Surfactant-enhanced liquid-liquid extraction in microfluidic channels with inline electric-field enhanced coalescence

Continuous microfluidic liquid-liquid extraction is realized in a microfluidic device by generating emulsions with large interfacial areas for mass transfer, and subsequently breaking these emulsions using electric fields into easily separated segments of immiscible liquids (plugs). The microfluidic device employs insulated electrodes in a potassium hydroxide-etched channel to create large electric fields (100 kV m(-1)) that drive coalescence of the emulsion phase. The result is a transition from disperse to slug flow that can then readily be separated by gravity. Extractions of phenol and p-nitrophenol from an aqueous to hexane-surfactant solution serve as model systems. In addition to the increased surface area in the emulsion, extraction efficiency is enhanced by reverse micelles resulting from the presence of surfactants. The surfactant concentration is varied approximately 1-10 wt% and a general two-parameter model is developed to quantify the extraction behavior and demonstrate the effectiveness of reverse micelle enhanced extraction.     

Effects of mucin addition on the stability of oil–water emulsions

In this work, bovine submaxillary gland mucin (BSM) was used as an emulsifier to stabilize oil–water emulsion systems. Prior to use, commercial BSM was purified by jacalin affinity chromatography. Emulsions consisting of 5% mineral oil in phosphate buffered saline (PBS) were prepared through the addition of different amounts of purified mucin followed by sonication using either of two methods: (1) low energy input for a long time (2 h), or (2) high energy input for a short time (20 s). The surfactancy property of mucin was investigated by surface tension measurements, which showed the BSM to greatly reduce the surface tension of PBS. Compared to several synthetic surfactants of the Pluronic® type, mucin showed comparable or better surface activity than F68, F88 and F108 products in dilute solutions. The formed emulsions had a mean droplet size that decreased monotonically with increasing concentration of mucin until a plateau was reached at concentrations around 0.1% by weight. The stability of these emulsions was evaluated by monitoring their average droplet size during a 33-day period. Emulsions with more than 0.25% mucin showed a constant mean size throughout the period. Specifically, an emulsion produced with 0.95% mucin showed a stable mean droplet size of about 300 nm. The stability of the mucin-emulsified systems was also evaluated by measuring turbidity changes with time, which allowed a comparison with similar emulsions stabilized by the Pluronic® surfactants in the same concentration. Thus, mucin showed its ability to establish more stable and more efficient oil–water emulsion systems. Since mucin is a glycoprotein, and hence biodegradable, our results suggest that mucin might serve as an ideal biological surfactant for the stabilization of emulsion systems intended for biomedical and pharmaceutical applications.     

One-step formation of w/o/w multiple emulsions stabilized by single amphiphilic block copolymers

Multiple emulsions are complex polydispersed systems in which both oil-in-water (O/W) and water-in-oil (W/O) emulsion exists simultaneously. They are often prepared accroding to a two-step process and commonly stabilized using a combination of hydrophilic and hydrophobic surfactants. Recently, some reports have shown that multiple emulsions can also be produced through one-step method with simultaneous occurrence of catastrophic and transitional phase inversions. However, these reported multiple emulsions need surfactant blends and are usually described as transitory or temporary systems. Herein, we report a one-step phase inversion process to produce water-in-oil-in-water (W/O/W) multiple emulsions stabilized solely by a synthetic diblock copolymer. Unlike the use of small molecule surfactant combinations, block copolymer stabilized multiple emulsions are remarkably stable and show the ability to separately encapsulate both polar and nonpolar cargos. The importance of the conformation of the copolymer surfactant at the interfaces with regards to the stability of the multiple emulsions using the one-step method is discussed.     

Emulsions and microemulsions with a fluorocarbon phase

A phase III clinical study of a perfluorooctyl bromide emulsion demonstrated reduction and avoidance of donor blood transfusion in surgery. Novel fluorocarbon-in-water emulsions are being investigated, including emulsions highly stabilized by fluorocarbon–hydrocarbon diblocks and targeted emulsions for molecular imaging, diagnosis and drug delivery. Reverse water-in-fluorocarbon emulsions and microemulsions that have potential for pulmonary drug delivery are also being studied. Microemulsions with highly fluorinated components are being actively investigated, with applications in polymerization technology and as research tools.     

Pickering stabilization of foams and emulsions with particles of biological origin

The focus in the study of Pickering foams and emulsions has recently been shifting from using inorganic particles to adopting particles of biological origin for stabilization. This shift is motivated by the incompatibility of some inorganic particles for food and biomedical applications, as well as their poor sustainability. This review focuses on major developments in foams and emulsions stabilized by particles of biological origin from the last 5 years. Recent reports in the literature have demonstrated the ability of particles derived from cellulose, lignin, chitin, starch, proteins (soy, zein, ferritin), as well as hydrophobic cells to stabilize biphasic dispersions. We review the use of such nano- and micron-sized particles of biological origin for the stabilization of foams and emulsions, summarize the current knowledge of how such particles stabilize these dispersions, provide an outlook for future work to improve our understanding of bio-derived particle-stabilized foams and emulsions, and touch upon how these systems can be used to create novel materials.     

Controllable preparation of monodisperse O/W and W/O emulsions in the same microfluidic device

This letter presents a simple way to prepare monodisperse O/W and W/O emulsions in the same T-junction microfluidic device just by changing the wetting properties of the microchannel wall with different surfactants. Highly uniform droplets ranging from 50 to 400 mum with a polydispersity index (sigma) value of less than 2% were successfully prepared. With the change in surfactants and surfactant concentrations, the interfacial tension and the wetting properties varied, and disordered or ordered two-phase flow patterns could be controllable. Monodisperse O/W and W/O emulsions were prepared under the action of a cross-flowing shear force or a perpendicular shear force by using an oil solution with 0.1-2.0 wt % Span 80 and an aqueous solution with 0.1-2.0 wt % Tween 20 as a continuous-phase flow, respectively. It gives a controllable method of preparing O/W and W/O emulsions in the same microfluidic device.     

Influence of microgel architecture and oil polarity on stabilization of emulsions by stimuli-sensitive core-shell poly(N-isopropylacrylamide-co-methacrylic acid) microgels: Mickering versus Pickering behavior?

Charged poly(N-isopropylacrylamide-co-methacrylic acid) [P(NiPAM-co-MAA)] microgels can stabilize thermo- and pH-sensitive emulsions. By placing charged units at different locations in the microgels and comparing the emulsion properties, we demonstrate that their behaviors as emulsion stabilizers are very different from molecular surfactants and rigid Pickering stabilizers. The results show that the stabilization of the emulsions is independent of electrostatic repulsion although the presence and location of charges are relevant. Apparently, the charges facilitate emulsion stabilization via the extent of swelling and deformability of the microgels. The stabilization of these emulsions is linked to the swelling and structure of the microgels at the oil-water interface, which depends not only on the presence of charged moieties and on solvent polarity but also on the microgel (core-shell) morphology. Therefore, the internal soft and porous structure of microgels is important, and these features make microgel-stabilized emulsions characteristically different from classical, rigid-particle-stabilized Pickering emulsions, the stability of which depends on the surface properties of the particles.     

Designer polymer-based microcapsules made using microfluidics

Filled microcapsules made from double emulsion templates in microfluidic devices are attractive delivery systems for a variety of applications. The microfluidic approach allows facile tailoring of the microcapsules through a large number of variables, which in turn makes these systems more challenging to predict. To elucidate these dependencies, we start from earlier theoretical predictions for the size of double emulsions and present quantitative design maps that correlate parameters such as fluid flow rates and device geometry with the size and shell thickness of monodisperse polymer-based capsules produced in microcapillary devices. The microcapsules are obtained through in situ photopolymerization of the middle oil phase of water-in-oil-in-water double emulsions. Using polymers with selected glass transition temperatures as the shell material, we show through single capsule compression testing that hollow capsules can be prepared with tunable mechanical properties ranging from elastomeric to brittle. A quantitative statistical analysis of the load at rupture of brittle capsules is also provided to evaluate the variability of the microfluidic route and assist the design of capsules in applications involving mechanically triggered release. Finally, we demonstrate that the permeability and microstructure of the capsule shell can also be tailored through the addition of cross-linkers and silica nanoparticles in the middle phase of the double emulsion templates.     

Effects of emulsifier charge and concentration on pancreatic lipolysis. 1. In the absence of bile salts

An in vitro study is performed with sunflower oil-in-water emulsions to clarify the effects of type of used emulsifier, its concentration, and reaction time on the degree of oil lipolysis, α. Anionic, nonionic, and cationic surfactants are studied as emulsifiers. For all systems, three regions are observed when surfactant concentration is scaled with the critical micelle concentration, C(S)/cmc: (1) At C(S) < cmc, α ≈ 0.5 after 30 min and increases up to 0.9 after 4 h. (2) At C(S) ≈ 3 × cmc, α ≈ 0.15 after 30 min and increases steeply up to 0.9 after 2 h for the cationic and nonionic surfactants, whereas it remains around 0.2 for the anionic surfactants. (3) At C(S) above certain threshold value, α = 0 for all studied surfactants, for reaction time up to 8 h. Additional experiments show that the lipase hydrolyzes molecularly soluble substrate (tributirin) at C(S) > cmc, which is a proof that these surfactants do not denature or block the enzyme active center. Thus, we conclude that the mechanism of enzyme inhibition by these surfactants is the formation of a dense adsorption layer on an oil drop surface, which displaces the lipase from direct contact with the triglycerides.     

Nonaqueous Emulsion Polycondensation Enabled by a Self-Assembled Cage-like Surfactant

Nonaqueous emulsions are crucial for a range of applications based on water-sensitive systems such as controlled polymerizations requiring anhydrous reaction conditions and the stabilization of readily hydrolyzable reagents or pharmacologically active components. However, defined molecular surfactants to stabilize such nonaqueous emulsions are scarce. We introduce a self-assembled coordination cage, decorated with cholesterol functionalities, to serve as a molecular surfactant for various oil-in-oil emulsions of immiscible organic solvents. While the positively charged cage forms the amphiphile's polar moiety, the non-polar cholesterol appendices can bend in a common direction to stabilize the emulsion. Templated by the droplets, polycondensation reactions were carried out to produce microstructured polyurethane and polyurea materials of different particle sizes and morphologies. Further, the amphiphilic cage can encapsulate a guest molecule and the resulting host-guest assembly was also examined as a surfactant. In addition, the aggregation behavior of the amphiphilic cage in an aqueous medium was examined.     

Droplet-based microfluidic platforms for the encapsulation and screening of Mammalian cells and multicellular organisms

High-throughput, cell-based assays require small sample volumes to reduce assay costs and to allow for rapid sample manipulation. However, further miniaturization of conventional microtiter plate technology is problematic due to evaporation and capillary action. To overcome these limitations, we describe droplet-based microfluidic platforms in which cells are grown in aqueous microcompartments separated by an inert perfluorocarbon carrier oil. Synthesis of biocompatible surfactants and identification of gas-permeable storage systems allowed human cells, and even a multicellular organism (C. elegans), to survive and proliferate within the microcompartments for several days. Microcompartments containing single cells could be reinjected into a microfluidic device after incubation to measure expression of a reporter gene. This should open the way for high-throughput, cell-based screening that can use >1000-fold smaller assay volumes and has approximately 500x higher throughput than conventional microtiter plate assays.     

疏水缔合聚合物稳定乳状液的研究*

本文综述了近年来疏水缔合聚合物稳定乳状液的研究进展。论述了疏水缔合聚合物水溶液的性质,由于其较复杂的分子结构以及其分子主链上疏水基团的缔合作用,使其水溶液增稠的能力比小分子表面活性剂的增稠能力强的多。另外,对疏水缔合聚合物单独稳定乳状液的研究现状进行了介绍,其稳定乳状液的机理与小分子表面活性剂不同。同时讨论了疏水缔合聚合物与表面活性剂的相互作用,此类聚合物可与小分子表面活性剂通过静电和疏水缔合发生强烈的相互作用形成复合体系,并评述了其复配体系稳定乳状液的情况。最后总结了疏水缔合聚合物稳定乳状液的机理。     

Water-in-carbon dioxide emulsions stabilized with hydrophobic silica particles

W/C emulsions were stabilized using hydrophobic silica particles adsorbed at the interface, resulting in average droplet diameters as low as 7.5 microm. A porous cross-linked shell was formed about a hydrophilic (colloidal and fumed) silica core with a trifunctional silylating agent, (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethyoxysilane, to render the particles CO(2)-philic. The stability of emulsions comprising equal weights of CO(2) and water was assessed with visual observations of settling fronts and the degree of emulsion coalescence, and the average drop size was measured by optical microscopy. The effect of CO(2) density on both emulsion stability and droplet size was determined quantitatively. The major destabilizing mechanism of the emulsions was settling, whereas Ostwald ripening and coalescence were not visible at any density, even over 7 days. Flocculation of the settling droplets did not occur, although gelation of the emulsions through particle interactions resulted after longer periods of time. CO(2)-philic particles offer a new route to highly stable W/C emulsions, with particle energies of attachment on the order of 10(6)kT, even at CO(2) densities as low as 0.78 g ml(-1). At these low densities, surfactants rarely stabilize emulsions as the result of poor surfactant tail solvation.     

Molecular dynamics simulation of cellulose-coated oil-in-water emulsions

The behaviors of cellulose chains and cellulose mini-crystal in oil-in-water emulsions were studied by molecular dynamics simulations to investigate the coating states and the structural features of cellulose in these emulsions. In oil-in-water emulsion, dispersed cellulose chains gradually assemble during the progress of the simulation, eventually surrounding the octane droplet. In case of a cellulose mini-crystal, the cellulose chain at the corner of the crystal first contacts with the octane droplet through its hydrophobic surface. The other cellulose chains along the crystal plane then gradually move toward the octane molecules. In both emulsions, the cellulose was found to interact with both water and octane surfaces with specific conformations that allow the CH groups of the glucose rings to contact with octane molecules, while the OH groups of these rings contact with water molecules to form hydrogen bonds. The cellulose chains on the octane droplet also contact with each other through lateral hydrogen bonding between chains. These interactions stabilize the emulsion formed by cellulose molecules as surfactants.     

Coalescence during emulsification. 2. Role of small molecule surfactants

An oil-soluble hexadecyl pyrene (HDP) probe is used to monitor coalescence of hexadecane oil-in-water emulsions, during emulsification, in stirred systems and in a high-pressure homogenizer (microfluidizer), when small molecule surfactants are used as emulsifiers. The effect of sodium dodecyl sulfate concentration and salt concentration on the amount of coalescence and final drop size is studied. The behavior of oil-soluble surfactants and mixtures of oil-soluble and water-soluble surfactants on emulsification performance is also discussed. For high-pressure homogenizers, the drop sizes obtained are found to depend mostly on the ability of surfactants to stabilize the drops against coalescence, rather than their ability to reduce the interfacial tension. Increasing oil phase fractions increase the coalescence rate, because of the increase in collision frequency, which, in turn, impacts the drop size of the homogenized emulsion.     

细菌纤维素纳米纤维稳定乳液的性能

采用超声和高压均质两种方式分散的细菌纤维素(BC)悬浮液制备了BC纳米纤维稳定的水包油型Pickering乳液, 并考察了纤维用量、 pH值和机械分散方式对乳液稳定性的影响. 结果表明, 乳液的稳定性随纳米纤维用量的增加而增加; 碱性条件比酸性条件制备的乳液稳定性高, 且在pH=12时达到最高. 用高压均质方式分散的BC稳定乳液的效果优于采用超声方式分散的BC的效果, 这是由于高压均质后的纤维较短, 可以提供更多的纳米纤维稳定乳液. 计算结果表明, BC纳米纤维在液体石蜡/水界面上的三相接触角为72.5°, 说明BC适合稳定水包油型乳液.     

Surfactants in droplet-based microfluidics

Surfactants are an essential part of the droplet-based microfluidic technology. They are involved in the stabilization of droplet interfaces, in the biocompatibility of the system and in the process of molecular exchange between droplets. The recent progress in the applications of droplet-based microfluidics has been made possible by the development of new molecules and their characterizations. In this review, the role of the surfactant in droplet-based microfluidics is discussed with an emphasis on the new molecules developed specifically to overcome the limitations of 'standard' surfactants. Emulsion properties and interfacial rheology of surfactant-laden layers strongly determine the overall capabilities of the technology. Dynamic properties of droplets, interfaces and emulsions are therefore very important to be characterized, understood and controlled. In this respect, microfluidic systems themselves appear to be very powerful tools for the study of surfactant dynamics at the time- and length-scale relevant to the corresponding microfluidic applications. More generally, microfluidic systems are becoming a new type of experimental platform for the study of the dynamics of interfaces in complex systems.     

Interface composition of multiple emulsions: rheology as a probe

We have investigated the dynamic rheological properties of concentrated multiple emulsions to characterize their amphiphile composition at interfaces. Multiple emulsions (W1/O/W2) consist of water droplets (W1) dispersed into oil globules (O), which are redispersed in an external aqueous phase (W2). A small-molecule surfactant and an amphiphilic polymer were used to stabilize the inverse emulsion (W1 in oil globules) and the inverse emulsion (oil globules in W2), respectively. Rheological and interfacial tension measurements show that the polymeric surfactant adsorbed at the globule interface does not migrate to the droplet interfaces through the oil phase. This explains, at least partly, the stability improvement of multiple emulsions as polymeric surfactants are used instead of small-molecule surfactants.     

New amphiphilic diblock copolymers: surfactant properties and solubilization in their micelles

Several series of amphiphilic diblock copolymers are investigated as macrosurfactants in comparison to reference low-molar-mass and polymeric surfactants. The various copolymers share poly(butyl acrylate) as a common hydrophobic block but are distinguished by six different hydrophilic blocks (one anionic, one cationic, and four nonionic hydrophilic blocks) with various compositions. Dynamic light scattering experiments indicate the presence of micelles over the whole concentration range from 10(-4) to 10 g x L(-1). Accordingly, the critical micellization concentrations are very low. Still, the surface tension of aqueous solutions of block copolymers decreases slowly but continuously with increasing concentration, without exhibiting a plateau. The longer the hydrophobic block, the shorter the hydrophilic block, and the less hydrophilic the monomer of the hydrophilic block is, the lower the surface tension is. However, the effects are small, and the copolymers reduce the surface tension much less than standard low-molar-mass surfactants. Also, the copolymers foam much less and even act as anti-foaming agents in classical foaming systems composed of standard surfactants. The copolymers stabilize O/W emulsions made of methyl palmitate as equally well as standard surfactants but are less efficient for O/W emulsions made of tributyrine. However, the copolymer micelles exhibit a high solubilization power for hydrophobic dyes, probably at their core-corona interface, in dependence on the initial geometry of the micelles and the composition of the block copolymers. Whereas micelles of copolymers with strongly hydrophilic blocks are stable upon solubilization, solubilization-induced micellar growth is observed for copolymers with moderately hydrophilic blocks.     

Stabilization of carbon dioxide-in-water emulsions with silica nanoparticles

Stable carbon dioxide-in-water emulsions were formed with silica nanoparticles adsorbed at the interface. The emulsion stability and droplet size were characterized with optical microscopy, turbidimetry, and measurements of creaming rates. The increase in the emulsion stability as the silica particle hydrophilicity was decreased from 100% SiOH to 76% SiOH is described in terms of the contact angles and the resulting energies of attachment for the silica particles at the water-CO(2) interface. The emulsion stability also increased with an increase in the particle concentration, CO(2) density, and shear rate. The dominant destabilization mechanism was creaming, whereas flocculation, coalescence, and Ostwald ripening played only a minor role over the CO(2) densities investigated. The ability to stabilize these emulsions with solid particles at CO(2) densities as low as 0.739 g/mL is particularly relevant in practical applications, given the difficulty in stabilizing these emulsions with surfactants, because of the unusually weak solvation of the surfactant tails by CO(2).