Surfactants And Interfacial Phenomena

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

https://absimage.aps.org/image/MAR06/MWS_MAR06-2005-004393.pdf

Novel emulsions stabilized by pH and temperature sensitive microgels

https://hal.archives-ouvertes.fr/hal-02330281/document

Pickering emulsions: Preparation processes, key parameters governing their properties and potential for pharmaceutical applications.

https://hal.umontpellier.fr/hal-03242593/document

Current Trends in Pickering Emulsions: Particle Morphology and Applications

The study of solid particles residing at fluid–fluid interfaces has become an established area in surface and colloid science recently, experiencing a renaissance since around 2000. Particles at interfaces arise in many industrial products and processes such as antifoam formulations, crude oil emulsions, aerated foodstuffs, and flotation. Although they act in many ways like traditional surfactant molecules, they offer distinct advantages also, and the area is now multidisciplinary, involving research in the fundamental science and potential applications. In this Feature Article, the flavor of some of this interest is given on the basis of recent work from our own group and includes the behavior of particles at oil–water, air–water, oil–oil, air–oil, and water–water interfaces. The materials capable of being prepared by assembling various kinds of particles at fluid interfaces include particle-stabilized emulsions, particle-stabilized aqueous and oil foams, dry liquids, liquid marbles, and powdered emulsions.

Colloidal Particles at a Range of Fluid–Fluid Interfaces

pH-responsive oil-in-water Pickering emulsions were prepared simply by using negatively charged silica nanoparticles in combination with a trace amount of a zwitterionic carboxyl betaine surfactant as stabilizer. Emulsions are stable to coalescence at pH ≤ 5 but phase separate completely at pH > 8.5. In acidic solution, the carboxyl betaine molecules become cationic, allowing them to adsorb on silica nanoparticles via electrostatic interactions, thus hydrophobizing and flocculating them and enhancing their surface activity. Upon increasing the pH, surfactant molecules are converted to zwitterionic form and significantly desorb from particles’ surfaces, triggering dehydrophobization and coalescence of oil droplets within the emulsion. The pH-responsive emulsion can be cycled between stable and unstable many times upon alternating the pH of the aqueous phase. The average droplet size in restabilized emulsions at low pH, however, increases gradually after four cycles due to the accumulation of NaCl. Experime...

/pdf/ph-responsive-pickering-emulsions-stabilized-by-silica-53k5f8xy2f.pdf

pH-Responsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with a Conventional Zwitterionic Surfactant.

A stable oil-in-water Pickering emulsion stabilized by negatively charged silica nanoparticles hydrophobized in situ with a trace amount of a conventional cationic surfactant can be rendered unstable on addition of an equimolar amount of an anionic surfactant. The emulsion can be subsequently restabilized by adding a similar trace amount of cationic surfactant along with rehomogenization. This destabilization–stabilization behavior can be cycled many times, demonstrating that the Pickering emulsion is switchable. The trigger is the stronger electrostatic interaction between the oppositely charged ionic surfactants compared with that between the cationic surfactant and the (initially) negatively charged particle surfaces. The cationic surfactant prefers to form ion pairs with the added anionic surfactant and thus desorbs from particle surfaces rendering them surface-inactive. This access to switchable Pickering emulsions is easier than those employing switchable surfactants, polymers, or surface-active par...

Switchable Pickering emulsions stabilized by silica nanoparticles hydrophobized in situ with a conventional cationic surfactant.

We put forward a simple protocol to prepare thermoresponsive Pickering emulsions. Using hydrophilic silica nanoparticles in combination with a low concentration of alkyl polyoxyethylene monododecyl ether (C12En) nonionic surfactant as emulsifier, oil-in-water (o/w) emulsions can be obtained, which are stable at room temperature but demulsified at elevated temperature. The stabilization can be restored once the separated mixture is cooled and rehomogenized, and this stabilization–destabilization behavior can be cycled many times. It is found that the adsorption of nonionic surfactant at the silica nanoparticle–water interface via hydrogen bonding between the oxygen atoms in the polyoxyethylene headgroup and the SiOH groups on particle surfaces at low temperature is responsible for the in situ hydrophobization of the particles rendering them surface-active. Dehydrophobization can be achieved at elevated temperature due to weakening or loss of this hydrogen bonding. The time required for demulsification decr...

/pdf/thermoresponsive-pickering-emulsions-stabilized-by-silica-1dudlu15iy.pdf

Thermoresponsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with Alkyl Polyoxyethylene Ether Nonionic Surfactant.

pH-Responsive Pickering foams stabilized by silica nanoparticles in combination with trace amount of dodecyl dimethyl carboxyl betaine

The invention relates to a stimulation-response surface activity grain taking pH as a trigger mechanism and belongs to the field of colloid and interface chemistry. The stimulation-response surface activity grain is composed of an unmodified nano-silicon dioxide grain and carboxyl glycine betaine amphiprotic compound, wherein the native grain size of the grain is 10-200nm, the use concentration is mass fraction 0.1%-2.0% (taking water phase as basis) and the use concentration of the carboxyl glycine betaine amphiprotic compound is 0.01-1.0 time of the critical micelle concentration (taking water phase as basis). In an acidic water medium, the amphiprotic compound is converted into positive ions and is attached to the surface of the grain with negative charges through electrostatic interaction, and the grain surface is subjected to in-situ hydrophobization so as to gain the surface activity. In an alkalescence medium, the amphiprotic compound is in electric neutrality. After the electrostatic interaction is removed, the amphiprotic compound can desorb from the grain surface and the grain surface loses the surface activity. By changing the pH value of the water phase, the grain can be circulated between the states of having surface activity and having no surface activity.

Kaihong Liu

Papers

pH-Responsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with a Conventional Zwitterionic Surfactant.

Switchable Pickering emulsions stabilized by silica nanoparticles hydrophobized in situ with a conventional cationic surfactant.

Thermoresponsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with Alkyl Polyoxyethylene Ether Nonionic Surfactant.

pH-Responsive Pickering foams stabilized by silica nanoparticles in combination with trace amount of dodecyl dimethyl carboxyl betaine

Stimulation-response surface activity grain taking pH as trigger mechanism