No abstract

Numerical solutions of the Poisson−Boltzmann equation for potentials in the electrical double layer surrounding a particle are used to derive a new relationship between the particle charge q and the surface potential ζ. Unlike the linear ζ−q relation for a particle in charge-free media, the new relation shows that as the particle charge increases, the initial linear increase of the potential slows down and asymptotes to a finite value. The asymptotic values of the potential at high particle charges are dependent on the Debye length (or the ionic charge density) of the media and are of the order of a few hundred millivolts for particles in typical nonaqueous dispersions, e.g., those used as liquid developers for electrographic images. Thus, with this relationship, the reported values of charge and the electrophoretic mobility determined experimentally for these dispersions correspond to physically reasonable values of ζ potentials, which are smaller than that expected from the linear relation by more than an order of magnitude. In addition, the variations of particle charges with ionic charge densities and particle concentrations are examined.

Using the concept of thermally activated mobility, a theory is developed to calculate the thermally stimulated current (TSC) in hopping systems as a function of temperature. Unlike the existing theories which describe only the band transport systems, the present theory is applicable to both band and hopping transport systems. Various features of this ’’mobility-induced’’ TSC are illustrated with numerical examples and compared with experiments performed on polymer films (e.g., PVK). These features form the basis of a new method (reported in an earlier paper) for determination of the activation energy, and a technique for studying very low mobilities and/or very low field transport phenomena. It is also found that an observation of shift in TSC peak temperature with applied field does not necessarily imply a field-dependent activation energy.

A calculation of intermolecular transfer integrals J for carbazole molecules shows that J varies by orders of magnitude when the molecular planes are rotated. The angular variation of J is used to estimate the corresponding changes in the transition rates. It is suggested that in certain cases the electronic transition occurs after thermally activated hindered rotation. Furthermore, the rotational energy barrier may be a principal component of the rather large electronic transport activation energy which is observed experimentally when carriers move by hopping in certain amorphous organic materials.

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