Interaction between stabilized droplets of different phases in the same continuous phase of an aqueous three-phase system.

Water-in-water (W/W) emulsions, also called aqueous two-phase systems, are formed by mixing two incompatible polymers in water that phase separate into two distinct phases. They can be stabilized by addition of colloidal particles. Droplets of the dispersed phase can be used to compartmentalize ingredients and induce localized reactions. By mixing more types of incompatible polymers, emulsions containing droplets of different phases can be formed that can potentially capture different ingredients. Here the interaction between dispersed droplets of different types was studied by gently mixing a W/W emulsion containing droplets rich in dextran (DEX) dispersed in a continuous phase rich in polyethylene oxide with an emulsion containing droplets rich in fish gelatin (GEL) dispersed in the same continuous medium. Bis-hydrophilic microgels (MG) consisting of DEX grafted with poly(N-isopropylacrylamide) were added and their effect on the stability of each binary emulsion was investigated. Interestingly, when two very stable emulsions were gently mixed, droplets of different types were observed with confocal scanning laser microscopy to coalesce immediately upon contact. In this manner, Janus-type droplets were formed containing a DEX and a GEL compartment with no MG at the GEL/DEX interface that further associated into strings of alternating droplets. Contact angles between the different phases in emulsions with and without MG were compared and used to determine the effect of the microgels on the interfacial tension between the phases.

pH-controlled breakup of fractal aggregates, microgels and gels formed by self-assembled amphiphilic triblock copolymers.

The degradation of (micro)gels and fractal aggregates based on self-assembled amphiphilic triblock copolymers has been investigated in water by confocal microscopy and light scattering respectively. The triblock copolymer consisted of a central hydrophilic poly(acrylic acid) (pAA) block and two hydrophobic end blocks that contained an equal amount of randomly distributed n-butyl acrylate (nBA) and AA units. These latter units helped at tempering the hydrophobic end blocks resulting in the control and the fine tuning of the dynamics of the self-assembled triblock through the pH. Starting from a pH where the dynamics is frozen, the rate of breakup of the macroscopic gels, microgels and of fractal aggregates was measured after increasing the pH to different values. The mechanism of the breakup was found to be independent of the pH, but its rate increased exponentially with increasing pH. The degradation proceeded through the release of the polymers from the bulk into the surrounding aqueous phase.

Elaboration and rheological characterization of nanocomposite hydrogels containing C60 fullerene nanoplatelets.

Nanocomposite hydrogels were elaborated that consisted of a physical network formed by an amphiphilic polymer in which C60 fullerene nanoplatelets were embedded. Characterization showed that the nanoplatelets within the polymer network were aggregated. The presence of these nanoplatelets led to an increase of the shear modulus of the hydrogels, that cannot be explained by a filler effect alone. The nanocomposite gels displayed similar rheological behavior, both in linear and non-linear domains, as neat hydrogels at higher polymer concentrations. We suggest that the particles reinforced the gels by forming additional connections between the polymer chains.

Effect of adding gelatin on the stability of water in water emulsions formed by mixtures of amylopectin and guar gum.

Stable water in water (W/W) emulsions of guar rich droplets dispersed in an amylopectin rich continuous phase (G/A) and the inverse (A/G) can be achieved by adding gelatin and inducing microphase separation of the latter by cooling. In this research, the effect of gelatin on the emulsion stability was further studied by storing the emulsions at 10, 20 and 25 °C. The visual aspect, the microstructure, and the viscosity of the emulsions were investigated at different times during storage at different temperatures and pH. It was found that depending on the conditions, the gelatin phase wetted the interface or formed small discrete microdomains that adsorbed at the interface and dispersed in the bulk phases. The observed differences in morphology and stability are related to the interplay of the rates of aggregation, phase separation of gelatin, which itself depend on the gelatin concentration, temperature and pH. Emulsions could be prepared in this manner that were stable for at least one week and remained visually homogeneous. We believe that this is a promising method to stabilize W/W emulsions as long as the components of the emulsion are incompatible with aggregated gelatin.

Effect of charge on the stabilization of water-in-water emulsions by thermosensitive bis-hydrophilic microgels.

HYPOTHESIS: Molecular surfactants are not able to stabilize water-in-water (W/W) emulsions, unlike nano or micro-particles, which can achieve this in some cases. However, the effect of electrostatic interactions between particles on the emulsion stability has rarely been investigated. We hypothesize that introducing charges modifies the stabilization capacity of particles and renders it both pH- and ionic strength-dependent. EXPERIMENTS: Charge was introduced into bis-hydrophilic and thermoresponsive dextran/polyN-isopropylacrylamide microgels by replacing a small fraction of polyN-isopropylacrylamide with acrylic acid groups. The size of the microgels was obtained by dynamic light scattering. The stability and microstructure of dextran/poly(ethyleneoxide)-based W/W emulsions, was studied as a function of pH, NaCl concentration and temperature using confocal microscopy and by analytical centrifugation. FINDINGS: The swelling degree of charged microgels depends on the pH, ionic strength and the temperature. In the absence of salt, charged microgels do not adsorb at the interface and have little stabilizing effect even after neutralization. However, the interfacial coverage and the stability increase with rising concentration of NaCl. Saltinduced stabilization of these emulsions was also observed at 50 °C. Increasing the temperature strongly influences the emulsion stability at low pH.

The effect of the contact angle on particle stabilization and bridging in water-in-water emulsions.

HYPOTHESIS: Water-in-water (W/W) emulsions formed by mixing incompatible polymers in aqueous solution can in some cases be stabilized by adding particles that adsorb spontaneously at the W/W interface. The importance of the contact angle of the particles with the interface on the stability of W/W emulsions is still an outstanding issue. We hypothesize that if the contact angle with the continuous phase is smaller than 90°, particles can bridge dispersed droplets, which enhances the stability of the emulsion. EXPERIMENTS: The W/W emulsions consisted of a dispersed poly(ethylene oxide) (PEO) phase in a continuous dextran phase or vice versa. Gelatin microgels were added and their contact angle was varied by varying the pH. The morphology during aging was observed by microscopy. FINDINGS: The contact angle of the microgels with the PEO phase varied between 110° close to neutral pH and 0° at pH 3 and pH 11. The W/W emulsions were stable only when the contact angle with the continuous phase was smaller than 90°. In this case, microgels could form bridges between dispersed droplets creating a network of droplets.

Poly(ethylene oxide)/Gelatin-Based Biphasic Photocrosslinkable Hydrogels of Tunable Morphology for Hepatic Progenitor Cell Encapsulation.

Macroporous hydrogels have great potential for biomedical applications. Liquid or gel-like pores were created in a photopolymerizable hydrogel by forming water-in-water emulsions upon mixing aqueous solutions of gelatin and a poly(ethylene oxide) (PEO)-based triblock copolymer. The copolymer constituted the continuous matrix, which dominated the mechanical properties of the hydrogel once photopolymerized. The gelatin constituted the dispersed phase, which created macropores in the hydrogel. The microstructures of the porous hydrogel were determined by the volume fraction of the gelatin phase. When volume fractions were close to 50 v%, free-standing hydrogels with interpenetrated morphology can be obtained thanks to the addition of a small amount of xanthan. The hydrogels displayed Young's moduli ranging from 5 to 30 kPa. They have been found to be non-swellable and non-degradable in physiological conditions. Preliminary viability tests with hepatic progenitor cells embedded in monophasic PEO-based hydrogels showed rapid mortality of the cells, whereas encouraging viability was observed in PEO-based triblock copolymer/gelatin macroporous hydrogels. The latter has the potential to be used in cell therapy.

Unimer Exchange Is not Necessary for Morphological Transitions in Polymerization-Induced Self-Assembly.

Polymerization-induced self-assembly (PISA) has established itself as a powerful and straightforward method to produce polymeric nano-objects of various morphologies in (aqueous) solution. Generally, spheres are formed in the early stages of polymerization that may evolve to higher order morphologies (worms or vesicles), as the solvophobic block grows during polymerization. Hitherto, the mechanisms involved in these morphological transitions during PISA are still not well understood. Combining a systematic study of a representative PISA system with rheological measurements, we demonstrate that-unexpectedly-unimer exchange is not necessary to form higher order morphologies during radical RAFT-mediated PISA. Instead, in the investigated aqueous PISA, the monomer present in the polymerization medium is responsible for the morphological transitions, even though it slows down unimer exchange.

Modification of grass pea protein isolate (Lathyrus sativus L.) using high intensity ultrasound treatment: Structure and functional properties.

In this study the effect of different high intensity ultrasound (HIU) amplitudes (25, 50 and 75%) and sonication times (5, 10 and 20 min) on the structure and functional properties of grass pea protein isolate (GPPI) was investigated. A higher sonication amplitude and longer time improved the protein solubility and surface hydrophobicity and reduced the particle size of GPPI. These physicochemical alterations in GPPI enhanced the protein adsorption at the oil-water interface, reduced the interfacial tension and increased the EAI and ESI values. SDS-page demonstrated that sonication did not change the primary structure of the protein. However, CD spectroscopy indicated a reduction in α-helix and an increase in the content of ß-sheet and random coil structures in the sonicated GPPI. The free SH groups content and UV-vis absorbance intensity increased after the sonication. However, prolonged sonication up to 20 min reduced the free SH content in GPPI due to the oxidation of susceptible SH groups. HIU increased the thermal degradation of GPPI and lowered the least concentration needed for gelatinization of GPPI (LGC). Therefore, less protein powder was needed to form a strong gel compared to the non-sonicated GPPI. Sonicated GPPIs showed higher gel strength especially when 75% amplitude used for 10 min. These results showed that the HIU is a promising approach for modification of the functional properties of GPPI for food applications.

Self-Assembly in water of C60 fullerene into isotropic nanoparticles or nanoplatelets mediated by a cationic amphiphilic polymer.

HYPOTHESIS: To disperse high concentration of C60 fullerene in water, we propose to use an emulsification-evaporation process in the presence of an amphiphilic polymer whose chemical structure has been chosen for inducing specific interaction with fullerene The viscosity enhancement provided by self-assembly of the amphiphilic polymers in water should result in high stability of the suspensions. The organic solvent has also to been chosen so as to maximize the initial fullerene concentration. EXPERIMENTS: The concentrations of polymer and fullerene, the solvent type and the volume fraction of the organic phase have been varied. Their influence on the concentration of the fullerene dispersions and on the size and shape of the resulting nanoparticles have been investigated by UV-Visible spectroscopy, light scattering and cryo-transmission electron microscopy experiments. FINDINGS: The resulting nanoparticles consist of aggregates of C60 fullerene stabilized by the cationic polymer with morphologies/sizes tunable through fullerene and polymer concentration. At high fullerene concentration, nanoplatelets are obtained that consist in thin 2D nanocrystals. Their suspensions are very stable with time due to the viscosity of the dispersing aqueous medium. The concentration of fullerene nanoparticles dispersed in water is as high as 8 g/L which corresponds to an upper limit that has never been reached so far.

Exploiting multiple phase separation to stabilize water in water emulsions and form stable microcapsules.

HYPOTHESIS: Water in water (W/W) emulsions are formed by mixing aqueous solutions of incompatible polymers. It is possible to add a third polymer solution that forms at the right conditions a phase that completely covers the dispersed droplets as a thin layer. Our hypothesis is that by gelling the third phase, W/W emulsions can be stabilized and that microcapsules can be formed that are stable against dilution. EXPERIMENTS: W/W emulsions were formed by mixing aqueous solutions of poly(ethylene oxide) (PEO) and dextran. Gelatin was added to form the third phase, gelation of which was induced by cooling. The morphology was observed by microscopy, and the rheological properties were investigated. FINDINGS: The compatibility of gelatin and PEO can be fine-tuned by the pH such that a continuous layer of the gelatin phase forms around the droplets of the dextran phase, with a thickness that can be varied. After cooling, the gelatin layer forms a gel and provides stabilization against coalescence. The gelatin microcapsule was found to be stable to dilution. The generality of the method was demonstrated by applying it to another, fully food-grade, W/W emulsion formed by mixing amylopectin and xyloglucan.

Thermo-induced inversion of water-in-water emulsion stability by bis-hydrophilic microgels.

HYPOTHESIS: Stabilization of water-in-water (W/W) emulsions resulting from the separation of polymeric phases such as dextran (DEX) and poly(ethyleneoxide) (PEO) is highly challenging, because of the very low interfacial tensions between the two phases and because of the interface thickness extending over several nanometers. In the present work, we present a new type of stabilizers, based on bis-hydrophilic, thermoresponsive microgels, incorporating in the same structure poly(N-isopropylacrylamide) (pNIPAM) chains having an affinity for the PEO phase and dextran moieties. We hypothesize that these particles allow better control of the stability of the W/W emulsions. EXPERIMENTS: The microgels were synthesized by copolymerizing the NIPAM monomer with a multifunctional methacrylated dextran. They were characterized by dynamic light scattering, zeta potential measurements and nuclear magnetic resonance as a function of temperature. Microgels with different compositions were tested as stabilizers of droplets of the PEO phase dispersed in the DEX phase (P/D) or vice-versa (D/P), at different concentrations and temperatures. FINDINGS: Only microgels with the highest DEX content revealed excellent stabilizing properties for the emulsions by adsorbing at the droplet surface, thus demonstrating the fundamental role of bis-hydrophilicity. At room temperature, both pNIPAM and DEX chains were swollen by water and stabilized better D/P emulsions. However, above the volume phase transition temperature (VPTT ≈ 32 °C) of pNIPAM the microgels shrunk and stabilized better P/D emulsions. At all temperatures, excess microgels partitioned more to the PEO phase. The change in structure and interparticle interaction induced by heating can be exploited to control the W/W emulsion stability.

Utilization of xanthan to stabilize water in water emulsions and modulate their viscosity.

Water in water emulsions were prepared by mixing aqueous solutions of dextran and poly(ethylene oxide) at three volume fractions. The xanthan was added to the emulsions up to 0.5 wt%. The stability of the emulsions was probed by measuring the time dependence of the transmission profiles at different centrifugal forces. At lower concentrations, xanthan partitioned to the dextran phase and strong shear-thinning was observed at higher concentrations. At lower concentrations, destabilization was caused by a combination of coalescence and creaming or sedimentation. Above 0.1 wt%, xanthan strongly increased the viscosity of the emulsions and stabilized them under gravity for at least one week. The time evolution of the emulsion microstructure was observed using confocal scanning laser microscopy. The effect of shear on the microstructure was investigated using a specific rheo-optical device. It showed the formation of thin strands that broke up into small drops after stopping the flow.

Effect of hydrophobicity and molar mass on the capacity of chitosan and κ-carrageenan to stabilize water in water emulsions.

A range of commercial chitosan samples with different molar masses and degrees of acetylation was tested for their capacity to stabilize water in water (W/W) emulsions formed by mixing aqueous solutions of dextran and poly (ethylene oxide). To further understand the effect of the acetylation degree, commercial samples were acetylated and deacetylated to different degrees. The effect of pH and chitosan concentration on the stability was investigated. The lowest investigated degree of acetylation (6%) was sufficient to inhibit coalescence, but higher degrees that were studied (up to 50%) led to faster stabilization resulting in smaller stable dispersed droplets that did not sediment for at least one week. The effect of hydrophobic acetyl units on the stability was confirmed for κ-carrageenan that could stabilize the W/W emulsion only after acetylation. For chitosan it was shown that the molar mass should be above a critical value independent of the degree of acetylation.

Effect of adding a third polysaccharide on the adsorption of protein microgels at the interface of polysaccharide-based water in water emulsions.

HYPOTHESIS: Water-in-water (W/W) emulsions are formed by mixing aqueous solutions of incompatible polymers and can in some cases be stabilized by addition of particles. The adsorption of particles at the interface of W/W emulsions is dictated by the interfacial tension between the two aqueous phases and between the particles and each phase. It should therefore be possible to induce and fine-tune adsorption by adding small amounts of a third polysaccharide that is compatible with one or both phases. EXPERIMENTS: W/W emulsions were formed by mixing aqueous solutions of pullulan (PUL), amylopectin (AMP), and protein microgels (MG). The microstructure and positioning of the MG were monitored using confocal laser scanning microscopy. The effect of adding small amounts of other types of polysaccharides on the adsorption of the MG at the interface and their partitioning between the phases was studied. FINDINGS: The addition of all polysaccharides led to a progressive shift of the MG from the PUL phase to the AMP phase and to adsorption of the MG at the interface when the partition was not extreme. The partition could be fine-tuned to be equal, in which case particles adsorbed at the interface even very close to the binodal. The findings were confirmed for another type of emulsion and particle.

Effect of the Interfacial Tension on Droplet Association in Aqueous Multiphase Systems.

Aqueous multiphase systems (AMPS) were formed by mixtures of three or more incompatible water-soluble macromolecules. Droplets formed by different phases in the water-in-water emulsions were found to associate and their morphology was studied using confocal laser scanning microscopy. By analyzing the angles between different associated phases it was possible to determine the relative interfacial tensions between phases with respect to each other. In this manner, the relative interfacial tension of 15 different pairs of polymers solutions was determined. The effect of the total polymer concentration on the relative interfacial tensions was found to be small as long as mixing of the polymers in the phases was small. The effect of adding protein microgels was studied for systems where they adsorb at the interface between the phases. It is shown that protein microgels can in some cases stabilize associated droplets in suspension.

Mixed iota and kappa carrageenan gels in the presence of both calcium and potassium ions.

The effect was studied of adding both KCl and CaCl2 on gelation of solutions of ι-carrageenan, κ-carrageenan and mixtures of both types. The gel temperature (Tg) of ι-car was found to be determined by the CaCl2 concentration and Tg of κ-car by the KCl concentration. At a given salt concentration, ι-car was stiffest with pure CaCl2, but κ-car gels and mixed carrageenan gels were stiffer when both KCl and CaCl2 were present. Gelation of κ-car increased the turbidity of mixed carrageenan gels in the presence of KCl or CaCl2, but when both salts were present it led to a drop of the turbidity. In mixed salt, K+ induces formation of a homogeneous κ-car network that causes the mixed network to become more homogeneous. Rheological and structural properties of carrageenan gels can be tuned for a given polymer and salt concentration by adding both KCl and CaCl2 to κ-car/ι-car mixtures.

Gelation of food protein-protein mixtures.

Gelation of proteins is one of the principal means to give desirable texture to food products. Gelation of individual proteins in aqueous solution has been investigated intensively in the past, but in most food products the system contains mixtures of different types of proteins. Therefore one needs to consider interaction between different proteins both before and during gelation. Most food proteins can be classified as globular proteins, but casein and gelatin are also important food proteins. In this review the focus is on gelation induced by heating or cooling, which is the most commonly used method. After briefly discussing general features of protein aggregation and gelation, the literature on gelation of mixtures of different types of globular proteins is reviewed as well as that of mixtures of globular proteins with gelatin or with casein. The effect on the gel stiffness and the microstructure of the gelled mixtures will be discussed in terms of different scenarios that can be envisaged: independent aggregation and gelation, co-aggregation and phase separation.

Stabilization of Water-In-Water Emulsions by Linear Homo-Polyelectrolytes.

The effect of adding a small quantity of linear polymers on the stability of water-in-water (W/W) emulsions was studied for emulsions of dextran-rich droplets in a continuous poly(ethylene oxide) (PEO) phase (D/P) and vice versa (P/D). It was found that out of 16 different polymers that were tested, three had a significant effect: chitosan (Chit), diethyl aminoethyl dextran (DEAED), and propylene glycol alginate (PGA). In the presence of Chit or PGA, P/D emulsions were much less stable than D/P emulsions, but DEAED stabilized both types of emulsion. Interactions of these polymers with PEO or dextran were investigated with light scattering, and the microstructure of the emulsions was studied with confocal laser scanning microscopy. The effect of pH, polymer concentration, interfacial tension, and ionic strength on the stability was investigated and was found to be different for the three polymer types. The results suggest that stabilization of W/W emulsions by linear polymers requires that they contain both charged and hydrophobic units.

Viscosity of mixtures of protein aggregates with different sizes and morphologies.

Protein aggregates were generated by thermal denaturation of whey protein isolates. Depending on the heating conditions, fractal aggregates of various sizes or microgels were obtained. The osmotic compressibility and correlation length of mixtures of fractal aggregates of different sizes were found to be close to the weighted averages of the individual components at the same concentration. The viscosity of these mixtures can be described by a logarithmic mixing law using the weight fraction and the viscosity of the individual components. The same mixing law describes the behavior of mixtures of fractal aggregates and microgels. The effect of the type of protein was investigated by mixing fractal aggregates formed by whey and soy protein isolates. It is suggested that the viscosity of the mixtures is determined by the cooperative movement over length scales much larger than the size of the aggregates.