engleski

Influence of Oppositely Charged Surfactants on the Structure of Polyelectrolyte Gels

The interactions of polyelectrolytes and oppositely charged surfactants, particularly the formation of complex vesicle-based structures, are of importance in both basic and applied science. The structure of the polyelectrolyte-surfactant complexes (PSC) is usually described as micelle-like aggregates residing within random coil of the polyelectrolyte. Specifically, mixtures of anionic polyelectrolyte, differently charged carrageenans (CAR) and dodecylammonium chloride (DDACl) are characterized by the formation of giant vesicles, i.e. lamellar ordering of the PSC even in the extremely dilute CAR solutions (1). At higher concentrations the κ - and ι -carrageenans (abbreviated as KC and IC) self-assemble in the gel starting with a coil to helix transition. In this work, rheological measurements, X-ray diffraction and confocal microscopy were used to study the influence of DDACl on the carrageenan gels. DDACl addition lead to the visible change of gel structure and the phase separation of white precipitate in micellar region. All gel samples displayed shear thinning and, at higher shear stress, Newtonian behaviour. With the addition of DDACl, more pronounced changes of the steady-state viscosity profiles were observed with IC gel compared to KC gel. Below the critical molar ratio, the gel network formation was related to the coil to helix transitions of carrageenan chains and electrostatic interactions between oppositely charged groups of both carrageenan and surfactant. Gels prepared without DDACl did not show the characteristic peaks at small angles. Addition of DDACl increased the ordering of gels and the appearance of microphase separated domains. Stacked bilayers and spherical aggregates were visualized by confocal microscopy. A three-dimensional, partly bilayered surfactant structures in conjuction with the carrageenan chains appeared as a result of the polyelectrolyte/surfactant complexation and the surfactant aggregation in the gel. Phase separation of white precipitate occurring beyond the critical molar ratio can be attributed to (i) the decrease of the fraction of the carrageenan chains available for helix formation and (ii) mutual aggregation of surfactant molecules. The appearance of a lamellar phase indicated the possibility of subsequent association of PSC into highly ordered arrangements. The increase of the carrageenan charge density improved the ordering as a result of increased electrostatic and hydrophobic interactions between carrageenan chains and DDACl. References 1. M. Vinceković, M. Bujan, I. Šmit, N. Filipović-Vinceković, Colloids Surface A, 2005, 255: 181.

biopolymers; surfactants; gelation

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