sodium-bromide and dodecyltrimethylammonium

The salt effect on complex formation of poly(acrylamide)-block-poly(acrylic acid) (PAM-b-PAA) and dodecyltrimethylammonium bromide (DTAB) at different NaBr concentrations, C(NaBr), was investigated by laser light scattering (LLS) and small angle neutron scattering (SANS). LLS and SANS clearly indicates that the aqueous solution of PAM-b-PAA and DTAB associate into colloidal complexes. For low surfactant-to-polymer charge ratio Z lower than the critical value Z(C), the colloidal complexes are single DTAB micelles dressed by a few PAM-b-PAA. Above Z(C), the colloidal complexes form a core-shell microstructure. The complex formation in the PAM-b-PAA/DTAB is enhanced by addition of salt: Z(C) decreases with increasing C(NaBr). This is considered to similar to the cmc behavior for the pure surfactant system. The core of the complex consists of densely packed surfactant micelles (DTA(+)), and PAA block chains bind to these micelles, displace their counteranions (Br-) and bridge them. The corona of the complex is constituted from the PAM. Since the interaction between polyelectrolyte and oppositely charged surfactant is primarily electrostatic in nature, the core radius and the intermicellar distance of the DTA(+) micelles inside the core depend on C(NaBr). The addition of salt screens the electrostatic attraction between oppositely charged PAA block and DTAB, which weakens the interaction. With increasing C(NaBr), therefore, the core of the colloidal complex is considered to swell, which leads to the increases in the core radius R(C) and the intermicellar distance of the DTA(+) micelles inside the core. The aggregation number expressed in terms of DTA(+) micelles per complex is also evaluated using the analogy with the homopolyelectrolyte/surfactant system.

Topics: Acrylic Resins; Bromides; Colloids; Drug Carriers; Hydrogen-Ion Concentration; Lasers; Magnetic Resonance Spectroscopy; Micelles; Neutron Diffraction; Quaternary Ammonium Compounds; Scattering, Small Angle; Sodium Compounds; Static Electricity; Surface-Active Agents

Small-angle X-ray scattering was used to elucidate the structure of crosslinked polyacrylate gel/dodecyltrimethylammonium bromide complexes equilibrated in solutions of varying concentrations of surfactant and sodium bromide (NaBr). Samples were swollen with no ordering (micelle free), or they were collapsed with either several distinct peaks (cubic Pm3n) or one broad correlation peak (disordered micellar). The main factor determining the structure of the collapsed complexes was found to be the NaBr concentration, with the cubic structure existing up to approximately 150 mM NaBr and above which only the disordered micellar structure was found. Increasing the salt concentration decreases the polyion mediated attractive forces holding the micelles together causing swelling of the gel. At sufficiently high salt concentration the micelle-micelle distance in the gel becomes too large for the cubic structure to be retained, and it melts into a disordered micellar structure. As most samples were above the critical micelle concentration, the bulk of the surfactant was in the form of micelles in the solution and the surfactant concentration thereby had only a minor influence on the structure. However, in the region around 150 mM NaBr, increasing the surfactant concentration, at constant NaBr concentration, was found to change the structure from disordered micellar to ordered cubic and back to disordered again.

Topics: Acrylic Resins; Bromides; Gels; Quaternary Ammonium Compounds; Scattering, Small Angle; Sodium Compounds; Surface-Active Agents