  The complexation between charged-neutral block copolymers and oppositely
charged surfactants was investigated by light scattering experiments and by
isothermal titration calorimetry (ITC). The copolymer was poly(sodium
acrylate)-b-poly(acrylamide) and the surfactant dodecyltrimethylammonium
bromide (DTAB). In a previous report, we had shown that the copolymers and the
surfactants co-assembled spontaneously into colloidal complexes. Depending of
the charge ratio Z = [DTA+]/[COO-], the complexes were either single surfactant
micelles decorated by copolymers, or core-shell hierarchical structures. ITC
was performed in order to investigate the thermodynamics of the complex
formation. Titrations of copolymers by surfactants and of surfactants by
copolymers revealed that the electrostatic co-assembly was an endothermic
reaction, suggesting a process dominated by the entropy of the counterions.
Here we found that the thermodynamic quantities associated with the reaction
depended on the mixing order. When surfactants were added stepwise to
copolymers, the titration was associated with the formation of single micelles
decorated by a unique polymer. Above a critical charge ratio, the micelles
rearranged themselves into 100 nm colloidal complexes in a collective process
which displayed the following features : i) the process was very slow as
compared to the timescale of Brownian diffusion, ii) the thermodynamic
signature was a endothermic peak and iii) the stoichiometry between the
positive and negative charges was modified from n = 0.48 (single micelles) to
0.75 (core-shell complexes). The amount of polyelectrolytes needed for the
complex formation exceeded the number required to compensate the net micellar
charge, confirming the evidence of overcharging in the complex formation.
