Abstract

Stable oil-in-water Pickering emulsions (PEs) are prepared by using hydrophilic and oppositely charged spherical (silica) and nonspherical (hematite) nanoparticle mixture. By varying aqueous dispersion pH, the surface potentials of the particles are tuned. For fixed sizes of silica and hematite, stable emulsions are formed at low concentrations of silica when the surface potential mismatch between the particles is high. Conversely, when the latter is small, stable emulsions are found up to high concentration of silica nanoparticles. These findings are explained based on the formation of silica-hematite heteroaggregates and on their surface-active characteristics. It is found that for a given pH and particle concentration (expressed as number ratio N_S-H, which is the ratio of the total number of spheres to the total number of spheroids), highly stable oil-in-water emulsions are formed only when the unstable aqueous binary mixture of colloids and the oil phase are emulsified. The result is presented as a state diagram, which shows the particle composition corresponding to the formation of stable emulsions, the composition range where the emulsions that result are unstable and the region where the emulsions do not form at all. As the formation of PE is kinetically controlled, we demonstrate that at conditions where emulsions formed via one-step emulsification for a given N_S-H are unstable, following a two-step emulsification procedure yields highly stable emulsions. The work highlights the role of (i) N_S-H, (ii) aqueous dispersion pH, (iii) surface potential mismatch and (iv) emulsification protocol on the formation and stability of solid particle stabilized emulsions.

摘要

通过使用亲水性和带相反电荷的球形（二氧化硅）和非球形（赤铁矿）纳米颗粒混合物制备稳定的水包油皮克林乳液 (PE)。通过改变水分散体的 pH 值，可以调节颗粒的表面电位。对于固定尺寸的二氧化硅和赤铁矿，当颗粒之间的表面电位失配高时，在低二氧化硅浓度下会形成稳定的乳液。相反，当后者较小时，在高浓度二氧化硅纳米粒子的情况下会发现稳定的乳液。这些发现的解释是基于二氧化硅-赤铁矿杂聚集体的形成及其表面活性特征。发现对于给定的 pH 值和颗粒浓度（表示为数比 N _S-H，这是球体总数与球体总数的比值），只有当胶体和油相的不稳定水性二元混合物被乳化时，才会形成高度稳定的水包油乳液。结果以状态图的形式呈现，其中显示了与稳定乳液形成相对应的粒子组成、所产生的乳液不稳定的组成范围以及根本不形成乳液的区域。由于 PE 的形成是动力学控制的，我们证明了在对给定的 N _S-H通过一步乳化形成的乳液不稳定的条件下，遵循两步乳化程序会产生高度稳定的乳液。这项工作突出了 (i) N _{S-H 的作用}, (ii) 水分散体 pH, (iii) 表面电位不匹配和 (iv) 关于固体颗粒稳定乳液的形成和稳定性的乳化协议。