Biocompatible Stimuli-Responsive W/O/W Multiple Emulsions Prepared by One-Step Mixing with a Single Diblock Copolymer Emulsifier

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

We present a modular single-step strategy for the formation of single and Pickering double emulsions (DEs). To this end, we consider the role of surface modification of particles and their dispersibility in different phases in the context of the design of Pickering emulsions by varying the volume fraction of oil in the oil-water mixture (ϕoil) used for emulsification. In particular, the experiments are performed by considering (a) model spherical and nonspherical colloids of different wettabilities which are tailored by oleic acid treatment, (b) immiscible liquids with or without particles, and (c) varying ϕoil from 0.1 to 0.9. We show that it is possible to affect a transition from (i) oil-in-water (O/W) emulsion to water-in-oil (W/O) emulsion and (ii) oil-in-water (O/W) to oil-in-water-in-oil (O/W/O) to water-in-oil (W/O) as ϕoil is systematically varied. We elucidate that the range of ϕoil at which particle stabilized DEs of the O/W/O type form can be tuned by engineering surface modification of particles to different extents. Furthermore, the arrangement of particles on the surface of droplets in the Pickering DEs is discussed. Our results conclusively establish that the differential wettability of particles is the key for the design of Pickering DEs. The versatility of the proposed strategy is established by developing DEs using a number of model colloidal systems.

Fabrication and stability of W/O/W emulsions stabilized by gum arabic and polyglycerol polyricinoleate

BACKGROUND

In order to study the effect of adsorption of surfactant at the two interfacial layers on emulsion stability, the kinetically stable water-in-oil-in-water (W/O/W) emulsion carriers were prepared using polyglycerol polyricinoleate (PGPR) and gum arabic (GA) as emulsifiers. The relationship between the adsorption of the surfactant and the stability mechanism of the emulsions was elucidated.

RESULTS

When the contents of PGPR and GA were low, the interfaces between oil and the inner and outer water phases, respectively, could not be completely covered. However, when the concentration of PGPR was higher than 60 g kg-1 , the excess PGPR was adsorbed on the interface between the oil phase and the outer water phase. When the concentration of GA reached 80 g kg-1 , more GA was adsorbed to the oil-in-water interface. Moreover, the presence of PGPR on the interface could reduce the adsorption capacity of GA. Two types of kinetically stable emulsions were obtained by optimizing the interface composition (60 g kg-1 GA/80 g kg-1 PGPR and 60 g kg-1 PGPR/80 g kg-1 GA). The kinetically stable W/O/W emulsions prepared in this study were successfully used to encapsulate a hydrophilic vitamin (vitamin B12) with an encapsulation efficiency (EE) of 80% and release efficiency (RE) of 95%. The interfacial adsorption GA can accelerate the hydrolysis of fat.

CONCLUSION

Overall, this study provides a new strategy for the preparation of W/O/W emulsions, which might be beneficial for application in food, cosmetic, chemical, and pharmaceutical industries. © 2023 Society of Chemical Industry.

Double Emulsions Stabilized by PGPR and Arabic Gum as Capsules: The Surprising Stabilizing Role of Inner Droplets

The encapsulation efficiency and stability over time of either vitamin B12, a model hydrophilic drug, or an aqueous suspension of Cydia pomonella granulovirus (CpGV), which is a biopesticide, using a water-in-sunflower oil-in-water (W1/O/W2) double emulsion, are studied. Two antagonistic stabilizers are used to prepare the double emulsion: the mainly lipophilic polyglycerol polyricinoleate (PGPR) and the mainly hydrophilic polysaccharide Arabic gum (AG). Combining ultraviolet-visible (UV-visible) titration, rheology, and oil globule size measurement allows assessing drug release, emulsion elasticity, and globule evolution as a function of time. A stability diagram is plotted as a function of two determining parameters: the nonadsorbed PGPR concentration in the oil and the inner water droplet fraction. To understand the presence of the nonstability domains, the influence of the two identified parameters on the outermost interfacial tension is examined. Surprisingly, the inner water drop volume fraction exhibits a stabilizing phenomenon that is discussed in terms of interfacial shielding to PGPR adsorption.

Double Emulsion Droplets as a Plausible Step to Fatty Acid Protocells

It is assumed that life originated on the Earth from vesicles made of fatty acids. These amphiphiles are the simplest chemicals, which can be present in the prebiotic soup, capable of self-assembling into compartments mimicking modern cells. Production of fatty acid vesicles is widely studied, as their growing and division. However, how prebiotic chemicals require to further yield living cells encapsulated within protocells remains unclear. Here, one suggests a scenario based on recent studies, which shows that phospholipid vesicles can form from double emulsions affording facile encapsulation of cargos. In these works, water-in-oil-in-water droplets are produced by microfluidics, having dispersed lipids in the oil. Dewetting of the oil droplet leaves the internal aqueous droplet covered by a lipid bilayer, entrapping cargos. In this review, formation of fatty acid protocells is briefly reviewed, together with the procedure for preparing double emulsions and vesicles from double emulsion and finally, it is proposed that double emulsion droplets formed in the deep ocean where undersea volcano expulsed materials, with fatty acids (under their carboxylic form) and alkanols as the oily phase, entrapping hydrosoluble prebiotic chemicals in a double emulsion droplet core. Once formed, double emulsion droplets can move up to the surface, where an increase of pH, variation of pressure and/or temperature may have allowed dewetting of the oily droplet, leaving a fatty acid vesicular protocell with encapsulated prebiotic materials.

One step generation of single-core double emulsions from polymer-osmose-induced aqueous phase separation in polar oil droplets

Water-in-oil-in-water emulsions (W/O/W) are aqueous droplet(s) embedded within oil droplets dispersed in a continuous water phase. They are attracting interest due to their possible applications from cosmetic to food science since both hydrosoluble and liposoluble cargos can be encapsulated within. They are generally prepared using a one-step or a two-step method, phase inversion and also via spontaneous emulsification. Here, we describe a general and simple one-step method based on hydrophilic polymers dispersed in polar oils to generate osmose-induced diffusion of water into oil droplets, forming polymer-rich aqueous droplets inside the oil droplets. Polyethylene glycol, but also other hydrophilic polymers (branched polyethylene imine or polyvinyl pyrrolidone) were successfully dispersed in 1-octanol or other polar oils (oleic acid or tributyrin) to produce an O/W emulsion that spontaneously transformed into a W1/O/W2 emulsion, with the inner aqueous droplet (W1) only containing the hydrophilic polymer initially dispersed in oil. By combining single drop experiments, with macroscopic viscosity measurements, we demonstrated that the double emulsion resulted of water diffusion, which amplitude could be adjusted by the polymer concentration. The production of high internal phase emulsions was also achieved, together with a pH-induced transition from multiple to single core double emulsion. We expect this new method for producing double emulsions to find applications in domains of microencapsulation and materials chemistry.

Formation and stabilization of multiple w/o/w emulsions encapsulating catechin, by mechanical and microfluidic methods using a single pH-sensitive copolymer: effect of copolymer/drug interaction

Multiple w/o/w emulsions (MEs) are promising systems for protecting fragile hydrophilic drugs and controlling their release. We explore the capacity of a single pH-sensitive copolymer, PDMS₆₀-b-PDMAEMA₅₀, and salts, to form and stabilize MEs loaded with sucrose or catechin by a one-step mechanical process or a microfluidic method. ME cytotoxicity was evaluated in various conditions of pH. Using the mechanical process, the most stable emulsions were obtained with Miglyol®812N and isopropyl myristate in a final pH range of 8-12 and [0.3 M-1 M] NaCl concentrations. Conversely, with the microfluidic method, isopropyl myristate at pH 3 without salt was more efficient. Catechin strongly affected the formation of droplets by the mechanical process but did not modify the conditions of stability of MEs obtained by the microfluidic method. The antioxidant power of catechin was preserved in the inner droplets, even in emulsions prepared by the mechanical method at pH 8. An incomplete release of sucrose and catechin from the emulsions was observed and attributed to the interaction of molecules with the copolymer through hydrogen bonding. This study highlights some of the barriers to break to formulate multiple emulsions stabilized by a PDMS-b-PDMAEMA copolymer or other polymers which can form hydrogen bonds interaction with encapsulated drugs.

Multiple Pickering emulsions stabilized by surface-segregated micelles with adaptive wettability

Surface-segregated micelles (SSMs) with adaptive wettability have considerable potential for application in Pickering emulsions and bioanalytical technology. In this study, spherical SSMs were prepared via polymerization-induced self-assembly co-mediated with a binary mixture of macromolecular chain transfer agents: pH-responsive poly(2-(dimethylamino) ethyl methacrylate) and hydrophobic polydimethylsiloxane. Using these SSMs as the sole emulsifier, we adjusted the pH to successfully produce both water-in-oil-in-water (W/O/W) and oil-in-water-in-oil (O/W/O) multiple emulsions through a single-step emulsification process. Moreover, we demonstrated that multiple emulsion systems with adjustable pH are suitable for the development of an efficient and recyclable interfacial catalytic system. Multiple emulsion microreactors increase the area of the oil–water interface and are therefore more efficient than the commonly used O/W and W/O emulsion systems.

Surface-segregated micelles (SSMs) with adaptive wettability have considerable potential for application in Pickering emulsions and microreactors.

Zwitterionic Ammonium Sulfonate Polymers: Synthesis and Properties in Fluids

Polymer zwitterions continue to emerge as useful materials for numerous applications, ranging from hydrophilic and antifouling coatings to electronic materials interfaces. While several polymer zwitterion compositions are now well established, interest in this field of soft materials science has grown rapidly in recent years due to the introduction of new structures that diversify their chemistry and architecture. Nonetheless, at present, the variation of the chemical composition of the anionic and cationic components of zwitterionic structures remains relatively limited to a few primary examples. In this article, we highlight the versatility of 4-vinylbenzyl sultone as a precursor to ammonium sulfonate zwitterionic monomers, which are then used in controlled free radical polymerization chemistry to afford "inverted sulfobetaine" polymer zwitterions. An evaluation of the solubility, interfacial activity, and solution configuration of the resultant polymers revealed the dependence of properties on the selection of tertiary amines used for nucleophilic ring-opening of the sultone precursor, as well as useful properties comparisons across different zwitterionic compositions. This article is protected by copyright. All rights reserved.

Tuning Supramolecular Polymers' Amphiphilicity via Host-Guest Interfacial Recognition for Stabilizing Multiple Pickering Emulsions

Supramolecular host-guest chemistry bridging the adjustable amphiphilicity and macromolecular self-assembly is well advanced in aqueous media. However, the interfacial self-assembled behaviors have not been further exploited. Herein, we designed a β-cyclodextrin-grafted alginate/azobenzene-functionalized dodecyl (Alg-β-CD/AzoC12) supra-amphiphilic system that possessed tunable amphiphilicity by host-guest interfacial self-assembly. Especially, supra-amphiphilic aggregates could be utilized as highly efficient soft colloidal emulsifiers for stabilizing water-in-oil-water (W/O/W) Pickering emulsions due to the excellent interfacial activity. Meanwhile, the assembled particle structures could be modulated by adjusting the oil-water ratio, resulting from the tunable aggregation behavior of supra-amphiphilic macromolecules. Additionally, the interfacial adsorption films could be partially destroyed/reconstructed upon ultraviolet/visible irradiation due to the stimuli-altering balance of amphiphilicity of Alg-β-CD/AzoC12 polymers, further constructing the stimulus-responsive Pickering emulsions. Therefore, the supramolecular interfacial self-assembly-mediated approach not only technologically advances the continued development of creative templates to construct multifunctional soft materials with anisotropic structures but also serves as a creative bridge between supramolecular host-guest chemistry, colloidal interface science, and soft material technology.

Redox-Driven Spontaneous Double Emulsion

The formation of spontaneous double emulsions is a peculiar phenomenon in emulsion systems. When compared to the traditional one-step and two-step methods for preparing double emulsions, spontaneous emulsification can not only steadily load uniform water droplets into an oil phase, but can also facilitate the preparation of emulsions with higher stability. However, the limited solubility of salts, which are typically used to modify osmotic pressure, in organic oils has inhibited the viability of this method for the preparation of W/O/W double emulsions. In this paper, a redox-driven spontaneous emulsification method is developed and investigated. Instead of employing oil-soluble salts, an oxidation reaction is implemented in the oil phase, which produces cation radicals and iodide counterions to generate osmotic pressure. Additionally, amphiphilic polymer chains are harnessed as stabilizers for the newly formed W/O interfaces. Various characterization methods have been used to elucidate the mechanism of both the oxidation reaction and the spontaneous formation of double emulsions.

The Horizon of the Emulsion Particulate Strategy: Engineering Hollow Particles for Biomedical Applications

With their hierarchical structures and the substantial surface areas, hollow particles have gained immense research interest in biomedical applications. For scalable fabrications, emulsion-based approaches have emerged as facile and versatile strategies. Here, the recent achievements in this field are unfolded via an "emulsion particulate strategy," which addresses the inherent relationship between the process control and the bioactive structures. As such, the interior architectures are manipulated by harnessing the intermediate state during the emulsion revolution (intrinsic strategy), whereas the external structures are dictated by tailoring the building blocks and solidification procedures of the Pickering emulsion (extrinsic strategy). Through integration of the intrinsic and extrinsic emulsion particulate strategy, multifunctional hollow particles demonstrate marked momentum for label-free multiplex detections, stimuli-responsive therapies, and stem cell therapies.

Emulsion Solvent Evaporation-Induced Self-Assembly of Block Copolymers Containing pH-Sensitive Block

A simple yet efficient method is developed to manipulate the self-assembly of pH-sensitive block copolymers (BCPs) confined in emulsion droplets. Addition of acid induces significant variation in morphological transition (e.g., structure and surface composition changes) of the polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) assemblies, due to the hydrophobic-hydrophilic transition of the pH-sensitive P4VP block via protonation. In the case of pH > pKa_(P4VP) (pKa _(P4VP) = 4.8), the BCPs can self-assemble into pupa-like particles because of the nearly neutral wetting of PS and P4VP blocks at the oil/water interface. As expected, onion-like particles obtained when pH is slightly lower than pKa_(P4VP) (e.g., pH = 3.00), due to the interfacial affinity to the weakly hydrophilic P4VP block. Interestingly, when pH was further decreased to ∼2.5, interfacial instability of the emulsion droplets was observed, and each emulsion droplet generated nanoscale assemblies including vesicles, worm-like and/or spherical micelles rather than a nanostructured microparticle. Furthermore, homopolymer with different molecular weights and addition ratio are employed to adjust the interactions among copolymer blocks. By this means, particles with hierarchical structures can be obtained. Moreover, owing to the kinetically controlled processing, we found that temperature and stirring speed, which can significantly affect the kinetics of the evaporation of organic solvent and the formation of particles, played a key role in the morphology of the assemblies. We believe that manipulation of the property for the aqueous phase is a promising strategy to rationally design and fabricate polymeric assemblies with desirable shapes and internal structures.

Formulating Polyethylene Glycol as Supramolecular Emulsifiers for One-Step Double Emulsions

One-step double emulsions via only one-step emulsification are leading to an attractive branch of emulsion research studies owing to the ease of preparation and reduced surfactant numbers. In addition to controlling the oil/water ratio, exploiting emulsifiers with desirable amphiphilicity that can stabilize both the inner and outer water/oil interfaces is crucial to the formation of one-step double emulsions. In particular, new emulsifiers with saving laborious efforts are highly preferred in consideration of low cost and practical applications. In this work, a commonly used homopolymer, polyethylene glycol (PEG), was attempted as emulsifiers to prepare emulsions via one-step emulsification. PEG is generally considered as a hydrophilic polymer and always anchored with a hydrophobic polymer to make the copolymer amphiphilic. In the water-chloroform binary system, PEG itself exhibits amphiphilic performance and tailors the formation of single emulsions or double W/O/W emulsions on the dependence of the oil/water ratio and the PEG concentration. A possible mechanism as explained by dissipative particle dynamics simulation was proposed to demonstrate the amphiphilic feature and emulsification capability of PEG. The amphiphilicity of PEG was further tuned by interacting with iodine as a result of the formation of a supramolecular complex, which, in turn, led to the conversion from single emulsions to O/W/O double emulsions. It is believed that this line of research provides inspiration for the preparation of controllable emulsions through supramolecular routes.