Binks Bernard P, Rodrigues Jhonny A, Frith William J
Surfactant and Colloid Group, Department of Chemistry, University of Hull, Hull, HU6 7RX, United Kingdom. b.p.binks@ hull.ac.uk
Langmuir. 2007 Mar 27;23(7):3626-36. doi: 10.1021/la0634600. Epub 2007 Feb 23.
Using a range of complementary experiments, a detailed investigation into the behavior of dodecane-water emulsions stabilized by a mixture of silica nanoparticles and pure cationic surfactant has been made. Both emulsifiers prefer to stabilize o/w emulsions. At high pH, particles are ineffective emulsifiers, whereas surfactant-stabilized emulsions become increasingly stable to coalescence with concentration. In mixtures, no emulsion phase inversion occurs although synergism between the emulsifiers leads to enhanced stability at either fixed surfactant concentration or fixed particle concentration. Emulsions are most stable under conditions where particles have negligible charge and are most flocculated. Freeze fracture scanning electron microscopy confirms the presence of particle flocs at drop interfaces. At low pH, particles and surfactant are good emulsifiers alone. Synergism is also displayed in these mixtures, with the extent of creaming being minimum when particles are most flocculated. Experiments have been undertaken in order to offer an explanation for the latter synergy. By determining the adsorption isotherm of surfactant on particles in water, we show that surfactant addition initially leads to particle flocculation followed by re-dispersion. Using suitable contact angle measurements at oil-water-solid interfaces, we show that silica surfaces initially become increasingly hydrophobic upon surfactant addition, as well as surfactant adsorption lowering the oil-water interfacial tension. A competition exists between the influence of surfactant on the contact angle and the tension in the attachment energy of a particle to the interface.
通过一系列补充实验，对由二氧化硅纳米颗粒和纯阳离子表面活性剂混合物稳定的十二烷 - 水乳液的行为进行了详细研究。两种乳化剂都倾向于稳定油包水乳液。在高pH值下，颗粒是无效的乳化剂，而表面活性剂稳定的乳液随着浓度增加对聚结变得越来越稳定。在混合物中，尽管乳化剂之间的协同作用导致在固定表面活性剂浓度或固定颗粒浓度下稳定性增强，但不会发生乳液相转变。乳液在颗粒电荷可忽略不计且絮凝程度最高的条件下最稳定。冷冻断裂扫描电子显微镜证实了液滴界面处存在颗粒絮凝物。在低pH值下，颗粒和表面活性剂单独都是良好的乳化剂。这些混合物中也表现出协同作用，当颗粒絮凝程度最高时乳析程度最小。已进行实验以解释后一种协同作用。通过测定表面活性剂在水中颗粒上的吸附等温线，我们表明添加表面活性剂最初会导致颗粒絮凝，随后再分散。通过在油 - 水 - 固体界面进行合适的接触角测量，我们表明添加表面活性剂时二氧化硅表面最初变得越来越疏水，并且表面活性剂吸附降低了油 - 水界面张力。表面活性剂对接触角的影响与颗粒附着到界面的附着能中的张力之间存在竞争。
