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Langmuir 2004, 20, 7766-7778
Formation and Properties of Positively Charged Colloids Based on Polyelectrolyte Complexes of Biopolymers Christophe Schatz,†,‡ Jean-Michel Lucas,‡ Christophe Viton,‡ Alain Domard,*,‡ Christian Pichot,† and Thierry Delair*,† Unite´ mixte CNRS-BioMe´ rieux, UMR 2142, ENS Lyon, 46, alle´ e d’Italie, 69364 Lyon Cedex 07, France, and Laboratoire des Mate´ riaux Polyme` res et des Biomate´ riaux, UMR CNRS 5627, ISTIL, Domaine Scientifique de la Doua, 15 Bd. Latarjet, 69622 Villeurbanne Cedex, France Received March 2, 2004. In Final Form: June 8, 2004 Formation of colloids based on polyelectrolyte complexes (PECs) was mainly studied with synthetic polyelectrolytes. In this study, we describe the elaboration of positively charged PEC particles at a submicrometer level obtained by the complexation between two charged polysaccharides, chitosan as polycation and dextran sulfate (DS) as polyanion. The complexes were elaborated by dropwise addition of default amounts of DS to excess chitosan. Quasi-elastic light scattering was used to investigate in detail the influence of the characteristics of components (chain length, degree of acetylation) and parameters linked to the reaction of complexation (molar mixing ratio, ionic strength, concentration in polymer) on the sizes and polydispersity of colloids. Chain length of chitosan is the major parameter affecting the dimensions of the complexes, high molar mass chitosans leading to the largest particles. Variations of hydrodynamic diameters of PECs with the molar mass of chitosan are consistent with a mechanism of particle formation through the segregation of the neutral and then hydrophobic blocks of the polyelectrolyte complexed segments. Resulting particles display probably a structure constituted by a neutral core surrounded by a chitosan shell ensuring the colloidal stabilization. Such a structure was evidenced by measurements of electrophoretic mobilities revealing that the positive charge of particles was decreasing with pH, in relation with the neutralization of excess glucosamine hydrochloride moieties.
1. Introduction In the field of polymeric materials, polyelectrolyte complexes (PECs) are of high relevance since they offer the possibility to combine physicochemical properties of at least two polyelectrolytes (PELs). In this way, selfassembly of PEL was recently developed to elaborate ultrathin films of well-defined composition1 by means of a layer-by-layer technique.2 The driving forces for the formation of PECs are the strong Coulombic interactions between oppositely charged polyelectrolytes leading to interpolymer ionic condensation and the concomitant release of counterions. Other inter-macromolecular interactions may be involved in the formation of PEC structures such as hydrogen bonding, hydrophobic interactions, van der Waals forces, or dipole-charge transfer. According to the structural features of the initial components, two main kinds of PEC structures may be defined. First, polyelectrolytes having weak ionic groups and large differences of molecular dimensions lead to the formation of water-soluble PECs when they are mixed in nonstoichiometric ratios.3 Such structures consisting of a long host molecule sequentially complexed with shorter guest polyions of opposite charge according to a ladder mechanism may be compared to block copolymers since they contain single-stranded hydrophilic and doublestranded hydrophobic sections.4 The water-soluble char* Corresponding authors:
[email protected] and
[email protected]. † Unite ´ mixte CNRS-BioMe´rieux, UMR 2142. ‡ Laboratoire des Mate ´ riaux Polyme`res et des Biomate´riaux, UMR CNRS 5627, ISTIL, Domaine Scientifique de la Doua. (1) Schuetz, P.; Caruso, F. Colloids Surf., A 2002, 207, 33. (2) Decher, G. Science 1997, 277, 1232. (3) Kabanov, V. A.; Zezin, A. B. Makromol. Chem., Suppl. 1984, 6, 259.
acter is preserved up to a critical value of the ratio between the concentrations of the guest and host polymers over which, water-soluble PECs coexist with insoluble PECs.3 Second, polyions with strong ionic groups and/or comparably high molar masses yield insoluble and highly aggregated complexes which were comprehensively studied by Dautzenberg.5 These PECs, structured according to the scramble-eggs model under incorporation of several polyions chains, follow in most cases a 1:1 stoichiometry well evidenced by the full release of counterions. Resulting systems are highly dependent on both the concentration of PEL solutions and their mixing ratios. Indeed, highly concentrated solutions of components (>10-2 g/mL) lead to a macroscopic flocculation whatever the mixing ratio whereas highly diluted solutions of opposite charge PEL (
