The effects of reverse micelles on charging behavior in apolar media
Michor, Edward LeRoy
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The study of particle charging behavior in apolar media is key to the understanding and design of electrophoretic displays and inks. The charging of colloidal particles in these apolar environments relies on the presence of surfactants and their reverse micelles. Reverse micelles can undergo disproportionation reactions with one another, resulting in bulk charges in the media and an increased system conductivity. Surfactant molecules adsorbed on particle surfaces can undergo acid-base interactions with the surface functional groups, resulting in the formation of charges. The now-charged surfactant molecule is then stabilized in a reverse micelle, leaving a net charge on the particle surface. Many factors influence the micellization and particle charging behavior of these surfactants – from environmental variables, such as temperature and humidity, to the surfactant, particle, and solvent chemistries and structures. We show that the micellization of the surfactant Aerosol OT becomes less favorable with increasing solvent dielectric constant. Surfactant chemistries can be tuned by performing simple ion-exchange reactions with the surfactant head group. We have shown that increasing the electronegativity of the cation in the head group results in more acidic surfactants, and therefore the ability to impart higher maximum charges to particle surfaces. The critical micelle concentrations (CMCs) of a surfactant series were investigated as a function of temperature, and found to increase with increasing temperature. This same series was then used to charge colloidal particles at three temperatures, which resulted in decreasing maximum particle charge with increasing temperature. The structures of these reverse micelles were investigated using small angle neutron scattering (SANS), and it was found that the core radii of the reverse micelles decreased with increasing surfactant tail length. Tri-tailed surfactants formed reverse micelles with the smallest core radii, of ~4Å. This study suggested that in order to impart the maximum charge to colloidal particles, the reverse micelles being used must have intermediate core sizes, in order to offer a large enough polar environment to stabilize charge, but not one too large such that disproportionation reactions in the bulk cause electrostatic screening of the particle charges. Applications of this research are discussed.
- Chemical engineering