Poly(acrylic acid) (PAA) and cetyltrimethylammonium bromide (C16TAB) are found to interact strongly in aqueous solution. The properties of the system are determined mainly by the parameter β, defined as [C16TAB]/[PAA]; the PAA concentration is expressed as monomers. Light scattering measurements in dilute solutions show that a single PAA chain binds several C16TAB micelles to form a complex. At β ≈ 1 (independent of the PAA concentration), the complex is saturated with surfactant. At higher β, free micelles coexist with the complexes. The hydrodynamic radius of the complex is smaller than (β < 1) or similar to (β > 1) the radius of the surfactant-free PAA coil. Estimates of the hydrodynamic virial coefficient show that the effective repulsion between complexes increases with β up to β ≈ 1 where a dramatic change is brought about by the screening effect of free micelles. Time-resolved fluorescence quenching shows that the aggregation number of the micelles in the complex increases from 50 at β = 0.35 to 90 at β = 1. (The aggregation number for 20 mM C16TAB in the absence of polymer is estimated as 107.) Pyrene lifetime measurements indicate that this change is accompanied by an increase of the apparent degree of bromide ion binding to the micelles from 50% to 95% of the value for free C16TAB micelles. Furthermore, the decreasing influence of the polymer on the surfactant assembly with increasing β is supported by the variation of the intramicellar quenching rate constant. Viscosity measurements confirm that no multipolymer complexes form in the studied concentration range. pH measurements show that the degree of dissociation of the polymer acidic groups increases when the polymer binds to the micelles.

Static and dynamic light scattering measurements have been made on poly(ethy1ene oxide) (PEO) in the presence of the ionic surfactant sodium dodecyl sulfate (SDS) in aqueous solution over a range of relative concentrations. The purpose was, in part, to demonstrate the polyelectrolyte effecte which arise in PEO on formation of the charged complex between the components in solution in which small SDS micelles are incorporated into the flexible PEO coil. The influence of the ionic strength of the medium was studied. Static light scattering measurements show that the radius of gyration changes as a function of SDS concentration and the ionic strength of the medium. Diffusion coefficients from dynamic light scattering (DLS) reflect changes in the pair interaction potential as the SDS and ionic strength are altered. Laplace inversion of the DLS time correlation function gives relaxation time distributions consisting of the PEO/SDS complex as the main component and free SDS micelles as the minor component. Changes in peak areas are related to the increase in the degree of binding between SDS and PEO as the relative concentrations change and also the increase in SDS micellar size with increase in the ionic strength of the medium.

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.

No abstract