Tuning TiO2 suspension stability with poly(α-alkyl carboxylic acids) via side chain length and chain stereochemistry

Abstract

Effects of hydrophobicity and stereochemistry of poly(α-alkyl carboxylic acids) on the properties of TiO2 suspensions were studied with experimental and in silico methods. Zeta potential measurements show that the addition of the more hydrophilic poly(acrylic acid) (PAA) to TiO2 suspensions shifts the isoelectric point of the nanoparticles towards lower pH the most, followed by poly(methacrylic acid) (PMA) and poly(ethacrylic) acid (PEA). Stereochemistry has a less pronounced effect on suspension stability, except at low pH, where isotactic PMA (iPMA) is not soluble in water. Dynamic light scattering measurements show that the aggregate size of the nanoparticles correlates well with their zeta potential and the aggregates are largest at the isoelectric point of the nanoparticles. Molecular dynamics (MD) and Monte Carlo (MC) computer simulations were used to study the interaction of polyacids with the surface of nanoparticles. MD simulations show that PAA forms the largest amount of hydrogen bonds with TiO2, followed by PMA. PEA forms very low numbers of hydrogen bonds with the nanoparticle surface. MC simulations were used to study the effect of pKa of COOH groups of polyacids on their adsorption on TiO2. It was found that even small differences in pKa between the polyacids have a strong effect.