Lazhar Benyahia

Stabilization of Water-in-Water Emulsions by Addition of Protein Particles

The effect of the addition of protein particles was investigated on the stability of water-in-water emulsions formed by mixing aqueous dextran and poly (ethylene oxide) solutions. Protein particles with hydrodynamic radii ranging from 15 to 320 nm were produced by heating globular proteins in controlled conditions. The structure of the emulsions was visualized with confocal laser scanning microscopy using different fluorescent probes to label the dextran phase and the protein particles. It is shown that contrary to native proteins, protein particles adsorb at the interface and can form a monolayer that inhibits fusion of emulsion droplets. In this way, water-in-water emulsions could be stabilized for a period of weeks. The effect of the polymer composition and the protein particle size and concentration was investigated.

Particles Trapped at the Droplet Interface in Water-in-Water Emulsions

Water-in-water emulsions were formed by mixing incompatible aqueous solutions of dextran and poly(ethylene oxide) (PEO) in the presence of latex or protein particles. It was found that particles with a radius as small as 0.1 μm become trapped at the interface between the PEO- and dextran-rich phases with interfacial tensions down to 10(-6) N/m. The particles were visualized at the interface of the emulsion droplets using confocal laser scanning microscopy (CLSM) allowing determination of the contact angle. Various degrees of coverage with particles could be observed. On densely covered droplets, the particles had a hexagonal crystalline order. At intermediate coverage, transient clustering of the particles was observed. The diffusion coefficient of the particles at the interface was determined using multiparticle tracking. Fusion of droplets was observed in all cases leading eventually to macroscopic phase separation.

pH-Responsive Water-in-Water Pickering Emulsions

The structure and stability of water-in-water emulsions was investigated in the presence of spherical, pH-sensitive microgels. The emulsions were formed by mixing aqueous solutions of dextran and PEO. The microgels consisted of cross-linked, synthetic polymers with a radius that steeply increased from 60 to 220 nm with increasing pH within a narrow range around 7.0. At all pH values between 5.0 and 7.5, the microgels were preferentially situated at the interface, but only in a narrow range between pH 7.0 and 7.5, the emulsions were stable for at least 1 week. The droplet size was visualized with confocal laser scanning microscopy and was found to be smallest in the stable pH range. Emulsions could be stabilized or destabilized by small changes of the pH. Addition of small amounts of salt led to a shift of the pH range where the emulsions were stable. The effects of varying the microgel concentration and the polymer composition were investigated.

Stabilization of Water-in-Water Emulsions by Polysaccharide-Coated Protein Particles

The phase diagram of mixtures of xyloglucan (XG) and amylopectin (AMP) in aqueous solution is presented. Water-in-water emulsions prepared from mixtures in the two-phase regime were studied in detail, and the interfacial tension was determined. It is shown that the emulsions can be stabilized by addition of β-lactoglobulin microgels (βLGm), but only at pH ≤ 5.0. Excess βLGm preferentially entered the AMP phase at pH > 5.0 and the XG phase at lower pH. The inversion was caused by adsorption of XG onto βLGm that started below pH 5.5. It is shown that modification of the surface of particles by coating with polysaccharides is a potential lever to control stabilization of water-in-water emulsions.

Structure and Viscoelasticity of Mixed Micelles Formed by Poly(ethylene oxide) End Capped with Alkyl Groups of Different Length

Poly(ethylene oxide) (PEO) end capped with an alkyl group is a highly asymmetric diblock copolymer that forms spherical micelles in aqueous solution resembling multiarm star polymers. The effect of varying the length of the alkyl end group on the structure and viscoelasticity was investigated for pure and mixed micelle suspensions. The aggregation number (p) of the micelles increased and the critical association concentration (CAC) decreased with increasing the length of the end group. At high concentrations a discontinuous reversible liquid-solid transition was observed below a critical temperature (Tc) that increased with increasing length of the end group. Mixing end-capped PEO with different alkyl lengths led first to formation of the micelles by polymers with the lowest CAC into which the other polymers were incorporated when the concentration was increased. The viscoelastic properties at high concentrations are the same for pure systems and mixtures with the same average length of the alkyl end group.

Synergistic effects of mixed salt on the gelation of κ-carrageenan

The effect of the addition of calcium or sodium ions on the potassium induced gelation of κ-carrageenan (κ-car) is investigated using oscillatory shear rheology and turbidimetry. Both the gelation kinetics and the steady state shear moduli are investigated. Gelation in mixed salt solutions is compared with that in pure potassium and calcium solutions. It is shown that the elastic shear modulus increases with increasing pure KCl concentration, but decreases with increasing pure CaCl2 concentration. In mixed salts, gelation of κ-car is induced by potassium and addition of CaCl2 leads to an increase of the elastic modulus with increasing CaCl2 concentration. κ-Car gelled at low mixed salt concentrations for which it remained liquid in pure salt. At equivalent ionic strengths, the effect of adding NaCl on potassium induced gelation is much weaker. In pure KCl solutions, κ-car gels are transparent, but in pure CaCl2 they become increasingly turbid with increasing CaCl2 concentration. The turbidity of gels formed in mixed salts is intermediate.

Influence of the Protein Particle Morphology and Partitioning on the Behavior of Particle-Stabilized Water-in-Water Emulsions

Protein fibrils, microgels, and fractal aggregates were produced by heating solutions of β-lactoglobulin (β-lg) under different conditions. The effect of the protein particle morphology on the stability and the structure of water-in-water (W/W) emulsions was studied for mixtures of poly(ethylene oxide) (PEO) and dextran. The protein particles partition to the dextran phase at pH 7.0 where they have a net negative charge, but they prefer the PEO phase at pH 3.0 where they have a net positive charge. The effect of partitioning on the stability and the structure of water-in-water (W/W) emulsions was studied by comparing emulsions at pH 3.0 with those at pH 7.0. The protein particle morphology and preference for one phase or the other are shown to have important consequences for the stability and the structure of the emulsions. Fibrils were found to be the most effective stabilizers at pH 7.0, whereas fractals were most effective at pH 3.0. The average droplet size obtained from confocal scanning laser microscopy was for most systems between 10 and 5 μm but was notably smaller for emulsions with fractals at pH 3.0.

Strain-Induced Droplet Retraction Memory in a Pickering Emulsion

We studied the deformation and relaxation of a water droplet covered with polystyrene latex particles (diameter ca. 200 nm) and embedded in an immiscible fluid after a large strain jump. We show that the presence of the solid particles at the droplet interface slows down the retraction kinetics in comparison with a pure water droplet and induces flow singularity not observed with pure water droplets. The terminal relaxation time of the retraction process, defined as the characteristic time required for the droplet to relax to its spherical equilibrium shape, increases linearly with the applied strain. This result implies a memory effect induced by the presence of solid particles at the droplet interface in a solid-stabilized or Pickering emulsion.

Dynamic mechanical characterization of the heat-induced formation of fractal globular protein gels

The frequency dependence of the shear modulus of heat denatured β-lactoglobulin gels at pH 7 at 0.1 M NaCl was studied during the gelation process. It can be characterized by two modes. The fast mode at different protein concentrations is successfully described in terms of the fractal gel model. The slow mode is characterized by a power law frequency dependence and is attributed to structural relaxation of the elastic backbone. The power law exponent is independent of heating time and decreases linearly with decreasing concentration. The evolution of the dynamic mechanical properties during gelation is independent of the heating temperature although the gelation rate increases strongly with increasing temperature.

Slow dynamics and structure in jammed milk protein suspensions.

The dynamic mechanical properties and the structure of dense suspensions of sodium caseinate were investigated using oscillatory shear theology and confocal laser scanning microscopy, respectively. Caseins are the most abundant milk proteins and form in the absence of calcium phosphate small star-like particles with radii of about 10 nm. The viscosity increases strongly with increasing protein concentration above -80 g L(-1) due to jamming of the particles. The viscosity increase is stronger at lower temperatures, caused by a strong decrease in the terminal relaxation time with increasing temperature. Addition of calcium ions introduces an attractive interaction that induces phase separation above a critical calcium concentration. Increasing the CaCl2 concentration leads to an increase in the terminal relaxation time, but to a decrease in the high frequency elastic modulus. The effect of adding CaCl2 is stronger at higher temperatures.