Salt-induced swelling transitions of a lamellar amphiphile—Polyelectrolyte complex

Abstract

We report salt-induced swelling transitions of a lamellar complex of the anionic polyelectrolyte, poly(acrylic acid sodium salt) (PAANa), and the cationic amphiphile, didodecyldimethylammonium chloride (DDAC). Increasing the concentration of NaCl in the solution is found to lead to a collapsed → swollen → collapsed transition of the complex. The swelling transition is driven by an abrupt increase in PAANa adsorption on DDAC bilayers above a threshold salt concentration. The lamellar periodicity of the swollen phase is not determined by the thickness of the adsorption layer, and additional mechanisms have to be invoked to understand the extent of its swelling. The swelling transition is not observed for the highest molecular weight of PAANa used, but a gradual transformation between the two collapsed structures is seen on increasing the salt concentration. The polyelectrolyte chains desorb from the bilayers at a very high salt concentration, in a process similar to the well-known destabilization of complexes of oppositely charged polyelectrolytes. However, unlike the PAANa chains, the polymer-free bilayers do not disperse uniformly in the solution. Instead, they form a collapsed lamellar stack containing very little water due to the van der Waals attraction between them. The occurrence of an abrupt swelling transition at intermediate salt concentrations in this system contrasts sharply with the gradual swelling reported in other complexes with increasing salt concentration. Furthermore, this behavior does not seem to have been anticipated by theories of complexation of oppositely charged macroions. More experiments are required for a clear understanding of the interactions stabilizing the different phases observed in this system.

Effect of adsorbed polyelectrolytes on the interactions and elasticity of charged surfactant bilayers

“…Since the electrostatic inter-bilayer repulsion is highly screened, van der Waals attraction dominates and the bilayers again form a lamellar phase with a very low periodicity, but with no polyelectrolyte in the intervening aqueous layer between the bilayers. Such salt-induced swelling and de-swelling transitions have been recently reported by us in complexes of the ionic surfactant didodecyldimethylammonium chloride (DDAC) and sodium salt of poly-acrylic acid (PAANa) [5].…”

“…Ionic amphiphiles, that self-assemble into bilayers, form a highly swollen lamellar phase in aqueous solutions, as a result of long-range electrostatic inter-bilayer repulsion [1][2][3]. Addition of an oppositely charged polyelectrolyte to the solution has a very pronounced effect on the inter-bilayer interactions, which depends sensitively on the salt concentration (c s ) in the solution [4,5]. At low c s the polyelectrolyte forms a heterogeneous adsorption layer on the bilayer surface, due to the electrostatic repulsion between the chains, as has been observed in the case of polyelectrolyte adsorption on oppositely charged rigid surfaces [6].…”

“…On increasing c s a homogeneous adsorption layer is formed on the bilayer surface due to the screening of inter-chain repulsion [4,6,9]. Bridging of neighboring bilayers is no longer possible [10] and the polyelectrolyte covered bilayers form a swollen complex, stabilized either by steric repulsion [11] between the adsorbed chains on adjacent bilayers or by undulation repulsion [12], if the bilayer bending rigidity, κ, is sufficiently low [5]. At high c s the polyelectrolyte desorbs from the bilayer surface.…”

“…κ is expected to decrease, whereas κ is expected to increase in the presence of an adsorption layer. The DDAC-water system forms a swollen L α phase when the electrostatic repulsion between the bilayers is screened by the addition of NaCl [5]. The lamellar periodicity of this phase is determined by undulation repulsion between the bilayers, which depends on κ [12,17].…”

“…If the swollen DDAC-PAANa complex is also stabilized by undulation repulsion between the bilayers, it should in principle be possible to ascertain the effect of polymer adsorption on κ by comparing the maximal lamellar periodicities of the swollen complex and the DDACwater-NaCl system. But it was not possible to address this issue unequivocally in [5], since the lamellar periodicity of the swollen complex showed a large spread, presumably as a result of the extremely slow equilibration due to the irreversible nature of the adsorption process [18].…”