Effects of dispersant molecular weight on dispersion and stability of zinc oxide nanoparticle aqueous suspension

Abstract

Aqueous suspensions of zinc oxide (ZnO) nanoparticles are prepared at 0.5 wt% solids content using polyacrylic acid (PAA) as a dispersant. Average primary particle sizes of ZnO powder are 65.31, 174.56 and 224.54 nm and PAA molecular weights are 1.8k, 450k and 3000k g/mol. Particle dispersion and stability of the suspensions are investigated. It is found that PAA(1.8k) exhibits the highest dispersant efficiency for all ZnO sizes. Well-dispersed and highly stable suspensions can be obtained at 0.5 wt% PAA(1.8k). Median agglomerate sizes are 0.18, 0.37 and 1.02 µm for suspensions prepared with ZnO powder of sizes 65.31, 174.56 and 224.54 nm, respectively. The highly stable suspensions are not completely precipitate up to 28 days. However, PAA(450k) and PAA(3000k) are also effective as dispersants when using at higher concentrations. Adsorption amounts of PAAs on ZnO surface are determined and the data exhibits Langmuir isotherm. Evaluated area occupied per molecule of PAAs on various ZnO sizes suggests that low molecular weight PAA adsorbed on particles as loops and tails while high molecular weight PAA mostly adsorbed as tails. Also, PAAs adsorbed as loops and tails on large particles while frequently tails on small particles. These differences in adsorption conformation could directly relate to dispersant efficiency of PAAs. In addition to the effects of PAAs on preparation of ZnO nanoparticle suspensions, its influence on characteristics of ZnO nanoparticles synthesized by precipitation method is also investigated. It is found that primary size of synthesized ZnO nanoparticles can be controlled by PAA concentrations. Increasing of PAA concentration narrow the distribution and shift the size to smaller particles. Average primary particle size of around 20 nm can be obtained with 1 wt% PAAs. Moreover, ZnO particles synthesized using PAAs exhibits higher zeta potential than the one without PAA, and the zeta potential values increase with increasing PAA concentration. It suggests that ZnO particles synthesized using 0.5-1 wt% PAAs should be well-dispersed in aqueous medium and provide a highly stable suspension, which is desired for most applications.