Electrostatic Charging of Nonpolar Colloids by Reverse Micelles

Abstract

Colloids dispersed in a nonpolar solvent become charged when reverse micelles are added. We study the charge of individual sterically stabilized poly(methyl methacrylate) spheres dispersed in micellar solutions of the surfactants sodium bis(2-ethyl 1-hexyl) sulfosuccinate [AOT], zirconyl 2-ethyl hexanoate [Zr(Oct)2], and a copolymer of poly(12-hydroxystearic acid)-poly(methyl methacrylate) [PHSA-PMMA]. Although the sign of the particle charge is positive for Zr(Oct)2, negative for AOT, and essentially neutral for PHSA-PMMA, the different micellar systems display a number of common features. In particular, we demonstrate that over a wide range of concentrations the particle potential is a constant, independent of the number of micelles added and independent of the colloid size. A simple thermodynamic model, in which the particle charge is generated by the competitive adsorption of both positive and negative micelles, is in good agreement with the experimental data.

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

“…33 The negative electrophoretic mobility agrees with previous measurements on this system. 5,6,8,11 The electrophoretic mobility of AOT inverse micelles in aliphatic solvents can be estimated to be µ = 3.8 × 10 −9 m 2 V −1 s −1 . 34 This is an order of magnitude greater than is measured for the PMMA latexes and shows that the mobility of the larger PMMA particles is being measured.…”

“…Two contrasts were measured: surfactant contrast-matched solvent, which highlights polymer-only scattering, and polymer contrast-matched solvent, which highlights surfactant-only scattering. Contrast has been obtained using mixtures of deuterium-labeled solvents (either dodecane-D 26 or toluene-D 8 ) and AOT-D 34 . By measuring these two contrasts, it is possible to determine whether the addition of the surfactant causes any difference to the structure of the polymer, as well as the structure of aggregated and adsorbed surfactant.…”

“…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. [5][6][7][8][9][10][11][12] In addi-tion to AOT, these particles can be charged with other CCAs, such as calcium octanoate, 13 sodium acetate, 14 or through the incorporation of ionizable monomers. [15][16][17] These model PMMA colloids, originally developed by Antl et al, 18 have been well-studied in nonpolar solvents, due to the ability to produce highly monodisperse particles that can behave essentially as hard-spheres.…”

Sulfosuccinate and Sulfocarballylate Surfactants As Charge Control Additives in Nonpolar Solvents

“…For reference, literature values for AOT1 in alkanes are ∼ 10 −5 . 36,37,40,49 This scales with the measured conductivity, as TC1 is less likely to dissociate than AOT1 and thus the conductivity is lower. Unlike the magnitude of the electrophoretic mobility, which increases with decreasing surfactant concentration, the ζ potential is independent of concentration.…”

“…54 However, it is reasonable to assume that the particles have a slight negative charge in agreement with literature. [36][37][38]40 Latex only SANS. Latex scattering.…”