Surfactant-Mediated Electric Charging Phenomena in Nonpolar Dispersions
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Electric charging of colloidal particles in nonpolar solvents plays a crucial role for many industrial applications and products. Although disfavored by the low solvent permittivity, particle charging can be induced by added surfactants, even nonionic ones, but the underlying mechanism is poorly understood, and neither the magnitude nor the sign of charge can generally be predicted from the particle and surfactant properties. The aim of this thesis is to achieve a better understanding on surfactant-mediated particle charging mechanisms in nonpolar dispersions, using a series of highly systematic approaches. We develop a method of characterizing the Lewis acid/base behavior of oil-borne surfactants, by which we can predict the propensity of charge transfer in polar interactions of the surfactants with other polar components in nonpolar media and examine some traditional speculations where surface charging was attributed to the direct polar interaction of the surfactants with particle surfaces. We synthesize and purify a series of custom surfactants under subtle variations of the chemical structure, and employ these surfactants as surface charging agents of several colloidal particles with well-defined surface polarity. We experimentally represent that surface charging is not likely a consequence of the single type of polar interaction between surfactant moieties and surface functional groups, disproving the past hypotheses, but a consequence of interplays between multiple charging pathways. In mechanistic interpretation of surface charging phenomena in these well-defined nonpolar systems, we prove that the ionization of surfactants is more preferred in the nonpolar liquid bulk via their polar interaction with oil-borne moisture, the third component, than at the particle surfaces via such interaction with surface moieties. We suggest that the inverse micellar ions, created by this surfactant-moisture polar interaction, can influence net surface charging significantly, as another (ionic) acids and bases adsorbing to the surfaces asymmetrically. We also claim that the asymmetric adsorption state may not only be determined by the surface’s chemical preference for a certain sign of micelle ions in terms of ion-dipole interaction, but also by the size asymmetry between the oppositely charged micellar ions in terms of minimizing the translational entropy loss associated with confining the continuous phase-soluble adsorbates. We show that the ionization of surface functional groups, which has been traditionally suggested as the only surface charging pathway, may only play a role as a contribution to net surface charging in the case where the surface is sufficiently wet by local aqueous bulk. Finally, we develop a new type of solid particle amphiphile promoting the electric conduction of nonpolar media, as an alternative to the molecular charging agent surfactants, based on the knowledge we gain from the prior mechanistic investigations.