| 摘要: | 一般來說，當粒子處於低介電係數溶劑中，因較強烈的庫倫作用力導致其反離子不易解離帶電。但是，電子紙卻是利用粒子在低介電係數溶液中解離並於施加電場後進行電泳。本論文主要利用毛細管電泳，研究在低介電係數溶液中，電滲流及電泳行為並配合理論推算，進而分析其解離原因、泳動速度及改善方法。 低介電係數的溶劑在毛細管電泳中施予高電壓時，只產生微弱電流，可避免因產生焦耳熱所導致黏度下降進而影響電滲流及電泳的mobility，推測在電子紙中亦可有效避免焦耳熱造成的影響。利用熔融矽(二氧化矽)材質之毛細管，以不同比例的去離子水，乙醇溶液、丙醇溶液及dioxan溶液可知，溶液黏度上升與介電係數降低造成電滲流mobility降低；並確認在低介電係數溶液下，毛細管可少量解離出離子產生電滲流，並經由數據回歸可得表面電荷密度與低介電係數溶劑之重量百分比成指數遞減關係，與介電係數成次方關係式。在電泳方面，利用理論計算與毛細管電泳實驗對照，證實處於低介電係數溶液中，粒子因表面電荷密度小，又為粒子半徑不大之關係，而使其泳動mobility緩慢，造成電滲流與電泳在毛細管電泳中之訊號重疊。 因此，由實驗及理論可得電泳mobility與黏度成反比，與粒子半徑及表面電荷密度成正比。因此，只要粒子具有足夠的大小，即使在低介電係數的溶液中亦可解離。在電子紙系統中，粒子泳動速度越快，畫面反應時間就越快速。因此，要增加其泳動速度除了找尋在該溶劑下之電荷密度較高的粒子材料外，亦可從增加粒子半徑著手。 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. |
