Electrolyte dependent particle aggregation by low frequency alternating electric fields.

摘要

Most solid surfaces in contact with solution are charged often as a result of dissociation of surface groups or adsorption of charged species. This results in the well known electric double layer consisting of the charged solid surface, adsorbed species and the diffuse layer, a region of excess mobile counterion charge in the solution near the interface. When an electric field acts on the ions in the diffuse layer, fluid flow can result. This is called eelectroosmosis. Oppositely, when fluid flow is applied, an electric current called the streaming current is generated in the diffuse layer. Streaming potentials can result if a high impedance external circuit is included. This thesis investigates phenomena relating to these fluid flow---electric field interactions.; A rotating disk sample in contact with solution generates a streaming potential when a high impedance measuring device, such as a voltmeter, is included in solution. The theory relating this streaming potential to the zeta potential of the disk is presented. The theory showed that the ideal positioning of two reference electrodes for measurement of the streaming potential is one on the axis of the disk near the disk surface with a second placed several radii from the disk surface where the streaming potential goes to zero. Theoretical predictions are shown to agree well with variation of rotation rate and position. Specifically, theory predicts a 3/2 power dependence of streaming potential on rotation rate. Zeta potential are currently measured as a one parameter fit to theory, most often with an oscillating rotation rate. Measurable streaming currents can be made in aqueous solutions at less than approximately 10 mM concentration.; Streaming currents are more common in a capillary device, but are not ideal using the rotating disk geometry. Surface current emanates from the edge of the disk and there is converted to ohmic bulk current. This current must flow through a low impedance external circuit such as an ammeter to make the measurement. A competing path independent of the external circuitry is also possible, that being the return of the current through the bulk solution back to the surface. Unfortunately at aqueous salt concentrations greater than approximately 0.1 mM the current prefers to flow through the bulk solution. At lower concentrations, streaming currents are measured, but are not as large as expected. Thus, a current collection efficiency is defined as the fraction of the current flowing through the measurement circuit. This is shown to be only geometry dependent at low concentrations. Streaming potentials are thus preferable to streaming current measurements.; The application of an alternating electric field in the vicinity of a particle leads to net electroosmotic flow around a particle at low frequencies. In the vicinity of an electrode, particles can aggregate or separate depending on the electrolyte. Fagan et al. previously proposed a model for this behavior based on a phase angle measured between the applied electric field and the height of the particle. The model predicted that particles would aggregate if this phase angle was greater than the expected 90° and separate if less than 90°. Experimental evidence in favor of this mechanism was based only on two salts above an indium tin oxide electrodes. This contribution extends this correlation to more electrolytes above indium tin oxide as well as platinum electrodes. A hypothesis for the origin of this phase angle is also presented based on changes to the zeta potential of the electrode surface while current passes through it.; Kim et al. presented a method of measuring the Debye length of a nonaqueous---surfactant system using capacitance measurement. Fu measured Debye lengths independently using total internal reflection microscopy in heptane---PIBS. Fu's results are shown to agree well with measurements made similar to those of Kim et al. The dielectric constant of the solution is al