Controlling colloid charge in nonpolar liquids with surfactants

Abstract

The formation of ions in nonpolar solvents (with relative permittivity ε r of approximately 2) is more difficult than in polar liquids; however, these charged species play an important role in many applications, such as electrophoretic displays. The low permittivities (ε r) of these solvents mean that charges have to be separated by large distances to be stable (approximately 28 nm or 40 times that in water). The inverse micelles formed by surfactants in these solvents provide an environment to stabilize ions and charges. Common surfactants used are sodium dioctylsulfosuccinate (Aerosol OT or AOT), polyisobutylene succinimide, sorbitan oleate, and zirconyl 2-ethyl hexanoate. The behavior of charged inverse micelles has been studied on both the bulk and on the microscopic scale and can be used to determine the motion of the micelles, their structure, and the nature of the electrostatic double layer. Colloidal particles are only weakly charged in the absence of surfactant, but in the presence of surfactants, many types, including polymers, metal oxides, carbon blacks, and pigments, have been observed to become positively or negatively charged. Several mechanisms have been proposed as the origin of surface charge, including acid-base reactions between the colloid and the inverse micelle, preferential adsorption of charged inverse micelles, or dissolution of surface species. While most studies vary only the concentration of surfactant, systematic variation of the particle surface chemistry or the surfactant structure have provided insight into the origin of charging in nonpolar liquids. By carefully varying system parameters and working to understand the interactions between surfactants and colloidal surfaces, further advances will be made leading to better understanding of the origin of charge and to the development of more effective surfactants.

The effect of solvent and counterion variation on inverse micelle CMCs in hydrocarbon solvents

“…This paper does not demonstrate any significant effect of varying solvent-surfactant interactions for ionic surfactants in alkanes, but these materials are very interesting to study, given their applications as microemulsion stabilizers [8], as nanoreactors [9], and as charge control agents [10]. This provides a strong motivation to gain further understanding into their aggregation.…”

“…This surfactant does not have a solvophobic increment like the surfactants shown in Figure 1, but AOT is an interesting and well-studied model surfactant that is important in numerous applications as, for example, a microemulsion stabilizer [8], as a micellar nanoreactor [9], or as a charge control agent [10]. Understanding how this surfactant forms inverse micelles at low concentrations is important for efficient and controllable use in applications.…”

The internal structure of poly(methyl methacrylate) latexes in nonpolar solvents

“…Dispersions of colloids in nonpolar solvents are prevalent in soft matter nanoscience [1][2][3][4][5][6][7][8][9][10][11][12]. The significance of such dispersions can be appreciated by considering the many various industrial sectors that make use of them, which include petrochemicals [13], lubricants [7,14,15], reprography [4,5], inkjet printing [5], magnetic recording media [4], rheological fluids [16,17], and electronic displays [18][19][20][21].…”

Interaction between Surfactants and Colloidal Latexes in Nonpolar Solvents Studied Using Contrast-Variation Small-Angle Neutron Scattering

“…1,2 The formation of charged species is important in many nonaqueous systems, including petroleum safety and the electrophoretic displays used in electronic paper devices. 3,4 A common system used to produce charged species in nonpolar solvents is by preparing dispersions of poly(methyl methacrylate) (PMMA) latexes with poly(12-hydroxystearic acid) (PHSA) brushes as steric stabilizers and the anionic surfactant, sodium dioctylsulfosuccinate (Aerosol OT or AOT), added as CCA.…”