Adsorption of oppositely charged polyelectrolyte/surfactant mixtures. Neutron reflection from alkyl trimethylammonium bromides and sodium poly(styrenesulfonate) at the air/water interface: The effect of surfactant chain length

摘要

The interactions between a series of cationic surfactants (alkyltrimethylammonium bromides) (C(n)TAB) and the anionic polymer sodium poly(styrenesulfonate) (NaPSS) have been studied using surface tension and neutron reflectivity. The effect of surfactant chain length on surfactant/polymer adsorption has been studied using chain lengths from 10 to 16 carbon atoms (i.e. C(10)TAB to C(16)TAB). The surface tension plots (gamma against ln c) have two types of shape. The C(10)TAB and C(12)TAB with NaPSS systems have plots with plateaus, while the longer chain C(14)TAB and C(16)TAB systems have a pronounced hump in the surface tension at surfactant concentrations just below the critical micelle concentration (cmc) of the free surfactant. Neutron reflection shows that there is a surfactant monolayer (thickness 20-22 Angstrom) adsorbed at dilute surfactant concentrations for the C(10)TAB, C(12)TAB, and C(14)TAB systems, but this changes to a thick layer (60-100 Angstrom) at concentrations closer to the cmc. The corresponding areas per adsorbed surfactant molecule vary from that typical for a close-packed monolayer to about half or a third of this value. For the C(16)TAB system only monolayer adsorption occurs. The structure of the thicker layer is a sandwich structure consisting of a surfactant monolayer and another surfactant rich layer below the surface, with an intermediate layer containing mainly solvent with some surfactant and some polymer. Both surface tension and neutron reflection results can be explained in terms of a model in which there are three kinds of polymer/surfactant complex. There is a complex of monolayer surfactant with the polymer backbone, and this complex (PSS) is present at the surface under all circumstances. A second complex (PS'(S)) of polyelectrolyte and a bilayer of surfactant may attach to the underside of the initial monolayer complex. Finally, there is a complex of surfactant aggregates with the polymer in the bulk solution (PSM). The monolayer complex PSS is formed at a much lower surfactant concentration than the PSM and determines the surface behavior at low surfactant concentrations. If the difference in stability of PSS and PSM is large, there may be secondary adsorption of PS'(S) to the PS'(S). The stability gap between PSS and PSM decreases as the surfactant chain length increases. Hence, layered structures are observed only for the three shorter chain surfactant systems because PSS and PSM become of comparable stability for C16TAB. The layered structures involving PS also stabilize the surface and tend to prevent the humps in surface tension. Hence, no such humps appear in the gamma-log c plots for C(10)TAB and C(12)TAB. [References: 21]