Microgel Particles at Interfaces: Phenomena, Principles, and

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Invited Feature Article

Microgel Particles at Interfaces: Phenomena, Principles and Opportunities in Food Sciences Man-hin Kwok, Guanqing Sun, and To Ngai Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04009 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 12, 2019

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Microgel Particles at Interfaces: Phenomena, Principles and Opportunities in Food Sciences Man-hin Kwok,1 Guanqing Sun,2* and To Ngai1,2* 1. Department of Chemistry, The Chinese University of Hong Kong, Shatin, NT, Hong Kong. 2. School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China.

Abstract: The use of soft microgel particles for stabilizing emulsions has captured increasing attention across a wide range of disciplines in the past decades. Being soft, the nanoparticles, which are spherical in solution, undergo a structure change when adsorbed at the oil-water interface. This morphology change leads to the special dynamic properties of interface layers and packing structures, which then alters the interfacial tension and rheological properties of the interface. In addition, emulsions stabilized by these particles, or known as Pickering emulsions can be triggered by changing a variety of environmental conditions which is especially desirable in industrial applications such as oil transportation process and biphasic catalysis, where the emulsions can be stabilized and destabilized on demand. Although many studies about the behaviours of soft microgel nanoparticles at interfaces have been reported, there are still many challenges in gaining full understanding of the structure, dynamics, and effective interactions between microgels at the interface. In this Feature Article, we would like to address some of the most important findings and problems in the field. They include the adsorption kinetics of soft microgel particle, particle conformation at the interface, pH and thermal responsiveness as well as the interfacial rheological properties of soft particle occupied interfaces. We also discuss some potential benefits of using emulsions stabilized by soft particles for food applications, as an alternative to conventional

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surfactant-based systems. We hope to encourage further investigation on these problems, which would be very beneficial to extend the knowledge to all other related soft matter systems.

Keywords:

Soft particles, Microgels, Particle at interface, Pickering emulsions, PNIPAM microgels

* To whom correspondence should be addressed. E-mail: [email protected] & [email protected].


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Introduction An emulsion is a heterogeneous mixture of two or more immiscible liquids. Intrinsically, the product of such mixing is thermodynamically unstable because of the increase in interfacial energy. Without a proper stabilizer, emulsions often undergo complete phase separation in the matter of seconds. Our everyday life is closely related to emulsions. Different emulsion systems can be easily found in food industry,1-4 pharmaceutical,5-9 cosmetics,5,

7, 10-13

petroleum industry,14-21

etc.22-23 Emulsion consists of a dispersed liquid in another immiscible liquid as a continuous phase.24 In order to stabilize emulsions, surfactants are often used to minimize the surface tension and provide extra stabilization to the emulsion droplets.25 Conventional surfactants with a hydrophilic head group and a hydrophobic tail can lower the interfacial energy at the oil/water interface and provide steric or electrostatic stabilization and thus emulsions can be kinetically stabilized. Although surfactants are effective, they are not ideal in personal care and pharmaceutical applications where surfactants often cause adverse effects such as irritancy and hemolytic behavior.26-30 Emulsion stabilizers are not limited only to small amphiphilic molecules. Pioneered by Ramsden and Pickering more than a century ago, colloidal particles are also interfacial active and capable of stabilizing emulsion droplets by providing a physical barrier against coalescence.31-32 These emulsions, which are stabilized by solid particles, are known as Pickering emulsions.33-34 Since then, it opened a new domain in the emulsion and interfacial science. Before the development of various kinds of polymeric materials, the studies of Pickering emulsions were limited mainly to inorganic particles.35-39 Metal oxides and hydroxides were described as “insoluble emulsifiers”.31 Other than the metal salts, clay particles40-43 and silica particles44-45 are also famous examples of inorganic Pickering stabilizers. Polymeric colloidal particles made of 3 ACS Paragon Plus Environment

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synthetic polymers have been intensively studied in the past decades because controlled syntheses of them are readily available. The most well-known examples of such polymeric particles include polystyrene (PS) latex46-48 and polymethylmethacrylate (PMMA) particles.49 For these solid particle stabilizers, they are considered to be “hard”, having high elastic moduli in the GPa range and are not easily deformed. On the other hand, some particles made of soft matter are usually soft with elastic moduli in the kPa range. They change dramatically with fluctuations in the surrounding, which is very different from the hard particles counterpart.50-51 It allows the design and preparation of interesting responsive systems and the potential applications in various fields are virtually unlimited. Besides of hard non-deformable nanoparticles, soft deformable nanoparticles, especially those derived from food-sourced nanoparticles or food colloids can also stabilize fluid interfaces.52 Historically, scientists have long realized the importance of particle stabilizers in food products and their role in stabilizing various interfaces has been intensively discussed by food scientists. 1, 53-54

Renaissance of increasing research interest in Pickering emulsions has also lead to better

understanding of food systems stabilized by proteins, fat crystals and so on. In 1986, Pelton and Chibante reported the preparation of a novel latex particle which was made of a thermal responsive polymer, poly(N-isopropylacrylamide) (PNIPAM).55 The polymeric gel particles are a wellknown material for soft, aqueous microgels. They undergo a structural transition at temperature above the lower critical solution temperature (LCST).56-57 At temperature below the LCST of the polymer, it is highly hydrophilic and the microgel is swollen by water.58 However, when the temperature is increased above the LCST, it becomes much less hydrophilic. The polymer collapses and expels its water content, which brings shrinkage along.59-60 The rapid volume phase transition happens in a matter of a few Kelvin. In addition, various olefin monomers can be easily


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co-polymerized with NIPAM so that the resulting particles can be pH-responsive,61-68 ionresponsive,66, 69-70 photo-responsive71-72 or even magnetic-responsive.73-74 In 2003 and 2004, more and more studies on pH-responsive Pickering emulsions were published.75-79 Nevertheless, these reports are all based on the modification of hard PS or PMMA particles as particulate stabilizers. Shortly after that, Ngai et al., reported on novel Pickering emulsions by pH and temperature sensitive microgels made of PNIPAM.80-81 These soft microgel particulate stabilizers not only provide high emulsion stability. The control of the emulsion stability can be easily achieved by changing the pH or temperature of the system because of the large response given by these particles. Since then, the investigation on soft particles at the interface has become even more popular and important. Before getting straight into the mechanism of emulsion and foam stabilization given by the soft microgels, the knowledge about the behaviours of soft nanoparticles at the interface is essential. Using microgel as a model system to investigate the interfacial science of soft materials is not just beneficial to the understanding of the mechanism of particle-stabilized emulsion systems but it is also of significance in a range of industrial (e.g, fuel production and oil transportation process) and emerging applications (e.g, biphasic catalysis and smart delivery).82-83 Imparting the knowledge obtained from the model microgel system to other soft particle stabilized systems such as colloids derived from natural resources used in food and pharmaceutical fields is receiving increasing interest.1, 84 Nevertheless, this is obviously not an easy task and therefore we would like to address the problems and challenges encountered in understanding the behaviours of microgels at the interface and discuss some potential benefits of using emulsions stabilized by microgels for food applications.


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Adsorption kinetics of microgel particles The adsorption of colloidal particles on to the fluid-fluid interfaces is generally considered an energy reduction process and the adsorbed particles should make a proper contact angle at the oilwater or water-air interface to obtain kinetically stable emulsions or foams (not close to 0° or 180°).85 For hard particles with a well-defined phase boundary, the adsorption of them can be understood in a relatively simple perspective. Although the origin is still controversial, air/water and air/oil interface is experimentally found to be negatively charged.86-88 The charged interface has a huge impact on adsorption of colloidal particles, as they usually gain colloidal stability by being charged. In addition, there is an aqueous film that separates the particle surface and the interface if the particles are dispersed in the aqueous phase. For the solid particles to be adsorbed onto the interface, the thin aqueous film must be broken and it exerts an energy barrier on the adsorption process.89 Just like hard particle counterparts, microgels can also adsorb to air/water and oil/water interfaces.90 Not many studies about the adsorption kinetics of microgels have been published. Nevertheless, the understanding of the adsorption process is quite important as it reveals the true nature of the microgels, which combine both particle and polymer properties in a single entity. Conventional precipitation polymerization of microgels does not control the cross-linking distribution within the particles.91-92 Therefore, the cross-linking density is completely determined by the fast-reacting cross-linker methylene-bis-acrylamide (MBA), giving the well-known gradual decreasing cross-linking inside the microgels. With numerous dangling polymer chains on the periphery, microgels possess strong polymeric properties. On the other hand, with the swollen gel structure which determines the particle equilibrium shape, microgels are still intact particles. Moreover, as microgels are highly swollen in water, it is very difficult to define the physical 6 ACS Paragon Plus Environment

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boundary of the particle at the interface. The force acting on the contact line is very complex,93 making microgels at interfaces a special and interesting system. Our group has first reported a systematic study on the adsorption kinetics of PNIPAM micorgels at oil-water interface.94 Pendant drop tensiometer was mainly used to characterize the surface pressure of a heptane/water interface with microgel dispersed in the aqueous phase. By measuring the interfacial tensions as a function of time, we discovered that the surface pressure increases linearly with the square root of time as shown in Figure 1(a). This is consistent with the expression derived by Ward and Tordai,95 which is shown in eq. 1, Π(t) = 2𝑅𝑇

𝐷

𝜋 𝐶𝑏𝑢𝑙𝑘

(1)

𝑡

where R is the ideal gas constant, T is the temperature of the system, D is the diffusion coefficient and Cbulk is the bulk concentration of the microgel. Therefore, the initial adsorption should be relative simple and is a diffusion-controlled process. However, when the surface pressure was increased above 25 mN/m, the increase in surface pressure slowed down significantly. We hypothesized the spreading of adsorbed microgels at the interface contributed to the further increase of surface pressure (Figure 1(b)). The study has demonstrated the good agreement between the measured change of surface pressure and the diffusion-controlled model. Nevertheless, the reason and the physical process behind were not deeply discussed.


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(a)

(b)

Figure 1. (a) Influence of PNIPAM microgel concentration on the evolution of dynamic surface pressure (Пt) vs t at 298 K. Dot lines are straight line fits. (b) Schematic illustration of PNIPAM microgels spreading at the oil/water interface. Reproduced with permission from ref 94. Copyright 2013, Royal Society of Chemistry. Another study done by Monteillet et al. focused more on the polymer-particle duality of microgels.89 The changes in interfacial tension and surface pressure were also measured by pendant drop tensiometry at different pH values and microgel concentrations. Normally, small surfactant molecules and non-cross-linked polymer chains can be adsorbed to the interface with negligible energy barrier. They argued that the observed fast spontaneous adsorption of microgels to the interface indicated the energy barrier was also very small or absent, just like small molecules, because of the surface dangling chains. However, for the slowdown found in the later stage of the adsorption, it might be related it to the insertion of microgels to the existing dense layer at the interface. A quadratic dependence for this final adsorption process on the microgel concentration was found and excluded the possibility of monolayer rearrangement. However, with just two set of data points in the deduction, there might still be rearrangements within the adsorbed layer. In


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addition, they discovered that the adsorption of charged microgel was slowed down because of increase in hydrophilicity.

Figure 2. (a) ln(1 - Γ / Γ m) as a function of time for various bulk concentration of microgel particles: (◊)0.10 g l-1, (Δ)0.20 g l-1, (□) 0.50 g l-1, (○)1.00 g l-1. Solid lines are straight line fits. (b) Adsorbed amount(Γ) as a function of the product Ct1/2 (C = concentration). The inset shows the individual curves of Γ vs. t1/2 for various bulk concentration of microgel particles: (◊)0.10 g l-1, (Δ)0.20 g l-1, (□) 0.50 g l-1, (○)1.00 g l-1. Solid lines are straight line fits and dashed lines are drawn with slopes calculated using diffusion constant D = DDLS (diffusion constant determined by dynamic light scattering). Reproduced with permission from ref 96. Copyright 2014, Royal Society of Chemistry. Deshmukh et al. studied the equation of state of soft microgels at the air/water interface.96 Just like the previous two studies, they recorded the change in surface pressure over time for different microgel concentrations, but this time using an air bubble instead of an oil droplet. Similar result was found in their study that there was a diffusion-controlled adsorption at the beginning and it was followed by a slowdown. An exponential relaxation model was derived based on the first order kinetic process. The experimental results agreed on their exponential relaxation model.


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Figure 2(a) shows that their initial adsorption results agree with the diffusion model and Figure 2(b) shows the fitting of an exponential relaxation model with the later stage of the adsorption. However, the rate constants determined could still not be interpreted well by the microgel bulk concentration. By considering these studies, we can see the big picture of the adsorption process is quite clear. An initial diffusion-controlled adsorption takes place with a negligible energy barrier and the change in surface pressure is determined by the transport of particles. Although the barrier-free adsorption has been attributed to the polymer nature of microgel, the physical picture of how these hydrated dangling chains greatly reduce the energy barrier, which should have been very large, is unclear. Moreover, the second stage of restricted adsorption, which consists of a relatively dense particle monolayer, is very complex. Because particle-particle interaction becomes significant in the adsorption process, there are many considerations when we investigate the process, such as conformation change of the adsorbed particles, rearrangement of the existing monolayer, formation of sublayer, etc. The kinetics and contribution of these processes are some of the essential components in the whole adsorption process, especially when the interface becomes crowded with particles. As Deshmukh et al. mentioned in their publication that, the process itself is rather complicated because the dependence between the rate constants and the bulk concentration is not linear.96 Nevertheless, further experiments investigating this dependence might be fruitful in understanding the process. In addition, the previous studies were mainly focused on the surface pressure only. If better imaging and tracking of particles at the interface can be achieved, these techniques will be very helpful in detecting any rearrangement or conformation change.


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Particle conformation at the interface Since the microgel stabilized Pickering emulsions were published,80-81 researches have been interested in developing a general understanding and mechanism of the stabilization giving by soft interfacial active particles. One of the biggest questions concerns the conformation of soft particles at the interface. Intuitively, flattened, spread soft microgel particles have a larger cross section area covering the interface and therefore should be better Pickering stabilizers. If this is a dominant factor for the stabilization for microgel stabilized Pickering emulsions, the dramatic reduction in emulsion stability above LCST can be easily explained. However, direct observation of adsorbed microgels is always a challenging task. Microgels are highly swollen in aqueous solution and so that the particles are nearly isorefractive with water. Combined with the size in sub-micron range, conventional optical microscopy is basically useless. Moreover, the size, conformation of the swollen gel particles and their surface dangling chains are highly dependent on the presence of solvent. Therefore, there are quite a lot of limitations in the corresponding studies. Destribats et al. first addressed the importance of particle deformability on emulsions stability.97 In the study, cryo-scanning electron microscopy (Cryo-SEM) was applied to observe the microgels at the dodecane/water interface. They proposed a “fried egg” conformation for the adsorbed microgels as shown in Figure 3(a). However, there might be several concerns in the study. Cryo-SEM required invasive sample preparation steps, including freezing and fracturing. Also, SEM images were not very good at observing the three-dimensional deformation there. Solvent sublimation and the interpretation of SEM contrast might also affect the results. Although cryoSEM provides the required resolution, the sample preparation procedures still introduce unknown effects on the emulsion samples. Schmidt et al. also observed microgels at the interface by cryo-


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SEM as shown in Figure 3(b).98 Particle deformation observed in their SEM images was not as significant. Nevertheless, they also found interconnecting filaments covering the interface.98 Geisel et al. observed the adsorbed particles at the oil phase instead (Figure 3(c)).99 They used freeze-fracture shadow casting (FreSCa) cryo-SEM. By the shadows casted by the protruded microgels, the protrusion height could be calculated with sufficient certainty. Also, core-corona morphology was found by looking from the oil phase but the origin of the corona was still not very clear. Although this method was fantastic for characterizing the protrusion height, it did not reflect much information about the deformation of the microgels as large part of the particle was still located in the aqueous phase. As mentioned above, SEM techniques always include invasive sample preparation procedures so that the sample can be fixed and transferred to the extreme condition in the SEM chamber. Often, fluorescence confocal microscopy images were included in these studies. However, the resolution was not comparable with that of SEM. In the study of adsorption behaviour of microgels by Monteillet et al., confocal images of emulsion droplets covered with PS-PNIPAM core-shell particles were shown in Figure 3(e).89 Those images were taken in aqueous solution and fluorescent dye was localized only in the hydrophobic PS core. By the correlation function, they found out that the composite particles were elongated, or shrunk at the decane/water instead of being flattened. They also explained the difference might be originated from the sample preparation. Although the interparticle distance could be determined, the actual conformation of the particles remained a mystery. Therefore, in one of our previous work, the focus has been placed on direct observation of microgels at the interface and in aqueous environment.100 Micron-sized microgels with a variety of structures were synthesized and the fluorescent labeling techniques were optimized under 12 ACS Paragon Plus Environment

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confocal laser scanning microscopy (CLSM). The lowest cross-linker content of our microgels reached 0.73 mol%. We discovered that unless the microgels were extremely swollen in charged state, the deformation of the main body of the particle was not significant, even for the sample with just 0.73 mol% cross-linking. As for the charged super swollen microgels, anisotropic flattening was observed at the interface. A portion of the particle deformed and fitted the curvature of the decane emulsion droplet (Figure 3(d)). The greatest weakness of our confocal study was that the outermost part did not show any contrast under CLSM. Nevertheless, the result confirmed that the major part of the microgels stayed spherical at the interface and we can be more focused in the outermost soft periphery. Later, Geisel et al. reported another study on microgel stabilized emulsions using transmission X-ray microscopy (TXM).101 TXM is also a novel, high resolution and non-invasive approach for observing microgels at the interface directly in aqueous solution. In addition, it does not require any fluorescent dye in the process and the essence of the system can be observed. 3D reconstruction of the microgel stabilized emulsion droplet was created using TXM. The droplet was loosely covered with microgels and there was no spreading of the microgels found as shown in Figure 3(f). Interestingly, their tomograms were very similar to our CLSM images. However, their investigation was also suffered from the low contrast between the outermost dangling chains and water. Style et al. studied the problem in a theoretical perspective.93 They took an amazing side view cryo-SEM image of the microgels adsorbed at the interface (Figure 3(g)). That image indicated that the main body of the microgel was not heavily flattened and there was a polymerlike layer covering the interface. A Neumann balance at the contact line was established and the theoretical calculation indicated that the deformation was dependent on the elastocapillary and 13 ACS Paragon Plus Environment

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plastocapillary. Recenly Snoeijer et al computed the shapes of soft elastic particles by molecular dynamics simulations and it is revealed that the shape is strongly influenced by the surface tension of the soft polymer microgel. In a later work by computer simulation, Potemkin et al also revealed the flattened morphology of soft microgels at fluid interfaces.102-103

Figure 3. (a) Cryo-SEM image of the interface of a heptane-in-water emulsion drop covered by 2.5 mol% BIS cross-linked microgels after sublimation. Reproduced with permission from ref 97. Copyright 2011, Royal Society of Chemistry. (b) P(NIPAM-co-MAA)-core/P(NIPAM)-shell microgels on the droplet surface of n-heptane/water (O/W) emulsions at pH 9. The white circles show the hydrodynamic diameters of the microgels in water. Reproduced with permission from Ref 98. Copyright 2011, American Chemical Society. (c) FreSCa image showing a clear shadow 14 ACS Paragon Plus Environment

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for a small ice crystal (circled in red) and the absence of it for the microgels (core microgel, pH 9). Reproduced with permission from Ref 99. Copyright 2012, American Chemical Society. (d) CLSM images of P(NIPAM-co-MAA) microgel (cross-linker content = 1.65 mol%) stabilized emulsions at pH 3. Excess decane was used, and the particles were not crowded. Reproduced with permission from Ref 100. Copyright 2016, Elsevier. (e) Organization of the composite microgels on the decane/water interface at pH 9.8. Reproduced with permission from Ref 89. Copyright 2014, Wiley-VCH. (f) TXM image of microgels on the surface of an oil droplet. Reproduced with permission from Ref 101. Copyright 2014, American Chemical Society. (g) PNIPAM particles at an oil–water interface, imaged using cryo-SEM. Reproduced with permission from Ref 93. Copyright 2015, Royal Society of Chemistry.

There are advantages and weakness in all the previously mentioned microscopy techniques. The non-invasive TXM and CLSM are quite promising in observing the main part of the microgels in aqueous solution. While only cryo-SEM techniques are capable of taking images of the low contrast polymer filaments. In order to detect the conformation of the hydrophilic dangling chains, development of novel labeling techniques is essential to provide sufficient contrast against the aqueous environment. Moreover, the effects on the interfacial properties brought by the labeling needed to be minimized. Understanding the equilibrium conformation of microgels at the interface is not only beneficial to figure out the stabilization mechanism of Pickering emulsions, it also allows us to come out with better hypotheses in the adsorption kinetics problems. In addition, investigation of the balance between the contact line tension and the elasticity of the microgels at the interface depends on accurate determination of the microgel conformation.


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Thermal responsive behaviour By looking into the previous two examples, the behaviours of microgels at the interface are complicated. Therefore, investigating the thermal responsive behaviour of microgels at the interface is even more challenging. PNIPAM microgel stabilized Pickering emulsions have been well-known for its responsiveness since they were first reported.80-81 By adjusting the temperature, the emulsion stability can be controlled conveniently. Demulsification, triggered release and even phase inversion can be achieved by adjusting the temperature. However, most of the examples reported in previous studies were quite case specific and it seems that the proposed mechanisms for the responsiveness can also be quite different. They include particle aggregation, particle detachment, change in particle partition, interfacial coverage, interfacial elasticity, etc. Although responsiveness comes with different mechanisms, the generalization of some of them can help us better predict the emulsion stability and responsiveness of microgel stabilized Pickering emulsions. Monteux et al. investigated the interfacial properties of PNIPAM microgels as a function of temperature. Although the change in microgel hydrodynamic diameter is monotonic with increasing temperature (from 20 °C to 40 °C), they discovered that there was a minimum of equilibrium interfacial tension around the LCST of microgel. In addition, microgels were still interfacial active in theirs collapsed state and the interfacial tension was still reduced by them significantly. However, complete phase separation of the emulsion was observed at temperature above the LCST. They suggested that the decrease in interfacial tension was caused by the more compact adsorbed particle layers. Once the microgels collapsed, they might form loosely packed aggregates at the interface and the protection against coalescence was compromised. However, this hypothesis was just based on the macroscopic aggregation observed.


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Destribats et al. studied the adhesion between microgel stabilized emulsion droplets at different temperatures.104 They proposed that as the microgel shrank at temperature above the LCST, the anchoring energy of the particles was considerably reduced. Also, the surface pressure of the monolayer was then enough to expel microgel particles away from the interface. Partial desorption of the microgels was observed as the turbidity increased in the aqueous phase. Interestingly, quite a lot of microgels stayed at the interface even at 41 °C, which was higher than the LCST of the microgel. If the emulsion was quenched to room temperature before significant phase separation, these adsorbed particles were able to provide remarkable emulsion stability by form a very dense protecting layer. Wu et al. studied the behaviour of thermal responsive copolymer microgels at the interface.105 They prepared different microgels made of different ratio of poly(N-vinylcaprolactam) (PVCL), PNIPAM and poly(N-isopropylmethacrylamide) (PNIPMAM). The adsorption kinetics of the microgels was studied at different temperatures. At temperatures above the LCST, the interfacial tension reduction was significantly slower compared to that at lower temperature. They attributed the results to the increased stiffness of the microgels at elevated temperatures. In order to provide good Pickering emulsion stability, a close packing interfacial structure is usually required. With lower particle mobility and deformability at temperature above the LCST, the time required to reduce the interfacial tension was significantly increased and they hindered good packing of microgels which protects emulsion droplets. We also studied the effect of temperature on microgels at the interface.106 Similar to the previously mentioned results, our measurements showed that the reduction in surface tension was much slower at temperature above the LCST (Figure 4).106 At the temperature above LCST, the soft microgels collapsed and the time needed for the nanoparticles to absorb onto the interface has 17 ACS Paragon Plus Environment

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increased and their capability to reduce the interfacial tension has been decreased. In addition, there was a minimum equilibrium surface tension found around the LCST of PNIPAM. We adopted some of the ideas proposed by Monteux et al.107 and a similar physical picture for the thermal responsive behaviour of microgels at the interface. However, we attributed the lowered surface pressure at temperature above LCST to the electrostatic repulsions by residue sulphate groups. Moreover, we performed single droplet experiment to study the effect of the cooling and heating processes. No matter it started at temperature below (Figure 5(a)) or above (Figure 5(b)) the LCST, the change in surface tension in the cooling and heating processes was not reversible at that rate (1 K min-1). The hysteresis of microgel thermal cycle is usually quite small in bulk solution. Therefore, it indicated that the interface might hinder the conformation change of microgels in certain state during the thermal cycle, which emphasized the complexity of the problem.

Figure 4. γ-time-temperature three dimensional plots of the heptane-water interface in the presence of PNIPAM microgels. Note that a minimum can be observed at temperature around


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VPTT (308.1 K). Reproduced with permission from Ref 106. Copyright 2014, Royal Society of Chemistry.

(a)

(b)

Figure 5. (a) The dynamic interfacial tension γ of single droplet, starting at 295 K, in the presence of PNIPAM microgels at various temperatures. (b) The dynamic interfacial tension γ of single droplet, starting at 320 K, in the presence of PNIPAM microgels at various temperatures. Reproduced with permission from Ref 106. Copyright 2014, Royal Society of Chemistry.

In these studies, there are some results which have been agreed among them. At temperature above the LCST, there is partial particle desorption and yet microgels are still significantly reducing the surface tension. All of these studies have located a minimum for the equilibrium surface tension at temperature near the LCST. On the other hand, the emulsion stability decreased dramatically once the temperature gets above the LCST. Therefore, equilibrium surface tension measurement might not be a direct method for investigating the reduction in emulsion stability above the LCST. In order to summarize the complete physical picture of the 19 ACS Paragon Plus Environment

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responsiveness, tracking of adsorbed microgels, viscoelasticity change of the interface and the adsorption energy change during the volume phase transition may hold the key pieces to put together the whole puzzle. pH-responsive behaviour Besides thermal responsive behaviours, pH-responsive properties are also important in microgel stabilized Pickering emulsions. However, the problem is even more complex because pHresponsive change of microgels relates various aspects. When the ionizable function groups of pHresponsive microgels are protonated or deprotonated, the electrostatic properties, hydrophilicity, size, deformability are all changed. Massé et al. studies the effect of charge of microgels on the stabilization of Pickering emulsions.108 In their study, acrylic acid (AA) and vinylacetic acid (VAA) were co-polymerized with NIPAM to give microgels with carboxylic function groups. Because of the difference in reactivity, AA was distributed rather evenly in PNIPAM-co-AA microgel and VAA was mostly located on the periphery of PNIPAM-co-VAA microgel. Therefore, the effect of surface charge of the deprotonated microgels could be compared. They characterized the Pickering emulsions through cryo-SEM images and the interparticle distance (centre-to-centre distance) obtained in the images. However, it is worthwhile to note that microgel packing usually contain defect. It may significantly affect the accuracy of the measurement of interparticle distance and lead to positive error. Nevertheless, they suggested that the deformability of charged microgels was reduced slightly because of the increase in the osmotic pressure. They found out that the spatial distribution of charge does not affect how microgels are adsorbed at the interface. Stable Pickering emulsions could be formed in the presence of 10 mM NaCl or with microgels bearing no carboxylic comonomer. On the other hand, emulsions stabilized with microgels bearing protonated, uncharged 20 ACS Paragon Plus Environment

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carboxylic groups were unstable and they suggested that intramolecular hydrogen bonding could reduce the Pickering emulsion stability. Nevertheless, the stabilization given by charged microgels should be a much more complicated problem. Schmidt et al. investigated the influence of microgel architecture by core-shell multiresponsive microgels.98 Methacrylic acid (MAA), a carboxylic acid similar to acrylic acid, was used as the co-monomer. Semi-batch syntheses were applied to enhance the core-shell structure of the resulting microgels. The internal architectures of core-shell microgels were characterized by electrophoretic mobility measurements. One of the microgels had MAA groups on the shell (MS) and the other had them in the core (MC). In their experiments, stable oil/water emulsions could be formed by both MC and MS microgel in alkaline condition but they could not stabilize emulsions in acidic condition. They suggested that the electrostatic repulsion between the interfacial microgels is not the origin of the extra stabilization in alkaline condition. As the presence of charge was critical to form stable emulsions, the stabilization was attributed to the enhanced interfacial viscoelasticity, which was backed by the observed interconnecting polymer filaments. In the previously mentioned freeze-fracture shadow-casting cryo-SEM study done by Geisel et al., the effects of pH-responsiveness on microgel interfacial properties were also reported.99 By analyzing the cryo-SEM images from the oil side, it was discovered that the protrusion height of microgel and the radial stretching at the interface were not strongly depended on the pH values. It indicated that interfacial activity and deformability dominate the microgel behaviour at the interface. However, the relation with Pickering emulsion stability was not main focus in this report. Previously, we have also investigated the mechanism and the stabilization of the Pickering emulsions stabilized by pH-responsive microgel.109 Two semi-batch core-shell microgels were 21 ACS Paragon Plus Environment

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prepared just like the work done by Schmidt et al. and the pH-responsiveness of them was enhanced by co-polymerizing a relatively high amount of MAA to the microgel. As shown in Figure 6, high density of MAA was located in the shell and the core of microgel-DS and microgel DC respectively. Similar to all other studies, both microgels were better Pickering stabilizer in theirs charged state (alkaline condition). We found out that the interfacial affinity of microgel-DC and microgel-DS and the corresponding emulsions stability was very similar in uncharged state. Therefore, the effect of intramolecular hydrogen bonding might not be a critical factor. Interestingly, in alkaline condition as shown in Figure 7, interfacial affinity of microgel-DC is much higher than that of microgel-DS, which possessed negative charge on the periphery.109 We suggested the increase in surface hydrophilicity reduced the interfacial affinity of microgel-DS. Surprisingly, with reduction in interfacial affinity, charged microgel-DS is still a better Pickering stabilizer compared to its uncharged state. Microgel-DS is also very deformable in charged state, which has been confirmed under CLSM. Moreover, although the negative charge of microgel-DC is in the core of the particle, the Pickering emulsion is much more stable compared to emulsions stabilized in its uncharged state. Therefore, we proposed that the combination of high interfacial affinity and high hydrophilicity provides the greatest stabilization to Pickering emulsions.


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Figure 6. Schematic illustration of the preparation of microgel-DS and microgel-DC by the combination

of

semi-batch

and

temperature-programmed

surface-free

precipitation

polymerization. Reproduced with permission from Ref 109. Copyright 2018, Elsevier.


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Figure 7. Emulsion droplets covered by microgel-DC and microgel-DS at pH 11, where the bright solid dots are microgel-DC and hollow particles are the microgel-DS. Emulsion droplets are mainly covered with microgel-DC rather than microgel-DS. Reproduced with permission from Ref 109. Copyright 2018, Elsevier. However, our study and some of the mentioned studies rely on exclusive method. Therefore, much work is still required to reveal the complex underlying mechanism of such microgel stabilized pHresponsive Pickering emulsions.

Interfacial rheological properties In the investigation of the emulsion stability of microgel stabilized Pickering emulsions, the viscoelasticity of the microgel covered interface has often been emphasized. Compared to the brittle interfacial monolayer formed by hard, solid particles, it is intuitive to assume microgel


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covered interfaces are more elastic and able to withstand larger force before coalescence occurs. Therefore, the understanding in the interfacial rheology of microgel adsorbed interface is essential in solving the previously mentioned problems, such as the mechanism of the responsiveness of the emulsions. In addition, it also reveals the inter-microgel interaction at the interface and is a good experimental model for other soft glassy materials. Brugger et al. studied the shear rheology and dilatational rheology of multi-responsive microgel covered heptane/water interface.110 Generally, the microgel covered interface shows elastic property. Nevertheless, this elasticity is highly depended on the pH value, which refers to the charge state of the microgels. From the interfacial shear rheology in acidic condition, the uncharged microgel layer exhibits viscous flow behaviour at a lower strain of 0.001. When it is at pH 9, the negatively charged microgel covered interface is much more viscoelastic, even the strain reaches 0.03. Dilatational rheology also indicated similar tendency in different pH values, that the microgel covered interface is more elastic in alkaline condition. It was proposed that the partially interconnecting microgel layer at pH 9 would provide a soft gel-like interface and enhance the Pickering emulsion stability. A surface elasticity study was also done by Pinaud et al. The elastic modulus of the microgel covered interface was determined as a function of surface pressure.111 Microgels with different cross-linking density were prepared by controlling the cross-linker content. They determined the elastic modulus and loss modulus of the interfacial layer during the microgel adsorption experiment. It exhibits a solid behaviour as the elastic modulus is larger than the loss modulus by ten times. Also, a maximum in elastic modulus was found at an intermediate surface pressure. However, the determined elastic modulus of interface covered by microgels with different cross-linking density are basically the same. 25 ACS Paragon Plus Environment

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Huang et al. investigated the interfacial rheology of microgel covered oil-water interface by magnetic bead microgrheology.112 Microrheology at the interface has been a challenge. Even the preparation of a stable microgel covered interface which could last for a sufficiently long period for the measurement is difficult. Nevertheless, the information obtained in such experiment is very valuable. With great imaging technique for the particles at the interface, the adsorption process was also studied. Partial microgel aggregation was observed. Instead of evenly distributed at the interface, adsorbed microgels tends to form clusters. The aging process of them was also observed by using the pair correlation function. Superparamagnetic beads were introduced at the interface and they could apply lateral forces on the microgel layer. The force-displacement response of the interfacial layer was non-linear under a slowly increasing, quasi-static lateral force. The deformation of the microgel covered interface is mainly plastic and only a small elastic recovery is observed when the stress is removal. Nevertheless, this plastic behaviour was observed in a relatively long timescale, which was around 10 minutes. Through another shearing rheology experiment of dumbbell shape magnetic particles at the interface, the deformation of the microgel layer propagated over 15 µm. It indicates that there should be microgel-microgel bond carrying the shear stress. Huang et al. later published another report on structure and rheology of microgel monolayer, which extended their previous findings.113 Rheology of a material is highly depended on the measurement timescale. In their previous study, the timescale of the experiments was quite long (minutes) and microgel rearrangement and plastic deformation was observed. In the second study, a concentration depended interfacial elasticity was discovered. When the microgels are in theirs compressed state at the interface, the storage modulus is much larger than that in relax state.


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They suggested the overlapping of the microgel main bodies causes the increase in storage modulus. The studies published by Huang et al. have been a very good demonstration of applying the microrheology techniques in the study of microgel covered interface. If their studies can be extended to microgels with different deformability, pH-responsiveness, and even at different temperatures, the comparison of their interfacial rheology will be extremely beneficial in understanding the effects and importance of interfacial viscoelasticity on the Pickering emulsion stability.

Soft particle stabilized interfaces in food systems Particulate species of proteins, polysaccharides, fat and so on are present in a variety of food products of the type of food emulsions or food foams. Most of these particulate species, or food colloids are soft micro- or nanoparticles in nature and the role of these soft particles in stabilizing the food emulsions and food foam has long been realized by scientists and researchers in the related fields.52, 114-115 However, as a food product in the type of emulsions or foams may contain various substances including small molecular surfactants, polymeric materials as well as these particulate species, it is nearly impossible to elucidate the mechanism of stabilizing capability of food colloids in real food products. The vast and intensive investigations of food colloids as interfacial stabilizers to elucidate its stabilization mechanism and to develop new food technology has also achieved increasing attention due to the renaissance of researches in particle-stabilized emulsions and foams during the past decades.53 Proteins derived from either plants or animals can be made into microgel particles and used as effective interfacial stabilizers in food emulsions or foams. Soy protein isolate (SPI) which is 27 ACS Paragon Plus Environment

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commercially available has long been used in food products and can be made into nanoparticles through sequential heat treatments.116 Liu et al. reported that the emulsion stability has been progressively enhanced due to the formation of a gel-like network and the oil phase can be structurally held inside it. de Folter et al. first demonstrated the use of zein protein nanoparticles to prepare stable o/w Pickering emulsions.117

They systematically investigate a series of

parameters that may influence the emulsion stability and morphology and the emulsions stabilized by zein nanoparticles are stable over a wide range of pH values. In combination with surfactant sodium stearate, the adsorption of zein nanoparticles onto the oil/water interface can be further enhanced and the aggregates can serve as steric barrier against coalescence which can lead to the formation of oil gel.118 To solve the problem of biocompatibility and biodegradability of inorganic or synthetic, high-internal-phase Pickering emulsions with an internal phase volume fraction of 87% stabilized by peanut protein isolate is prepared.119 When the internal phase is edible oil, it can be directly used as food products and it can be used as template to prepare porous materials when the internal phase is volatile solvents. Whey proteins which exist in milk are intrinsically surfaceactive and these proteins have also been to prepare microgel particles and subsequent to stabilize emulsions. It is shown that the emulsions are stable within the investigated pH ranges and salt cannot damage the emulsion stability against coalescence. Using cryo-SEM, they found that uncharged particles were highly aggregated and formed a continuous 2-D network at the interface while charge particles formed individual aggregates separated by particle-free regions.120 Polysaccharides contain starch, chitin, and chitosan and so on and all these materials can be made into colloid particles for emulsion stabilization.121 Tan reported the fabrication of starch based microparticles by nanoprecipitation to stabilize Pickering emulsion with a phase inversion trigger.122 Both catastrophic inversion and transitional inversion can be induced either by oil-water


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ratio or by tuning the pH in the aqueous. It is also possible to stabilize high internal phase emulsions with starch nanocrystals with certain pH ranges.123 For emulsions stabilized by fat crystals, the emulsion type is usually w/o due to the difficulty of dispersing them in aqueous phase and the emulsion stability is usually attributed to Pickering stabilization effect as well as network formation within the oil phase or a combination of both.124 Rousseau has classified fat crystal stabilized emulsion into three types depending on the stabilization mechanism.

125

In the three

types of fat crystal stabilized emulsions, type II is most similar to the case discussed in this article and the interfacial stability is attributed to the adsorption of nano- or microparticles at the interface.

Conclusions and Outlook All of these studies have provided us valuable insight in understanding the behaviour of soft microgel nanoparticles at the interfaces and this is the foundation for its use in related industrial fields. Nevertheless, because of the complexity of the problem, more experiments and simulations123 would be necessary to verify the current models and hypotheses developed. The previously mentioned problems are intertwined with each other. For example, if the equilibrium conformation of microgel particles at the interface can be accurately determined through observation and simulation, there will be more information to develop better understanding in the adsorption kinetics problem and their interfacial rheology. Only with all these mysteries resolved, a general and precise understanding of soft particle at interfaces can then be well established. Although there are some researches on the particle stabilizers in complex oil-water systems containing various surfactants and amphiphilic polymers, future challenges still remain in obtaining using information based on these fundamental results. Compared with particles with low deformability, soft microgels are more universally present in food and pharmaceutical products 29 ACS Paragon Plus Environment

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and therefore soft particles at the interfaces play a vital role in food emulsion stability and food products development. As food emulsions and foams usually consist of different types of soft deformable particles as well as hard particles and many other surface-active species, the interplay between these particles，surfactants and polymers is crucial for the stabilization of food products. We believe the studies of soft particles has provided useful information to understand food systems. In the future, the research into complex systems such as mixture of soft and hard particles and soft particles and biopolymers will offer more insightful understand of food system stabilizations.

Acknowledgements The financial support of this work by the Hong Kong Special Administration Region (HKSAR) General Research Fund (CUHK14306617, 2130535), the Direct Grant for Research of the Chinese University of Hong Kong (4053272), and the National Natural Science Foundation of China (21703085 & 21574110) is gratefully acknowledged.


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21. Fortuny, M.; Oliveira, C. B.; Melo, R. L.; Nele, M.; Coutinho, R. C.; Santos, A. F., Effect of salinity, temperature, water content, and pH on the microwave demulsification of crude oil emulsions. Energy & Fuels 2007, 21 (3), 1358-1364. 22. Hofstetter, K.; Lutz, J.; Lang, I.; Witholt, B.; Schmid, A., Coupling of biocatalytic asymmetric epoxidation with NADH regeneration in organic–aqueous emulsions. Angewandte Chemie International Edition 2004, 43 (16), 2163-2166. 23. Wiese, S.; Spiess, A. C.; Richtering, W., Microgel‐Stabilized Smart Emulsions for Biocatalysis. Angewandte Chemie 2013, 125 (2), 604-607. 24. Becher, P., Emulsions: theory and practice. 1965. 25. Binks, B. P., Modern aspects of emulsion science. Royal Society of Chemistry: 1998. 26. Blake-Haskins, J.; Scala, D.; Rhein, L.; Robbins, C., Predicting surfactant irritation from the swelling response of a collagen film. J Soc Cosmet Chem 1986, 37 (4), 199-210. 27. Wilhelm, K.-P.; Freitag, G.; Wolff, H. H., Surfactant-induced skin irritation and skin repair: evaluation of the acute human irritation model by noninvasive techniques. Journal of the American Academy of Dermatology 1994, 30 (6), 944-949. 28. Venhuis, S. H.; Mehrvar, M., Health effects, environmental impacts, and photochemical degradation of selected surfactants in water. International Journal of photoenergy 2004, 6 (3), 115-125. 29. Cserháti, T.; Forgács, E.; Oros, G., Biological activity and environmental impact of anionic surfactants. Environment international 2002, 28 (5), 337-348. 30. Lewis, M. A., Chronic and sublethal toxicities of surfactants to aquatic animals: a review and risk assessment. Water Research 1991, 25 (1), 101-113. 31. Pickering, S. U., Cxcvi.—emulsions. Journal of the Chemical Society, Transactions 1907, 91, 2001-2021. 32. Ramsden, W., Separation of solids in the surface-layers of solutions and ‘suspensions’(observations on surface-membranes, bubbles, emulsions, and mechanical coagulation).—Preliminary account. Proceedings of the royal Society of London 1904, 72 (477-486), 156-164. 33. Chevalier, Y.; Bolzinger, M.-A., Emulsions stabilized with solid nanoparticles: Pickering emulsions. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2013, 439, 23-34. 34. Aveyard, R.; Binks, B. P.; Clint, J. H., Emulsions stabilised solely by colloidal particles. Advances in Colloid and Interface Science 2003, 100, 503-546. 35. Schulman, J. H.; Leja, J., Control of contact angles at the oil-water-solid interfaces. Emulsions stabilized by solid particles (BaSO 4). Transactions of the Faraday Society 1954, 50, 598-605. 36. Pickering, S. U., CXLVIII.—Organic ferric salts. Journal of the Chemical Society, Transactions 1913, 103, 1358-1368. 37. Wiley, R., Limited coalescence of oil droplets in coarse oil-in-water emulsions. Journal of Colloid Science 1954, 9 (5), 427-437. 38. Pickering, S. U., CV.—Metallo-compounds of cobalt and nickel. Journal of the Chemical Society, Transactions 1915, 107, 942-954. 39. Mukerjee, L.; Srivastava, S., Finely divided solids as emulsifiers part III. Kolloid-Zeitschrift 1957, 150 (2), 144-148. 40. Ashby, N.; Binks, B., Pickering emulsions stabilised by Laponite clay particles. Physical Chemistry Chemical Physics 2000, 2 (24), 5640-5646. 41. Guillot, S.; Bergaya, F.; de Azevedo, C.; Warmont, F.; Tranchant, J.-F., Internally structured pickering emulsions stabilized by clay mineral particles. Journal of colloid and interface science 2009, 333 (2), 563-569. 42. Yan, N.; Masliyah, J. H., Adsorption and desorption of clay particles at the oil-water interface. Journal of colloid and interface science 1994, 168 (2), 386-392. 43. Nonomura, Y.; Kobayashi, N., Phase inversion of the Pickering emulsions stabilized by plateshaped clay particles. Journal of colloid and interface science 2009, 330 (2), 463-466. 44. Frelichowska, J.; Bolzinger, M.-A.; Chevalier, Y., Pickering emulsions with bare silica. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2009, 343 (1-3), 70-74. 32 ACS Paragon Plus Environment

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For Table of Contents Only

Biography 37 ACS Paragon Plus Environment

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To Ngai has received his B.Sc. in chemistry at the Chinese University of Hong Kong (CUHK) in 1999. In 2003, he obtained his Ph.D. in chemistry in the same university, where he worked on light scattering and polymer interaction in solution. He moved to BASF (Ludwigshafen, Germany) in 2003 as the postdoctoral fellow for two years, working on colloids and surface chemistry. After a short postdoctoral training in the Chemistry Department at the University of Minnesota in 2005, he joined the Chemistry Department at CUHK in 2006 as a research assistant professor. He has been appointed as an assistant professor in 2008, and early promoted to associate professor in 2012. In 2017, he was promoted to Professor. His current research interests center around the colloids, surface chemistry, polymers and soft matter.

Guanqing Sun is currently working as an associate professor in Jiangnan University, Wuxi, China. He received his B.Eng degree in Polymer Science in 2009 from Zhejiang University. He then went to Hong Kong for his Ph.D studies. Under the supervision of Prof. Ngai To, he obtained his Ph.D degree in Chemistry from The Chinese University of Hong Kong in 2014. His research interests mainly focus on interfacial-related phenomena, colloidal systems and advanced coating materials developed based on these fundamental principles.


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Man-hin Kwok currently is a post-doctoral research associate in the department of Macromoleclar Science and Engineering in Case Western Reserve University. He received his B.Sc. in Chemistry in The Chinese University of Hong Kong in 2012. He won the Hong Kong PhD Fellowship Award and worked on structure and behavior of soft microgel particles assembled at the liquid interfaces. He received the PhD in Chemistry in the Chinese University of Hong Kong in 2016.


Microgel Particles at Interfaces: Phenomena, Principles, and

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