Dejun Sun

Highly stable concentrated nanoemulsions by the phase inversion composition method at elevated temperature.

Oil-in-water nanoemulsions were produced in the system water/Span 80-Tween 80/paraffin oil via the phase inversion composition (PIC) method at elevated temperature. With the increase of preparation temperature from 20 to 70 °C, we found that the emulsion droplet diameter decreases from 10.3 μm to 51 nm, proving the formation of nanoemulsions. The viscosity of nanoemulsions clearly increases with droplet volume fraction, φ, but the droplet size changes less. Significantly, at φ ≤ 0.5, the size distribution of nanoemulsions can be kept unchangeable more than 5 months. These results proved that the highly viscous paraffin oil can hardly be dispersed by the PIC method at 25 °C, but the increase in preparation temperature makes it possible for producing monodisperse nanoemulsions. Once the nanoemulsion is produced, the stability against Ostwald ripening is outstanding due to the extremely low solubility of the paraffin oil in the continuous phase. The highly stable nanoemulsions are of great importance in practical applications.

Double Phase Inversion of Emulsions Containing Layered Double Hydroxide Particles Induced by Adsorption of Sodium Dodecyl Sulfate

A liquid paraffin-water emulsion was investigated using layered double hydroxide (LDH) particles and sodium dodecyl sulfate (SDS) as emulsifiers. Both emulsifiers are well-known to stabilize oil-in-water (o/w) emulsions. Surprisingly, a double phase inversion of the emulsion containing LDH particles is induced by the adsorption of SDS. At a constant LDH concentration, the emulsion is o/w type when SDS concentrations are low. At intermediate SDS concentrations, the first emulsion inversion from o/w to w/o occurs, which is attributed to the enhanced hydrophobicity of LDH particles caused by the desorption of the second layer of surfactant, leaving a densely packed SDS monolayer on the LDH exterior surfaces. The second inversion from water-in-oil (w/o) to o/w occurs at higher SDS concentrations, which may be due to the competitive adsorption at the oil/water interfaces between the LDH particles modified by the SDS bilayers and the free SDS molecules in the bulk solution, but the free SDS molecules dominate and determine the emulsion type. Laser-induced fluorescent confocal micrographs clearly confirm the adsorption of LDH particles on the surfaces of the initial o/w and intermediate w/o emulsion droplets, whereas no LDH particles were adsorbed on the final o/w emulsion droplet surfaces. Also, transmission electron microscopy (TEM) observations indicate that the shape of the final o/w emulsions is similar to that of the monomeric SDS-stabilized emulsion but different from that of the initial o/w emulsions. The adsorption behavior of SDS on LDH particles in water was investigated to offer an explanation for the emulsion double phase inversion. The zeta potential results show that the particles will flocculate first and then redisperse following surfactant addition. Also, X-ray diffraction (XRD) measurements indicate that SDS adsorption on the LDH interior surfaces will be complete at intermediate concentrations.

Pickering emulsions stabilized by a lipophilic surfactant and hydrophilic platelike particles.

Liquid paraffin-water emulsions were prepared by homogenizing oil phases containing sorbitan oleate (Span 80) and aqueous phases containing layered double hydroxide (LDH) particles or Laponite particles. While water-in-oil (w/o) emulsions are obtained by combining LDH with Span 80, the emulsions stabilized by Laponite-Span 80 are always o/w types regardless of the Span 80 concentration. Laser-induced fluorescent confocal micrographs indicate that particles are absorbed on the emulsion surfaces, suggesting all the emulsions are stabilized by the particles. The difference of the particle-stabilized emulsion type may be explained by comparing particle contact angles and the oil-water interfacial tensions, indicating that more Span 80 molecules are adsorbed on the LDH particles than on Laponite. Apparently, the LDH particles are rendered more hydrophobic by Span 80, resulting in the formation of w/o emulsions. The long-term stability of the emulsions was also compared. Emulsions stabilized by Span 80 alone completely separate into two bulk phases of oil and water after 3 months. However, emulsion stability is greatly enhanced with the addition of LDH or Laponite particles. This synergism was accounted for by an increase of the dilational viscoelasticity modulus of the oil-water interface after particles were added to the aqueous phase. This increase indicates that the gel-like particle layer stays at the oil-water interface and resists emulsion coalescence. Scanning electron microscope (SEM) images display the presence of a firm layer surrounding the emulsion droplets and a three-dimensional particle network which extends into the bulk phase aiding emulsion stability.

A simple one-pot synthesis of single-crystalline magnetite hollow spheres from a single iron precursor.

This paper presents a facile route using a simple solvothermal reaction to synthesize monodisperse and single-crystalline Fe(3)O(4) hollow spheres. Fe(3)O(4) hollow spheres with a mean diameter of 200 nm are fabricated using the coordination compound [Fe(urea)(6)]Cl(3) as the sole iron source, in the absence of any other additives. TEM, SEM and HRTEM results show that single-crystalline Fe(3)O(4) hollow spheres are composed of well-aligned nanoparticles. The as-prepared hollow spheres have a Brunauer-Emmett-Teller (BET) surface area of about 16.251 m(2) g(-1) with an average pore size of 3.537 nm. The hollow spheres display obvious ferromagnetism at room temperature with a saturation magnetization of 79.58 emu g(-1), a remanent magnetization of 19.1 emu g(-1) and coercivity of 133.5 Oe. The growth mechanism of single-crystalline Fe(3)O(4) hollow spheres is attributed to the cooperation of oriented aggregation and Ostwald ripening.

Pickering emulsions stabilized by paraffin wax and Laponite clay particles.

Emulsions containing wax in dispersed droplets stabilized by disc-like Laponite clay particles are prepared. Properties of the emulsions prepared at different temperatures are examined using stability, microscopy and droplet-size analysis. At low temperature, the wax crystals in the oil droplets can protrude through the interface, leading to droplet coalescence. But at higher temperatures, the droplet size decreases with wax concentration. Considering the viscosity of the oil phase and the interfacial tension, we conclude that the wax is liquid-like during the high temperature emulsification process, but during cooling wax crystals appear around the oil/water interface and stabilize the droplets. The oil/water ratio has minimal effect on the emulsions between ratios of 3:7 and 7:3. The Laponite is believed to stabilize the emulsions by increasing the viscosity of the continuous phase and also by adsorbing at the oil/water interface, thus providing a physical barrier to coalescence.

Double Inversion of Emulsions Induced by Salt Concentration

The effects of salt on emulsions containing sorbitan oleate (Span 80) and Laponite particles were investigated. Surprisingly, a novel double phase inversion was induced by simply changing the salt concentration. At fixed concentration of Laponite particles in the aqueous phase and surfactant in paraffin oil, emulsions are oil in water (o/w) when the concentration of NaCl is lower than 5 mM. Emulsions of water in oil (w/o) are obtained when the NaCl concentration is between 5 and 20 mM. Then the emulsions invert to o/w when the salt concentration is higher than 50 mM. In this process, different emulsifiers dominate the composition of the interfacial layer, and the emulsion type is correspondingly controlled. When the salt concentration is low in the aqueous dispersion of Laponite, the particles are discrete and can move to the interface freely. Therefore, the emulsions are stabilized by particles and surfactant, and the type is o/w as particles are in domination. At intermediate salt concentrations, the aqueous dispersions of Laponite are gel-like, the viscosity is high, and the transition of the particles from the aqueous phase to the interface is inhibited. The emulsions are stabilized mainly by lipophilic surfactant, and w/o emulsions are obtained. For high salt concentration, flocculation occurs and the viscosity of the dispersion is reduced; thus, the adsorption of particles is promoted and the type of emulsions inverts to o/w. Laser-induced fluorescent confocal micrographs and cryo transmission electron microscopy clearly confirm the adsorption of Laponite particles on the surface of o/w emulsion droplets, whereas the accumulation of particles at the w/o emulsion droplet surfaces was not observed. This mechanism is also supported by the results of rheology and interfacial tension measurements.

Ca2+ ion responsive pickering emulsions stabilized by PSSMA nanoaggregates.

A novel Ca(2+) ion responsive particulate emulsifier, which is based on copolymer nanoaggregates, is reported in this work. Results from dynamic light scattering (DLS) and cryo-transmission electron microscopy (cryo-TEM) indicate that the formation of poly (4-styrenesulfonic acid-co-maleic acid) sodium salt (PSSMA) nanoaggregates is strongly dependent on Ca(2+) concentration. The PSSMA copolymer only aggregates above a critical Ca(2+) concentration (0.2 M) with an average diameter of 10-40 nm. After dilution with water, PSSMA nanoaggregates are rapidly redissolved again. On the basis of the properties of PSSMA nanoaggregates, Ca(2+) ion responsive Pickering emulsions were successfully prepared. At high Ca(2+) concentrations, the emulsions with high stability against coalescence can be prepared with the size in the submicrometer range as determined by DLS. Cryo-TEM and dynamic interfacial tension results confirm the adsorption of PSSMA nanoaggregates at the interface, which is the key to the stability of the emulsions. More importantly, rapid demulsification can be achieved by dilution with water on demand. It is because, upon dilution with water, PSSMA nanoaggregates undergo a transition from stable nanoaggregates to individual polymer chains, which leads to interfacial desorption of nanoaggregates and rapid demulsification of emulsions. Thus, this finding presents a new manipulation on emulsion stability and is expected to provide a useful guidance in the fields of oil recovery, food science, environment protection, and so on.

Removal of petroleum sulfonate from aqueous solutions using freshly generated magnesium hydroxide

Freshly generated magnesium hydroxide (FGMH), produced by adding water-soluble magnesium salts to highly alkaline solutions, was used to remove anionic surfactant petroleum sulfonate (PS) from aqueous solutions. Adsorption experiments were carried out to investigate the effects of pH, adsorbent dosage, contact time, PS concentration, and temperature. The results showed that FGMH displayed excellent treatment efficiency for PS in the pH range 12.0-13.0. The maximum PS removal efficiency was reached within 60 s. The best dosage of magnesium chloride was 2.0 g/L. The adsorption capacity of FGMH for PS decreased as the temperature increased from 303 K to 333 K. The adsorption process was exothermic. The removal mechanism of PS by FGMH may be a coagulation-adsorption process involving a combination of flocculation, adsorption, charge neutralization, and netting catch affection. The results of this study showed that FGMH can be effectively used to treat surfactant wastewaters.

Phase Behavior of Mixtures of Positively Charged Colloidal Platelets and Nonadsorbing Polymer

We investigated the effect of nonadsorbing polymer on the phase behavior of suspensions of positively charged Mg2Al layered double hydroxide (LDH) platelets by birefringence observations and rheological measurements. We show that the depletion attraction, induced by the addition of a high-molecular-weight polyvinyl pyrrolidone (PVP), enriches the phase behavior of these electrostatically stabilized suspensions. At intermediate LDH and polymer concentration, two isotropic phases (I1-I2) coexist, nematic-nematic (N1-N2) demixing occurs, and a sediment phase is observed, with the appearance of two-, three-, four-, and even six-phase coexistence. Upon increasing the polymer concentration, the I-N phase transition and the sol-gel transition shift to lower LDH concentrations; meanwhile, the I-N coexistent samples enter the purely nematic phase. We explain the richness of the phase behavior in such LDH-PVP mixtures by discussing the interactions among PVP-induced depletion attraction, particle polydispersity, and particle sedimentation.

Phase Inversion of Emulsions Containing a Lipophilic Surfactant Induced by Clay Concentration

Emulsions stabilized by clay particles and sorbitan monooleate (Span 80) were investigated, and an abnormal phase inversion was observed by increasing the concentration of clay particles in the aqueous phase. At a fixed concentration of Span 80 in the oil phase, the emulsions were oil-in-water (o/w) when the concentration of clay particles in the aqueous phase was low. Surprisingly, the emulsion inverted to water-in-oil (w/o) when the concentration of the hydrophilic clay particles was increased. On the basis of the results of rheological measurements and laser-induced fluorescent confocal microscopy observation, we suggest that this phase inversion is induced by the gel structures formed at high concentration of clay particles. The effects of clay concentration on the stability and the droplet size of these emulsions were also investigated.