Investigation of Structures in Polyelectrolyte/Surfactant Complexes by X-ray Scattering

Abstract

Structural investigation in systems of anionic polyelectrolytes and dodecyl-(DPC) and cetylpyridinium chlorides (CPC) were performed at various surfactant to polyelectrolyte (S/P) ratios using synchrotron X-ray scattering. The polyelectrolytes used were sodium poly(styrenesulfonate) (NaPSS), poly(acrylate) (NaPA), and poly(methacrylate) (NaPMA). From the Bragg peaks emerging in the scattering curves, different types of organization of the surfactant in conjunction with the polyion are proposed. They depend on the surfactant chain length, on the polyelectrolyte chemistry, and on the S/P value: (1) NaPSS/DPC (all S/P values), NaPSS/CPC (S/P < 1), and NaPA(NaPMA)/DPC (S/P < 1) complexes produced a micellelike organization of the surfactant along the polyion chain. The NaPSS-induced micelle is smaller in size than the ordinary one because of the inclusion of the aromatic rings on the PSS chain into the hydrophobic interior of the micelle. The size of the ordered elements in complexes with the hydrophilic NaPA and NaPMA corresponds to the radius of an ordinary globular micelle together with the thickness of the polyelectrolyte chain that surrounds it. (2) In NaPSS/CPC precipitate (S/P g 1), a hexagonal phase is observed with a unit cell parameter equal to 39.5 Å. (3) The multiple reflections in the scattering curves of NaPA(NaPMA)/DPC complexes with S/P g 1 and of NaPA(NaPMA)/CPC ones for all S/P values point to some cubic structure. The cell constants of these mesophases correspond approximately to 2.5 diameters of a globular surfactant micelle. (4) In addition to a cubic phase, a well-pronounced hexagonal phase with a unit cell parameter of 40.5 Å develops in the NaPA/CPC case with S/P g 1.

Polyelectrolyte–surfactant complexes with long range order

“…The two phase appearance before the point of global charge neutrality in mixed polymer/surfactant systems made with one unmodified polyelectrolyte and one oppositely charged surfactant was found previously in other systems [6] and the observation in Fig. 1 that phase separation is seen at a later stage with the hydrophobically modified polymers is also an expected result and similar observations have previously been reported in the literature [6,21,22].…”

“…Frequently association and complex formation between polyelectrolytes and oppositely charged surfactants leads to precipitation of an almost neutral complex, being in equilibrium with an aqueous phase comprising excess polymer or surfactant [4]. These complexes sometimes present a long range order (lamellar, hexagonal, or cubic) [5,6]. Carboxymethyl cellulose (CMC) is a cellulose based negatively charged polyelectrolyte (Scheme 1), which is found in formulations for applications to modify viscosity and to stabilize emulsions.…”

Interactions in mixed cationic surfactants and dextran sulfate aqueous solutions

“…Investigations of the surfactant chain length dependence on surfactants binding to dextran sulfate revealed a highly cooperative character of binding due to a large effect of the hydrophobic interactions between bound surfactants [16]. It has been shown that the free energy of binding [16], the concentration region where the binding proceeded [8,18,28,29], and the structure of polylelectrolyte-surfactant complexes [30,31] strongly depend on the length of the alkyl chain. The increase of carbon atoms in the surfactant chains enhances hydrophobic association, shifts the cac toward lower surfactant concentrations, and change the structure of complexes, both in the bulk solution and in the solid state [29,31,32].…”

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

“…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.…”