Water-soluble nonstoichiometric complexes between sodium poly(styrenesulfonate) and cetylpyridinium chloride in...

Complexes formed between a cationic surfactant cetylpyridinium chloride, CPC, and an anionic polyelectrolyte sodium poly(styrenesulfonate), NaPSS, in aqueous 0.1 M NaCl solutions were studied by static and dynamic light scattering and by ζ-potential measurements in a broad region of surfactant cation, CP(+), to polyanion, PSS(-), charge ratio, S/P. Two NaPSS samples were used, NaPSS-L with a lower molar mass, M(w) = 1.4 × 10(5) g/mol, and NaPSS-H with a considerably higher M(w) (= 2.6 × 10(6) g/mol), to elucidate the effect of the polyion chain length on the behavior of the aggregates. In the polyelectrolyte-rich regime (S/P < 1), CPPSS complexes are soluble up to rather high S/P values (around 0.72, irrespective of the polyion chain length), which is attributed to a specific interaction between the hydrophobic benzene groups on the polyion and the surfactant micelle, which leads to a less efficient charge screening. The addition of surfactant causes chain contraction. The obtained data suggest a pronounced effect of the NaPSS chain length on the structural properties of the CPPSS complexes. The CPPSS-L complexes are small and dense (the radius of gyration, R(g), is 7.3 nm at S/P = 0.7), and their shape is close to spherical. In contrast, the CPPSS-H complexes (R(g) around 73 nm at S/P = 0.7) have no well-defined structure and reveal a stronger tendency toward intermolecular association when the nominal charge of the complex is reduced, although they remain pretty monodisperse. A model of a temporary network-like association in which surfactant micelles serve as cross-links for polyion chains is proposed to explain the behavior in CPPSS-H solutions. The forces responsible for such labile intermolecular association are weak, in contrast to strong specific interaction involved in the formation of the primary complex. The redissolution of the complex by adding excess surfactant (S/P > 1: the surfactant-rich regime) depends strongly on the polyion chain length and is a slow process, taking several days (CPPSS-L) or even weeks (CPPSS-H). It is achieved only at very high S/P values (around 240 and 2400 in the CPPSS-L and CPPSS-H cases, respectively) by hydrophobic binding of surfactant to the CPPSS complex. The predominating species in these solutions are free surfactant micelles. Similarly, a large excess of NaCl (almost 390 and 690 mol per 1 mol of CPPSS-L and CPPSS-H, respectively) is needed to disintegrate the stoichiometric CPPSS (S/P = 1) complex into free polyion chains and free surfactant micelles.