A Small-Angle X-ray Scattering Study of Complexes Formed in Mixtures of a Cationic Polyelectrolyte and an Anionic Surfactant

Abstract

The internal structure of the solid phase formed in mixtures of the anionic surfactant sodium dodecyl sulfate (SDS) and a range of oppositely charged polyelectrolytes with different side chains and charge density has been investigated using small-angle X-ray scattering. Polyelectrolytes with short side chains ([3-(2-methylpropionamido)propyl]trimethylammonium chloride, MAPTAC, and poly{[(2-propionyloxy)ethyl]trimethylammonium chloride}, PCMA) form a 2-dimensional hexagonal structure with SDS, whereas a polyelectrolyte without side chains (poly(vinlyamine), PVAm) forms a lamellar structure. The hexagonal structure of MAPTAC is retained either when a neutral monomer (acrylamide, AM) is included in the polymer backbone to reduce the charge density or when a nonionic surfactant is admixed to the SDS/polyelctrolyte complex. The unit cell length of AM-MAPTAC increases with decreasing charge density from a = 47.7 Å (MAPTAC, 100% charge density) to 58.5 Å (AM-MAPTAC, 30% charge density). The unit cell length in the lamellar SDS/PVAm complex (a = 36.1 Å) is significantly smaller than for the different hexagonal structures. It is conjectured that the cylinders in the hexagonal structure and the bilayers in the lamellar structure are based on self-assembled surfactant aggregates with the polyelectrolyte mainly located in the aqueous region adjacent to the charged surfactant headgroups.

Trapped non-equilibrium states in aqueous solutions of oppositely charged polyelectrolytes and surfactants: effects of mixing protocol and salt concentration

“…We suggest that the much smaller size of the PCMA-SDS complex at low surfactant concentrations is due to the extra induction of hydrophobicity by SDS to the complex, which together with a reduction in the charge density, enable denser packing in the interior of the polyelectrolyte-surfactant complex. In fact, SANS and SAXS studies [29,31] have shown that the interior of the aggregates formed close to the charge neutralization point consists of cylindrical SDS micelles wrapped by polyelectrolyte arranged in a hexagonal organization. The water layer thickness, including the surfactant head group and the polyelectrolyte, between these structural units has been estimated to be 1.6 nm [31].…”

“…In fact, SANS and SAXS studies [29,31] have shown that the interior of the aggregates formed close to the charge neutralization point consists of cylindrical SDS micelles wrapped by polyelectrolyte arranged in a hexagonal organization. The water layer thickness, including the surfactant head group and the polyelectrolyte, between these structural units has been estimated to be 1.6 nm [31].…”

“…Let us rationalize this by considering the internal structure of the aggregates. This has previously been determined with SANS [29] and SAXS [31] for the PCMA-SDS system prepared by the PTS-method without added salt. It was shown that the internal structure of the aggregates is characterized by a 2D-hexagonal organization due to packing of cylindrical SDS micelles wrapped by the polyelectrolyte.…”

“…oppositely charged polymers or polyelectrolyte-surfactant mixtures. What is known is that in certain concentration ranges very stable and surprisingly monodisperse polyelectrolyte-surfactant aggregates are formed [26,27], and that the internal structures of some of these aggregates are well defined [18,[28][29][30][31][32], e.g. consisting of rod-like surfactant micelles wrapped by polyelectrolyte, with the micelles ordered in hexagonal symmetry.…”

Interaction of Sodium Dodecyl Sulfate with Poly(ethyleneimine) in Bulk Solution and at the Air-Solution Interface

“…[9,10] Ionic polymers having similar charge with the surfactant molecules show moderate effect however, polymers having an opposite charge may cause more important effects in surfactant solutions. [1,11] Among the branched polymers, polyethyleneimine (PEI) is of special interest because it is widely used as dispersion stabilizer, flocculation agent, adhesive, and in gene delivery therapy. [12,13] Similarly, SDS is an anionic surfactant that is commonly used in mineral processing, flotation, and manufacturing many chemical products.…”