used as a criterion for the geometric assessment of the opiate activity. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1462–1468, September, 2006.     

Friction force spectroscopy as a tool to study the strength and lateral diffusion of protein layers

We present a method to study the strength of layers of biological molecules in liquid medium. The method is based on the Friction Force Spectroscopy operation mode of the Atomic Force Microscope. It works by scratching the sample surface at different applied loads while registering the evolution of the sample topography and of the friction between probe and sample. Results are presented for BSA and β-casein monolayers on hydrophobic surfaces. We show how the simultaneous monitoring of topography and friction allows detecting differences not only between the strength of both types of layers, but also between the lateral diffusion of the proteins within these layers. Specifically, β-casein is shown to form stronger layers than BSA. The yield strengths calculated for both of these systems are in the range 50-70 MPa. Moreover, while no lateral diffusion is observed for BSA, we show that β-casein diffuses along the hydrophobic substrates at a rate higher than the scan velocity of the tip (16 μm s(-1) in our case).     

Investigation of the interfacial properties of water-in-diluted-bitumen emulsions using micropipette techniques

The interfacial properties of water-in-diluted bitumen emulsions were studied using micropipette techniques. It was observed that, as bitumen concentration in the bulk phase (C0) increased, the interfacial tension on the water droplet surfaces decreased. In addition, there was a small effect on the interfacial tension when different solvent mixtures were used. Mixtures of toluene and heptane in different ratios were used as solvents for bitumen dilution. Crumpling of the interface was influenced by bitumen concentration and type of solvent. No crumpling was found for bitumen content less than 0.01% for all solvents used. Crumpling was observed at higher bitumen concentrations when deionized water (pH 5.4-5.6) was used. Setting "heptol[A]" to be the mixture of toluene and heptane, with the volume percent of toluene being A, the following were concluded. Crumpling disappeared at C0 > 1% and when heptol[100] was used, and also at C0 > 10% and when heptol[30] was used. Crumpling was strongly affected by the water pH. In the case of heptol[50], at a higher pH, the crumpling region that normally occurred at C0 > 0.01% disappeared. The micropipette technique proved to be useful in studying the interfacial properties of micrometer-sized emulsion drops.     

Methyl group dynamics as a probe of the protein dynamical transition

Hydrated proteins undergo a dynamical transition around 200 K from glasslike to liquidlike motion. Molecular dynamics simulations have been used to study the temperature dependence of the dynamics of ribonuclease A in the hydrated crystal, a model dehydrated powder, and aqueous solution. Changes in the dynamics accompanying the transition throughout the protein have been quantified in terms of the mean-squared fluctuations (MSFs) of methyl hydrogen atoms on the 100 ps time scale. In solution at 300 K the MSFs span a broad distribution, consistent with NMR relaxation measurements. The MSF distribution in the hydrated crystal at 300 K is qualitatively similar to the solution result, except for a slight shift to lower values, and dehydration results in a dramatic shift of the MSFs to lower values. As the temperature is lowered, the whole distribution of methyl group fluctuations in the hydrated crystal shifts to lower values. Most of the methyl groups in the hydrated protein display a nonlinear temperature dependence with a dynamical transition at approximately 200 K, but most methyl groups do not undergo a transition in the dehydrated protein. We conclude that the dynamical transition occurs throughout most of the protein and that solvent is required for the transition.     

Fourier transform rheology as a universal non-linear mechanical characterization of droplet size and interfacial tension of dilute monodisperse emulsions

A new protocol to gain interfacial tension and droplet size of dilute monodisperse emulsions from Fourier Transform Rheology (FTR), is proposed. Specifically, a universal dimensionless quantity E was found at small strain amplitudes to correlate with the droplet size of the emulsion where E is inversely related to the square of the capillary number Ca and directly proportional to the relative intensities of the fifth and third harmonics, I(5)/I(3). The limiting value E(0) at small strain deformations can be used as a universal parameter to calculate different emulsion properties. Different morphological constitutive models for emulsions were used to establish the universality of the parameter E(0). Preliminary analysis on experimental data confirms the validity of this approach for the characterization of emulsion properties, including the estimation of interfacial tension and droplet radius.     

Rheological properties and interfacial slip of a multilayer structure under dynamic shear

Interfacial slip between polymer melt under steady shear has been studied using a simplified multilayer structure. In this investigation, interfacial slip under dynamic shear was studied by calculating the angular displacements of the multilayer structure and its component layers. On the basis of the angular displacements, a slip index was defined to quantify the degree of interfacial slip. A relationship governing the rheological behavior of the multilayer structure under slip and nonslip condition was established. These results were correlated with equations derived from consideration of energy equilibrium in the multilayer structure. Polymer multilayer structures of high‐density polyethylene/polystyrene and liquid crystal polymer(LCP)/poly ethylene naphthalate(PEN) were investigated. Of all the polymers investigated, large interfacial slip was found at LCP/PEN interface under dynamic shear. The high rigidity and alignment along the interface of LCP molecules was believed to prevent chain entanglement in the interfacial layer and therefore promote interfacial slip at the interface. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2683–2693, 2005     

Novel green strategy to improve the hydrophobicity of cellulose nanocrystals and the interfacial elasticity of Pickering emulsions

The development of Pickering emulsions as ecologically correct stabilized with bio-based material by substituting synthetic petroleum-derived tensoactives assumed a very attractive level, representing the current guideline of the global market for homecare industry, food and beverage applications. In this wor, cellulose nanocrystals (CNCs), a hierarchically advanced biomaterial, were produced to stabilize innovative emulsions formulated with western soapberry Sapindus saponaria L. oil (SO). Besides, green surfactants (triterpene saponins extracted from S. saponaria L. pericarp; SAP) were also investigated to stabilize the oil/water interface. The synergistic combination between cellulose nanowhiskers and the bioactive glycosides has never been reported in the literature. Dynamic interfacial tensions of SAP and SO were firstly investigated, and their capacity to form a plastic membrane at oil/water interface was revealed. Response surface methodology (RSM) was employed to study the influence of the binary systems (CNC:SAP) on the stability of emulsified systems, such as size and zeta potential. In addition, a new calculation was proposed to determine the coverage of the oil droplets formed by the mixture of cellulose crystallites and natural surfactants. The optimal nanoemulsion composition was determined to be 60 w/w (%) of water, 23.905 w/w % of SO, 5 w/w % of CNC and 8.095 w/w% of SAP to produce of smallest droplet (165.1 nm) combined with higher zeta potential module (?46.7 mV). Results highlight the potential of Sapindus saponins and cellulose nanowhiskers for efficient producing label-friendly nanoemulsions applicable for drug, cosmeceutical or edible delivery systems.

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On the formation of concentrated stable w/o emulsions

The conditions for the formation of concentrated w/o emulsions based on Aerosol OT and aliphatic hydrocarbons were studied. It was found that high stability is obtained if the attraction between the emulsion droplets is kept to a minimum. This is achieved by matching the refractive index of the dispersed phase and the organic phase. Furthermore, it is shown that the mere presence of salt does not render any stability. Hence, the speculated mechanism that salt stabilises against Ostwald ripening does not hold for these emulsions.     

Rheology of semi-dilute emulsions: viscoelastic effects caused by the interfacial tension

The steady state rheological properties of viscous emulsions are discussed in the dilute and semi-dilute concentration regions. In these systems the first normal stress differences can be measured as well. Such data have been collected over a wide range of ratios of droplet over matrix viscosity. In this manner data became available to evaluate the Choi-Schowalter model. Application of the latter to the normal stresses requires that the droplet diameter be known. At high shear rates the droplet diameter changes nearly inversely proportional to the shear rate. This results in a first normal stress difference proportional to shear rate and hence a ‘normal viscosity’ can be defined. This is used to compare the data with the available theoretical predictions. At low shear rates deviations from a constant normal viscosity can be observed. They are associated with a hysteresis region, where no single steady state droplet size can be defined anymore. Slightly viscoelastic components have been used as well to investigate whether this would result in deviations from the behaviour observed for mixtures of Newtonian fluids.