Phase inversion of the Pickering emulsions stabilized by plate-shaped clay particles

We investigated the phase inversion of Pickering emulsions stabilized by plate-shaped clay particles. Addition of water induced a phase inversion from a water-in-oil (W/O) emulsion to an oil-in-water (O/W) emulsion when the amount of the oil phase exceeded a limiting amount of oil absorption to solid particles. On the other hand, a phase inversion from a powdery state to an O/W emulsion state through an oil-separated state is observed when the amount of an oil phase is less than the limiting amount of the oil absorption. Interestingly, the oil separated is re-dispersed as emulsion droplets into the O/W emulsion phase. This type of phase inversion, which is a feature of the Pickering emulsions stabilized by the clay particles, is caused by a change in the aggregate structures of particles.     

Pickering emulsion polymerization of graphene oxide-stabilized styrene

Polystyrene (PS) particles were prepared via Pickering emulsion polymerization using graphene oxide (GO) as the stabilizer. The results show that pH is an important factor in the stability of Pickering emulsions. The effects of two different phase initiators, the water phase initiator potassium persulfate and the oil phase initiator azobisisobutyronitrile, on the morphology of PS particles in Pickering emulsion polymerization had been investigated in detail. Wrinkled particles were prepared using the water phase initiator, and spherical particles were prepared using the oil phase initiator. In addition, hexadecane was used as the auxiliary stabilizer in the polymerization, which narrowed the diameter distribution of the PS spheres, and the hollow PS spheres were fabricated. The size of the GO particles also influenced the final morphology of the particles. Nano-sized polymer particles were grafted onto the surface of micro-sized GO. Small GO particles were suitable for Pickering emulsion polymerization to prepare the composite particles. The thermogravimetric analysis of the prepared particles confirmed that they were PS/GO composite particles, which could have a wide range of potential applications, such as in catalysts, sensors, environmental remediation, and energy storage.     

Synthesis of Core-Shell Particles of Polystyrene and Poly(methyl methacrylate) Using Emulsion Photopolymerization

Summary: Submicron core-shell particles of polystyrene (PS) and polystyrene-co-poly(methyl methacrylate) (PS-co-PMMA) coated with PMMA were obtained by emulsion photopolymerization. The seeds of PS or PS-co-PMMA were prepared by emulsion polymerization with or without emulsifier and a ratio of functional monomer and crosslinker (SVBS/EDGMA) in order to obtain different surfaces for the subsequent coating with PMMA. At each stage, the evolution of the average particle size were monitored by using photon correlation spectroscopy (DLS) and the final polymer particles was analyzed via transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The core-shell morphology was identified as the increase of the average particle size in the second stage by DLS technique and by the direct observation by TEM of the differentiation between PS core and PMMA shell, and by the presence of two glass transition temperatures (T_g) as a consequence of the existence of two partially miscible phases.     

Preparation of novel suspensions of ZnO/living block copolymer latex nanoparticles via pickering emulsion polymerization and their long term stability

In this work, we present the first Pickering emulsion polymerization with a controlled/living character. Pickering emulsion polymerization in the presence of a novel suspension of zinc oxide/poly(sodium 4‐styrenesulfonate) (ZnO/PSS^?) nanocomposite particles was applied to prepare ZnO/living block copolymer latexes. In the emulsion system, 1,1‐diphenylethene (DPE)‐controlled radical polymerization of poly(methyl methacrylate)‐b‐poly(butyl acrylate) (PMMA‐b‐PBA) was proceeded in oil phase. The nanocomposite particles of ZnO/PSS^? with an average diameter of 20 nm and negatively charged zeta potential around ?30 mV were synthesized via hydrothermal method then served as an effective emulsion stabilizer at the oil/water interface. Living polymerization was carried out using DPE‐capped PMMA as the macroinitiator and PMMA‐b‐PBA block copolymer latex was successfully prepared with coverage of ZnO/PSS^? nanoparticles. Narrow size distributions of the droplets as well as latex particles were obtained, and the livingness of block copolymers was comparable to that of emulsions stabilized by conventional surfactants. The controlled/living character in Pickering emulsion polymerization was slightly influenced by the amount of PSS^? immobilized into the ZnO/PSS^? nanoparticles, whereas it was significantly influenced by the weight ratios between ZnO/PSS^? and oil phase. The Pickering latexes showed excellent long term stability against either coalescence or sedimentation over several months. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Fat crystals and water-in-oil emulsion stability

Products such as cosmetics, pharmaceuticals, and crude oil often exist as water-in-oil (W/O) emulsions during their processing or in final form. In many cases, their dispersed aqueous phase is encased in a crystal network and/or by interfacially-adsorbed (‘Pickering’) particles [paraffins, triacylglycerols, polymers, etc.] that promote emulsion kinetic stability by hindering droplet–droplet contact, coalescence and macroscopic phase separation. In processed foods, important questions remain regarding whether a continuous phase fat crystal network or Pickering crystal provides better stabilization. This review explores the following factors related to crystal-stabilized W/O emulsions: i) the key properties dictating fat crystal spatial distribution (at the interface or in the continuous phase); ii) how temperature and freeze–thaw emulsion destabilization are intimately linked with fat crystal spatial distribution, and; iii) why oil-soluble surfactant interactions with the continuous oil phase influence fat crystal wettability and emulsifier efficacy. It is shown that these parameters strongly govern W/O emulsion formation and stability.     

Study of emulsion polymerization stabilized by amphiphilic polymer nanoparticles

Submicron-sized polystyrene (PS) microspheres with a relatively narrow particle size distribution can be easily produced through emulsion polymerization induced by γ-ray at room temperature using a new type of amphiphilic cross-linked poly(stearyl methacrylate-co-acrylamide-co-acrylic acid) particles as stabilizer. The properties of these amphiphilic particles were described, including morphology, size, ζ potential, and contact angles. The effect of the pH value and the content of amphiphilic particles on the formation and stability of emulsions were also investigated. Meanwhile, the obtained PS microspheres were characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. In addition, through observing the morphology and size of emulsion droplets at different times under an optical microscope, we found it is interesting that Pickering emulsions formed initially disappeared gradually, which is different from the common Pickering emulsions stabilized by inorganic particles. Thus, the mechanism was further discussed.     

SiO2纳米颗粒与聚氧乙烯对Pickering乳液的协同稳定作用简

研究了聚氧乙烯(PEO)与SiO2纳米颗粒对水/二甲苯体系Pickering乳液的协同稳定作用. 实验发现,PEO的存在减小了乳液液滴的平均直径,抑制了乳液的相反转,有效阻止了乳液的熟化,使乳液具有更好的稳定性. 进一步对纳米颗粒膜的流变性质进行研究,结果表明,PEO高分子促进了纳米颗粒形成更大尺寸的聚集结构,提高了其在界面上的吸附性,增强了颗粒膜的力学性能,在较小颗粒用量条件下使得Gibbs稳定性判据得到满足.     

Effects of Surface Properties on Rheological and Interfacial Properties of Pickering Emulsions Prepared by Fumed Silica Suspensions Pre-Adsorbed Poly(N-Isopropylacrylamide)

The hydrophobic fumed silica suspensions physically pre-adsorbed poly(N-isopropylacrylamide) (PNIPAM) in water could prepare oil dispersed in water (O/W) Pickering emulsion by mixing of silicone oil. The resulting Pickering emulsions were characterized by the measurements of volume factions of emulsified silicone oil, adsorbed amounts of the silica suspensions, oil droplet size, and some rheological responses, such as stress-strain sweep curve and dynamic viscoelastic moduli as a function of the added amount of PNIPAM. Moreover, their characteristics were compared with those of the O/W Pickering emulsions prepared by the hydrophilic fumed silica suspensions pre-adsorbed PNIPAM. For the emulsions prepared by the hydrophobic silica suspensions, an increase in the added amount of PNIPAM led to (1) a decrease in the volume fraction of the emulsified oil in the emulsified phase, (2) both the size of oil droplets and the adsorbed amount of the corresponding silica suspensions being almost constant, except for the higher added amounts, and (3) both the storage modulus (G′) and the yield shear strain being constant. The term of 1 is the same for the emulsions prepared by the hydrophilic silica suspensions, whereas both the adsorbed amount of the corresponding silica suspension and the G′ value increase and both the droplet size and the yield shear strain decrease with an increase in the added amount of PNIPAM. The differences between the rheological properties of the emulsions prepared by the hydrophilic silica suspensions and those by the hydrophobic ones are attributed to the hydrophobic interactions of the flocculated silica particles in the Pickering emulsions.     

Monitoring the formation of polystyrene/silica nanocomposites from vinyl triethoxysilane containing copolymers

Random copolymers of polystyrene-co-polyvinyl triethoxysilane (PS-co-PVTES) were prepared via semi-batch emulsion polymerization with different feed monomer compositions and evaluated as precursors of polystyrene (PS)/silica nanocomposites. Small-angle X-ray scattering (SAXS) profiles acquired from 20 °C to 180 °C showed that, at temperatures higher than glass transition temperature (T _g) of PS, the latex particles aggregate. On thermal annealing at 180 °C, silica-rich domains are formed, as corroborated by scanning electron microscopy. Infrared spectroscopy and differential scanning calorimetry analyses showed a reduction of the silanol concentration and an increase in the T _g value, respectively. The silica long domain spacing, measured by SAXS, depends on the concentration of vinyl triethoxysilane (VTES) in the feed; this value varied from 35 to 57 nm when the weight ratio of the monomers (styrene/VTES) was 50:50 and 90:10, respectively.     

Adsorption of poly(N-isopropylacrylamide-co-4-vinylpyridine) onto core-shell poly(styrene-co-methylacrylic acid) microspheres

Adsorption of the thermoresponsive copolymer of poly(N-isopropylacrylamide-co-4-vinylpyridine) (PNIPAM-co-P4VP) onto the core-shell microspheres of poly(styrene-co-methylacrylic acid) (PS-co-PMAA) is studied. The core-shell PS-co-PMAA microspheres are synthesized by one-stage soap-free polymerization in water. The copolymer of PNIPAM-co-P4VP is synthesized by free radical polymerization of N-isopropylacrylamide and 4-vinylpyridine in the mixture of DMF and water using K₂S₂O₈ as initiator. Adsorption of PNIPAM-co-P4VP onto the core-shell PS-co-PMAA microspheres results in formation of the composite microspheres of PS/PMAA-P4VP/PNIPAM. The driven force to adsorb the copolymer of P4VP-co-PNIPAM onto the core-shell PS-co-PMAA microspheres is ascribed to hydrogen-bonding and electrostatic affinity between the P4VP and PMAA segments. The resultant composite microspheres of PS/PMAA-P4VP/PNIPAM with surface chains of PNIPAM are thermoresponsive in water and show a cloud-point temperature at about 33 °C.     

Fabrication of macroporous polystyrene/graphene oxide composite monolith and its adsorption property for tetracycline

Macroporous polystyrene microsphere/graphene oxide(PS/GO) composite monolith was first prepared using Pickering emulsion droplets as the soft template. The Pickering emulsion was stabilized by PS/GO composite particles in-situ formed in an acidic water phase. With the evaporation of water and the oil phase(octane), the Pickering emulsion droplets agglomerated and combined with each other, forming a three-dimensional macroporous PS/GO composite matrix with excellent mechanical strength. The size of the macrospores ranged from 4 mm to 20 mm. The macroporous PS/GO composite monolith exhibited high adsorption capacity for tetracycline(TC) in an aqueous solution at p H 4–6. The maximum adsorption capacity reached 197.9 mg g 1at p H 6. The adsorption behaviour of TC fitted well with the Langmuir model and pseudo-second-order kinetic model. This work offers a simple and efficient approach to fabricate macroporous GO-based monolith with high strength and adsorption ability for organic pollutants.     

High‐Internal‐Phase Pickering Emulsions Stabilized Solely by Peanut‐Protein‐Isolate Microgel Particles with Multiple Potential Applications

High‐internal‐phase Pickering emulsions have various applications in materials science. However, the biocompatibility and biodegradability of inorganic or synthetic stabilizers limit their applications. Herein, we describe high‐internal‐phase Pickering emulsions with 87 % edible oil or 88 % n‐hexane in water stabilized by peanut‐protein‐isolate microgel particles. These dispersed phase fractions are the highest in all known food‐grade Pickering emulsions. The protein‐based microgel particles are in different aggregate states depending on the pH value. The emulsions can be utilized for multiple potential applications simply by changing the internal‐phase composition. A substitute for partially hydrogenated vegetable oils is obtained when the internal phase is an edible oil. If the internal phase is n‐hexane, the emulsion can be used as a template to produce porous materials, which are advantageous for tissue engineering.     

Synergetic Effect of Reactive Surfactants and Clay Particles on Stabilization of Nonaqueous Oil-in-Oil (o/o) Emulsions

Although surfactants and particles are often used together in stabilization of aqueous emulsions, the contribution of each species to such stabilization at the oil-water interface is poorly understood. The situation becomes more complicated if we consider the nonaqueous oil-oil interface, i.e, the stabilization of nonaqueous oil-in-oil (o/o) emulsions by solid particles and reactive surfactants which, to our knowledge, has not been studied before. We have prepared Pickering nonaqueous simple (o/o) emulsions stabilized by a combination of kaolinite particles and a nonionic polymerizable surfactant Noigen RN10 (polyoxyethylene alkylphenyl ether). Different pairs of immiscible oils were used which gave different emulsion stabilities. Using kaolinite with equal volumes of paraffin oil/formamide system gave no stable emulsions at all concentrations while the addition of Noigen RN10 enhanced the emulsion stability. In contrast, addition of Noigen RN10 surfactant to silicon oil-in-glycerin emulsions stabilized by kaolinite resulted in destabilization of the system at all concentrations. For all systems studied here, no phase inversion in simple emulsion was observed by altering the volume fraction of the dispersed phase as compared to the known water-based simple Pickering emulsions.      

CO2-triggered switchable Pickering emulsion stabilized by guanidine-functionalized silica particles

The guanidine group-modified silica particles were used as emulsifier to obtain a CO₂-responsive Pickering emulsion. To compare the wettability effect of the particles on the stability of the emulsion, both guanidine and alkyl chain were attached on the surface of silica particles. The influences of tension, particles concentration, oil-water fraction, NaCl concentration, and CO₂ on Pickering emulsion properties were investigated. Although the particles did not decrease the surface and interfacial tensions of the air/oil-water interfaces, they attached on the oil–water interfaces and stabilized the emulsions at room temperature for at least 4 weeks. Addition of salt increased the emulsion stability and induced phase inversion at high salt concentration. The stabilization–destabilization cycles of the emulsion could be successively controlled by alternative CO₂/heating triggers due to the protonation-deprotonation of guanidine groups on the particle surfaces.     

A Three-Step Model for Submicron W/O Emulsion Formation in a Transitional-Phase Inversion Process

A three-step model of the transitional phase inversion (TPI) process for the formation of water-in-oil (W/O) emulsions is presented. Three types of emulsions exist in an emulsification process at different oil–water ratios and hydrophilic–lipophilic balance (HLB). A stable W/O emulsion was obtained using Sorbitan oleate (Span 80) and polyoxyethylenesorbitan monooleate (Tween 80) with a specified HLB and oil volume fraction. Oil was added into water, which contained the water-soluble surfactant, to dissolve the oil-soluble surfactant. This route allowed TPI to occur, and an interesting emulsification process was observed by varying the HLB, which corresponded to the change in the oil–water ratio. Two types of emulsions in the emulsification process were found: transition emulsion 1 (W/O/W high internal phase emulsion) and target emulsion 2 (W/O emulsion with low viscosity). This study describes the changes that occurred in the emulsification process.     

Synthesis of bilayer oleic acid-coated Fe3O4 nanoparticles and their application in pH-responsive Pickering emulsions

Fe(3)O(4) nanoparticles coated with oleic acid bilayer (a diameter about 12 nm) were synthesized. The structure and composition of the particles were analyzed by TEM, FTIR and TGA. The TGA experiments of the bilayer-coated particles show a distinct two-stage mass loss. Partition experiments show that the modified Fe(3)O(4) nanoparticles are affected by aqueous dispersion pH and ion strength. Accordingly, the Pickering emulsions stabilized by modified Fe(3)O(4) particles are also sensitive to pH and ion strength. The phase inversion of the emulsions occurs when 1.0013.50. The phase inversion of emulsions also can be adjusted by the ion strength. In interfacial adsorption experiments, the hydrophobic Fe(3)O(4) nanoparticles form particle clusters, while the hydrophilic particles form uniform multilayers.     

Synthesis, characterization, and temperature-dependent colloidal stability of poly(N-isopropylacrylamide)-grafted polystyrene/poly(styrene-co-4-vinylbenzyl N, N-diethyldithiocarbamate) hairy particles

Poly(N-isopropylacrylamide)-grafted polystyrene/poly(styrene-co-4-vinylbenzyl N, N-diethyldithiocarbamate) [PNIPAM-grafted PS/P(St-co-VBDC)] hairy particles were synthesized by photo-polymerizing N-isopropylacrylamide monomer in the presence of PS/P(St-co-VBDC) core particles. Here, the VBDC unit, which was incorporated into the surface of core particles by seeded soap-free emulsion copolymerization, acted as a photo-iniferter. By varying the polymerization conditions, a series of hairy particles having different grafting heights and grafting densities was successfully obtained. The hairy particles exhibited well-defined core/shell morphology. PS/P(St-co-VBDC) formed the core which was surrounded by PNIPAM shell. The determination of critical coagulation concentration (CCC) indicated that the hairy particles were stabilized via both electrostatic and steric mechanisms (i.e., electrosteric mechanism) at a temperature lower than LCST of PNIPAM. However, these particles gave much lower CCCs when heated to the temperature higher than LCST, exhibiting temperature-dependent colloidal stability.     

Pickering乳液模板法制备Janus粒子

本文以SiO₂粒子稳定的水包油(O/W)型Pickering乳液作为模板, 在乳液连续相进行SI-ATRP, 将聚合物刷接枝到SiO₂粒子外半表面, 破乳得到半修饰的Janus粒子.     

Surfactant-Free Multiple Pickering Emulsions Stabilized by Combining Hydrophobic and Hydrophilic Nanoparticles

A series of W/O/W or O/W/O emulsion stabilized solely by two different types of solid nanoparticles were prepared by a two-step method. We explored the option of particular emulsifiers for the multiple Pickering emulsions, and a variety of nanoparticles (silica, iron oxide, and clay) only differing in their wettability was used. The primary W/O emulsion was obtained by the hydrophobic nanoparticles, and then the hydrophilic nanoparticles were used as emulsifier in the secondary emulsification to prepare the W/O/W emulsion. In a similar way, the primary O/W emulsion of the O/W/O emulsion was stabilized by the hydrophilic nanoparticles, while the secondary emulsification to prepare the O/W/O emulsion was effected with the hydrophobic nanoparticles. The resultant multiple Pickering emulsion was stable to coalescence for more than 3 months, except the W/O/W emulsions of which the secondary emulsion stabilized by clay nanoparticles became a simple O/W emulsion in a day after preparation. Moreover, the temperature and pH sensitive poly(N-isopropylacrylamide-co-methacrylic acid) (P(NIPAm-co-MAA)) microgels were introduced as an emulsifier for the secondary emulsification to obtain the stimulus-responsive multiple W/O/W emulsion. Such microgel-stabilized multiple emulsions could realize the efficient controlled release of water-soluble dye, Rhodamine B (RB) on demand in a multiple-emulsion delivery system.      

纳米SiO2与阳离子表面活性剂的相互作用及其诱导的正辛烷-水乳状液的双重相转变

研究了3种不同结构的水溶性阳离子表面活性剂对纳米二氧化硅颗粒的原位表面活性化作用, 它们分别是单头单尾的十六烷基三甲基溴化铵(CTAB)、单头双尾的双十二烷基二甲基溴化铵(di-C12DMAB)和双头双尾的Gemini型阳离子三亚甲基-二(十四酰氧乙基溴化铵)(II-14-3), 并通过测定Zeta电位、吸附等温线及接触角等参数对相关机理进行了阐述. 结果表明, 阳离子表面活性剂吸附到颗粒/水界面形成以疏水基朝向水的单分子层, 从而增强了颗粒表面的疏水性是原位表面活性化的基础. 通过吸附CTAB和II-14-3, 颗粒的疏水性适当增强, 能吸附到正辛烷/水界面稳定O/W(1)型乳状液; 而通过吸附di-C12DMAB所形成的单分子层更加致密, 颗粒的疏水性进一步增强, 进而使乳状液从O/W(1)型转变为W/O型; 当表面活性剂浓度较高时, 由于链-链相互作用, 表面活性剂分子将在颗粒/水界面形成双层吸附, 使颗粒表面变得亲水而失去活性, 但此时体系中游离表面活性剂的浓度已增加到足以单独稳定O/W(2)型乳状液的程度. 因此当采用纳米二氧化硅和di-C12DMAB的混合物作乳化剂时, 通过增加di-C12DMAB的浓度即可诱导乳状液发生O/W(1)→W/O→O/W(2)双重相转变.