Water Separation
4.5 Applications
salt. at this temperature, the salt was securely encapsulated in the pickering emulsion. when temperature was increased to 22 °C, the encapsulation was still quite secure. however, when the temperature was increased to 30 °C, the rate of salt release increased rapidly. it took less than 1 hour to release 5%
of the salt. Finally, when the temperature was increased to 55 °C, complete release of salt could be finished in around 200 seconds.
4.5.2 Petroleum Industry
in the petroleum industry, emulsion science is always related to the pro- cessing of oil. For example, this includes reservoir flooding, oil recovery, oil transportation, etc. Some surface-active molecules like naphthenic acids and asphaltenes exist naturally in crude oil.102 they are capable of forming very stable water-in-oil emulsions. there are ionic salts dissolved in the water-in-oil emulsions, which damage the facilities for oil processing. on the other hand, reservoir flooding processes also rely on emulsion science.
Conventionally, surfactants are added to the water for reservoir flooding processes. Surfactants can effectively lower the interfacial tension between the flooding solution and crude oil.103 the efficiency of crude oil extraction from a reservoir can be enhanced. however, after extracting the oil from the ground, the removal and recycling of these surfactants is not easy and it requires a lot of effort. during the oil recovery process, the emulsions are required to be collected and broken. By considering these features, it seems that responsive pickering emulsion systems will be suitable for such applications.
wang and co-workers have developed a recyclable ph-responsive mag- netic nanoparticle system.104 For their particles, the preliminary magnetic cores were surrounded by silica shells. then, the ph-responsive polymer of poly(dimethylaminoethyl methacrylate) (pdMaeMa) was grown on the silica surface. these particles were used in an oil–water separation process. Figure 4.34 shows the isolation of diesel oil from a diesel-in-water emulsion. First, their hybrid magnetic nanoparticles were added to a diesel-in-water emul- sion. then, the magnetic nanoparticles were adsorbed onto the oil–water interface. they could be collected by a static magnetic field. therefore, the clear aqueous phase was removed separately. in addition, they recycled the magnetic nanoparticles and repeated this separation six times. this demon- stration suggested that such a pickering emulsion system was capable of being applied in oil recovery processes in petroleum industry.
Besides crude oil extraction and recovery, emulsions can also be applied in heavy crude oil transportation.105 in order to transport crude oil by pipe lines, the viscosity of the crude oil must be lowered under a certain value.
the formation of crude oil-in-water emulsions can achieve this objective.
however, properties of such emulsions are also very important. For exam- ple, the stability of the emulsions is required to be high in both dynamic and static conditions. in addition, after the transportation, the emulsions can be conveniently broken before entering the refinery facilities. therefore, the excellent stability of pickering emulsions makes it a nice choice for this
application. Moreover, the destabilization of the responsive pickering emul- sions can easily be triggered after the transportation.
4.5.3 Extraction
responsive pickering emulsions are gaining attractiveness to be used in extraction. this is because they have excellent stability compared to conven- tional emulsions and can be demulsified on demand. the extraction con- tent can be conveniently collected by destabilizing the emulsion while the remaining stabilizers can be reused.
Monteillet et al. have developed a novel system which applies ionic liquid emulsions in extraction.106 ionic liquids are often considered to be green extraction solvents for extracting proteins, lipids and other molecules. the use of emulsions is desirable in such applications because emulsions offer a very Figure 4.34 Schematic illustration of the oil–water separation process using magnetic nanoparticles prepared by wang et al.104 reprinted from X. wang, Y. Shi, r. w. graff, d. Lee and h. gao, developing recyclable ph-responsive magnetic nanoparticles for oil-water separation, Poly- mer, 2015, online published, Copyright 2015, with permission from elsevier.
large specific surface area. therefore, the extraction efficiency can be greatly improved. in their work, pnipaM/polystyrene composite particles were pre- pared. the polystyrene (pS) cores were synthesized with a fluorescent dye so that the particles could be easily seen under a confocal microscope. then, pni- paM-co-Maa polymer shell was polymerized onto the pS core. as discussed in the previous sections, pnipaM-co-Maa was a ph- and thermo-responsive polymer. this gave the composite particles responsiveness. it was discovered that the type of emulsion was dependent on the composition of the ionic liquid. when the cationic component of the ionic liquid was hydrophobic [p+6,6,6,14], water-in-ionic liquid emulsions were usually formed. however, when it was replaced by [n+1,8–10,8–10,8–10], the microgel particles stayed in the aque- ous phase. the resulting emulsion was ionic liquid-in-water. they mentioned that the permeability of the interface was very important. in Figure 4.35, the permeability of the interface was tested by an ionic liquid-in-water emulsion prepared by [p6,6,6,14][dCa]. then, β-carotene and a hydrophobic fluorescent dye were added to the continuous aqueous phase. they found that the dye was extracted to the ionic liquid within minutes. this was also confirmed by their confocal images, which are shown in Figure 4.35. the red dye was extracted into the ionic liquid even when the interface was fully covered by the com- posite microgel particles. in some of the previous examples, the collecting of emulsion droplets was demonstrated by applying a mild static magnetic field.
in this study, instead of using magnetic particles, the ionic liquid itself was chosen to be paramagnetic, where [p6,6,6,14][FeCl4] was used. therefore, the emulsion droplets could be simply collected by a state magnetic field. Finally, the temperature-induced destabilization of the emulsions was also success- fully demonstrated by them. when the temperature was increased above the LCSt of pnipaM, the emulsion was broken as expected.
another example was reported by Chen et al.107 the particles prepared by them were core–shell particles with magnetic Fe3o4 cores and pnipaM shells. their responsive pickering emulsion system was designed for effec- tive oil harvesting. in Figure 4.36, the responsiveness of the emulsions is shown. the core–shell particles were capable of stabilizing toluene-in-water emulsions. these oil droplets could be collected by a magnetic, just like the other examples which were discussed previously. then, the destabilization of the emulsions was achieved by increasing the temperature above the LCSt of pnipaM. their system showed potential applications in oil spill accidents and as a treatment for industrial oily wastewater.
4.5.4 Catalysis
in the previous section, a photo-responsive pickering emulsion prepared by Chen et al. was discussed.97 however, they also performed a bipha- sic enantioselective biocatalysis with their emulsion. they converted (R,S)-mandelonitrile to R-(−)-mandelic acid by A. faecalis atCC 8750.
(R,S)-Mandelonitrile was more soluble in the oil phase while R-(−)-man- delic acid and A. faecalis atCC 8750 remained in the aqueous phase. Con- ventionally, monophasic or biphasic approaches were often considered
for the reaction. however, there were disadvantages for both monopha- sic and biphasic reactions. in monophasic reactions, the concentrations of the reactants were limited because of the low solubility. on the other hand, the reaction rate was limited in biphasic reactions as the catalyst and the reactants stayed in two different phases. the transportation of materials to different phases limited the rate of reaction.
therefore, Chen and co-workers applied their responsive pickering emul- sions to the hydrolysis reaction. For this biphasic system, the interfacial area was greatly increased because of the formation of emulsion droplets.
Figure 4.37 compares the rates of conversion in different conditions. it was Figure 4.35 (a) photos of [p6,6,6,14][dCa] in a water emulsion stabilized by compos-
ite microgel particles.106 (b) β-Carotene was adsorbed by the ionic liq- uid. (c) and (d) were the confocal images of the emulsion droplets.
reproduced from ref. 106 with permission from the royal Society of Chemistry.
discovered that at low mandelonitrile concentration (50 mM), monophasic and emulsion systems shared similar efficiency. however, when the concen- tration of mandelonitrile was increased to 100 mM and 300 mM, the picker- ing emulsion system was much more desirable than the other two systems for the hydrolysis. the rate of conversion could be greatly improved. Fur- thermore, the inversion capability of the responsive emulsion benefited the product separation after the hydrolysis. after the reaction, near-infrared radi- ation could change the emulsions from w/o to o/w. the pickering emul- sion stabilizers and biocatalyst could be recovered. in addition, the use of near-infrared instead of UV radiation for the inversion did less damage to the biocatalyst. therefore, the efficiency of the biocatalyst did not deteriorate quickly. the application of responsive pickering emulsions in catalysis was also reported by many different researchers.4,108,109
4.5.5 Pickering Emulsion Polymerization
emulsion polymerization is a well-known technique for preparing polymeric colloidal particles.110 Conventionally, surfactant molecules are added to the reaction mixture in order to provide stabilization to the newly formed col- loidal particles. the application of pickering emulsions in emulsion polym- erization has attracted more and more attention. these surfactant-free Figure 4.36 toluene-in-water emulsion could be obtained by shaking. the emul-
sion droplets could then be collected by a magnet. a thermal triggered demulsification was also demonstrated.107 reprinted (adapted) with permission from Y. Chen, Y. Bai, S. Chen, J. Ju, Y. Li, t. wang and Q.
wang, ACS Appl. Mater. Interfaces, 2014, 6, 13334–13338. Copyright 2014 american Chemical Society.
emulsions are considered to be more environmentally friendly as they are less toxic and the stabilizing particles are easily recyclable. Colloidal parti- cles can often be used as an antifoaming agent.111 Foam reduction is also an advantage of using particulate stabilizers.
wei et al. used lignin extracted from furfural residues in emulsion polymer- ization of polystyrene.112 Figure 4.38 showed the cycle of using lignin particles as particulate stabilizers. First, lignin was dissolved in an alkaline solution.
then, lignin colloidal particles could be formed by addition of acid. these par- ticles act as the emulsion stabilizers in the emulsion polymerization of sty- rene. after the polymerization, alkali was added and the lignin became soluble again and the whole process could be repeated for next batch of styrene. the recycling of stabilizer is not easy for a conventional surfactant approach.