Effect of Adsorption of Long Chain Alcohol Molecules on Silica Particles on Toner Charging

Abstract

The adsorption of long chain alcohols on the surfaces of hydrophobic silicas has been studied using 1-hexadecanol on R972 silica as a model system. Both DSC and IR spectral data suggest that 1-hexadecanol is in the gaseous state on the silica surface at concentrations ≤ 4.8% by weight. At 9.4% loading, which is equivalent to half of a theoretical monolayer on the silica surface, results suggest that the adsorbed 1-hexadecanol forms a hydrocarbon protective layer on the surface. The formation of the layer implies that the hydrocarbon chains in the adsorbed 1-hexadecanol are interacting with each other, presumably by folding the hydrocarbon chains back toward the silica surface. As the concentration of 1-hexadecanol increases, the space occupied by the folded hydrocarbon chains is replaced by the added 1-hexadecanol, up to one theoretical monolayer (17%). Beyond this concentration, crystallization of 1-hexadecanol occurs. The effect of the chain length of the adsorbed alcohol is studied at a theoretical monolayer coverage for a series of normal alcohols, from C12 to C22. While protective hydrocarbon layers are formed for all the normal alcohols studied, IR spectral data suggest that the layers formed from 1-octadecanol, 1-eicosanol and 1-docosanol are less organized. The surfaces of these modified silicas may be more hydrophilic as compared to those from 1-dodecanol, 1-tetradecanol and 1-hexadecanol. The charging properties of alcohol-treated silicas were studied by first blending them with 9 μm unpigmented SPAR toner at 0.5% by wt. concentration, followed by charging the resulting toners with metal beads. The tribo data reveal that adsorption of long chain alcohols on the silica surface enhances the negative tribocharge of the resulting toner at both low (20%) and high (80%) relative humidity (RH). High tribo with minimal RH sensitivity, as judged from the tribo ratio from 20% to 80% RH, are obtained when R972 is covered with one theoretical monolayer of 1-hexadecanol. The attainment of optimal charging result is shown to correlate to the proposed molecular structure of the adsorbed alcohol layer. The important role of the hydrophobicity of the silica surface in toner charging is discussed.