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

Abstract

: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 polymersurfactant 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 finitesize 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 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 ± 0.06 and 0.38 ± 0.02. I -IntroductionThe complexation between ion-containing polymers and oppositely charged macroions is currently attracting much attention in the field of soft condensed matter 1,2 . Associations based on the electrostatic Coulomb interactions are present in many applications and for highly charged systems the elementary mechanisms such as adsorption and self-assembly are only partially understood. The complexation between oppositely charged species is found e.g. in the treatment of waste water, the formulation of personal care products, the purification of proteins or in gene and drug delivery. In biophysics, comprehensive studies have been dedicated to the cooperative condensation of DNA with multivalent counterions [3][4][5] or with cationic liposomes 1,[6][7][8][9] . In material science, electrostatic layer-by-layer deposition yielding polyelectrolyte multilayers have been achieved for encapsulation purposes and colloidal stabilization [10][11][12] .When an ion-containing polymer solution is mixed to a dispersion of oppositely charged colloids, a phase separation usually follows [13][14][15][16][17][18][19][20][21][22][23] . At the mixing, the solution becomes turbid, and after centrifugation it displays two separated phases. The bottom phase appears as a precipitate and its chemical analysis reveals that it contains most of the polymers and colloids. The supernatant is fluid and transparent, and contains the solvent and the counterions released d...

Stability and Adsorption Properties of Electrostatic Complexes: Design of Hybrid Nanostructures for Coating Applications

“…In aqueous solutions at neutral pH, the chains are dispersed and in a state of unimers. Light scattering performed in the dilute regime have revealed a molecular weight MW = 35 000 g mol -1 and an hydrodynamic diameter DH = 11 nm [10,11]. Complexation schemes were realized using cerium oxide nanoparticles or surfactant micelles that were of opposite charge with respect to that of the polymers.…”

“…Anionic sodium dodecylsulfate (SDS) were purchased from Sigma and used without further purification. In the concentration range explored, SDS self-assemble into micelles with an aggregation number of 50 [11]. The nanoparticles investigated were a dispersion of cerium oxide nanocrystals (nanoceria).…”

“…In aqueous solutions at neutral pH, the chains are dispersed and in a state of unimers. Light scattering performed in the dilute regime have revealed a molecular weight MW = 35 000 g mol -1 and an hydrodynamic diameter DH = 11 nm [10,11]. [19]).…”

Superstructures par agrégation contrôlée de nanocolloïdes : caractérisation structurale par diffusion de neutrons aux petits angles et simulation numérique

“…Le polymère étudié est le poly(acide acrylique)-b-poly(acrylamide) de poids moléculaire 6500 g mol −1 et 37000 g mol −1 pour chacun des blocs. Ce polymère est abrégé par PANa 6.5K -b-PAM 37K dans la suite [8] Ces polymères ont été synthétisés par Rhodia en utilisant la technologie Madix en vue d'applications pour les formulations de produits de « Personal Care » [9]. Le tensioactif cationique est le bromure de dodecyltrimethylammonium (DTAB), qui est caractérisé par une chaîne aliphatique de douze atomes de carbone.…”

“…(2). Nous avons pu la mesurer en ajustant un facteur de forme sur la courbe expérimentale [8], et nous avons trouvé que les micelles de tensioactifs de DTAB ont un nombre d'agrégation de 53 ± 3 et un diamètre de 3.63 nm. Notons aussi que la diffusion aux grands angles correspond à la diffusion par la surface des micelles.…”