Dynamics of Adsorption of an Oppositely Charged Polymer−Surfactant Mixture at the Air−Water Interface:  Poly(dimethyldiallylammonium chloride) and Sodium Dodecyl Sulfate

Abstract

The dynamic adsorption behavior of mixtures of the cationic polymer poly(dimethyldiallylammonium chloride) [poly(dmdaac)] and the anionic surfactant sodium dodecyl sulfate (SDS) has been studied at the expanding liquid surface of an overflowing cylinder. A combination of ellipsometry and external reflection Fourier transform infrared spectroscopy was used to measure the adsorbed amounts of poly(dmdaac) and SDS as a function of the bulk surfactant concentration for various polymer concentrations in the range 0-0.2 g dm-3. Laser Doppler velocimetry was used to determine the surface age, which was approximately 1 s for solutions where the polymer adsorbed. The interfacial behavior is rationalized in terms of competition between surface activity and mass transport to the expanding surface. At low surfactant concentrations, adsorption of both poly(dmdaac) and SDS is enhanced as a result of the formation in solution of polymer-surfactant complexes that are more surface active than either component alone. The rate of adsorption of these complexes is diffusion-controlled, and their interfacial composition remains constant at three dmdaac units per SDS molecule over a 5-fold change in the surfactant concentration. For the higher polymer concentrations studied, the complexes saturate the air-water interface: the adsorbed amount is independent of the polymer concentration and remains constant also over a factor of 5 in the surfactant concentration. Once the number of bound surfactant molecules per dmdaac monomer exceeds 0.3, the complexes begin to form large aggregates, which are not surface active due to their slower mass transport. The adsorbed amount decreases rapidly on approach to the equivalence point (one SDS molecule per dmdaac monomer), and when it is reached, only a very small amount of material remains at the interface. At still higher surfactant concentrations, the free SDS adsorbs but there is no adsorbed poly(dmdaac). The dynamic adsorption data are compared with equilibrium measurements of the same system by Staples et al. (Langmuir 2002, 18, 5147), which show very different surface compositions and no significant change in surface coverage at the equivalence point.

The Adsorption Behavior of Ionic Surfactants and Their Mixtures with Nonionic Polymers and with Polyelectrolytes of Opposite Charge at the Air–Water Interface

“…If these species also require greater amounts of surfactant for precipitation, which is plausible, this would cause the observed discrepancy. Support for this comes from the dynamic experiments of Campbell et al 16 Campbell et al 16 used the overflowing cylinder to study the surface of PDMDAAC−SDS−NaCl on a 1 s time scale. They found a constant PE−S ratio in the adsorbed layer over the whole concentration range up to the equivalence point and a broad dip down to zero adsorption at the equivalence point.…”

“…45 The interest in such systems is the evolution of wetting of the surface near the upper consolute temperature, but we are here interested only in the simpler behavior. Wetting is governed by Antonow's rule, which is that the interfacial tensions of the three potential interfaces in the two liquid phase + vapor system obey the relation (16) where γ denotes air and the αγ interface is taken to have the highest interfacial tension and α is the upper bulk liquid phase. The rule requires that at equilibrium the αγ surface is coated by a wetting film of the lower surface energy β phase, 46 but if it is not obeyed, a drop of β phase forms only a lens on the α phase.…”

Complexes of surfactants with oppositely charged polymers at surfaces and in bulk

“…Campbell et al [45] studied the kinetics of adsorption of these mixtures in an overflowing cylinder (OFC), which is a device for generating a continuously expanding flat liquid surface under steadystate conditions [46]. The surface of the OFC expands uniformly with a strain rate, θ, in the range of 0.5 -5 s -1 ; the equivalent surface age is given approximately by (2θ) -1 .…”

Polymer–surfactant interactions: Binding mechanism of sodium dodecyl sulfate to poly(diallyldimethylammonium chloride)

“…It was usually observed that at certain surfactant concentrations such interaction leads to precipitation of a polymersurfactant complex. This precipitation was characterized for various oppositely charged polyelectrolyte-surfactant systems [12][13][14][15][16][17]. To prevent or reduce the phase separation in such systems, the addition of a salt is often required [18,19].…”