Water Separation
4.4 Responsiveness of Emulsions
4.4.1 Thermal Stimulation
responsive pickering emulsions have drawn more and more attention since the report of a multi-responsive pickering emulsion.13 this was stabilized by ph and thermal responsive microgel particles. the first category of respon- sive emulsion in our discussion is thermo-responsive pickering emulsions, which can be obtained by using thermo-responsive particulate stabilizers.
the properties of these particles are very sensitive to changes in tempera- ture. therefore, their ability to stabilize emulsions also changes with the temperature.
in the previous sections, thermo-responsive particles were discussed.
they are capable of being particulate stabilizers. Figure 4.19 shows emul- sions stabilized by one of our microgel samples at different temperatures.
the microgel particles were made of pnipaM and 3% wt/wt N,N′-methylen- ebisacrylamide (MBa) was used as the cross-linking agent. the emulsion was stable at temperatures below the LCSt of the pnipaM polymer and did not undergo significant phase separation in at least six months, even though creaming occurred right after the agitation. however, the emulsion was unstable when the temperature was increased above the LCSt. the mechanism of this thermo-responsiveness was studied by many different researchers.12,80 First, the microgel particles expel theirs water content and shrink at high temperature.64 Surface area provided by the microgel particles is reduced significantly. if the diameter of the microgel shrinks to half of the original value, then the surface area of microgel is just one-fourth of the original. it leads to an obvious consequence. Some of the emulsion droplets might be exposed instead of being covered by the particles. as a result, the
Figure 4.19 a typical thermo-responsive pickering emulsion stabilized by pnipaM microgel particles.
droplets will be vulnerable to coalescence when the droplets meet each other.
destribats et al. studied the relationship between the deformability of micro- gel particles and the resulting emulsion stability.81 they found out that soft, more deformable microgel samples, which possessed less cross-linker con- tent, are better stabilizers. Combining their observation under the cryogenic scanning electron microscopy (Cryo-SeM), they attributed the emulsion sta- bility of microgel-stabilized pickering emulsions to the lateral overlapping and interfacial elasticity. therefore, it may also explain the reduction in sta- bility of microgel-stabilized emulsion when the temperature is increased above the LCSt. as the microgel particles shrink at increased temperature, the polymer chains in the polymeric network hold each other strongly and resist any deformation. hashmi et al. reported that the fully shrunk microgel particle could be ten times stiffer than the fully swollen microgel particle.82
eqn (4.3) in Section 4.2.2 predicts that the desorption energy of each parti- cle decreases as the radius of it is reduced and increases as the contact angle approaches 90°. nevertheless, changes in desorption energy might not be that significant in the discussion. this is because the desorption energy of individual particles is still in a way larger than the thermal energy. however, there is one more major factor which causes particle desorption – the aggre- gation of microgel particles. as the temperature increases above the LCSt of pnipaM, polymer–polymer interactions would be more favourable compared to polymer–solvent interactions. then the microgel particles may coagulate and even form macroscopic aggregates. this actually disables the stabiliza- tion given by the microgel dispersion. Figure 4.20 shows a pnipaM microgel dispersion at different temperatures. the microgel was a stable pale light dispersion at room temperature. when the temperature was increased to just above the LCSt, the scattering intensity of the solution increased. as the temperature was further increased, the microgel particles finally aggregated to form observable aggregates.
Besides microgel particles, thermo-responsive composite particle dis- persions are also popular choices for preparing responsive pickering
Figure 4.20 the thermo-responsiveness of a typical pnipaM microgel dispersion.
emulsions. tsuji and Kawaguchi studied the thermo-sensitive pickering emulsions with microgel particles and pnipaM-carrying polystyrene par- ticles.83 Stability of emulsions prepared by different oils was compared at different temperatures. their microgel particles were just synthesized by conventional precipitation polymerization with no special features. nev- ertheless, they grafted pnipaM polymer onto polystyrene particles. the hydrodynamic diameters of the two particles were similar and around 800 nm. it was discovered that the hairy responsive particles showed better ther- mo-responsiveness than the conventional microgel particles. therefore, the emulsions stabilized by hairy responsive particles also demonstrated better thermo-responsiveness. the emulsions stabilized by conventional microgel particles only showed partial coalescence of droplets at elevated temperature. Figure 4.21 shows the thermo-responsiveness of emulsions stabilized by their hairy particles. the responsiveness of emulsions pre- pared by different oils was different. nevertheless, all of them demonstrated the thermal-induced destabilization when the temperature was increased above the LCSt of pnipaM.
as mentioned in the previous section, polysaccharide, which is not ther- mo-responsive, can also be modified so that it can be used as a responsive particulate stabilizer. Zoppe et al. prepared cellulose nanocrystals grafted with thermo-responsive polymer brushes [poly(nipaM)-g-CnCs].56 their cellulose nanocrystals were prepared from ramie fibres. the emulsions sta- bilized by these particles can be stable for more than four months, if the temperature was not raised above the LCSt of pnipaM. however, when the pickering emulsions were heated to temperature above 35 °C for one min- ute, phase separation was observed for their emulsions. grafted nanoparticle aggregation was also observed by them. therefore, they attributed the desta- bilization to the desorption and aggregation of the modified nanoparticles.
Figure 4.21 thermo-responsive emulsions prepared by tsuji and Kawaguchi.83 hairy pnipaM grafted polystyrene particles were the particulate sta- bilizers. different types of oil were used in their experiment. hp, hd, tCe and tL referred to heptane, hexadecane, trichloroethylene and toluene. reprinted (adapted) with permission from S. tsuji and h.
Kawaguchi, Langmuir, 2008, 24, 3300–3305. Copyright 2008 american Chemical Society.
in the previous section, Janus particles prepared by tanaka et al. were discussed.79 in Figure 4.22a, the microscope image of a 1-octanol droplet is shown. it was observed that the hydrophilic pdM grafted polymer faced the aqueous phase. at ph 7.2, a stable emulsion could be formed at room tem- perature. however, when the temperature was increased to 60 °C, the Janus particles desorbed and entered the oil phase. the emulsion underwent com- plete phase separation.