Sprecher
Beschreibung
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 –CH$_2$ –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 otential reveals a 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.
Macromolecules in press 2024
