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Adsorption characteristics of charged and nonionic small molecules to colloidal alumina.
Journal of Colloid and Interface Science 2018 Februrary 16
Dense fluorescent pigments used for inkjet printing of UV and IR-readable non-photobleaching security features require stabilizers to prevent aggregation/sedimentation and inkjet head clogging at high resolution. A study of small molecule adsorption to α-alumina, a model system for security pigments, is presented. Alumina is dispersed by two methods yielding different zeta potentials but identical isoelectric points. Essentially complete dispersion is obtained in water at pH 3 but aggregation occurs at pH 6 where the surface charging is lower. Adsorption studies focus on the naphthyl-phosphate, -sulfate, and hydroxyl (triethylene glycol) groups. Phosphate adsorption was strongest with a 1.2 molecules/nm2 plateau, close to the titratable exchange capacity of 1.3 OH groups/nm2 on the alumina surface with ΔHadsorption =-7.58±1.63kJ/mol determined by calorimetry. Sulfate adsorption was weaker with a more linear adsorption isotherm. The adsorption/exchange process yields a rise in pH that is correlated with the binding strength. Hydroxyl binding is weakest, being driven by hydrogen bonding, and showed no rise in pH during adsorption. A polyphosphate-poly(ethylene glycol) block copolymer is expected to be advantageous for the dispersion of such inkjet colloids.
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