Surface Charged Polymeric Micelles-A Tunable Model System Studied by SANS

We investigate surface charged "starlike" micelles in aqueous solution formed by carboxy terminated n-octacosyl-poly(ethylene oxide) block copolymers, C-28-PEO5-COOH with 5 the PEO molar mass in kg/mol, by small angle neutron scattering (SANS), zeta-potential measurements, and rheology. The -COOH end group was introduced by selective oxidation of the -CH2-OH end group of a C-28-PEO5-OH precursor using Bobbitt's salt. Micellar solutions of different concentrations in the dilute and semidilute range were investigated at pH 2, 6, and 12 to vary ionic strength and the number of effective surface charges Z(eff). Z(eff) was further varied by using mixtures of C-28-PEO5-COOH and C-28-PEO5-OH at different mixing ratios. SANS measurements reveal that the intramicellar form factor P(Q) is identical at the different pH values, which implies that the individual micellar structure is unaffected by the number of surface charges. On the contrary, the intermicellar structure factor S(Q) and the phase behavior show a strong dependence on Z(eff). In particular, we observe a distinct shift of the liquid-fcc crystal phase boundary. A quantitative analysis in terms of a screened Hard Sphere Yukawa potential reveals very good agreement between experiment and theory. Because of this consistency and of the tunability of the n-alkyl-PEO starlike micelles, we consider this system to be an excellent model for further studies on the interplay between steric and electrostatic interactions in soft colloids.