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Organization of Polymer-Surfactant Mixtures at the Air-Water Interface: Sodium Dodecyl Sulfate and Poly(dimethyldiallylammonium chloride) E. Staples,† I. Tucker,† J. Penfold,*,‡ N. Warren,§ R. K. Thomas,§ and D. J. F. Taylor§ Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, United Kingdom, ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, United Kingdom, and Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, United Kingdom Received January 9, 2002. In Final Form: April 8, 2002 Specular neutron reflection and surface tension have been used to investigate the composition and structure of the surfactant-polymer mixture of sodium dodecyl sulfate, SDS, and the cationic polymer poly(dimethyldiallylammonium chloride) at the air-water interface. The variation of surface tension with SDS concentration shows a complex behavior, with a marked increase between the concentrations normally associated with the critical aggregation concentration and the critical micellar concentration. The neutron reflectivity measurements show that this change in surface tension is associated with changes in the amount of SDS and polymer at the interface. The changes are attributed to the competition between the formation of surface and solution surfactant-polymer complexes.
Introduction Polymers in aqueous surfactant solutions are extensively used in many important technological applications, as viscosity modifiers, stabilizers, and deposition aids, and in applications such as fabric conditioners, hair shampoos, detergents, foams, emulsions, and mineral recovery. Systems of neutral polymers with an anionic surfactant and of a charged polymer with a single surfactant of opposite charge have been extensively studied and well documented.1,2 However, such studies have focused predominantly on the bulk solution behavior, and the interfacial properties are much less well understood. Recent studies3 have shown that the bulk behavior, as revealed through measurements such as surface tension, is not necessarily a reliable indication of the interfacial properties. Adsorption at the air-water interface involves a balance between complex formation in bulk and at the interface and also a modification of surfactant monomer concentration adsorbed at the interface. This can give rise to a complex pattern of surface tension behavior, which can often be rationalized qualitatively but is difficult to interpret in detail. Although a variety of different techniques have been used to study polymer-surfactant complexes,4,5 none give a complete picture of adsorption at interfaces. The interpretation of conventional experimental methods, such as surface tension, is complicated by this formation of complexes in bulk and at interfaces. The resulting †
Unilever Research, Port Sunlight Laboratory. ISIS Facility, Rutherford Appleton Laboratory. § Physical and Theoretical Chemistry Laboratory, Oxford University. ‡
(1) Interaction of surfactants with Polymers and Proteins; Goddard, E. D., Ananthapandmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Polymer-surfactant systems; Kwak, J. C. T., Ed.; Surfactant Science Series, Vol. 77; Marcel Dekker: New York, 1998. (3) Purcell, I. P.; Lu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold, J. Langmuir 1998, 14, 1637. (4) Chari, K.; Hossain, T. J. Chem. Phys. 1991, 95, 3302. (5) Bain, C. D. Curr. Opin. Colloid Interface Sci. 1998, 3, 287.
variation in surfactant activity is difficult to measure and renders the Gibbs equation difficult to apply. Furthermore, such techniques provide no direct structural information. In recent years, a number of new techniques, which offer considerable potential in this area, have emerged.5,6 We have demonstrated that one of these, neutron reflectivity, is a powerful method for determining adsorbed amounts of surfactants at interfaces,7 especially for multicomponent mixtures,8,9 and for obtaining detailed surface structure.10 Of particular relevance to this paper is the use of neutron reflectivity3,11,12 and the complementary X-ray reflectivity13 to study the adsorption of polymer-surfactant mixtures at the air-water interface. Of direct relevance to this paper, we have previously reported the preliminary use of neutron reflectivity to study the adsorption of the mixed surfactants of sodium dodecyl sulfate, SDS, and hexaethylene glycol monododecyl ether, C12E6, at the air-water interface, in the presence of the cationic copolymer poly(dimethyldiallylammonium chloride (dmdaac)-acrylamide) and the cationic homopolymer dimethyldiallylammonium chloride.11,14 A wide range of different polymer-SDS systems have been studied, including cationic and nonionic polymers. A variety of techniques, including surface tension, rheology, and light scattering, have been used to characterize (6) Thomas, R. K.; Penfold, J. J Phys.: Condens. Matter 1990, 2, 1369. (7) Simister, E. A.; Thomas, R. K.; Penfold, J.; Aveyard, R.; Binks, B. P.; Cooper, P.; Fletcher, P. D. I.; Lu, J. R.; Sokolowski, A. J. Phys. Chem. 1992, 96, 1383. (8) Penfold, J.; Staples, E.; Cummins, P.; Tucker, I.; Thompson, L.; Thomas, R. K.; Simister, E. A.; Lu, J. R. J. Chem. Soc., Faraday Trans. 1996, 92, 1773. (9) Penfold, J.; Staples, E.; Thompson, L.; Tucker, I.; Hines, J.; Thomas, R. K.; Lu, J. R. Langmuir 1995, 11, 2496. (10) Penfold, J.; Staples, E.; Cummins, P.; Tucker, I.; Thompson, L.; Thomas, R. K.; Simister, E. A.; Lu, J. R. J. Chem. Soc., Faraday Trans. 1996, 92, 1539. (11) Creeth, A. M.; Staples, E.; Thompson, L.; Tucker, I.; Penfold, J. J. Chem. Soc., Faraday Trans. 1996, 92, 589. (12) Jean, B.; Lee, L. T.; Cabane, B. Langmuir 1995, 15, 7585. (13) Stubenrauch, C.; Albouy, P. A.; Klitzing, R. V.; Langevin, D. Langmuir 2000, 16, 3206. (14) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K. J. Phys.: Condens. Matter 2000, 12, 6023.
10.1021/la020034f CCC: $22.00 © 2002 American Chemical Society Published on Web 05/22/2002
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Figure 1. Schematic surface tension plots for SDS/poly-dmdaac (solid line) and the classical form for a nonionic polymer/anionic surfactant (dashed line).
their bulk solution properties and complex formation. Recent studies include cationic starch/SDS,15 poly(N,N,Ntrimethyl ammonio) ethyl acetate chloride/SDS,16 poly(ethylene glycol)/SDS,17 poly(ethylene imine)/SDS,18 hydrophobically modified cellulose (ethyl hydroxyethyl cellulose)/SDS,19 and hydrophobically modified gelatin/ SDS.20 Different patterns of behavior in the surface tension and adsorption dependence on surfactant concentration arise and are a consequence of the strength of the interaction between the polymer and surfactant. For the neutral polymers (PEO, PVP), the strength of the interaction is weak and arises from the hydrophobic interaction. These systems give rise to the classical surface tension behavior (see Figure 1) where the two sharp breaks in the surface tension variation with surfactant concentration, T1 and T2/T3, are associated with the critical aggregation concentration (cac) and the critical micellar concentration (cmc). In this case, the cac denotes the concentration at which micelles start to form on the polymer, and the cmc is the point at which the polymer is saturated and free micelles form in solution. For charged polymers and oppositely charged surfactants, the interaction is much stronger and of electrostatic origin. This gives rise to a different surface tension behavior, dependent upon the strength of the interaction, and will be discussed in more detail later in this paper. Recent neutron reflectivity studies on mixtures of SDS and neutral polymers21-24 show the extent to which (15) Merta, J.; Stenius, P. Colloid Polym. Sci. 1995, 273, 924. (16) Fundin, J.; Brown, W.; Vethamutha, M. S. Macromolecules 1996, 29, 1195. (17) Ballerat-Busserolles, K.; Roux-Desgranges, G.; Roux, A. H. Langmuir 1997, 13, 1946. (18) Winnik, M. A.; Bystryak, S. M.; Chassenieux, C.; Strasshko, V.; Macdonald, P. M.; Siddiqui, J. Langmuir 2000, 16, 4495. (19) Lauten, R. A.; Kjoniksen, A. L.; Nystrom, B. Langmuir 2000, 16, 4478. (20) Griffiths, P. C.; Fallis, I. A.; Teeraponchaisit, P.; Grillo, I. Langmuir 2000, 17, 2594. (21) Cooke, D. J.; Dong, C. C.; Lu, J. R.; Thomas, R. K.; Simister, E. A.; Penfold, J. J. Phys. Chem. B 1998, 102, 4912. (22) Cooke, D. J.; Blondel, J. A. K.; Lu, J. R.; Thomas, R. K.; Wang, Y.; Han, B.; Han, H.; Penfold, J. Langmuir 1998, 14, 1990. (23) Purcell, I. P.; Lu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold, J.; Langmuir 1998, 14, 1637. (24) Cooke, D. J.; Dong, C. C.; Thomas, R. K.; Howe, A. M.; Simister, E. A.; Penfold, J. Langmuir 2000, 16, 6546.
detailed information about the surface layer, which complements surface tension data, can be obtained. The data for M-DS (for M ) Li, Na, K) and poly(ethylene oxide)21,22 show competitive adsorption between the M-DS and poly(ethylene oxide), PEO, at the interface and evidence for some interaction between the surfactant and polymer at the interface. However, for the poly(vinyl pyrrolidone) (PVP)/SDS mixture, the PVP enhances the SDS adsorption at low SDS concentrations, whereas the SDS adsorption is suppressed at higher SDS concentrations. For both systems, the combination of surface tension and neutron reflectivity provides evidence of a surface interaction, the formation of surface polymer-surfactant complexes. Similar studies on the mixture of SDS/gelatin show further evidence for a strong surface polymersurfactant interaction. Between the cac and the cmc, the surface layer is considerably thicker due to polymersurfactant complex formation at the interface. Neutron reflectivity measurements on the mixture of SDS and poly(N-isopropyl acrylamide), PNIPAM,12,25 show a surface behavior similar to that for SDS/PEO. Above the cac, the polymer is displaced from the surface by SDS, and this is attributed to solution complex formation.25 Experimental Details (i) Surface Tension. The surface tension measurements were made using a Kruss K10T maximum pull digital tensiometer, with a du Nouy ring. The platinum/iridium alloy ring was rinsed in UHQ water and flamed in a Bunsen burner before measurements. Repeated measurements were taken until readings were within 0.03 mN/m. The equilibration time was found to depend on the type of solution, and all measurements were made to equilibrium. (ii) Neutron Reflectivity. The specular reflection of neutrons provides information about inhomogeneities normal to an interface or surface, and the technique is described in detail elsewhere.6 The basis of a neutron reflectivity experiment is that the variation in specular reflection with Q (the wave vector transfer normal to the surface, defined as Q ) (4π/λ) sin θ where λ is the neutron wavelength and θ is the grazing angle of incidence) is simply related to the composition or density profile in a direction (25) Richardson, R. M.; Pelton, R.; Cosgrove, T.; Zhang, J. Macromolecules 2000, 23, 6269.
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normal to the interface. In the kinematic or Born approximation,26 it is just related to the square of the Fourier transform of the scattering length density profile, F(z),
R(Q) )
16π2 | F(z)e-iQz dz|2 Q2
∫
(1)
where F(z) ) ∑i ni(z)bi, ni(z) is the number density of the ith nucleus, and bi is its scattering length. The key to the use of the technique for the study of surfactant adsorption is the ability to manipulate the scattering length density or neutron refractive index profile (where the neutron refractive index is defined as n ) 1 - λ2F(z)/2π) at the interface using hydrogen (H)/deuterium (D) isotopic substitution (where H and D have vastly different scattering powers for neutrons). The specular neutron reflectivity measurements were made on the SURF reflectometer27 at the ISIS pulsed neutron source at the Rutherford Appleton Laboratory, U.K. The measurements were made using a single detector at a fixed angle, θ, of 1.5°, using neutron wavelengths, λ, in the range 0.5-6.8 Å to provide a Q range of 0.048-0.5 Å-1. The reflectivity at high Q (Q > 0.2 Å-1) is dominated by sample-dependent background. This arises primarily from incoherent scattering from the bulk solution and is constant in Q. The background is determined from the reflectivity in the limit of high Q and has been subtracted from the data before any subsequent analysis. This has been shown to be a valid procedure providing there is no pronounced offspecular or small-angle scattering from the bulk solution, and this has been verified here by making off-specular measurements (either side of the specular reflection). The absolute reflectivities were calibrated with respect to D2O.28 For a deuterated surfactant in a null reflecting water, nrw (92 mol % H2O-8 mol % D2O has a scattering length of zero, that is, the refractive index of air), the reflectivity arises only from the adsorbed layer at the interface. This reflected signal can be analyzed in terms of the adsorbed amount at the interface and the thickness of the adsorbed layer. The most direct procedure for determining the surface concentration of surfactant is to assume that it is in the form of a single layer of homogeneous composition. The measured reflectivity can then be fitted by comparing it with a profile calculated using the optical matrix method for this simple structural model.29 The parameters obtained for such a fit are the scattering length density, F, and the thickness, τ, of the layer. The area per molecule is then
A)
∑ b /Fτ i
(2)
where ∑bi is the scattering length of the adsorbed surfactant molecule. This has been shown to be an appropriate method for surfactants. The sources of error (arising from errors in the measurements due to calibration or background subtraction or because the model is too simple) have been discussed in detail elsewhere and give rise to an error of typically (2 Å2 at an area per molecule of 50 Å2 .7 The simplest model, a single layer of homogeneous thickness, describes the data with sufficient accuracy. The data are not increased to sufficiently high Q for models with different functional forms (for example, a layer with a Gaussian distribution of scattering length density) or for the inclusion of interfacial roughness to be required. It has been shown that although the value of the layer thickness, τ, will be sensitive to the choice of model, the adsorbed amount depends on the product Fτ and so is largely independent of the detailed model.7 (26) Lekner, J. Theory of reflection; Martinus Nijhoff: Dordrecht, 1987. (27) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E.; Roser, S. J.; McLure, I. A.; Hillman, A. R.; Richards, R. W.; Staples, E. J.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899. (28) Lee, E. M.; Thomas, R. K.; Penfold, J.; Ward, R. C. J. Phys. Chem. 1989, 93, 381. (29) Penfold, J. In Neutron, X-ray and Light Scattering; Lindner, P., Zemb, T., Eds.; Elsevier: New York, 1991.
In the evaluation of the adsorbed amounts of the SDS at the interface, it is assumed that for the polymer/surfactant mixtures discussed here, the polymer has a scattering length sufficiently close to zero and that its contribution is negligible. We have been unable to synthesize the deuterium-labeled poly-dmdaac polymer and cannot get an estimate of the amount of polymer at the interface by a direct extension of eq 2. However, the amount of polymer at the interface can be estimated indirectly by making the measurements in a different way. For measurements with both surfactants deuterated in a D2O subphase, any deviation from the reflectivity of pure D2O (the deuterated surfactants are here closely matched to the D2O) arises from the polymer at the interface, and simple modeling can be used to estimate its amount and its spatial extent at the interface. Analyzing such reflectivity data as a single layer of uniform composition will give a layer thickness τ and a scattering length density F. The difference between the measured scattering length density, F, and that of D2O, FD2O, is assumed to be due to the presence of polymer such that
F ) (1 - φp)FD2O
(3)
where φp is the polymer volume fraction in the layer. (iii) Materials. The protonated SDS was obtained from BDH. The deuterated SDS (d-SDS, CD3(CD2)11SO4Na) was synthesized and purified by methods previously described.30 The chemical purity of the surfactants was assessed by surface tension measurements and thin layer chromatography, TLC. The cationic polymer poly-dmdaac was synthesized from the monomer, dimethyldiallylammonium chloride, by free radical polymerization, as described by Negi et al.31 The molecular weight used in this study was 100K and was evaluated from viscometry measurements.32 Deuterium oxide (D2O) was supplied by Fluorochem, and high-purity water (Elga Ultrapure) was used throughout. The glassware and poly(tetrafluoroethylene) troughs used for the neutron measurements were cleaned using alkaline detergent (Decon 90), followed by copious washing in high-purity water. The neutron reflectivity measurements were made from the surfactant-polymer mixtures of SDS and poly-dmdaac in 0.1 M NaCl solution at a temperature of 25 °C at the air-solution interface. The measurements were made predominantly in nrw using d-SDS and h-poly-dmdaac. Measurements to estimate the amount of polymer at the interface were made using D2O and d-SDS. The surface tension measurements were made in the SDS concentration range from 2 × 10-6 to 2 × 10-2 M and for polymer concentrations in the range from 10 to 80 ppm. The neutron reflectivity measurements were made in the concentration range from 10-5 to 2 × 10-3 M and for polymer concentrations of 10 and 80 ppm.
Results (i) Surface Tension Data. The variation in surface tension with SDS concentration for the SDS/poly-dmdaac mixture is shown in Figure 2. For pure SDS, the surface tension shows the expected behavior, with an abrupt change in the slope at the surfactant cmc (∼10-3 M). The addition of poly-dmdaac results a marked change in the surface tension behavior. The initial decrease in surface tension with increasing surfactant concentration is now shifted to much lower concentrations and appears to be independent of the polymer concentration. At some intermediate concentration, the surface tension increases abruptly, to a value close to its value at much lower surfactant concentrations. The concentration at which the abrupt increase occurs increases with increasing polymer (30) Lu, J. R.; Morrocco, A.; Su, T. J.; Thomas, R. K.; Penfold, J. J. Colloid Interface Sci. 1993, 158, 303. (31) Negi, Y.; Harada, S.; Ishizuk, O. J. Polym. Sci., Part A: Polym. Chem. 1967, 5, 1951. (32) Asnacios, A.; Langevin, D.; Argillier, J. F. Macromolecules 1996, 29, 7412.
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Figure 2. Surface tension versus SDS concentration for SDS/poly-dmdaac/0.1 M NaCl for different polymer concentrations: (2) 0 ppm, (O) 10 ppm, (4) 20 ppm, (0) 50 ppm, and (3) 80 ppm. Lines are a guide to the eye only. Table 1. Values of C1-C4, Derived from the SDS/ Poly-dmdaac/0.1 M NaCl Surface Tension Data polymer concn (ppm) 10.0 20.0 50.0 80.0
C1 (M)
C2 (M)
C3 (M)
C4 (M)
6 × 10-5
8 × 10-5 1.4 × 10-4 2.5 × 10-4 4.1 × 10-4
9.8 × 10-5 1.5 × 10-5 2.7 × 10-4 4.9 × 10-4
1.4 × 10-3 1.5 × 10-3 1.7 × 10-3 1.9 × 10-3
concentration. Further increases in the SDS concentration then result in the surface tension decreasing (with a slope slightly steeper than that for pure SDS) to a final constant value, which is marginally smaller than for pure SDS. The increasing slope with increasing polymer concentration is indicative of an increase in adsorption, and the lower final value of the surface tension suggests that the surface layer at higher concentrations in the presence of the polymer is more surface active than for a pure SDS monolayer. The pattern of surface tension variation with surfactant concentration is markedly different from the classical behavior that has been routinely reported for nonionic polymer/anionic surfactant systems,21,22,24 and these differences are summarized in the schematic diagram in Figure 1. For the nonionic polymer/anionic surfactant mixture, three break points are usually found: T1, which represents the onset of micellization on the polymer (cac); T2, which represents the point at which the polymer becomes saturated with surfactant (which is in general not very well-defined); and T3, the point at which free micelles begin to form (cmc). For the SDS/poly-dmdaac surface tension curves, there are four break points, which are denoted C1-C4. The SDS concentrations for those different regions as a function of polymer concentration are summarized in Table 1. (ii) Neutron Reflectivity Data. Neutron reflectivity measurements at the air-water interface have been used to evaluate the amount of SDS and poly-dmdaac at the interface. Measurements in nrw, and with the SDS deuterated, provide an estimate of the amount of SDS at the interface, using eq 2 as described earlier in the experimental section. The poly-dmdaac has a scattering
length density sufficiently close to zero that its contribution to the reflectivity introduces a negligible error. Neutron reflectivity and surface tension measurements of polydmdaac in the absence of SDS showed that it was not surface active without surfactant. The variation of the amount of SDS adsorbed (in mol cm-2) as a function of SDS concentration is shown in Figure 3a for the two polymer concentrations, 10 and 80 ppm. The variation in the amount of polymer at the interface (expressed as a volume fraction) is shown in Figure 3b. The amount of polymer is estimated using eq 3, from measurements in D2O with the SDS deuterated, such that the deviation in reflectivity from the reflectivity of pure D2O arises from the polymer at the interface. For a polymer concentration of 10 ppm, the amount of SDS at the interface is initially fairly constant, with a slight increase with increasing SDS concentration. At the SDS concentration corresponding to the abrupt increase in the surface tension, the amount of SDS at the interface drops suddenly and then increases gradually with increasing surfactant concentration. A similar trend is observed for the polymer concentration of 80 ppm but is less clearly associated with the peak in the surface tension curve for that polymer concentration. Over the SDS concentration range measured, there is a significant amount of polymer at the interface. At low surfactant concentrations, it is initially constant with surfactant concentration, and the amount at the interface decreases substantially in the region where the surface tension starts to increase. At surfactant concentrations of ∼cmc, there is still ∼20 vol % of polymer at the interface. The neutron reflectivity data also provide some important structural information about the adsorbed layer. The straightforward measurement of the thickness of the adsorbed layer already provides an important insight which reinforces the conclusions that can be drawn from the information about adsorbed amounts. The measurements of d-SDS/h-poly-dmdaac/nrw provide a direct estimate of the extent of the surfactant at the interface. This is shown in Figure 3c for both polymer concentrations (10 and 80 ppm). The thickness of the surfactant layer is ∼18-20 Å and is independent of the surfactant or polymer
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Figure 3. (a) SDS adsorption (Γ × 10-10 mol cm -2) at the air-water interface as a function of SDS concentration for (O) SDS/10 ppm poly-dmdaac, (b) SDS/80 ppm poly-dmdaac, and (4) pure SDS (without electrolyte or polymer). (b) Polymer volume fraction at the interface as a function of SDS concentration for (O) SDS/10 ppm poly-dmdaac and (b) SDS/80 ppm poly-dmdaac. (c) Thickness of the adsorbed SDS layer as a function of SDS concentration for (O) SDS/10 ppm poly-dmdaac and (b) SDS/80 ppm poly-dmdaac. (d) Thickness of the adsorbed polymer layer as a function of SDS concentration for (4) SDS/10 ppm poly-dmdaac and (2) SDS/80 ppm poly-dmdaac. Lines are a guide to the eye only. The positions of C2 are indicated on (a) and (b).
concentration. This is the typical thickness obtained for a surfactant monolayer of a C12 alkyl chain surfactant and is consistent with the polymer-surfactant complex at the interface containing surfactant in monomeric form, and not as micelles. This suggests that if C1 is equivalent to a cac, then there may well be micelle/polymer complexes in solution which affect the SDS monomer concentration, but not at the interface. This is similar to the observations from neutron reflection measurements on the SDS/gelatin mixture,24 where between T1 and T2 the extent of the adsorbed layer was consistent with polymer/surfactant complexes at the interface containing surfactant in monomeric form and at concentrations beyond T2 the thickness of the adsorbed layer corresponded to that of a surfactant layer. The reflectivity measurements for d-SDS/ h-poly-dmdaac/D2O provide an estimate of the extent of the polymer layer at the interface, and this is shown in Figure 3d. It is roughly constant for surfactant concentrations of
