Effect of dispersion pH on the formation and stability of Pickering emulsions stabilized by layered double hydroxides particles

Using positively charged plate-like layered double hydroxides (LDHs) particles as emulsifier, liquid paraffin-in-water emulsions stabilized solely by such particles are successfully prepared. The effects of the pH of LDHs aqueous dispersions on the formation and stability of the emulsions are investigated here. The properties of the LDHs dispersions at different pHs are described, including particle zeta potential, particle aggregation, particle contact angle, flow behavior of the dispersions and particle adsorption at a planar oil/water interface. The zeta potential decreases with increasing pH, leading to the aggregation of LDHs particles into large flocs. The structural strength of LDHs dispersions is enhanced by increasing pH and particle concentration. The three-phase contact angle of LDHs also increases with increasing pH, but the variation is very small. Visual observation and SEM images of the interfacial particle layers show that the adsorption behavior of LDHs particles at the planar oil/water interface is controlled by dispersion pH. We consider that the particle-particle (at the interface) and particle-interface electrostatic interactions are well controlled by adjusting the dispersion pH, leading to pH-tailored colloid adsorption. The formation of an adsorbed particle layer around the oil drops is crucial for the formation and stability of the emulsions. Emulsion stability improves with increasing pH and particle concentration because more particles are available to be adsorbed at the oil/water interface. The structural strength of LDHs dispersions and the gel-like structure of emulsions also influence the stability of the emulsions, but they are not necessary for the formation of emulsions. The emulsions cannot be demulsified by adjusting emulsion pH due to the irreversible adsorption of LDHs particles at the oil/water interface. TEM images of the emulsion drops show that a thick particle layer forms around the oil drops, confirming that Pickering emulsions are stabilized by the adsorbed particle layers. The thick adsorbed particle layer may be composed of a stable inner particle layer which is in direct contact with the oil phase and a relatively unstable outer particle layer surrounding the inner layer.     

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.     

Sustainable stabilization of oil in water emulsions by cellulose nanocrystals synthesized from deep eutectic solvents

There is an urgent global need to develop novel types of environmentally safe dispersing chemicals from renewable resources in order to reduce the environmental impact of oil spills. For this goal, cellulose, the most abundant natural polymeric source, is a promising green, nontoxic alternative that could replace the current synthetic surfactants. In this study, cellulose nanocrystals (CNC) synthesized using a deep eutectic solvent (DES) and two commercially available cellulose nanocrystals were used as marine diesel oil–water Pickering emulsion stabilizers. In particular, oil in water (o/w) emulsion formation and stability of emulsified oil during storing were addressed using a laser diffraction particle size analyzer, image analysis, and oil emulsion volume examination. The particle size of the o/w reference without CNCs after dispersing was over 50 µm and coalescence occurred only a few minutes after the emulsifying mixing procedure. All three investigated CNCs were effective stabilizers for the o/w system (oil droplets size under 10 µm) by preventing the oil droplet coalescence over time (6 weeks) and resulting in a stable creaming layer. The CNCs prepared using green DES systems boasted performance comparable to that of commercial CNCs, and they showed effectiveness at 0.1% dispersant dosage.     

Dispersion stability and aggregation behavior of TEMPO-oxidized cellulose nanofibrils in water as a function of salt addition

Dispersion stability of TEMPO-oxidized cellulose nanofibrils (TOCNs) in water was investigated through both experimental and theoretical analyses to elucidate the critical aggregation concentration of different salts. The 0.1 wt% TOCN/water dispersions with various NaCl concentrations were evaluated by measuring light transmittance, viscosity under steady-shear flow, and the weight fraction of TOCN that had aggregated. Homogeneous TOCN/water dispersion turned to gel as the NaCl concentration increased. The TOCN dispersion maintained its homogeneous state up to 50 mM NaCl, but aggregated gel particles were formed at 100 mM NaCl. The mixture became separated into two phases (gel and supernatant) at ≥200 mM NaCl. Theoretical analysis using ζ-potentials of TOCN elements in the dispersions revealed that the aggregation behavior upon NaCl addition could be explained well in terms of the interaction potential energy between two cylindrical rods based on the Derjaguin–Landau–Verwey–Overbeek theory. The experiments were extended to analyze critical aggregation concentrations of MgCl₂ and CaCl₂ for the 0.1 wt% TOCN dispersion. In the case of divalent electrolytes, TOCN elements began to form aggregated gel particles at salt concentrations of 2–4 mM, corresponding to the critical aggregation concentration predicted by the empirical Schultz-Hardy rule.     

不同形态双金属氢氧化物颗粒水分散体系及其稳定的Pickering乳液

采用共沉淀法制备了3种形态的MgAl双金属氢氧化物颗粒的水分散体系, 并以其为乳化剂制备了Pickering乳液. 比较了3种颗粒的分散体系及其稳定的Pickering乳液的性质. X射线衍射(XRD)和透射电子显微镜(TEM)表征结果表明, 低结晶度的颗粒以形状不规则、 结构疏松、 表面粗糙的絮状体形式分散于水中, 且颗粒尺寸随高速搅拌分散时间的延长而减小; 而良好结晶的颗粒以形状规则、 结构致密、 表面平滑的六角片存在于水中. Zeta电位测试表明, 3种颗粒在水中均带正电荷, NaCl可降低颗粒的Zeta电位而使其发生絮凝, 但良好结晶颗粒的分散体系在更高NaCl浓度时才出现明显沉淀. 分别采用3种双金属氢氧化物颗粒/NaCl水分散体系制备了水包油(O/W)型Pickering乳液, 并比较了乳液的稳定性. 结果表明, NaCl的引入在一定程度上可提高3类乳液的稳定性; 良好结晶颗粒稳定乳液的能力强于低结晶度的颗粒; 对于低结晶度颗粒, 大颗粒稳定乳液的能力比小颗粒更强.     

NiFe2O4与TiO2稳定的多重Pickering乳液的制备

由沉淀法和溶胶-凝胶法分别制备了Ni Fe_2O_4和Ti O_2,并用XRD和SEM对两种粒子进行了表征,结果表明成功制备了粒径大小较为均匀的目标产物。用CTAB对Ni Fe_2O_4进行了改性,并由IR和Zeta电位测试予以确认。以改性Ni Fe_2O_4和Ti O_2作为稳定粒子,由两步法制备了W/O/W型多重Pickering乳液,采用数码照片和光学显微照片观察所制备的乳液的宏观与微观形貌。研究表明,制备的单重Pickering乳液粒径较为均匀,多重Pickering乳液粒径范围稍宽,但两者稳定性能都非常良好。     

The role of bentonite addition in UF flux enhancement mechanisms for oil/water emulsion

The main problem in treating oil/water emulsion from car wash waste-water by ultrafiltration (UF) is fouling caused by oil adsorption on the membrane surface and internal pore walls. This study demonstrates that the addition of bentonite clay can reduce the adsorption layer on cellulose acetate UF membrane, resulting in a reduction of total membrane resistance (R_t). Experiments were conducted to identify and describe three possible mechanisms: (i) bulk oil emulsion concentration reduction; (ii) particle aggregation and (iii) detachment of the adsorbed gel layer by shear force. Adsorption of oil emulsion by bentonite can lead to a significant reduction of bulk oil emulsion concentration, one of the major causes of flux enhancement. Results show that contact of oil emulsion with bentonite forms larger particles resulting in flux increment. An optimum particle size of 37 μm, corresponds with a bentonite concentration of 300 mg/l and provided the highest flux. Beyond this limiting concentration, flux improvement gradually declined, possibly due to the formation of packed cake of particles on the membrane surface. The presence of bentonite in the oil emulsion promotes high shear stress which acts against the gel layer. This high shear stress, caused by bentonite particles and cross-flow velocity, reverses the adsorbed gel layer to the bulk of the liquid phase.     

Fabrication of macroporous foam and microspheres of polystyrene by Pickering emulsion polymerization

Poly(styrene-co-methacrylic acid) (PS-co-MAA) particles were synthesized via surfactant-free emulsion polymerization and then used as particulate emulsifiers for preparation of Pickering emulsions. Our results showed that adjusting the solution pH can tune the wettability of PS-co-MAA particles to stabilize either water-in-oil (W/O) or oil-in-water (O/W) Pickering emulsions. Stable W/O emulsions were obtained with PS-co-MAA particles at low pH values due to their better affinity to the dispersed oil phase. In contrast, increasing the pH value significantly changed the stabilizing behavior of the PS-co-MAA particles, leading to the phase inversion and formation of stable O/W emulsions. We found that the oil/water ratio had a significant influence on pH value of the phase inversion. It decreased with decreasing the oil/water ratio, and no phase inversion occurred when the styrene volume fraction reduced to 10 %. Additionally, macroporous polystyrene (PS) foam and PS microspheres were obtained via polymerization of Pickering high internal phase emulsion (Pickering HIPE) and O/W Pickering emulsion, respectively.     

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.     

Effect of sulfonated 3-pentadecyl phenyl acrylate as surfmer in the emulsion polymerization of styrene: synthesis and polymer properties

In this work, the evaluation of a newly developed anionic polymerizable surfactant (surfmer), viz., sulfonated 3-pentadecyl phenyl acrylate, in the emulsion polymerization of styrene and its effect on the polymer properties is reported. The results were compared with the commercially available non-reactive anionic surfactant sodium lauryl sulfate. The surfmer has a low critical micellar concentration value of 40.11 mg/L (8.7?×?10^?5?mol/L) in comparison to 2,400 mg/L (8.28?×?10^?3?mol/L) for sodium dodecyl sulfate. Nanosized polystyrene dispersions with varying concentration of this surfmer were prepared and characterized for conversion, particle size, and size distribution at a fixed monomer/water ratio of 0.1. The particle radii decreased from 560 nm for the surfactant-free dispersions to 45 nm for the dispersion with 2.3 mol% surfmer. Increasing surfmer content above this concentration did not further affect the particle size but increased the width of the particle size distribution. Transmission electron microscopy results along with particle size data show that with increasing surfmer content the particle size distribution broadens, and film formation is facilitated. The microstructure analysis of the copolymers using infrared and ¹H-NMR spectroscopy confirms that the surfmer is chemically attached to the polymer chains. The effect of the ionic sulfonate groups and the alkyl chains of the surfmer moieties on the polymer properties have been studied through measurement of dilute solution viscosity and thermal and viscoelastic properties. These results indicate that the behavior of surfmer-containing polymers resembles that of plasticized ionomers.     

Ca2+ Responsive Microgel-Stabilized Pickering Emulsions

As an ionic cross-linker that can change the size of poly(N-isopropylacrylamide-co-acrylic acid) microgel, Ca²⁺ is applied as a trigger to demulsify microgel-stabilized oil/water Pickering emulsions. The influence of Ca²⁺ induced intra-particle ionic cross-linking and inter-particle aggregation on the stability of microgel-stablized “Pickering” emulsion is described. At low and mediate concentration of Ca²⁺, ionic cross-linking can change the internal elasticity of the microgel, and cause the coarsening of the oil droplets. At high concentration of Ca²⁺, microgels flocculate due to the salt out effect and the emulsion is destabilized. This work provide a facile method to control the stability of the Pickering emulsions at ambient condition.     

In situ synthesis of plate-like Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles in porous cellulose films with obvious magnetic anisotropy

Nanocomposite cellulose films with obvious magnetic anisotropy have been prepared by in situ synthesis of plate-like Fe₂O₃ nanoparticles in the cellulose matrix. The influence of the concentrations of FeCl₂ and FeCl₃ solutions on the morphology and particle size of the synthesized Fe₂O₃ nanoparticles as well as on the properties of the composite films has been investigated. The Fe₂O₃ nanoparticles synthesized in the cellulose matrix was γ-Fe₂O₃, and its morphology was plate-like with size about 48 nm and thickness about 9 nm, which was totally different from those reported works. The concentration of FeCl₂ and FeCl₃ solution has little influence on the particle size and morphology of the Fe₂O₃ nanoparticles, while the content of Fe₂O₃ nanoparticles increased with the increase of the concentration of the precursor solution, indicating that porous structured cellulose matrix could modulate the growth of inorganic nanoparticles. The unique morphology of the Fe₂O₃ nanoparticles endowed the composite films with obvious magnetic anisotropy, which would expand the applications of the cellulose based nanomaterials.     

Magnetically loaded poly(methyl methacrylate-co-acrylic acid) nano particles

Magnetically loaded polymeric nano-particles carrying functional groups on their surface were prepared by a two-stage process. In the first stage, super-paramagnetic magnetite (Fe₃O₄) nano-particles were produced by a co-precipitation method from the aqueous solutions of FeCl₂·4H₂O and FeCl₃·6H₂O using a NaOH solution. The smallest size obtained was 40.9 nm with poly-dispersity index of 0.194 obtained by using a Zeta Sizer. The effects of Fe²⁺/Fe³⁺ molar ratio, stirring rate, temperature, base concentration, and pH on the particle size/size distribution and stability of the dispersions were examined. Increasing the relative concentration of Fe²⁺ ion and decreasing the stirring rate and pH increased the particle size, while the concentration of NaOH and temperature did not change the particle size significantly. Polymer coating was achieved by emulsion polymerization at high surfactant to monomer ratio of methyl methacrylate (MMA) and acrylic acid which were used as comonomers (comonomer ratio: 90/10 weight) with high surfactant to monomer ratio. The surfactant and initiator were SDS and KPS, respectively. Nano-particles in the range of 115 and 300 nm in diameter were produced depending on recipe. Increasing the Fe₃O₄/monomer and surfactant/monomer ratios, the KPS concentration caused a decrease in the average diameter. Magnetic properties of the nano-particles were obtained by electron spin resonance and vibrating-sample magnetometer. Most of the polymer-coated nano-particles exhibited super paramagnetic behavior.An erratum to this article can be found at     

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.     

New Pickering emulsions stabilized by bacterial cellulose nanocrystals

We studied oil in water Pickering emulsions stabilized by cellulose nanocrystals obtained by hydrochloric acid hydrolysis of bacterial cellulose. The resulting solid particles, called bacterial cellulose nanocrystals (BCNs), present an elongated shape and low surface charge density, forming a colloidal suspension in water. The BCNs produced proved to stabilize the hexadecane/water interface, promoting monodispersed oil in water droplets around 4 μm in diameter stable for several months. We characterized the emulsion and visualized the particles at the surface of the droplets by scanning electron microscopy (SEM) and calculated the droplet coverage by varying the BCN concentration in the aqueous phase. A 60% coverage limit has been defined, above which very stable, deformable droplets are obtained. The high stability of the more covered droplets was attributed to the particle irreversible adsorption associated with the formation of a 2D network. Due to the sustainability and low environmental impact of cellulose, the BCN based emulsions open opportunities for the development of environmentally friendly new materials.     

Semibatch emulsion polymerization of methyl methacrylate using different polyurethane particles

Aqueous acrylic‐polyurethane (AC–PU) hybrid emulsions were prepared by semibatch emulsion polymerization of methyl methacrylate (MMA) in the presence of four polyurethane (PU) dispersions. The PU dispersions were synthesized with isophorone diisocyanate (IPDI), 1000 and 2000 molecular weight (MW) poly(neopentyl) adipate, 1000 MW polytetramethyleneetherglycol, butanediol (BD), and dimethylol propionic acid (DMPA). MMA was added in the monomer emulsion feed. We studied the effect of the use of different PU seed particles on the rate of polymerization, the particle size and distribution, the number of particles, and the average number of radicals per particle. The PU rigidity was controlled by varying the polyol chemical structure, the polyol MW (M_n), and by adding BD. The monomer feed rate was varied to study its influence on the process. It was observed that the PU particles that had been prepared with a higher MW polyol swelled better with MMA before the monomer‐starved conditions occurred. There seemed to be no significant discrepancies between the series with different PU seeds in the monomer‐starved conditions. The overall conversion depended on the monomer addition rate, and the polymerization rate acquired a constant value that was comparable to the value of the monomer addition rate. The instantaneous conversion increased slightly. The average particle size increased, and the total particle number in the reactor was constant and similar to the number of PU particles in the initial charge. The average number of radicals per particle increased. The differences between the system with a constant particle number and average number of radicals per particle and the system with a fixed radical concentration are discussed. The semibatch emulsion polymerization of MMA in the presence of PU particles studied was better compared to the system with a fixed radical concentration. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 844–858, 2005     

Extraction of Cr (VI) by pickering emulsion liquid membrane using amphiphilic silica nanowires (ASNWs) as a surfactant

Pickering emulsion is the replacement of surfactants with solid, often nano-sized particles. The particle-stabilized emulsions have good thermodynamic and kinetic stability. Pickering emulsion liquid membrane (PELM) was prepared using mahua oil as a diluent, aliquat 336 (Trioctyl methylammonium chloride) as a carrier and amphiphilic silica nanowires (ASNWs) (10–40?ml ethanol addition) as a surfactant. Sodium hydroxide (NaOH) was used as stripping phase in the concentration range from 0.1 to 0.5?M for the extraction of hexavalent chromium [Cr (VI)] from aqueous solution. The variety of edible and non-edible oils was investigated for the stability of water in oil emulsion. Factors that influence silica-stabilized Pickering emulsions are pH, agitation speed, stripping phase concentration, the volume ratio of membrane to stripping phase (M/S), initial feed concentration, treat ratio(feed to emulsion volume ratio) and surfactant concentration for better PELM stability. And also, the extraction efficiency of Cr (VI) was investigated using aliquat as a carrier. The physicochemical properties of ASNWs were studied using Scanning Electron Microscopy (SEM), Fourier Transforms Infrared Spectroscopy (FTIR) and Dynamic Light Scattering (DLS) techniques. At an optimum condition, 99.69% of Cr (VI) removal from aqueous solution was obtained.     

聚氨酯脲-丙烯酸酯水分散液的粒径及形态研究

研究了羧基含量、异氰酸酯指数（［ － ＮＣＯ］／［ － ＯＨ］） 、聚氨酯脲与聚丙烯酸酯组成比（ＰＵＵ／ＰＡ） 以及一系列制备工艺因素对聚氨酯脲—丙烯酸酯（ＰＵＡ） 水分散液粒子尺寸及形态的影响。结果表明：羧基含量和ＰＵＵ／ＰＡ 组成比增大或异氰酸酯指数减小都会导致分散液粒径减小;ＰＵＡ 中ＰＡ 含量越大,ＰＵＡ 水分散液粒子形态越不规整;工艺因素如搅拌强度、升温速率等对ＰＵＡ 水分散液粒子尺寸及形态的影响不符合传统乳液聚合的规律。     

Pickering emulsions stabilized solely by layered double hydroxides particles: the effect of salt on emulsion formation and stability

The formation and stability of liquid paraffin-in-water emulsions stabilized solely by positively charged plate-like layered double hydroxides (LDHs) particles were described here. The effects of adding salt into LDHs dispersions on particle zeta potential, particle contact angle, particle adsorption at the oil-water interface and the structure strength of dispersions were studied. It was found that the zeta potential of particles gradually decreased with the increase of salt concentration, but the variation of contact angle with salt concentration was very small. The adsorption of particles at the oil-water interface occurred due to the reduction of particle zeta potential. The structural strength of LDHs dispersions was strengthened with the increase of salt and particle concentrations. The effects of particle concentration, salt concentration and oil phase volume fraction on the formation, stability and type of emulsions were investigated and discussed in relation to the adsorption of particles at the oil-water interface and the structural strength of LDHs dispersions. Finally, the possible stabilization mechanisms of emulsions were put forward: the decrease of particle zeta potential leads to particle adsorption at the oil-water interface and the formation of a network of particles at the interface, both of which are crucial for emulsion formation and stability; the structural strength of LDHs dispersions is responsible for emulsion stability, but is not necessary for emulsion formation.     

Synthesis of millimeter‐sized hydrogel beads by inverse Pickering polymerization using starch‐based nanoparticles as emulsifier

The preparation of millimeter‐sized poly(acrylamide‐co‐acrylic acid) hydrogel beads via inverse Pickering emulsion polymerization using starch‐based nanoparticles (SNPs) as stabilizers is reported. Amphiphilic starch is fabricated by the introduction of butyl glycidyl ether groups and palmitate groups, and the hydrophobically modified SNPs are fabricated by a nanoprecipitation process. The obtained SNPs could adsorb at oil‐water interfaces to stabilize an inverse Pickering emulsion, and the effects of oil/water volume fraction ratio and SNP concentration on emulsions are comprehensively studied. Poly(acrylamide‐co‐acrylic acid) hydrogel beads with a size of approximately 1 mm are obtained by inverse Pickering emulsion polymerization stabilized by SNPs. The morphology and structure of hydrogel beads are extensively investigated, which confirms that SNPs locate on the surface of hydrogel beads and act as emulsifiers and network structures present inside the beads. Polymerization is also detected to investigate the potential formation mechanism of hydrogel beads. The pH‐responsive property of hydrogel beads and its potential application for drug delivery are also explored.