Competitive adsorption of (phosphorylated) ethoxylated styrene oxide polymer and polyacrylic acid on silica coated iron oxide pigment

The colloidal stabilization in waterbased paint is poorly understood due to its complexity in composition, usually containing mixtures of particles and of surface active agents ("dispersants"). In this study we make a step forward by analyzing the competitive adsorption of a few widely used dispersants on a typical inorganic pigment (70 nm sized Fe2O3-based red pigment; surface treated with silica; negative zetapotential at pH ~ 7). The supposition is that any particle type in paint needs sufficient adsorbed dispersant in order to be stable. Thus, we investigate, for two combinations of two dispersants, how they mutually affect their adsorption at that pigment. Also the "single" adsorption of these dispersants was investigated, thus in the absence of the other. The dispersants are an anionic, polyacrylic acid sodium salt ("PANa"; MW = 15,000 Da) in combination with a MW = 1500 Da blockcopolymer of styrene oxide (SO) and ethoxylene (EO), either or not endcapped by a phosphate group (P). The adsorption behavior was analyzed by size exclusion chromatography of the processed supernatant of the pigment dispersion. PANa and SO–EO–P adsorb for electrostatic reasons while SO–EO has affinity only to an organic surface. PANa and SO–EO–P show regular single adsorption with a plateau starting at the critical micelle concentration (cmc). SO–EO shows single adsorption only beyond its cmc based on the adsorption of full micelles. When in competition, with SO-EP-P/PANa the adsorption of SO–EO–P is lowered by ~65% while that of PANa is unchanged. With SO–EO/PANa the surface active species behave like in single adsorption below the cmc, but beyond the cmc a complicated phase separation occurs that cannot be based on mixed micelles. The adsorption data of PANa are compromised by depletion of PANa from interstices between particles. The anionic dispersants adsorb to the silicium oxide coated iron oxide pigment with negative surface potential because of the presence of a pH dependent relatively small number of positive iron oxide surface sites