Adsorption of poly(methacrylic acid) onto differently charged silica nanoparticles and its consequences on particles clustering

This work aims at gaining a comprehensive picture of the interactions between three differently functionalized silica nanoparticles and a polyacid - PMAA namely - in aqueous media. Native silica nanoparticles and silica nanoparticles functionalized with amine or quaternary amine groups are either negatively or positively charged with various charge densities whereas PMAA chains display an increasing negative charge density as the pH is increased from 3 to 9. Adsorption isotherms were obtained by Total Organic Carbon (TOC). It was shown that native silica interacts only weakly with PMAA while stronger adsorptions were evidenced for the two aminefunctionalized silica. Whereas electrostatic attractive interactions between positively-charged surfaces and negatively-charged PMAA are driving the adsorption at pH larger than 3, hydrophobic interactions between the propyl moieties of the grafts at the silica surfaces and the methyl groups of the PMAA hypercoils are dominating at low pH value. In this last case, the more hydrophobic the silica surface is, the higher the adsorption. Contrary to expectations, hydrophobic interactions (dominating at low pH) seem to be stronger than attractive electrostatic interactions (dominating at pH larger than 3) as adsorbed amounts are larger in the first case. Small-angle X-ray scattering experiments were performed on PMAA/silica dispersions under the condition of saturation adsorption in order to correlate the extent of particle dispersion with polymer/surface interactions. The stronger the polymer/surface interactions, the more compact aggregates are formed.