Structure of colloidal complexes obtained from neutral/poly- electrolyte copolymers and oppositely charged surfactants

Abstract

We report on the phase behavior and scattering properties of colloidal complexes made from block copolymers and surfactants. The copolymer is poly(sodium acrylate)-b-poly(acrylamide), hereafter abbreviated as PANa-PAM, with molecular weight 5000 g/mol for the first block and 30000 g/mol for the second. In aqueous solutions and neutral pH, poly(sodium acrylate) is a weak polyelectrolyte, whereas poly(acrylamide) is neutral and in good-solvent conditions. The surfactant is dodecyltrimethylammonium bromide (DTAB) and is of opposite charge with respect to the polyelectrolyte block. Combining dynamical light scattering and small-angle neutron scattering, we show that in aqueous solutions PANa-PAM diblocks and DTAB associate into colloidal complexes. For surfactant-to-polymer charge ratios Z lower than a threshold (Z(C) approximately 0.3), the complexes are single surfactant micelles decorated by few copolymers. Above the threshold, the colloidal complexes reveal an original core-shell microstructure. We have found that the core of typical radius 100-200 A is constituted from densely packed surfactant micelles connected by the polyelectrolyte blocks. The outer part of the colloidal complex is a corona and is made from the neutral poly(acrylamide) chains. Typical hydrodynamic sizes for the whole aggregate are around 1000 A. The aggregation numbers expressed in terms of numbers of micelles and copolymers per complex are determined and found to be comprised between 100-400, depending on the charge ratio Z and on the total concentration. We have also shown that the sizes of the complexes depend on the exact procedure of the sample preparation. We propose that the driving mechanism for the complex formation is similar to that involved in the phase separation of homopolyelectrolyte/surfactant systems. With copolymers, the presence of the neutral blocks prevents the macroscopic phase separation from occurring.

Stabilization and controlled association of inorganic nanoparticles using block copolymers

“…4 quantitatively, we assume for the nanoparticlepolymer complexes a core-shell structure similar to the one found with surfactant micelles [13,15]. In this model, the core is a dense assembly of particles, which dominates the scattering intensity in the wave-vector range of interest (q > 0.01 Å -1 ).…”

“…%). The arrow indicates the position of the correlation peak arising from particles interactions located in the cores [13,15]. …”

Evidence of overcharging in the complexation between oppositely charged polymers and surfactants

“…The two systems are very similar in terms of molecular weights and structures but they differ by their electrostatic charge. For charged-neutral diblocks and surfactants with opposite charges, we have established the thermodynamical phase diagrams and found broad regions where hierarchical aggregates spontaneously form via electrostatic selfassembly [34][35][36][37][38][39][40] . These aggregates are termed colloidal complexes because they form through complexation and because of their colloidal nature 21,41 .…”

The many facets of polyelectrolytes and oppositely charged macroions complex formation

“…More subtle effects seen for protein binding [7] were attributed to charge heterogeneity, which introduces repulsive forces between polyanions and protein negative domains and also alters the nature of the polyanion-binding site. Complexes involving polyelectrolyte-neutral block copolymers and oppositely charged micelles were recently investigated by Berret [8][9][10][11][12]. The structure of such complexes was elucidated by small-angle neutron scattering and it was found that these complexes exhibit a core-shell microstructure, where the core is a dense coacervate microphase of micelles surrounded by neutral blocks.…”