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

Abstract

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...

The Compressibility of pH‐Sensitive Microgels at the Oil–Water Interface: Higher Charge Leads to Less Repulsion

“…It was even shown that oppositely charged droplets that are covered by microgels do not coalesce when they are mixed. [3] Furthermore, direct electron microscopy of the microgelcovered interface with freeze-fracture shadow-casting (FreSCa) cryogenic scanning electron microscopy (cryo-SEM) [25] revealed the three-dimensional structure of the microgels at the interface. The particles are strongly deformed at the interface and protrude very little into the oil phase.…”

Isostructural solid–solid phase transition in monolayers of soft core–shell particles at fluid interfaces: structure and mechanics

“…In addition to these applications, they find increasing use as emulsion stabilizers 31 , where the effects of softness and responsiveness combine to impart new functionalities to emulsions, including extreme compliance and stability under flow 32 or triggered rupture and coalescence 33,34 . The use of microgels as emulsion stabilizers is driven by their strong propensity to adsorb spontaneously at oil-water (o/w) interfaces.…”

Surface modification of MPEG-b-PCL-based nanoparticles via oxidative self-polymerization of dopamine for malignant melanoma therapy

“…This could be attributed to the fact that dry NPs were prone to shrinking and collapsing. 13,51 The in vitro drug release profiles of the PTX-loaded NPs and PTX-loaded NPs@PDA in the first 30 days are shown in Figure 5. As can be seen from Figure 5, similar drug release curves of PTX-loaded NPs and PTX-loaded NPs@PDA were observed at the same time.…”