Behavior of a Weak Polyelectrolyte at Oil-water Interfaces under Different Environmental Conditions

Abstract

Understanding the behavior of macromolecules adsorbed to a surface or interface under different environmental conditions is very important in stabilization of colloidal formulations, coating, drug delivery and lubrications. However, the experimental understanding on polyelectrolyte behavior at interfaces under different environmental conditions is lacking. Here, we probe the conformational changes of a weak polyelectrolyte (Polyacrylic acid, PAA) adsorbed at an oil-water interface, under different physiochemical condition such as pH and salt of various concentrations and valences, using a simple spectroscopic approach. The conformational change of PAA adsorbed at magnetically polarizable emulsions droplets is manifested from the Bragg spectral shift or interdroplet spacing change from an array of droplets. In addition, the intermolecular repulsive interaction between the polyelectrolyte covered droplets is studied in details. In the pH range of 3–11.8, at zero salt concentration, the interdroplet spacing increased from 40 to 173nm, indicating a large extended conformation of adsorbed PAA, which undergoes a sharp collapse in the presence of multi valence salt. The decay length obtained from the force measurement results, at zero salt concentration and at low pH, suggest that the adsorbed PAA exhibits a compact structure at an oil-water interface whereas it adopts an extended structure at high pH. Interestingly, the scaling exponents observed from the force decay length during swelling and collapse of the adsorbed PAA at different pH and salts are in good agreement with mean field theory and counter-ion fluctuation theory, though the interface and anchoring behavior of polyelectrolyte are quite different. Our results provide new experimental insights into macromolecular conformation at oil-water interfaces under different practical situations and offer new possibilities in probing macromolecular behavior at interfaces, for creating stimuli responsive nanostructures (pH, temperature, ion etc), lubrication and better stabilization of colloids.