Electrokinetic study of adsorption of ionic surfactants on titania from organic solvents

Charging of Oxide Nanoparticles in Media of Intermediate Dielectric Constant

The use of surfactants to charge colloidal particles in solvents of intermediate dielectric constants (5 < ε < 40) is investigated. While particle charging mechanisms in aqueous (ε = 80) and apolar (ε < 5) media are well understood, the interplay of these different charging mechanisms, which can all occur in solvents of intermediate dielectric constants (sometimes referred to as "leaky dielectrics"), remains to be fully explored. Conductometric titrations determining the critical micelle concentration (CMC) of the surfactant (aerosol-OT) confirm the existence of reverse micelles in intermediate dielectrics and show that as the solvent dielectric constant decreases, the CMC decreases as well. Electrophoretic mobility measurements of three oxide particles (SiO₂, TiO₂, and MgO) highlight various charging mechanisms that arise from particle-solvent, particle-surfactant, and solvent-surfactant interactions in a solvent series of alcohols and ketones. The results show that a combination of donor-acceptor particle-solvent interactions, surfactant ion adsorption, and reverse micelle-mediated acid-base interactions can all charge oxide particles in intermediate dielectrics. Furthermore, the results show that the dielectric constant of the solvent affects the relative magnitudes of each charging mechanism.

Molecular catalysis at polarized interfaces created by ferroelectric BaTiO3

Colloidal suspensions of ferroelectric BaTiO₃ nanoparticles act as a dispersible polarized interface that can influence the selectivity of non-faradaic reactions.

The local environment at polarized solid–liquid interfaces provides a unique medium for chemical reactions that could be exploited to control the selectivity of non-faradaic reactions. Polarized interfaces are commonly prepared by applying a voltage to an electrode in an electrolyte solution, but it is challenging to achieve high surface charge densities while suppressing faradaic reactions. Ferroelectric materials have permanent surface charge densities that arise from the dipole moments of ferroelectric domains and can be used to create polarized solid–liquid interfaces without applying a voltage. We studied the effects of ferroelectric oxides on the selectivity of a Rh porphyrin-catalyzed carbene rearrangement. The addition of ferroelectric BaTiO₃ nanoparticles to the reaction solution changed the product ratio in the same direction and by a similar magnitude as performing the reaction at an electrode–electrolyte interface polarized by a voltage. The results demonstrate that colloidal suspensions of BaTiO₃ nanoparticles act as a dispersible polarized interface that can influence the selectivity of non-faradaic reactions.

Investigation of charging behavior of PS particles in nonpolar solvents

The charging behavior of PS (polystyrene) particles dispersed in nonpolar solvent containing surfactant AOT (sodium di-2-ethylhexylsulfosuccinate) was researched by phase angle light scattering (PALS). The effects of the AOT concentration, the particle concentration and the particle size on the zeta potential of the particles were analyzed systemically. The results showed, at different particle concentrations (expressed in the volume fraction of 10(-5)-10(-3)), that the zeta potential could be adjusted by surfactant AOT over a wide concentration range of 0.001-100 mM. An obvious difference of zeta potential on particle concentration was revealed between the high AOT concentrations (beyond 10 mM) and the low ones (below 1 mM). Meantime, it was found that the relationship of particle size to zeta potential showed a great discrepancy between the dilute particle concentrations (below 10(-4)) and the concentrated ones (beyond 6 × 10(-4)). These findings were consistent with the mechanism of preferential adsorption of the charged micelles in nonpolar solvent.

Solvents, in which ionic surfactants do not affect the zeta potential

The influence of solvent polarity on adsorption behavior of DOSS (anionic surfactant) has been studied by means of electroacoustic method. DOSS substantially affected the zeta potential of titania and alumina dispersed in methanol and in hydrocarbons. In 1-propanol, 2-propanol, and 1-butanol, which have intermediate polarity between methanol and hydrocarbons, the effect of the ionic surfactant on the zeta potential of solid particles was less significant.