No abstract

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.

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.

Water-in-water (W/W) emulsions formed by mixing incompatible water-soluble polymers cannot be stabilized with molecular surfactants. However, they can be stabilized by particles through the so-called Pickering effect. Recently, it was shown that its stabilization can be achieved also with nanoplates. Here, we show for the first time that even nanorods in the form of cellulose nanocrystals (CNCs) can efficiently stabilize W/W emulsions. Static light scattering and confocal microscopy techniques were used to determine the surface coverage by CNCs. In the presence of 50 mM NaCl very weak gels were formed by excess CNCs in the continuous phase. In this way creaming of the dispersed phase could be arrested. The nontoxicity, sustainability, and low cost of CNCs and the abundant availability of cellulose render these nanorods potentially highly suited for preparing W/W emulsions.

No abstract

No abstract

Cross-correlation dynamic light scattering was used to measure the structure factor of heated solutions of the globular protein β-lactoglobulin at pH 7 and 0.1 M NaCl. After 24 h heating at 80 °C finite size aggregates are formed at protein concentrations below 15 g/L, whereas at higher concentrations turbid gels are formed. The gels may be considered as collections of randomly close packed “blobs” with a self-similar structure characterized by a fractal dimension d f = 2.0 ± 0.1. The concentration dependence of the structure factor was compared with that of the elastic shear modulus. The results for this particular protein system were consistent with predictions of the so-called fractal gel model. Limits of validity of the model are discussed.

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.