Tailoring Pickering Double Emulsions by in Situ Particle Surface Modification

Abstract

Fundamental studies on the formation and stability of Pickering double emulsions are crucial for their industrial applications. Available methods of double emulsion preparation involve multiple tedious steps and can formulate a particular type of double emulsion, that is, water-in-oil-in-water (w/o/w) or oil-in-water-in-oil (o/w/o). In this work, we proposed a simple single-step in situ surface modification method to stabilize different types of double emulsions using hematite and silica particle systems which involves the addition of oleic acid. In the emulsification studies, we use (i) a combination of hematite and oleic acid, which is termed the binary system, and (ii) a mixture of hematite and silica particles together with oleic acid, which is designated as the ternary system. The wettability of hematite particles is tuned by direct or sequential addition of oleic acid to the water–decane medium. The direct surface modification (which involves the addition of a known quantity of oleic acid to the oil–water mixtures at once) of hematite particles in both binary and ternary systems shows transitional phase inversion from oil-in-water (o/w) to water-in-oil (w/o) emulsions. However, sequential surface modification results in the transition of a single emulsion to double emulsions. In the case of the binary system, the sequential surface modification of the hematite-particle-stabilized o/w emulsion can be converted into double emulsions of o/w/o type. However, in the case of the ternary system, i.e., in the presence of silica particles, sequential surface modification of hematite particles stabilizes both single (o/w) and double (w/o/w and o/w/o) emulsions. The critical concentration of oleic acid required to form a double emulsion is observed to be dependent on the ratio of the surface area of the silica particle to the total surface area of particles (S) and mixing protocols. A study of the size distribution of oil and water droplets of double emulsions shows that droplet size can be controlled by oleic acid concentration and magnitude of S. The arrangements of the particles at interfaces are visualized by SEM imaging. In this way, we developed an easy and novel single-step method of double emulsion preparation and provide a strategy to tailor the formation of different types of emulsions with a single/binary particle system by sequential in situ surface modification of the particles.

Single-Step Formation of Pickering Double Emulsions by Exploiting Differential Wettability of Particles

“…The arrangement of particles on the surface of the inner oil drops and the surface of larger water drops in the O/W/O emulsions were visualized by SEM. The emulsion samples were first freeze-dried (Christ, Alpha 1–2 LDplus, Germany), sputter-coated with gold for 20 s, and finally observed under a scanning electron microscope …”

“…The emulsion samples were first freeze-dried (Christ, Alpha 1−2 LDplus, Germany), sputter-coated with gold for 20 s, and finally observed under a scanning electron microscope. 30…”

“…DEs are classified as either water-in-oil-in-water (W/O/W) or oil-in-water-in-oil (O/W/O), depending on the order of the phases addition and the type of primary emulsion used. ,− Several studies have exploited the two-stage emulsification method for the formation of DEs stabilized by different materials such as poly­(ethylene imine) modified silica particles, cellulose nanofibers, block copolymers, carboxymethyl chitosan, combination of hydrophobic and hydrophilic nanoparticles, Fe 2 O 3 nanoparticles and hydrophobic fumed silica, CaCO 3 , and SDS . Tiwari et al recently demonstrated the use of a combination of oppositely charged particles along with oleic acid as strategy to render in situ particle surface modification as a simple strategy for the design of Pickering DEs which involves the sequential addition of oleic acid and multiple emulsification steps.…”

One-Step Formation of Pickering Double Emulsion Costabilized by Hydrophobic Silica Nanoparticles and Sodium Alginate

“…Due to its ease of use and effectiveness, one-step emulsification has recently attracted increasing attention for preparing Pickering double emulsions in situations near phase inversion . Typically, this emulsification uses a single particle stabilizer, mainly including inorganic particles and synthetic polymer particles (micelles, microgel, and latex), with either intermediate hydrophobicity or a distribution of wettability. − For example, Bai et al prepared O/W/O Pickering double emulsions using palmitoyl chloride to increase the hydrophobicity of diatomite particles . A W/O/W Pickering double emulsion with amphiphilic Janus particles that have a hydrophilic area rich in acrylic acid and a hydrophobic area rich in styrene was described by Lee et al Nonomura et al found that silicone particles shaped like a bowl with varying wettability caused by contact angle hysteresis due to solvent entrapment can stabilize Pickering double emulsions .…”

Bottom-Up Synthesis of Multicompartmentalized Microreactors for Continuous Flow Catalysis

“…Multiple Pickering emulsions that are stabilized by colloidal particles are emerging as a promising strategy for constructing multicompartmentalized microreactors. , Because the high adsorption energy of colloidal particles at the droplet surface renders them more stable than usual surfactant-based soft material systems . Apart from the stability, fabrication of Pickering emulsions has a broad range of liquids for choice since any two immiscible liquids can in principle form Pickering emulsions .…”