| Summary: | 碩士 === 國立中央大學 === 化學工程與材料工程研究所 === 98 === In general, the dissociation of counterion from charged particle is difficult in a medium with low dielectric constant due to strong Coulomb attraction. However, in electronic paper, charged particles exist in low dielectric medium and migrate by application of an electric filed, so called electrophoresis. In this work, the electrophoretic mobility and the origin of counterion dissociation from charged particle are investigated by using capillary electrophoresis system through electro-osmosis in a salt-free and low dielectric constant medium, in which the Joule heat effect is insignificant under strong electric field due to low electric current.
Owing to strong Coulomb interaction, the surface charge density (?) of silica particles in a salt-free and low dielectric constant medium is very small compared to that in water. Therefore, the electrophoretic mobility is essentially zero. Intuitively, it is anticipated that the electro-osmotic flow (EOF) is absent in a fused silica capillary because of the lack of counterions. We consider organic solvents with low dielectric constant (?), including ethanol (? = 24), propanol (? = 20), butanol (? = 19), tetrahydrofuran (? = 7), and dioxane (? = 2), and their mixtures with water. The dielectric constant of the mixture can be controlled by tuning the concentrations (c). It is interesting to find that EOF always occurs regardless of the dielectric constant. The mobility generally declines with increasing viscosity or decreasing dielectric constant. The relationship between surface charge density and dielectric constant can be described by surface charge decsity ~ exp(-ac) ~ (dielectric constant)^b.
In conclusion, even in a salt-free and low dielectric constant medium, counterion dissociation always takes place and thus electrokinetic phenomena occur, as long as particles’ sizes are large enough. An example is EOF, which involves a very large surface area of an inner tube wall. Consequently, in order to increase the response time in e-paper applications, the electrophoretic mobility can be increased by increasing the surface charge density or particle size.
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