Surface potential, change with number layers

The popular applications of the adsorption potential measurements are those dealing with the surface potential changes at the water/air and water/hydrocarbon interface when a monolayer film is formed by an adsorbed substance. " " " Phospholipid monolayers, for instance, formed at such interfaces have been extensively used to study the surface properties of the monolayers. These are expected to represent, to some extent, the surface properties of bilayers and biological as well as various artificial membranes. An interest in a number of applications of ordered thin organic films (e.g., Langmuir and Blodgett layers) dominated research on the insoluble monolayer during the past decade. 

The potential in the diffuse layer decreases exponentially with the distance to zero (from the Stem plane). The potential changes are affected by the characteristics of the diffuse layer and particularly by the type and number of ions in the bulk solution. In many systems, the electrical double layer originates from the adsorption of potential-determining ions such as surface-active ions. The addition of an inert electrolyte decreases the thickness of the electrical double layer (i.e., compressing the double layer) and thus the potential decays to zero in a short distance. As the surface potential remains constant upon addition of an inert electrolyte, the zeta potential decreases. When two similarly charged particles approach each other, the two particles are repelled due to their electrostatic interactions. The increase in the electrolyte concentration in a bulk solution helps to lower this repulsive interaction. This principle is widely used to destabilize many colloidal systems. 

The expected Tafel slope of 60mV/decade is not always found. There are a number of reasons for this, aside from kinetic effects in the bulk of the semiconductor. The kinetic effects associated with faradaically active surface states is of considerable significance, as shown below, but another common problem is that part of the potential change may appear across the Helmholtz layer rather than across the depletion layer. A well-known case in point is germanium, for which the surface is slowly converted from "hydride to "hydroxylic forms as the potential is ramped anodically. This conversion gives rise to a change in the surface dipole and hence Aij/ AT. In fact, the anodic dissolution of p-germanium is found to follow a law [106]... 

The existence of this process demonstrates that the structure of the amorphous intercrystalline layers in a semicrystalline polymer is different from a polymer melt. The reason can be easily seen All the chain sequences are fixed with their ends in the crystallites and, furthermore, the concentration of entanglements is enhanced. As a consequence, the mean chemical potential of the units is higher than in a melt and varies with the layer thickness. The direction of change is obvious. The numbers of entanglements and points of chain entry into the crystallites are constant. The motional restrictions thus become diminished if the layer thickness increases, which implies a decrease in the chemical potential. Under such conditions each change in temperature leads to a new local equilibrium between crystallites and amorphous regions, via a surface crystallization or melting process. 

Fig. 1 Charge inversion for polyelectrolyte multilayers represented as the surface potential dependence on the number of adsorbed layer, N. a Charge inversion as it was obtained by the Kelvin probe for (PDADMAC + PSS) multilayers adsorbed from a NaCl solution of concentration 50 mM onto a flat surface. Adapted from Ref. [80] with permission from The Royal Society of Chemistry, b Zeta potential of (PSS + PAH)n multilayers deposited onto colloidal microparticles of methylformamide with a positive bare charge. Adapted with permission from Ref. [52]. Copyright (1998) American Chemical Society, c Zeta potential changes evaluated using Streaming potential for multilayers built with PSS and PAH, the results are represented as zeta potential against number of measurement. The number of measurements is related to the number of times that each single layer was measured and it shows as accumulative number with the increase of the layer number. Adapted with permission from Ref. [83]. Copyright (2000) American Chemical Society...

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