Water Separation
4.4 Responsiveness of Emulsions
4.4.3 Magnetic Stimulation
For industrial applications, sometimes it is not desirable to control the emulsion stability by temperature or ph. this is because this may take large amounts of chemicals or energy to achieve. therefore, magnetic responsive emulsion stabilizers are introduced. the control of the pickering emulsion can be easily achieved by turning on and off the magnetic field. as the syn- theses of different kinds of particles have been well studied, the prepara- tion of magnetic particles are also much more accessible than before. these magnetic particles can be modified, just like other types of particles, to form composite stabilizers.
Zhou et al. prepared a magnetic pickering emulsion using superparamag- netic Fe3o4 nanoparticles.91 it seems that Fe3o4 particles are a popular choice for magnetic nanoparticles. Fe3o4 was not an expensive material and the syn- theses of such magnetic nanoparticles were very accessible. they prepared Figure 4.28 Fraction of free water and paraffin oil released three months after the emulsification. the emulsions were stabilized by modified Fe2o3
nanoparticles.89 reprinted from Q. Lan, C. Liu, F. Yang, S. Liu, J. Xu and d. Sun, Synthesis of bilayer oleic acid-coated Fe3o4 nanoparticles and their application in ph-responsive pickering emulsions, J. Colloid Interface Sci., 2007, 310, 260–269, Copyright 2007, with permission from elsevier.
the Fe3o4 nanoparticles by conventional co-precipitation of iron(ii) and iron(iii) salts in alkaline conditions. the diameter distribution of the result- ing nanoparticles was not very monodispersed. nevertheless, these particles were still capable of stabilizing pickering emulsions. as mentioned in the previous section, inorganic salts were usually hydrophilic; this was also true for the Fe3o4 nanoparticles. they investigated o/w pickering emulsions with different kinds of oil. the Fe3o4 nanoparticles could stabilize o/w pickering emulsions well when the oil was less polar. however, if the polarity of the oil was high, stable emulsions could not be formed using their Fe3o4 nanopar- ticles. in Figure 4.29, it shows that their particles were not effective in stabi- lizing butyl butyrate and decanol. the three phase contact angles were also studied for different oils. when oil with high polarity was considered, the contact angle was quite small. therefore, stable o/w emulsions could not be formed. however, the magnetic properties of their magnetic pickering emul- sions were not described in detail in the study.
in other research done by Melle, Lask and Fuller, carbonyl iron particles were used to prepare magnetic responsive pickering emulsions.92 the diam- eters of the particles were around a few micrometres. in Figure 4.30, the effects of a static magnetic field are clearly shown. when the magnetic field was turned on, the oil droplets were attracted to the bottom of the vial. they
Figure 4.29 emulsions prepared by Zhou et al. different oil fractions were stud- ied.91 also four oils were used in their experiment. they are (a) dodec- ane, (b) pdMS, (c) butyl butyrate and (d) decanol. reprinted (adapted) with permission from J. Zhou, X. Qiao, B. p. Binks, K. Sun, M. Bai, Y. Li and Y. Liu, Langmuir, 2011, 27, 3308–3316. Copyright 2011 american Chemical Society.
found that the critical field strength was a function of particle concentra- tion. as the concentration increased, the magnetic field required to move the oil droplets was reduced. in addition, they demonstrated at stronger mag- netic fields that the pickering emulsion could be destabilized and undergo complete phase separation. in this case, the critical magnetic field strength was independent of the particle concentration. this can be understandable because the force required to pull the particles away from the interface should be the same for different particle concentrations. Moreover, they proved that this stabilization destabilization cycle was reversible by agitation. therefore, we can see that the stabilization and destabilization of emulsions, separation and recycling of the magnetic particles could be easily controlled by a static magnetic field. these features are very desirable in industrial applications.
Brugger and richtering have reported their stimuli-responsive particle emulsifiers.93 their stimuli-responsive particles were sensitive to magnetic and thermal stimuli. nipaM, Maa and 2-hydroxyethylmethylacrylate (heMa) were used to prepare the primary microgel. then, iron(ii,iii) oxide magnetic nanoparticles were synthesized inside the microgel particles, which was pre- viously reported by Zhang et al.94 Many small magnetic nanoparticles with a diameter of 20 nm were formed within a larger microgel particle. in their study, the direct destabilization was not performed by applying a strong mag- netic field. on the contrary, a moderate magnetic field was applied so that the emulsion droplets could be attracted and gathered by the field. More importantly, the field strength was not strong enough to cause coalescence and phase separation in their work.
in Figure 4.31, the capture of the emulsion droplets by a static magnetic field is demonstrated. when the static magnetic field was removed, the oil droplets were released and the density difference caused them to rise again.
the size distribution of the pickering emulsion was compared before and after the magnetic treatment. it was almost unchanged. this was because they would like to demonstrate the thermo-induced destabilization by induc- tion heating rather than using a static magnetic field. a high frequency mag- netic field was applied to the pickering emulsion. induction heating of the Figure 4.30 a magnetic responsive pickering emulsion prepared by Melle et al.92 By controlling the strength of the magnetic field, the emulsion droplets could be attracted and broken. reprinted (adapted) with permission from S. Melle, M. Lask and g. g. Fuller, Langmuir, 2005, 21, 2158–2162.
Copyright 2005 american Chemical Society.
magnetic particles in the microgel increased the temperature of the emul- sion. as the microgel stabilizers were thermo-responsive, the destabilization of the emulsion could be done by this remote controlled localized heating technique. it was a great example for combining the features of a thermo- responsive microgel with magnetic particles.