  We report on the complexation between charged-neutral block copolymers and
oppositely charged surfactants studied by small-angle neutron scattering. Two
block copolymers/surfactant systems are investigated, poly(acrylic
acid)-b-poly(acrylamide) with dodecyltrimethylammonium bromide and
poly(trimethylammonium ethylacrylate methylsulfate)-b-poly(acrylamide) with
sodium dodecyl sulfate. The two systems are similar in terms of structure and
molecular weight but have different electrostatic charges. The neutron
scattering data have been interpreted in terms of a model that assumes the
formation of mixed polymer-surfactant aggregates, also called colloidal
complexes. These complexes exhibit a core-shell microstructure, where the core
is a dense coacervate microphase of micelles surrounded by neutral blocks.
Here, we are taking advantage of the fact that the complexation results in
finite-size aggregates to shed some light on the complexation mechanisms. In
order to analyze quantitatively the neutron data, we develop two different
approaches to derive the number of surfactant micelles per polymer in the mixed
aggregates and the distributions of aggregation numbers. With these results, we
show that the formation of the colloidal complex is in agreement with the
overcharging predictions. In both systems, the amount of polyelectrolytes
needed to build the core-shell colloids always exceeds the number that would be
necessary to compensate the charge of the micelles. For the two
polymer-surfactant systems investigated, the overcharging ratios are 0.66 and
0.38.
