Article pubs.acs.org/Langmuir
Dynamic Adsorption of Weakly Interacting Polymer/Surfactant Mixtures at the Air/Water Interface Anna Angus-Smyth,†,‡ Richard A. Campbell,† and Colin D. Bain‡,* †
Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France Department of Chemistry, Durham University, South Road, Durham DH1 3LE, United Kingdom
‡
S Supporting Information *
ABSTRACT: The dynamic adsorption of polymer/surfactant mixtures containing poly(ethylene oxide) (PEO) with either tetradecyltrimethylammonium bromide (C14TAB) or sodium dodecyl sulfate (SDS) has been studied at the expanding air/ water interface created by an overflowing cylinder, which has a surface age of 0.1−1 s. The composition of the adsorption layer is obtained by a new approach that co-models data obtained from ellipsometry and only one isotopic contrast from neutron reflectometry (NR) without the need for any deuterated polymer. The precision and accuracy of the polymer surface excess obtained matches the levels achieved from NR measurements of different isotopic contrasts involving deuterated polymer, and requires much less neutron beamtime. The PEO concentration was fixed at 100 ppm and the electrolyte concentration at 0.1 M while the surfactant concentration was varied over three orders of magnitude. For both systems, at low bulk surfactant concentrations, adsorption of the polymer is diffusioncontrolled while surfactant adsorption is under mixed kinetic/diffusion control. Adsorption of PEO is inhibited once the surfactant coverage exceeds 2 μmol m−2. For PEO/C14TAB, polymer adsorption drops abruptly to zero over a narrow range of surfactant concentration. For PEO/SDS, inhibition of polymer adsorption is much more gradual, and a small amount remains adsorbed even at bulk surfactant concentrations above the cmc. The difference in behavior of the two mixtures is ascribed to favorable interactions between the PEO and SDS in the bulk solution and at the surface.
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INTRODUCTION Mixtures of polymers and surfactants are extensively used in industrial formulations. As a consequence, their behavior both in the bulk and at interfaces is widely studied, although bulk phase studies are predominant. Generally, polymer/surfactant mixtures are discussed in terms of the strength of the interactions between the polymer chains and the surfactant molecules, which can be electrostatic or hydrophobic in nature, depending on the structures and properties of the two components. When polymers and surfactants interact in the bulk solution, complexes are formed. The surfactant in these complexes may be in the form of either monomers1,2 or micelles3−5 depending on the system and the solution conditions. Several comprehensive reviews of the bulk5,6 and interfacial behavior7−9 of such systems have been published in recent years. Most studies of adsorption from polymer/surfactant systems at the air/water interface have been performed under static conditions. Such studies tell us much about the structure and composition of the material at the interface; however, they are limited in their ability to explain how the adsorbed layer forms. They are also restricted to states of the interface close to equilibrium, while applications of polymer/surfactant mixtures in commercial systems are frequently under dynamic conditions © 2012 American Chemical Society
where the interface is far from equilibrium. Dynamic measurements have been performed on some polymer/surfactant systems by the oscillating barrier and oscillating drop methods.10,11 Such measurements yield information about the coupling of the bulk and interfacial properties in polymer/ surfactant systems. Another experimental platform that has been used to study air/water interfaces far from equilibrium is the overflowing cylinder (OFC).12 An OFC has a large, flat, continually expanding surface with a surface age typically in the range 0.1− 1 s. The surface is at steady-state, permitting its study by a wide-range of surface-sensitive techniques, including neutron reflectometry (NR)13−15 and ellipsometry,14,16,17 which are the two techniques used here. A previous study of the polymer/ surfactant system poly(dimethyldiallylammonium chloride) (PDMDAAC)/sodium dodecyl sulfate (SDS)17 showed that neither the amount nor the composition of the material at the expanding surface of the OFC bore any simple relationship to those measured in the static case. PDMDAAC/SDS is, however, a strongly interacting system in which kinetically Received: March 28, 2012 Revised: June 28, 2012 Published: July 2, 2012 12479
dx.doi.org/10.1021/la301297s | Langmuir 2012, 28, 12479−12492
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electrolyte. The questions that we seek to answer include whether competitive adsorption of the polymer and surfactant is sufficient to explain the adsorption kinetics, whether the weak interaction between PEO and SDS results in qualitatively different behavior from the PEO/C14TAB mixtures, and whether adsorption is under diffusion or kinetic control. A system is described as being under diffusion, kinetic, or mixed control depending on whether the overall rate of adsorption is dictated by diffusion, convection, and migration in the bulk phase or by the free energy barrier for adsorption to the interface. On the OFC, the relationship between the diffusion coefficient (D), bulk and subsurface concentrations (cb and cs), surface expansion rate (θ), and surface excess (Γ) is given by the steady-state solution of the convective diffusion equation:36
trapped aggregates in the bulk lead to nonequilibrium, historydependent behavior. To develop a general understanding of adsorption kinetics in polymer/surfactant solutions far from equilibrium we need to start with much simpler systems. Mixtures of the nonionic polymer poly(ethylene oxide) (PEO) with sodium dodecyl sulfate (SDS) or tetradecyltrimethylammonium bromide (C14TAB) are the model systems upon which our current understanding of equilibrium properties of polymer/surfactant systems in the bulk and at interfaces was first built.18−21 Consequently, in this paper we address the dynamic adsorption of these two polymer/surfactant mixtures at the surface of the OFC. PEO/C14TAB is a good approximation to a noninteracting polymer/surfactant pair in the bulk solution, owing to the weakness of the hydrophobic interaction between PEO and hydrocarbon chains and the absence of any electrostatic attraction between the two species.22 PEO has previously been shown to be surface-active alone, with a monolayer of polymer segments forming even at low bulk concentrations. This surface activity has been attributed to the hydrophobicity of the CH2−CH2 groups of the monomer segments.23−26 PEO/C14TAB shows none of the deviations in the surface tension isotherms that are usually attributed to polymer/ surfactant interactions in the bulk solution.20 Studies of adsorption from mixtures of PEO and cationic surfactants are rare, as it is commonly accepted that there no interactions in such systems,21 and therefore it is expected that adsorption will be simply competitive on the basis of excluded volume considerations. In contrast, PEO/SDS interacts weakly in the bulk solution. This interaction has been traditionally attributed either to ion− dipole interactions or to interactions between the CH2 groups of the polymer and the surfactant chains,19 although several studies22,27,28 have suggested that the polymer/surfactant interaction is ionic in nature, mediated by metal cations complexed to the PEO chains. This interaction leads to the formation of bulk complexes consisting of SDS micelles attached to individual polymer chains above a bulk concentration commonly known as the cac (critical aggregation concentration).18,19,29−33 Previous studies on the equilibrium adsorption of PEO/SDS at the air/water interface have shown that although polymer adsorbs at low bulk surfactant concentrations due to its inherent surface activity, it is displaced from the surface as the bulk surfactant concentration is increased. The adsorption of the two species is therefore commonly described as competitive,20,34,35 although there is thought to be a cooperative component.21 The competitive nature of the adsorption has been attributed to increasing surface pressure upon surfactant adsorption, forcing polymer from the interface, or to bulk complex formation which removes polymer from the surface, although the latter seems unlikely especially as polymer appears to be significantly displaced below the cac.21,34 Recent simulations of Darvas et al.35 suggest a competitive adsorption mechanism in which polymer segments are displaced stepwise from the interface by the adsorption of surfactant molecules, and where polymer is not fully displaced from the interface until a monolayer of surfactant forms. The present study explores the adsorption kinetics of both PEO/surfactant systems on the OFC on time scales of 0.1−1 s. The polymer concentration is fixed at 100 ppm, and the surfactant concentration is varied over 3 orders of magnitude (typically ∼0.01−6 mM), all with 0.1 M of added inert
Γ=
2D (cb − cs) πθ
(1)
For charged surfactants, cs is interpreted as the concentration at the edge of the electrical double layer. Under diffusion control, adsorbed surfactant is locally in equilibrium with surfactant in the subsurface; that is, the relationship between the static surface excess and the bulk concentration is equal to the relationship between the dynamic surface excess and the subsurface concentration: Γdyn(cs) = Γeq(cb). For a pure surfactant, saturation coverage of the interface is not reached until cs = ccmc, which does not occur until cb > ccmc on the OFC. If the barrier to adsorption from the subsurface to the interface is rate-determining, then cs = cb and diffusion plays no part in the adsorption kinetics. If the system is under mixed kinetic/ diffusion control, cs is greater than it would be for a given Γ when the system is under diffusion control, but less than cb. A barrier to adsorption may arise from (i) a lack of empty sites at the interface, (ii) reorientation of the surfactant for adsorption, (iii) electrostatic or steric repulsions, or (iv) slow break-up of micelles or polymer/surfactant complexes. In a binary surfactant system, the surface composition does not generally match the bulk composition, even under diffusion control,37 for a combination of thermodynamic reasons (one component is more surface active than the other) and kinetic reasons (one surfactant diffuses faster than the other). If there are interactions between the components, one additionally needs to know the interaction parameters at the surface and, if applicable, in the bulk: the resulting adsorption kinetics can become very complicated.38,39 When the surface is far from equilibrium (cs ≪ cb), interactions at the surface cease to be important and mass transport to the surface is determined by eq 1 with cb − cs ≈ cb. Surface tensiometry and NR are the most common techniques used to characterize the interfacial behavior of polymer/surfactant mixtures; however, quantitative interpretation of tensiometry data in terms of surface composition is complicated by interactions within the bulk solution that make it difficult to determine the chemical potentials of the two components. A powerful feature of NR is isotopic substitution: the analysis of data obtained for chemically equivalent interfaces in different isotopic contrasts allows the determination of the adsorbed amounts of polymer and surfactant and provides information on structure, neither of which is accessible by tensiometry. A key objective of the present work is to validate a new quantitative approach for obtaining the composition of polymer/surfactant mixtures at the air/water 12480
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center of the OFC and NR measurements averaged over the central 3−4 cm is therefore expected to be
