Structural evolution of complexes of poly(styrenesulfonate) and cetyltrimethylammonium chloride

Structural evolution is tracked as a charged surfactant, cetyltrimethylammonium chloride (CTAC), is titrated into a solution of oppositely charged poly electrolyte, sodium poly(styrenesulfonate), to form poly electrolyte-surfactant complexes. At surfactant-to-polymer molar charge ratio [r] less than 0.5, small-angle neutron scattering of the soluble complexes reveals bound spherical CTAC micelles of radii 21-24 Å, slightly less than for pure CTAC. At such small [r], the number of spherical micelles per chain grows with [r] but never becomes large (<2). At [r] ∼ 0.5, if salt concentration is low (<100 mM), cylindrical micelles of radii 20-22 Å and aspect ratio at least 5:1 replace the spherical micelles. Irrespective of salt concentration, at [r] ∼ 0.7 precipitation occurs, with X-ray scattering revealing CTAC arranged into hexagonally close-packed cylinders. At even higher [r], the hexagonal phase transforms into the Pm3n cubic phase. Raising ionic strength diminishes NaPSS-CTAC attraction, delaying onset of insolubility and disrupting precipitate order.