Salt-Resistant Switchable Pickering Emulsions Stabilized by Mesoporous Nanosilica Hydrophobized In Situ by pH-Insensitive Surfactants

Abstract

Novel oil-in-water (O/W) Pickering emulsions (PEs) were prepared using mesoporous nanosilica in combination with a pH-insensitive cationic surfactant as a stabilizer and show an interesting sensitivity to acids and bases. Adding a suitable amount of NaOH (n NaOH /n cationic surfactant ≥ 1) led to prompt demulsification within 10 s. Upon further adding HCl solutions (n HCl / n NaOH = 1), stable PEs re-formed after homogenization. These emulsions remained stable for over 30 days after 60 cycles, switching from stable to unstable and back to stable states, and showed a high salt tolerance. A mechanism for the switching of the Pickering emulsion (PE) to unstable and back to stable states was derived and involved anionic and neutral forms of hydroxyl groups at the mesopores of the mesoporous silica nanoparticles (MSNPs). This work reveals a switchable PE system involving a pH-insensitive surfactant, in which the species of oils and cationic surfactants can be arbitrarily selected, a feature that greatly expands the applicability of PEs.

Pickering Emulsions Stabilized by Mesoporous Nanoparticles with Different Morphologies in Combination with DTAB

“…Most of the DTAB molecules were absorbed into the inner mesopores of the MSNPs. These results proved that the previous proposed mechanism 14 worked, regardless of the morphology of the MSNPs. Thus, the unique responsive behavior of Pickering emulsions stabilized by thread-like MSNPs and DTAB following addition of NaOH should be attributed to the difference in morphology.…”

“…In the presence of a cationic surfactant, these MSNPs can be hydrophobized in situ and act as effective emulsifiers to stabilize the emulsions. 1 , 14 , 42 , 43 For spherical MSNP systems, as shown in Figure 2 A, the Pickering emulsions were unstable at DTAB concentrations below 0.2 mmol·L –1 . Phase separations occurred within 2 days.…”

“…In the reported nanosilica Pickering emulsions, the average diameters of droplets were typically between 10 and 100 μm. 14 , 18 , 23 , 45 , 46 …”

“…Compared with the outer surface, there were more silica hydroxyl groups in the inner surface of MSNPs. 39 , 40 , 52 Due to the large specific surface areas of MSNPs, most of the DTAB molecules were adsorbed into the inner channels 14 (see the schematic illustration in Figure 6 ). As a result, the MSNPs became hydrophilic and were thus unable to stabilize the emulsions.…”

“…In our previous work, Pickering emulsions stabilized by spherical mesoporous silica nanoparticles and the pH-insensitive cationic surfactant cetyltrimethylammonium bromide (CTAB) showed novel responsiveness to the addition of bases and acids. 14 The rich pore structure of MSNPs plays a critical role in their novel stimuli-response behavior. However, the effects of MSNP morphology are still unclear.…”