Polarizability and complex conductivity of dilute suspensions of spherical colloidal articles with uncharged (neutral)polymer coatings.

摘要

The polarizability of polymer-coated colloidal particles,as measured via dielectric relaxation spectroscopy,reflects on the degree to which convection,diffusion,and electromigration deform the equilibrium double layer.With a polymer coating,convection and electro-osmosis ae resisted byhydrodynamic drag on the polymer segments.The electro-osmotic flow near th eunderlying bare surface is therefore diminished.Characteristics of the particles and the adsorbed polymer cna,in principle,be inferred bymeasuring the frequency-dependent polarizability.In this work,"exact" numerical solutions of the electrokinetic equations are used to examine how adsorbed polymer changes the particle polarizability and,hence,the conductivity and dielectric constant increments of dilute suspensions.For neutral polymer coatings,the conductivity and dielectric constant increments are found to be very similar to those of the underlying bare paticles,so the response depends mostly on the underlying bare particles.These observations suggest that dielectric spectroscopy is best used to determine the underlying surface charge,withcharacteistics of the coating inferred form the electrophoretic or dynamic mobility,together with the hydrodynamic radius obtained from sedimentation or dynamic light scattering.Addressed briefly are the effects of added counterions and nonspecific adsorption.The electrokinetic modle explored in this work can be used to guide experiments(frequency and ionic strength,for example) to eithe rminimize or maximize the sensitivity of the complex conductivity to the coating thickness or permeability.