Polymer Engineering and Colloid Science (PECS) Laboratory, Department of Chemical Engineering , Indian Institute of Technology Madras , Chennai 600 036 , India.
Langmuir. 2018 Nov 6;34(44):13312-13321. doi: 10.1021/acs.langmuir.8b02913. Epub 2018 Oct 23.
Solid-stabilized emulsions commonly known as Pickering emulsions offer unique benefits such as superior stability and controlled permeability compared to conventional surfactant stabilized emulsions. In this article, the effect of pH, the electrolyte and particle concentration, homogenization speed, and volume fraction of oil on the formation, stability, and the microstructure of emulsion droplets stabilized by micron-size peanut-shaped hematite particles are investigated. The influence of surface charge of particles on emulsification is studied by varying the pH of the dispersing medium, the addition of an electrolyte or a combination of both. Stable O/W emulsions are formed only when the aqueous dispersions at intermediate pH between 4 and 11, and decane (2:1 volume ratio) are vigorously mixed. However, emulsions are not formed when the particles are highly charged that is, at pH 2 and 12. The presence of monovalent salt or high-speed homogenization assists the emulsion formation at pH 3, whereas their combination helps in emulsification at pH 2. However, neither the addition of an electrolyte nor the high-speed homogenization or their combination facilitates the formation of emulsions at pH 12. We show that the image-charge repulsion and the surface charge induced wettability change can explain the influence of both pH and salt concentrations on the formation of Pickering emulsions. Although oil-in-water emulsions typically cream because of the density difference, microscopy observations revealed the presence of a large number of small particle-covered oil droplets in the sediments of the emulsified samples. These drops are observed to be entrapped in dense-particle networks. This leads to a considerable reduction in the number of particles available for the stabilization of floating emulsion droplets and thus influences their size and surface coverage. The possibility of tailoring the stability, droplet size and, the surface coverage discussed in this article can play a crucial role in situations that demand controlled release of active components.
固-稳乳液通常被称为 Pickering 乳液，与传统表面活性剂稳定乳液相比，具有优异的稳定性和可控渗透性等独特优势。本文研究了 pH 值、电解质和颗粒浓度、均质速度以及油的体积分数对微米级花生形赤铁矿颗粒稳定的乳液滴的形成、稳定性和微观结构的影响。通过改变分散介质的 pH 值、添加电解质或两者的组合来研究颗粒表面电荷对乳化的影响。只有在 pH 值为 4 到 11 之间的中间值且将水相与 decane（体积比为 2:1）剧烈混合时，才会形成稳定的 O/W 乳液。然而，当颗粒带高度电荷时，即 pH 值为 2 和 12 时，乳液则无法形成。单相盐或高速均质化有助于在 pH 值为 3 时形成乳液，而两者的组合有助于在 pH 值为 2 时乳化。然而，无论是添加电解质还是高速均质化或两者的组合，都无法促进 pH 值为 12 时乳液的形成。我们表明，空间电荷排斥和表面电荷诱导的润湿性变化可以解释 pH 值和盐浓度对 Pickering 乳液形成的影响。尽管油包水乳状液通常由于密度差异而出现分层，但显微镜观察表明，在乳化样品的沉积物中存在大量的小颗粒覆盖的油滴。这些液滴被观察到被包裹在密集颗粒网络中。这导致可用于稳定浮油乳液滴的颗粒数量大量减少，从而影响其尺寸和表面覆盖率。本文讨论的稳定性、液滴尺寸和表面覆盖率的可调节性在需要控制活性成分释放的情况下可能会起到关键作用。
