Novel emulsions stabilized by pH and temperature sensitive microgels

Ngai, To; Behrens, Sven H.; Auweter, Helmut

doi:10.1039/b412330a

Cited by 349 publications

(343 citation statements)

References 17 publications

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“…Microgels are nano-to micrometer-sized porous polymer particles, which consist of a cross-linked polymer network that is swollen with solvent. pH and temperature-sensitive microgels are able to change their size by swelling or shrinking in response to changes in their environment, and they provide a new way to stabilize emulsions, which has been studied only in recent years (26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39).…”

Section: Resultsmentioning

confidence: 99%

Non-coalescence of oppositely charged droplets in pH-sensitive emulsions

Liu

Seiffert

Thiele

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

100

102

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Like charges stabilize emulsions, whereas opposite charges break emulsions. This is the fundamental principle for many industrial and practical processes. Using micrometer-sized pH-sensitive polymeric hydrogel particles as emulsion stabilizers, we prepare emulsions that consist of oppositely charged droplets, which do not coalesce. We observe noncoalescence of oppositely charged droplets in bulk emulsification as well as in microfluidic devices, where oppositely charged droplets are forced to collide within channel junctions. The results demonstrate that electrostatic interactions between droplets do not determine their stability and reveal the unique pH-dependent properties of emulsions stabilized by soft microgel particles. The noncoalescence can be switched to coalescence by neutralizing the microgels, and the emulsion can be broken on demand. This unusual feature of the microgel-stabilized emulsions offers fascinating opportunities for future applications of these systems.stimuli-sensitive emulsions | limited coalescence | poly (N-isopropylacrylamide) microgels | Pickering emulsions | lab-on-a-chip E mulsions are multiphase systems, which consist of two immiscible liquids, one dispersed in another in the form of small droplets with submicron to micron sizes. Emulsions are used in food, cosmetics, medicine, coatings, paints, and a large number of other technical applications (1). Due to their large interfacial area, emulsions are thermodynamically unstable and have a tendency to undergo coalescence, a process during which two droplets merge into a bigger one, minimizing the surface energy (2). Certain molecules or materials, such as surfactants or solid particles, can be added to prevent coalescence, making emulsions kinetically stable. These emulsion stabilizers usually provide electrostatic repulsion, steric repulsion, and/or strength to the interfacial layer of the droplets, according to widely accepted theories. For example, when emulsion droplets are covered with an ionic surfactant, they carry like charges; these charges repel each other due to electrostatic interaction, which prevents droplet aggregation or coalescence. By contrast, when oppositely charged droplets are generated in an emulsion, they attract each other and coalescence is hastened; this phenomenon is the key event in processes like electrocoalescence (3, 4).In contrast to this classical behavior, unexpected noncoalescence of two oppositely charged droplets has been reported very recently (5, 6); these investigations show that under the influence of an electric field of sufficient strength, two oppositely charged water droplets recoil after contact rather than coalesce, depending on the curvature and local capillary pressure at the liquid bridge between two adjacent droplets. However, these studies were conducted with drops that were formed temporarily under the influence of an electric field in a continuous phase of air rather than with drops that are dispersed in a liquid. By contrast, stable emulsions, which consist of oppositely charg...

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Section: Resultsmentioning

confidence: 99%

Non-coalescence of oppositely charged droplets in pH-sensitive emulsions

Liu

Seiffert

Thiele

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

100

102

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“…These form easily [6,10,11], and can be tuned in situ, for instance by altering solvent properties, such as temperature and pH [12][13][14][15][16][17]. This has been demonstrated with emulsions created with poly(Nisopropylacrylamide) (pNIPAM) particles.…”

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confidence: 99%

Adsorption of soft particles at fluid interfaces

2015

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Soft particles can be better emulsifiers than hard particles because they stretch at fluid interfaces. This deformation can increase adsorption energies by orders of magnitude relative to rigid particles. The deformation of a particle at an interface is governed by a competition of bulk elasticity and surface tension. When particles are partially wet by the two liquids, deformation is localized within a material-dependent distance L from the contact line. At the contact line, the particle morphology is given by a balance of surface tensions. When the particle radius R L, the particle adopts a lenticular shape identical to that of an adsorbed fluid droplet. Particle deformations can be elastic or plastic, depending on the relative values of the Young modulus, E, and yield stress, σp. When surface tensions favour complete spreading of the particles at the interface, plastic deformation can lead to unusual fried-egg morphologies. When deformable particles have surface properties that are very similar to one liquid phase, adsorption can be extremely sensitive to small changes of their affinity for the other liquid phase. These findings have implications for the adsorption of microgel particles at fluid interfaces and the performance of stimuli-responsive Pickering emulsions.Emulsions are vital in many fields, including foods, cosmetics, pharmaceuticals, and oil recovery [1][2][3][4]. Emulsions are typically stabilised by molecular surfactants, but they can also be stabilised with microparticles [5,6]. The resulting Pickering emulsions have many benefits over regular emulsions. In particular, they are highly stable and avoid the use of potentially harmful or irritating surfactants [5].A recent development is the use of soft colloidal particles (typically microgels) to make Pickering emulsions [6][7][8][9]. These form easily [6,10,11], and can be tuned in situ, for instance by altering solvent properties, such as temperature and pH [12][13][14][15][16][17]. This has been demonstrated with emulsions created with poly(Nisopropylacrylamide) (pNIPAM) particles. Since pNI-PAM undergoes a reversible swelling/shrinking transition at a temperature close to body temperature, they have potential for stimuli-responsive release of encapsulated active ingrediants for drug delivery [6].A key difference between soft and hard particles as Pickering emulsifiers is that soft particles stretch as they adsorb [8,18]. For example, microgel particles can have a much larger diameter at the interface than they do in the bulk. The extent of stretching depends on the crosslinking ratio of the hydrogel [18,23,24]. Figure 1 shows a side view of a p(NIPAM) microgel at a water/n-decane interface using cryo-SEM after freeze-fracture [25]. While these soft particles have a spherical shape in solution, they are strongly deformed at the interface. The central region of the particle remains somewhat spherical, especially the part exposed to the bulk water phase. However, the particles are pulled strongly outward at the contact line, and a thin film of ...

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“…5,6 For this purpose particles have to be coated or grafted with functional groups which can be transformed between hydrophilic and lipophilic in response to appropriate triggers. 6 Currently several triggers have been developed, including light irradiation, 7-10 CO 2 /N 2 , 11-14 temperature, [15][16][17] pH, 18-24 redox, 25,26 magnetic eld, 27,28 and dual triggers like pH-temperature, 29,30 light-temperature 31 and magnetic eld intensity-temperature.…”

Section: 6mentioning

confidence: 99%

CO₂/N₂ triggered switchable Pickering emulsions stabilized by alumina nanoparticles in combination with a conventional anionic surfactant

Zhang

Pei

et al. 2017

RSC Adv.

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Stable n-decane-in-water Pickering emulsions were prepared using positively charged alumina nanoparticles in combination with a trace amount of the anionic surfactant sodium dodecyl sulfate (SDS) as a stabilizer. Particles were hydrophobized in situ by adsorption of surfactant enhancing their surface activity. Emulsions can be readily demulsified by addition of an equal amount of a switchable surfactant,, which can be transformed between a surface-active amidinium/cationic form and a surface-inactive amidine/neutral form by bubbling CO 2 or N 2 ,respectively. Following addition of cationic DDAA which prefers to form ion pairs with SDS, desorption of SDS from particle surfaces occurs and alumina particles are rendered hydrophilic resulting in demulsification of the emulsion. However, by bubbling N 2 into the demulsified mixture, DDAA molecules are converted to the amidine/neutral form leading to collapse of the ion pairs and re-establishment of the in situ hydrophobization of particles. Stable Pickering emulsions can be prepared again following homogenization. This simple demulsification/re-stabilization cycle can be repeated several times.Experimental evidence including measurement of the adsorption isotherm, zeta potentials, extent of particle adsorption at droplet interfaces in emulsions and microscopy is given to support the postulated mechanisms.

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Novel emulsions stabilized by pH and temperature sensitive microgels

Abstract: Surfactant-free oil-in-water emulsions prepared with temperature and pH sensitive poly(N-isopropylacrylamide)(PNIPAM) microgel particles offer unprecedented control of emulsion stability.

Cited by 349 publications

References 17 publications

Non-coalescence of oppositely charged droplets in pH-sensitive emulsions

Non-coalescence of oppositely charged droplets in pH-sensitive emulsions

Adsorption of soft particles at fluid interfaces

CO₂/N₂ triggered switchable Pickering emulsions stabilized by alumina nanoparticles in combination with a conventional anionic surfactant

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Novel emulsions stabilized by pH and temperature sensitive microgels

Abstract: Surfactant-free oil-in-water emulsions prepared with temperature and pH sensitive poly(N-isopropylacrylamide)(PNIPAM) microgel particles offer unprecedented control of emulsion stability.

Cited by 349 publications

References 17 publications

Non-coalescence of oppositely charged droplets in pH-sensitive emulsions

Non-coalescence of oppositely charged droplets in pH-sensitive emulsions

Adsorption of soft particles at fluid interfaces

CO2/N2 triggered switchable Pickering emulsions stabilized by alumina nanoparticles in combination with a conventional anionic surfactant

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Product

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About

CO₂/N₂ triggered switchable Pickering emulsions stabilized by alumina nanoparticles in combination with a conventional anionic surfactant