Surface stability, electrostatic condition

According to classical electrostatics, polar surfaces are thus unstable. However, it can be shown [2,94] that the macroscopic dipole moment can be cancelled out by modifications of the charge densities on the outer layers. Assuming that m outer layers have a charge density aj different from the bulk ( cTj 7 (7 for 1 < j < m) and that the layer spacing is alternatively equal to Ri and R2, the electrostatic condition for surface stability reads ... 

A semi-infinite crystal cut along a polar direction has an infinite electrostatic energy, because the electrostatic field has a non-zero mean value in the material. Yet, under some specific conditions that we will now consider, the macroscopic field may be cancelled out and the surface stabilized. The following arguments rely upon a macroscopic analysis, which means that ... 

The stability of many colloidal solutions depends critically on the magnitude of the electrostatic potential ( /q) at the surface of the colloidal particles. One of the most important tasks in colloid science is therefore to obtain an estimate of V /o under a wide range of electrolyte conditions. In practice, one of the most convenient methods for obtaining /o uses the fact that a charged particle will move at some constant, limiting velocity under the influence of an applied electric field. Even quite small particles (i.e. <1 pm) can be observed using a dark-field micro-... 

As we noted above, the evaluation of W for given values of dispersion properties such as surface potential, Hamaker constant, pH, electrolyte concentration, and so on, forms the goal of classical colloid stability analysis. Because of the complicated form of the expressions for electrostatic and van der Waals (and other relevant) energies of interactions, the above task is not a simple one and requires numerical evaluations of Equation (49). Under certain conditions, however, one can obtain a somewhat easier to use expression for W. This expression can be used to understand the qualitative (and, to some extent, quantitative) behavior of W with respect to the barrier against coagulation and the properties of the dispersion. We examine this in some detail below. 

The results showed that all batch polymerizations gave a two-peaked copolymer compositional distribution, a butyl acrylate-rich fraction, which varied according to the monomer ratio, and polyvinyl acetate. All starved semi-continuous polymerizations gave a single-peaked copolymer compositional distribution which corresponded to the monomer ratio. The latex particle sizes and type and concentration of surface groups were correlated with the conditions of polymerization. The stability of the latex to added electrolyte showed that particles were stabilized by both electrostatic and steric stabilization with the steric stabilization groups provided by surface hydrolysis of vinyl acetate units in the polymer chain. The extent of this surface hydrolysis was greater for the starved semi-continuous sample than for the batch sample. 

ASOs are synthesized as complex mixtures of diastereomers. In the solid state, they are amorphous, electrostatic, hygroscopic solids with low-bulk densities, possessing very high surface areas, and poorly defined or no melting points. Their good chemical stability allows them to be stored as lyophilized or spray-dried powders, or as concentrated, sterile solutions. For example, the 21-mer ASO Vitrave-ne (fomivirsen sodium intravitreal injectable) is approved in the USA for a storage condition from 2 to 25 °C [2]. 

Once the system of eqs 3 and 4 is solved under the boundary conditions (7a—d), the total free energy of the system can be calculated by adding the van der Waals interactions between the surfaces to the double layer free energy composed of electrostatic, entropic, and chemical contributions,11 and the stability ratio can be calculated in the usual manner.18... 

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