Dispersity Of Fumed Silica And Interactions Between Fumed Silica And Quaternary Ammonium Gemini Surfactants

Fumed silica is a special fine amorphous power with narrow ranges offluctuation in the particle size distributions (PSD). It has small primary particles,large specific surface area, high surface activity and copious surface silanol groups,and shows special dispersity, rheology and adsorption performances in medium,therefore，it has been widely employed in academic research and application fields.The performances of fumed silica directly related to the form of surface silanolgroups, and cationic surfactants are important material for surface modification offumed silica. Quaternary ammonium Gemini surfactant is a kind of cationicsurfactant with novel structure, which consists of two traditional cationic surfactantslinked at the head groups by spacer and has a lot of special properties that traditionalsurfactant doesn’t have. The spacer length of Gemini surfactants intensivelyinfluences the adsorption behaviors of surfactants at solid/liquid interface, and thiswill definitely exert influences on dispersity and rheological properties of fumedsilica suspensions. So the study on adsorption of quaternary ammonium Geminisurfactants on fumed silica surface and interactions between fumed silica andquaternary ammonium Gemini surfactants has an important theoretical and practicalsignificance. In this thesis, three quaternary ammonium Gemini surfactants withdifferent spacer length C12-s-C12·2Br (s=2,4,6) have been selected to investigatethe interactions between fumed silica and quaternary ammonium Gemini surfactantsby many methods, such as dynamic light scattering (DSL), rheology, conductivityand fluorescence probe technique. In addition, a conventional quaternary ammoniumsurfactant C16TABr with critical micelle concentration (cmc) near to C12-s-C12·2Br (s=2,4,6) has also been selected for purpose of comparison. The main results in thisthesis are described as follows:1. The influences of shear rate and shear time on average hydrodynamic diameter of fumed silica particles (d) in fumed silica suspensions were investigatedby dynamic light scattering (DSL) technology. At a certain shear rate, the d rapidlydecreases with increasing shear time and then appears a plateau region. The d inplateau region decreases with increasing shear rate within investigated condition.2. The dispersity of fumed silica in different pH mediums were investigated bydynamic light scattering (DSL) and electrophoresis technology. The results showedthat isoelectric point and titration end point of surface silanol groups of fumed silicaare present in pH (IEP)=2.09and pH (TE)=7.47respectively in fumed silicaaqueous suspensions. The particles of fumed silica suspension become big by addingH2SO4, while fumed silica suspension is stable by adding NaOH.3. The dispersity of fumed silica in quaternary ammonium surfactantsC12-s-C12·2Br (s=2,4,6) and C16TABr solutions were investigated by dynamic lightscattering (DSL). Silica suspensions exhibit a process of dispersion–flocculation–redispersion by adding a light C16TABr or C12-2-C12·2Br, while no flocculation ofsilica suspensions occurs by adding C12-4-C12·2Br or C12-6-C12·2Br.4. The adsorption and aggregation process of surfactants on fumed silicasurface were investigated by pH, conductivity, and pyrene fluorescence probetechnology. The results showed that C12-s-C12·2Br (s=2,4,6) and C16TABr areintensively adsorbed to fumed silica surface. The adsorption and aggregation processof surfactants exhibit four stages, they are ion-exchange, electrostatic repulsion,formation of adsorption layer by hydrophobic interaction and formation of micelle inthe bulk.5. Schematic phase diagrams of the respective silica/C16TABr and silica/C12-2-C12·2Br suspensions were obtained. The results showed that both C16TABrand C12-2-C12·2Br can prompt fumed silica suspensions form gels at lowerconcentrations. Rheological characteristics (flow pattern, thixotropy) of fumed silicasuspensions with fumed silica content are4%, C16TABr and C12-2-C12·2Brconcentrations are1.5mM in gel-state regimes were investigated, and comparedwith the corresponding silica/C12-4-C12·2Br and silica/C12-6-C12·2Br suspensions. The results showed that both silica/C16TABr and silica/C12-2-C12·2Br suspensionsshow typical pseudoplastic-flow behavior, while silica/C12-4-C12·2Br and silica/C12-6-C12·2Br suspensions nearly show Newtonian-flow behavior.6. Dynamic viscoelasticity of silica/C16TABr and silica/C12-2-C12·2Brsuspensions in gel-state regimes were investigated. At C16TABr and C12-2-C12·2Brconcentrations of Cs=1.5mM,4%silica suspensions mainly show elasticcharacteristics，while At Cs=4mM,4%silica suspensions mainly show viscouscharacteristics.