Behavior of a strong polyelectrolyte, poly(diallyldimethylammonium chloride) physisorbed at oil-water interface under different environments : A comparison with a weak polyelectrolyte

We compare the behavior of a strong polyelectrolyte (PE), poly(diallyldimethylammonium chloride) (PDDA), physisorbed at oil-water interface with that of a weak PE, poly(acrylic acid) under different pH and salt by measuring the interaction forces, zeta potential and hydrodynamic size measurement. The change in interdroplet separation is manifested from the Bragg peak position suggests that PDDA adopts an extended conformation at oil-water interface at zero salt concentration that remains unchanged in the pH range 2-10 but collapses completely at pH > 10. At a constant pH, the extended conformation of PDDA remains unaltered at low salt concentration but collapsed completely at high salt content. In case of a weak PE, the adsorbed molecules undergo coil to extended transition with increase in solution pH but swells weakly at very low salt concentration. It collapses strongly at high salt concentration due to condensation of ions over PAA backbones that result in enhanced neutralization of surface group. However, washing of PAA emulsion with lower concentration of monovalent ion leads to desorption of condensed electrolyte ions and re-swelling of PAA. The rate of collapse observed from force measurement in both systems is in agreement with electrophoretic data. Interestingly, the rate of swelling and collapse of the physisorbed weak and strong PE in the presence of salt is remarkably similar to that of end-grafted PE, though the interfaces are entirely different. These results show that interaction of PE's at O/W interface can be used to probe biologically important phenomenon such as macromolecules at cell membrane-fluid interface, binding of protein, adsorption of DNA and protein folding.