Nanoparticle Charge Control in Nonpolar Liquids: Insights from Small-Angle Neutron Scattering and Microelectrophoresis

Abstract

Electrostatic forces are typically produced in low polarity solvents by the addition of surfactants or charge-control additives. Although widely used, there is no consensus on the mechanism by which surfactants control the level of particle charge. We report an investigation using highly sensitive, single particle optical microelectrophoresis measurements combined with a small-angle neutron scattering study to establish the mechanism of charging by the surfactant AOT in the nonpolar solvent n-dodecane. We show that polymer-grafted particles with no chemically bound surface charges only charge above the critical micellar concentration of the surfactant. The surface potential increases gradually with increasing surfactant concentration c, before finally saturating at high c. The increase in the surface potential is correlated to the amount of surfactant adsorbed onto the surface of the particle. Using deuterated AOT and contrast variation techniques, we demonstrate that the surfactant is adsorbed within the polymer layer surrounding the particle core, probably as individual molecules rather than surfactant aggregates. A simple thermodynamic model accounts for the concentration dependence of the observed surface potential.

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.…”

“…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.…”

“…The only previous study proposed that AOT surfactant adsorbs as monomers in tightly-bound layers around the PMMA cores. 11 This study was limited by the use of only one particle type and one surfactant concentration achieving sufficient scattering intensity resolution. Given the number of previous studies of PMMA latexes using SANS and chemical characterization, 13,[20][21][22][23][24][25][26][27][28] it is known that latexes and analogues for the stabilizing polymers can be readily generated for precise characterization.…”

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.…”