Department of Chemical Engineering, University of Washington, Seattle, Washington 98195-1750, United States.
Langmuir. 2012 Aug 7;28(31):11633-8. doi: 10.1021/la302806y. Epub 2012 Jul 27.
This paper presents an investigation of the effects of trace water on the charging of silica (SiO(2)) particles dispersed in a nonpolar medium. There are a growing number of applications that seek to use electrostatic effects in apolar media to control particle movement and aggregation stability in such systems. One factor that is often overlooked in the preparation of nonpolar colloidal dispersions is the amount of water that is introduced to the system by hygroscopic particles and surfactants. The amount and location of this water can have significant effects on the electrical properties of these systems. For nonpolar surfactant solutions it has been shown that water can affect the conductivity, and it has been speculated that this is due to swelling of the polar cores of inverse micelles, increasing the fraction of them that are charged. Some studies have suggested that particle surface charging may also be sensitive to water content, but a clear mechanism for the process has not been fully developed. The situation with particles is further complicated by the fact that it is often unclear whether the water resides on the particle surfaces or in the polar cores of inverse micelles. The current work explores not only the effect of water content on reverse micelle and particle charging but seeks to differentiate between water bound to the particles and water located in the micelles. This is accomplished by measuring the solution conductivity and the electrophoretic mobility of silicon dioxide particles dispersed in solutions of Isopar-L and OLOA 11000. The water content is determined for both the dispersion and the supernatant after centrifuging the particles out. It is found that at equilibrium the majority of the water in the system adsorbs to the surface of the hygroscopic silica particles. In addition, the effect of water on particle electrophoretic mobility is found to be dependent on surfactant concentration. At small OLOA concentrations, additional water results in an increase in particle mobility due to increased particle charging. However, at large OLOA concentrations, additional water leads to a decrease in particle mobility, presumably as a result of increased electrostatic screening or neutralization. Thus, particle charging and electrophoretic mobility in an apolar surfactant solution are found to be highly sensitive to both the total water content in the system and to its concentration relative to the amount of surfactant present.
本文研究了痕量水对分散在非极性介质中的二氧化硅（SiO2）颗粒荷电的影响。越来越多的应用试图在非极性介质中利用静电效应来控制这些系统中颗粒的运动和聚集稳定性。在制备非极性胶体分散体时，往往会忽略一个因素，即吸湿性颗粒和表面活性剂引入系统的水量。这些水量的多少和位置会对这些系统的电学性质产生重大影响。对于非极性表面活性剂溶液，已经表明水可以影响电导率，并且有人推测这是由于反胶束的极性核膨胀，增加了带电的反胶束的比例。一些研究表明，颗粒表面充电也可能对含水量敏感，但对于这一过程的明确机制尚未完全开发。颗粒的情况更加复杂，因为通常不清楚水是存在于颗粒表面还是存在于反胶束的极性核中。目前的工作不仅探讨了含水量对反胶束和颗粒充电的影响，而且试图区分与颗粒结合的水和位于胶束中的水。这是通过测量分散在异烷烃和 OLOA11000 溶液中的二氧化硅颗粒的溶液电导率和电泳迁移率来实现的。通过离心除去颗粒后，确定分散体和上清液中的含水量。结果发现，在平衡状态下，系统中的大部分水吸附在吸湿性二氧化硅颗粒的表面上。此外，发现水对颗粒电泳迁移率的影响取决于表面活性剂的浓度。在 OLOA 浓度较低时，由于颗粒充电增加，额外的水会导致颗粒迁移率增加。然而，在 OLOA 浓度较高时，额外的水会导致颗粒迁移率降低，这可能是由于静电屏蔽或中和增加所致。因此，在非极性表面活性剂溶液中，颗粒的充电和电泳迁移率对系统中的总含水量及其与存在的表面活性剂量的浓度比都非常敏感。
