Microgel Dispersions Research Articles

Proposing moderate unfolding of interfacial proteins to enhance the techno-functional features of complex microgel particles formed by whey protein-chitooligosaccharide conjugation

To elucidate the roles of interfacial protein microstructure in regulating techno-functional attributes of complex microgel, this work explored the effects of transglutaminase cross-linking alone or in combination with ultrasonication on conformation and functional properties of whey protein isolate-chitooligosaccharide microgel dispersion (WPI–COS). It was found that transglutaminase-crosslinked WPI-COS microgels presented improved interfacial properties when a moderate unfolding process was induced by sonication, validating the existence of optimum flexibility of complex Pickering particles during unfolding/refolding of interfacial proteins. Particularly, TG induced excessive crosslinking of WPI-COS to form aggregates, causing a lower surface hydrophobicity. In contrast, ultrasound was inclined to increase the surface hydrophobicity for TG-catalyzed and uncatalyzed samples, suggesting differential unfolding degrees of WPI induced by transglutaminase and ultrasonication. Correspondingly, interfacial properties of ultrasonicated WPI-COS complexes, as revealed by percentage of adsorbed protein, interfacial tension and three-phase contact angle and emulsifying properties, were improved significantly. Confocal laser scanning microscopy and transmission electron microscopy consistently observed WPI-COS tightly adsorbed at the oil-water interface, with smaller emulsion droplets, more uniform distribution and thicker interfacial films at the oil-water interface in WPI-COS microgel particles with sonication-induced unfolding. This investigation demonstrated the feasibility of conformational regulation of interfacial proteins in tuning techno-functional features of microgel particles.

Dynamic properties of the layers of cupin-1.1 aggregates at the air/water interface

Spread layers of amorphous aggregates of the structural domain of plant protein vicilin, cupin-1.1, at the water – air interface were studied by the surface tensiometry, dilational surface rheology, Brewster angle and atomic force microscopy. The layer properties differed strongly from the results for the layers of previously studied proteins. The dependency of the dynamic elasticity of the layer on surface pressure had two local maxima with the second peak being four times higher than the first one. In the region of the first maximum the obtained results are similar to those for dispersions of polymer microgels with a hairy corona. At the beginning of surface compression separate threads of the corona are stretched along the surface and the surface elasticity increases. The further compression results in the formation of loops and tails leading to a decrease of the elasticity. The second local maximum of the dynamic surface elasticity is presumably caused by the interactions of the rigid cores of the aggregates leading finally to the formation of multilayer structures at high surface pressures. In this case, the surface elasticity starts to decrease as a result of the segment exchange between different layers at the interface.

A facile method to determine the molar mass of soft nanoparticles

The determination of the molar mass of soft nanoparticles is essential to estimate their (molar) concentration in dispersions. Measuring this quantity by conventional methods, however, often proves challenging. We describe a facile approach to determine the molar mass of (soft) nanoparticles via counting their number per volume using a widefield fluorescence microscope. The method is exemplified on a microgel dispersion as a model system, while it is applicable to other types of stainable nanoparticles. For this, covalent labeling or modification of the nanoparticles is not required. The dispersion is simply mixed with a Nile Red solution in a defined ratio and measured in an optical fluorescence microscope accessible to most researchers in the field.

Temperature-Responsive Pickering Double Emulsions Stabilized by Binary Microgels.

Microgels are excellent emulsifiers that can self-assemble to reduce interfacial tension and form a steric barrier at an oil-water interface. Herein, we report a two-step emulsification approach to prepare oil-in-water-in-oil (O/W/O) Pickering double emulsions through the dispersion of microgels in two immiscible phases. The stabilization mechanism depends on the uneven distribution and adsorption of hydrophilic water-swollen microgels and hydrophobic octanol-swollen microgels on either outer water droplets or inner oil droplets. Our results reveal that binary microgels outperformed single microgels in terms of interfacial tension reduction and emulsion stabilization. Notably, the binary microgel-stabilized Pickering double emulsions show excellent temperature responsiveness owing to the intrinsic thermal sensitivity of microgels. Consequently, the selective and rapid release of encapsulated substances in different phases can be achieved through the adjustment of the ambient temperature.

Physicochemical investigations on the interaction of an anionic surfactant with cellulose based polymer microgel

Abstract This work focuses on the interaction of cellulose-based colloidal microgel dispersions with the anionic surfactant sodium dodecyl sulfate (SDS) at different concentrations. First, a Cellulose-P(NIPAAm-MAA)-based responsive microgel sample was prepared by a radical polymerization method using a one-pot process. The samples thus obtained were purified, characterized and used to study microgel-surfactant interactions. To this end, SDS solutions with different concentrations were prepared and the interactions of SDS in the premicellar and micelle regions with polymer microgel were investigated using different physicochemical techniques. Experiments were also performed at different temperatures to obtain the degree of binding of SDS to microgel in gel surfactant mixtures. Due to the temperature-sensitive properties of microgel, we observed significant changes in various properties of microgel-surfactant mixtures when the solution temperature changed. The overall results showed that various experimental variables, such as the polymer gel/surfactant ratio and temperature, affect the gel–surfactant interaction quantitatively and qualitatively. With increasing surfactant concentration, conductance, viscosity and absorbance values ​​increased, but a decrease in surface tension was observed. This is due to the adsorption of SDS at the air-water interface. The micellar activity of surfactants is enhanced by the addition of microgels. It was found that the strength and nature of the interaction depended not only on the gel/surfactant ratio, solvent type and test temperature, but also on the chemical structure of the surfactant used. Furthermore, the visual stability of the colloidal particles in the mixture of microgel and SDS was also noticeable over time.

Inline Particle Size Analysis during Technical-Scale Processing of a Fermented Concentrated Milk Protein-Based Microgel Dispersion: Feasibility as a Process Control

Particle size is not only important for the sensory perception of fat-free fermented concentrated milk products, but also for processing operations because of the direct relationship with apparent viscosity. The aim of this study was to apply inline particle size analysis using focused beam reflectance measurement (FBRM) to obtain real-time information regarding the particle size of a fat-free fermented concentrated milk product, namely, fresh cheese. By comparing inline particle size data to offline particle size, apparent viscosity, protein content and processing information, the potential to use inline particle size analysis as a process monitoring and control option during fresh cheese production was assessed. Evaluation of inline particle size after fermentation and before further processing, e.g., after a buffering tank, shows promise as a means to control variance of product entering downstream processing and, thus, improve final product consistency over time. Measurement of inline particle size directly before filling could allow for precise control of final product characteristics by the use of mechanical or mixing devices placed before the inline measurement. However, attention should be given to the requirements of the inline measurement technology for accurate measurement, such as product flow rate and pressure.

Designing ultra-stable linseed oil-in-water Mickering emulsions using whey protein isolate cold-set microgels containing marjoram aqueous extract: Effect of pH and extract on rheological, physical, and chemical properties

In this study, whey protein isolate (WPI) cold-set microgels containing marjoram (Origanum majorana) aqueous extract (MAE) were prepared at different pHs (4.0, 5.0, and 6.0). After characterization, the microgel dispersion was used to stabilize linseed oil-in-water Mickering emulsions (MEs). The resultant MEs were then characterized in terms of physicochemical and rheological properties under the effect of pH and MAE addition. The morphology, particle size, zeta potential, and interfacial tension of microgels were affected by pH and MAE. XRD patterns showed the amorphous structure. Microgel-stabilized MEs did not reveal any significant sign of instability under gravity during 6 months of storage. All MEs had dominant elastic character. Despite the lowest zeta potential values, MEs prepared at pH 4 showed the highest physical stability against gravity but the lowest centrifugal stability against oiling off, which indicated that both viscous and elastic components are required for MEs stability. This sample had the highest apparent viscosity and the strongest viscoelastic properties. Rheological data were best fitted with Herschel-Bulkley and Power Law models. An increase in pH and presence of MAE improved the oxidative stability of MEs. The results of this study showed that WPI microgels are appropriate candidate for long-term stabilization of linseed oil-in-water MEs. The presence of MAE is useful in designing special emulsions in which the aqueous phase is partially replaced by the aqueous extract of medicinal plants.

Concentration and temperature dependent interactions and state diagram of dispersions of copolymer microgels.

We investigate by means of small angle neutron scattering experiments and numerical simulations the interactions and inter-particle arrangements of concentrated dispersions of copolymer poly(N-isopropylacrylamide)-poly(ethylene glycol methyl ether methacrylate) (PNIPAM-PEGMA) microgels across the volume phase transition (VPT). The scattering data of moderately concentrated dispersions are accurately modeled at all temperatures by using a star polymer form factor and static structure factors calculated from the effective potential obtained from simulations. Interestingly, for temperatures below the VPT temperature (VPTT), the radius of gyration and blob size of the particles significantly decrease with increasing the effective packing fraction in the non-overlapping regime. This is attributed to the presence of charges in the system associated with the use of an ionic initiator in the synthesis. Simulations using the experimentally corroborated interaction potential are used to explore the state diagram in a wide range of effective packing fractions. Below and slightly above the VPTT, the system undergoes an arrest transition mainly driven by the soft repulsion between the particles. Only well above the VPTT the system is found to phase separate before arresting. Our results highlight the versatility and potential of copolymer PNIPAM-PEGMA microgels to explore different kinds of arrested states balancing attraction and repulsion by changing temperature and packing fraction.

Fabrication of Au-polymer hybrid colloids via a pH-modulated in situ reduction process for improved catalytic activity

Here, we reported a novel strategy for the controllable synthesis of Au nanoparticles within functional microgels. By simply mixing Au(Cl)4- ions with a microgel dispersion at room temperature for several hours, Au(III) ions were reduced into Au(0) nanoparticles on the surface of the microgels. Without the use of any additional reductant, the reduction of the Au(III) ions was realized and controlled by tuning the volume of the base solution as a result of the unique reductive 3-carbonyl-N-vinylcaprolactam structure inside the microgels. Moreover, the hybrid microgels showed efficient catalytic activities for the model reduction reaction of 4-nitrophenol (Nip). These results revealed that the synthesis strategy of fabricating Au-polymer hybrids possesses great potential in the field of wastewater treatment.

Synthesis of Magneto-Controllable Polymer Nanocarrier Based on Poly(N-isopropylacrylamide-co-acrylic Acid) for Doxorubicin Immobilization.

In this work, the preparation procedure and properties of anionic magnetic microgels loaded with antitumor drug doxorubicin are described. The functional microgels were produced via the in situ formation of iron nanoparticles in an aqueous dispersion of polymer microgels based on poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-PAA). The composition and morphology of the resulting composite microgels were studied by means of X-ray diffraction, Mössbauer spectroscopy, IR spectroscopy, scanning electron microscopy, atomic-force microscopy, laser microelectrophoresis, and static and dynamic light scattering. The forming nanoparticles were found to be β-FeO(OH). In physiological pH and ionic strength, the obtained composite microgels were shown to possess high colloid stability. The average size of the composites was 200 nm, while the zeta-potential was -27.5 mV. An optical tweezers study has demonstrated the possibility of manipulation with microgel using external magnetic fields. Loading of the composite microgel with doxorubicin did not lead to any change in particle size and colloidal stability. Magnetic-driven interaction of the drug-loaded microgel with model cell membranes was demonstrated by fluorescence microscopy. The described magnetic microgels demonstrate the potential for the controlled delivery of biologically active substances.

Catalytic degradation of organic dyes using Au-poly(styrene@N-isopropylmethacrylamide) hybrid microgels

Poly([email protected]N-isopropylmethacrylamide) [P([email protected]NPM)] microgel particles were produced by seed facilitated free radical precipitation polymerization technique and laterly gold (Au) nanoparticles (NPs) were integrated into the crosslinked polymeric network of shell of P([email protected]) core shell microgel particles to form gold-P([email protected]) hybrid microgel system by reduction of HAuCl4.3H2O within microgel dispersion. The synthesized microgel and hybrid microgel particles were characterized with FTIR, UV–Vis., DLS, TEM, and XRD methods. Then gold-P([email protected]) system was used as catalyst for reduction/degradation of rhodamine-B (RhB), congo red (CoR), methyl orange (MeO), eriochrome black T (ErBT), methylene blue (MeB), methyl red (MeR), and eosin (ES) dyes in the presence of NaBH4 at ambient temperature. Their reduction/degradation kinetics was also studied. Au-P([email protected]) was also applied as catalyst to reduce/degrade the wastewater sample containing various dyes to study the real application of the catalytic system. The Au-P([email protected]) is recyclable without any substantial loss of activity.

Crystallization kinetics of charged PNIPAM microgels dispersions at low volume fractions

This work studies the kinetics of crystallization of charged microgels suspensions of Poly-N-Isopropylacrylamide (PNIPAM) at low ionic strength. The liquid-crystal transition is induced by suddenly decreasing the temperature of the microgel dispersion, and the crystallization process is monitored by measuring the temporal evolution of the static structure factor of the dispersion using light scattering. We find that the crystal growth rate, indicated by the temporal evolution of the crystallinity factor, strongly depends on the quenching temperature.

Magnetic Field-Assisted Fast Assembly of Microgel Colloidal Crystals.

Compared with the colloidal crystals (CCs) of hard spheres, large-scale, high-quality CCs of soft microgel spheres are easier to be assembled because they are more tolerant to defects. However, to assemble microgel CCs, a microgel dispersion should first be concentrated and then allowed to crystallize, which is tedious and time-consuming. Herein, we demonstrated that a magnetic poly(N-isopropylacrylamide) (PNIPAM) microgel with an Fe3O4 core and a PNIPAM shell can be assembled into CCs quickly by simply applying an external magnetic field to the diluted microgel dispersions. The resulting CCs are highly ordered as revealed by their iridescent color, laser diffraction pattern, and confocal characterization. They display a sharp Bragg peak on their reflection spectra, which shifts to lower wavelength when heated because of the thermosensitivity of the PNIPAM shell. The magnetic assembly is not only simple and fast but also allows control of the CC structure in both horizontal and vertical directions. Using spatially varying magnetic fields, patterned microgel CCs were facilely assembled. More importantly, magnetic microgel spheres with different sizes can be assembled in a layer-by-layer manner by adding them sequentially, and the thickness of each layer can be simply controlled by the amount of spheres added.

Comparison of oral tribological performance of proteinaceous microgel systems with protein-polysaccharide combinations

Polysaccharides are often used as rheology modifiers in multiphasic protein-rich food systems. Recently, proteinaceous microgels have garnered research attention as promising lubricating agents. However, whether proteinaceous microgels can be used to replace polysaccharides in a tribological context remains poorly understood. In this study we compared the flow and oral-tribological behaviour of Newtonian solutions of the polysaccharide dextran (D, 1–11 wt%) when combined with a dispersion of whey protein isolate (W, 1–13 wt%) or whey protein microgels (WPM, 41.7 vol%) and compared with microgels of D conjugated to W (Conj(D[11] + W[5])MG) or dispersions of WPM in W solutions. W and WPM alleviated frictional forces between elastomeric surfaces as well as biomimetic tongue-like surfaces in the boundary lubrication regime. Despite the negligible influence of D on the thin-film lubricity, its impact on viscous-facilitated lubricity was significant. The importance of measurements with the tongue-mimicked setup emerged where Conj(D[11] + W[5])MG did not show significant lubricity enhancement despite its outstanding performance with conventional tribo-testing setups. By optimising a combination of WPM and non-microgelled W, we demonstrate that a combined viscous and thin-film lubricity could be achieved through a single-protein-component without the need of polysaccharides. The dispersions of WPM (41.7 vol%) deliver the same flow and viscous-friction behaviour to that of 5 wt% D and excel in thin-film lubricity. These findings pave the way towards design of processed foods with clean labels, taking advantage of using a single proteinaceous moiety whilst delivering enhanced lubricity and viscosity modification without the need of any additional thickener.

Selective penetration behavior of microgels in superpermeable channels and reservoir matrices

Gel treatment is an effective way to attack excessive water production in many mature oilfields around the world. Selective penetration is desired for successful gel treatments. That is, gel materials should easily penetrate the target zones (i.e., channeling features such as superpermeable channels) without entering/damaging the nontarget zones (i.e., reservoir matrices or oil zones). This study revealed that presence of threshold penetration pressure (ΔPth) was responsible for selective penetration behavior of tested microgels. The concept of ΔPth was utilized to figure out favorable working conditions for effective gel treatments. Microgel dispersions were injected into superpermeable (super-k) sandpacks (mimicking super-k channels in reservoirs, 60–221 darcies), heterogeneous models with super-k channels (79–230 darcies), and sandstone cores (mimicking reservoir matrices, 50–5000 md). The results demonstrated that a minimum differential driving pressure (i.e., threshold penetration pressure, ΔPth) was required to push microgel particles to penetrate channels or matrices. The critical penetration behavior was closely related to the particle/pore size ratio. Low ΔPth at smaller particle/pore ratios was beneficial to allow easy penetration of gel materials into the channeling zones. On the contrary, high ΔPth at larger particle/pore ratios was desirable to prevent gel materials from massively invading and damaging the matrices. Instead, the gel particles accumulated at the inlet surface, and a gel cake was gradually formed. The cake further prevented the invasion of the gels. The cake could be removed by chemical breakers to resume the injectivity/productivity of the matrices. Correlations were developed to describe the relationship between ΔPth and particle/pore ratio. A distinct transition was identified at the particle/pore ratio of about 3. This work could help identify the favorable conditions to achieve successful gel treatments. In an effective conformance treatment, the particle/pore ratio in the channel should be sufficiently low to allow easy penetration of gel materials into the channel (e.g., particle/pore ratio<2 in this study). Meanwhile, the particle/pore ratio in the matrix should be large enough to support a high ΔPth and thus prevent massive gel invasion into the matrix. This study advances the current pore scale studies (a single particle passing through a single channel) to Darcy-scale characterization.

Tailoring the Textural Characteristics of Fat-Free Fermented Concentrated Milk-Protein Based Microgel Dispersions by Way of Upstream, Downstream and Post-Production Thermal Inputs.

There is a growing demand for new strategies to tailor the texture of fat-free fermented concentrated milk products, also referred to as milk protein-based (MPb) microgel dispersions. Methods should be easy to incorporate into the production scheme, offer labelling without added components and be cost-efficient. Thermal treatments are traditionally used upstream (milk heating) and downstream (pre-concentration heating) in the production of these dispersions, though there is little knowledge as to the effects that combinations of different thermal input levels have on final texture. Therefore, this study investigated combinations of thermal input at different intensities and steps in the production scheme at the pilot scale and the relationships with texture. We demonstrated that increasing the intensity of upstream milk heat treatment, in combination with downstream pre-concentration heating, increases gel firmness and apparent viscosity. Downstream pre-concentration heating produces final fat-free fermented concentrated MPb microgel particles that are resistant to post-heating aggregation. On the other hand, omission of downstream pre-concentration heating results in smaller particles that are sensitive to post-heating aggregation. Furthermore, gel firmness and apparent viscosity increase with post-heating. Consequently, combining different levels of thermal inputs upstream, downstream (pre-concentration) and post-production, can produce fat-free fermented concentrated MPb microgel dispersions with a range of different textures.

Viscosity of macromolecules with complex architecture

It is well-known that the architecture of macromolecules plays an important role in the hydrodynamics and viscosity of its semi-diluted solutions. However, the systematic study of the rheology of macromolecules with complex topology, such as microgels, remains a difficult task. In this work, we use the computer simulations methods of non-equilibrium molecular dynamics to study the viscous properties of randomly cross-linked microgels and compare it to multiple reference systems (linear chains, 10-arm star polymers and hard spheres). We show that the microgel cross-linking density and, thus, the particle shape, plays crucial role in its shear viscosity. Also, in contrast to a simpler polymer systems, microgel dispersions in good solvent show much less shear thinning.

β-lactoglobulin microgel layers at the surface of aqueous solutions

The spread and adsorbed layers of β-lactoglobulin (BLG) microgel were investigated by the dilational surface rheology, atomic force microscopy and optical microscopy. It is shown that only careful purification of the BLG microgel dispersion gives a possibility to observe distinctions between the properties of spread layers of BLG nano- and micro particles on the one hand, and the properties the layers of BLG molecules on the other hand. The BLG particles do not have a thick and soft corona like poly(N-isopropylacryl) microgel. The fluctuations of the surface pressure of the BLG microgel layers at high surface compressions are a consequence of a special collapse mechanism, which has not observed earlier for the layers of both soft and rigid nano- and microparticles.

How the interplay of molecular and colloidal scales controls drying of microgel dispersions

Bringing an aqueous dispersion or solution into open air leads to water evaporation. The resulting drying process initiates the buildup of spatial heterogeneities, as nonvolatile solutes and colloids concentrate. Such composition gradients associate with mesostructure gradients, which, in turn, impact flows within these multicomponent systems. In this work, we investigate the drying of microgel dispersions in respect to two reference systems, a colloidal dispersion and a polymer solution, which, respectively, involve colloidal and molecular length scales. We evidence an intermediate behavior in which a film forms at the air/liquid interface and is clearly separated from bulk by a sharp drying front. However, complex composition and mesostructure gradients develop throughout the drying film, as evidenced by Raman and small-angle X-ray scattering mapping. We show that this results from the soft colloidal structure of microgel, which allows them to interpenetrate, deform, and deswell. As a result, water activity and water transport are drastically decreased in the vicinity of the air/liquid interface. This notably leads to diffusional drying kinetics that are nearly independent on the air relative humidity. The interplay between water fraction, water activity, and mesostructure on water transport is generic and, thus, shown to be pivotal in order to master evaporation in drying complex fluids.

Fat-free fermented concentrated milk protein-based microgel dispersions manufactured at technical scale: Production parameters as drivers of textural properties

Fat-free fermented concentrated milk products, recently classified as dispersions of (fat-free) fermented concentrated milk protein-based microgel particles, are basically produced by upstream, fermentation, and downstream processing. Development of production parameters aimed at, e.g., increasing yield, shelf-stability, and reducing acid whey, has led to a variety of possible parameters at each production stage. Though the influences of some individual parameters on the final textural properties are well understood, combinations of parameters and outcomes in large technical scale production environments are not well documented. Production parameters of six fat-free fermented concentrated milk products from technical scale production were reviewed and parallels were drawn between chemical composition, serum release, particle size, and rheological properties. In order of importance, the five parameters of downstream standardisation, pre-heating, concentration method, addition of rennet during fermentation and fermentation temperature/time were identified as main textural drivers, by initiating changes of microgel particle voluminosity, surface properties, size, and composition.