Interactions between a 30% charged polyelectrolyte and an anionic surfactant in bulk and at a solid-liquid interface

The association between a 30% charged cationic polyelectrolyte and an anionic surfactant, sodium dodecyl sulfate (SDS), in 10 mM 1:1 electrolyte was investigated using surface force measurements and dynamic light scattering. The polyelectrolyte employed was a random copolymer of the neutral acrylamide and cationic [3-(2-methylpropionamide)propyl]trimethylammonium chloride (AM-MAPTAC-31). Light scattering measurements show that upon progressive addition of SDS to an AM-MAPTAC-31 solution the single coil size decreases until precipitation occurs at an SDS/MAPTAC ratio of just above 0.4. At SDS/MAPTAC ratios at or above 2, redispersion of the aggregates takes place. The interfacial behavior of AM-MAPTAC-31/SDS complexes was investigated in two ways. In one set of experiments a droplet containing a mixture of SDS and AM-MAPTAC-31 was placed between the surfaces and adsorption was allowed to occur from the aqueous mixture. It was found that the range of the steric force decreased when the SDS/MAPTAC ratio was increased from 0 to 0.4, indicating adsorption in a less extended conformation due to a decreased repulsion between the polyelectrolyte segments. At a ratio of 0.6 a compact interfacial complex was formed and the measured force was attractive over a small distance regime. A further increase in SDS/MAPTAC ratio resulted in precipitation of large aggregates at the surface, and reproducible force data could not be obtained. At an even higher SDS/AM-MAPTAC ratio of 4, individual aggregates were once again adsorbed at the surface. Hence, we find a good correspondence between association in bulk and at the solid surface. In another set of experiments the polyelectrolyte was first preadsorbed to mica surfaces and then SDS was added to the polyelectrolyte-free solution surrounding the surfaces. In this way precipitation of large SDS-polyelectrolyte aggregates onto the surfaces was avoided. Addition of SDS up to a concentration of 0.1 mM hardly affected the long-range interaction but gave an increased compressed layer thickness. A further increase in SDS concentrations to 1 mM results in a dramatic increase in the range of the force, suggesting formation of strongly negatively charged polyelectrolyte-surfactant complexes.