Role of concentration and hydrophobic nature of weak polyelectrolytes on adsorption structure and thermodynamics at oil-water interface: Study of several carboxylate polymers

Abstract

Intermolecular structure, conformations, solvation and adsorption characteristics of four partially ionized polycarboxylic acids syndiotactic-poly(acrylic acid) (s-PAA), syndiotactic-poly(methacrylic acid) (s-PMA), isotactic-poly(methacrylic acid) (i-PMA) and atactic-poly(ethacrylic acid) (a-PEA) at oil (CCl4) - water interface is studied by molecular dynamics simulations over the range from dilute to monolayer coverage. Linearity and slope of Θ-Γ curves are correlated to conformational invariance and extent of adsorption (partial vs complete) respectively, within the range of concentration of study. Adsorption of i-PMA and a-PEA chains occurs deeper into the oil-side of interface region as driven by the greater hydrophobicity of these polymers. Higher tendency of highly extended s-PMA to desorb from the interface as compared to strong adsorption of compact coiled i-PMA is driven by their solvation behavior dictated by aspects of structural partitioning of chemical groups with respect to the solvents of the two phases. Hydrophobic side-group in the polyelectrolyte chain is responsible for the slower dynamics of the backbone. The observed variation in thickness of adsorbed layer and fractional interface coverage with concentration suggest partial adsorption of chain segments beyond saturation of interface rather than coiling of chains to accommodate additional chains. While a-PEA exhibits greater variation in interfacial tension, the change in electric potential difference across the interface indicates weak adsorption by s-PMA. The influence of the tacticity of PMA on dynamics and distribution of counter-ions is in agreement with experimental results. Results provide a significant advancement in our understanding of polyelectrolyte and polymer adsorption at oil-water liquid interface.