Anjali Thriveni G, Basavaraj Madivala G
Polymer Engineering and Colloid Science (PECS) Laboratory, Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai - 600 036, India.
Phys Chem Chem Phys. 2017 Nov 22;19(45):30790-30797. doi: 10.1039/c7cp04665k.
The superior stability of Pickering emulsions can be detrimental in several applications. There is therefore growing interest in using stimuli responsive particle emulsifiers that can considerably reduce the cost of demulsification. Among the possible triggers for the remote control of emulsion stability, pH sensitive systems are advantageous because of the simplicity and ease of implementation. Herein, the destabilization mechanism of pH-switchable emulsions stabilized solely by particles of different shapes and surface properties - namely - hematite, silica and polystyrene - are discussed. The surface activity of all these particles can be tuned by controlling the surface charge density via dispersing them in aqueous solutions of different pH. Hematite particles of cuboidal, spherocylindrical, peanut and ellipsoidal shapes stabilized O/W emulsions at pH 6.5, but they were completely destabilized when the pH of the continuous phase was adjusted to either 2 or 12. Similarly, the O/W emulsions stabilized by silica rods and spheres at pH 2 were destabilized upon adjusting the pH to 6.5 and 12. The detachment of particles from the droplet surfaces resulted in partially covered drops that coalesced, leading to the destabilization of emulsions as confirmed by direct visualization via optical microscopy. The destabilization mechanism of the Pickering emulsions stabilized solely by pH responsive particles was observed to be general, and was mediated by the detachment of particles because of the pH induced wettability change. Furthermore, the pH responsive behaviour was found to be reversible. Since stable emulsions are formed in several particle-water-oil systems, the possibility of conveniently changing the pH of the continuous phase can be exploited to tune the particle wettability and thus the destabilization in applications that demand controlled demulsification.
皮克林乳液的超强稳定性在某些应用中可能会产生不利影响。因此，人们越来越关注使用刺激响应型颗粒乳化剂，这种乳化剂可以大幅降低破乳成本。在可用于远程控制乳液稳定性的可能触发因素中，pH敏感系统因其简单易行而具有优势。本文讨论了仅由不同形状和表面性质的颗粒（即赤铁矿、二氧化硅和聚苯乙烯）稳定的pH可切换乳液的失稳机制。通过将这些颗粒分散在不同pH值的水溶液中控制表面电荷密度，可以调节所有这些颗粒的表面活性。长方体、球柱体、花生形和椭圆形的赤铁矿颗粒在pH 6.5时稳定了水包油乳液，但当连续相的pH值调整为2或12时，乳液完全失稳。同样，由二氧化硅棒和球体在pH 2时稳定的水包油乳液在将pH值调整为6.5和12时也会失稳。颗粒从液滴表面脱离导致部分覆盖的液滴聚结，通过光学显微镜直接观察证实这导致了乳液的失稳。观察到仅由pH响应颗粒稳定的皮克林乳液的失稳机制具有普遍性，并且是由pH诱导的润湿性变化导致颗粒脱离介导的。此外，发现pH响应行为是可逆的。由于在几种颗粒-水-油体系中都能形成稳定的乳液，因此可以利用方便地改变连续相pH值的可能性来调节颗粒的润湿性，从而在需要可控破乳的应用中实现失稳。
