Organization of Polymer−Surfactant Mixtures at the Air−Water

The data are not measured sufficiently high Q for models with different ..... Staples et al.22 and Green at al.,34 from neutron reflectivity measureme...
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Langmuir 2002, 18, 5139-5146

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Organization of Polymer-Surfactant Mixtures at the Air-Water Interface: Poly(dimethyldiallylammonium chloride), Sodium Dodecyl Sulfate, and Hexaethylene Glycol Monododecyl Ether E. Staples,† I. Tucker,† J. Penfold,*,‡ N. Warren,§ and R. K. Thomas§ 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 December 28, 2001. 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, hexaethylene glycol monododecyl ether, C12E6, and the cationic polymer poly(dimethyldiallylammonium chloride), dmdaac, at the air-water interface. The variation of surface tension with surfactant concentration shows a similar complex behavior to that observed and previously reported for SDS/poly-dmdaac. There is a marked increase in the surface tension between the concentrations normally associated with the critical aggregation concentration (cac) and the critical micellar concentration (cmc). The extent of the increase depends strongly upon the solution composition and is most pronounced for solutions rich in SDS. The neutron reflectivity measurements show that this marked increase in surface tension is strongly associated with surfactant composition at the surface and with the amount of polymer at the interface. In this region of surface tension increase, between nominally the cac and cmc, the amount of SDS and polymer at the surface decreases markedly. This is attributed to the competition between the formation of surface and solution surfactantpolymer complexes that are rich in SDS.

Introduction Polymers in aqueous surfactant solutions are extensively used in many important technological applications, as viscosity modifiers, stabilizers, and deposition aids. 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 Most technological applications, however, involve the use of surfactant mixtures, and the bulk and interfacial properties of mixed surfactant-polymer complexes have also attracted much recent interest3,4 but are less well understood. This is particularly true of the interfacial properties, as most of the studies to date have involved investigations of bulk properties using light scattering and related techniques.3-8 Recent studies9 have shown that the bulk behavior, as revealed through measurements such as surface tension, †

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) Buckingham, J. H.; Lucassen, J.; Holloway, F. J. Colloid Interface Sci. 1978, 67, 423. (3) Li, Y.; Xia, J.; Dubin, P. L. Macromolecules 1994, 27, 7049. (4) Dubin, P. L.; The´, S. S.; McQuigg, D. W.; Chew, C. H.; Gan, L. M. Langmuir 1989, 5, 89. (5) Dubin, P. L.; Rigsbee, D. R.; McQuigg, D. W. J. Colloid Interface Sci. 1985, 105, 509. (6) Dubin, P. L.; Chow, C. H.; Gan, L. M. J. Colloid Interface Sci. 1987, 128, 516. (7) Dubin, P. L.; Curran, M. E.; Hua, J. Langmuir 1990, 6, 707. (8) McQuigg, D. W.; Kaplan, J. I.; Dubin, P. L. J. Phys. Chem. 1992, 96, 1973. (9) Purcell, I. P.; Lu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold, J. Langmuir 1998, 14, 1637.

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. A variety of different techniques have been used to study polymersurfactant complexes,10,11 but none give a complete picture of adsorption at interfaces. The formation of complexes in bulk and at interfaces can give rise to a complex pattern of surface tension behavior, which can be difficult to analyze quantitatively. In recent years, a number of new techniques, which offer considerable potential in this area, have emerged.11,12 We have demonstrated that one of these, neutron reflectivity, is a powerful method for determining adsorbed amounts of surfactants at interfaces,13 especially for multicomponent mixtures,14,15 and for obtaining detailed surface structure.16 Of particular relevance to this paper is the use of neutron reflectivity9,17,18 and the complementing X-ray reflectivity19 to study the adsorption of (10) Chari, K.; Hossain, T. J. Chem. Phys. 1991, 95, 3302. (11) Bain, C. D. Curr. Opin. Colloid Interface Sci. 1998, 3, 287. (12) Thomas, R. K.; Penfold, J. J. Phys:. Condens. Matter 1990, 2, 1369. (13) 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. (14) 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. (15) Penfold, J.; Staples, E.; Thompson, L.; Tucker, I.; Hines, J.; Thomas, R. K.; Lu, J. R. Langmuir 1995, 11, 2496. (16) 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. (17) Creeth, A. M.; Staples, E.; Thompson, L.; Tucker, I.; Penfold, J. J. Chem. Soc., Faraday Trans. 1996, 92, 589. (18) Jean, B.; Lee, L. T.; Cabane, B. Langmuir 1995, 15, 7585.

10.1021/la011863o CCC: $22.00 © 2002 American Chemical Society Published on Web 05/22/2002

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polymer-surfactant mixtures at the air-water interface. We have previously reported some results of the 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. The anionic-nonionic surfactant mixture of SDS/C12E6 has been extensively studied in the absence of the polymer.15 Its adsorption at the air-water interface has been characterized over a broad concentration and composition range and was shown to be broadly consistent with the predictions of regular solution theory.20 We have demonstrated, in the results published earlier,17 that the addition of the cationic polymer produces a surface more rich in the anionic surfactant, SDS. The copolymer and dmdaac homopolymer behave in a similar way and illustrate that the polymer-surfactant interaction is primarily with the cationic dmdaac block of the copolymer. The variation in adsorption with surfactant and polymer concentration is complex, and the initial measurements21 were rationalized in terms of the competition between the formation of surface and bulk polymer-surfactant complexes. We have also previously shown that for the SDS/poly-dmdaac mixtures22 this competition gives rise to a complex variation of surface tension with surfactant concentration. Similar variations in surface tension, where a pronounced maximum is observed at concentrations between the critical aggregation concentration (cac) and the critical micellar concentration (cmc), have been observed in other studies.23,32-34 For the SDS/poly-dmdaac mixture,22 this marked increase in surface tension was associated with changes (desorption) in the amount of polymer and surfactant at the interface. The changes were attributed to the competition between the formation of surface and solution surfactant-polymer complexes. 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.12 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 normal to the interface. In the kinematic or Born approximation,24 it is just related to the square of the Fourier transform of the scattering length density profile, F(z),

R(Q) )

2

∫

16π | F(z)e-iQz dz|2 Q2

(1)

where F(z) ) ∑i ni(z)bi, ni(z) is the number density of the ith (19) Stubenrauch, C.; Albouy, P. A.; Klitzing, R. V.; Langevin, D. Langmuir 2000, 16, 3206. (20) Holland, P. M. Colloids Surf. 1986, 19, 171. (21) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K. J. Phys.: Condens. Matter 2000, 12, 6023. (22) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K. Langmuir, in press (23) Merta, J.; Stenius, P. Colloids Surf. 1999, 149, 367.

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 reflectometer25 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.26 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.27 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

(2)

i

where ∑bi is the scattering length of the adsorbed 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 Å2.13 The simplest model, a single layer of homogeneous thickness, describes the data with sufficient accuracy. The data are not measured 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.13 It is straightforward to extend this approach to the determination of the surface composition of a multicomponent mixture.28 By selective deuteration of each component in turn, the surface excess of each component can be determined. For example, for a binary mixture eq 2 becomes

F)

∑ b /A τ + ∑ b /A τ l

l

2

2

(3)

(24) Lekner, J. Theory of reflection; Martinus Nijhoff: Dordrecht, 1987. (25) 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. (26) Lee, E. M.; Thomas, R. K.; Penfold, J.; Ward, R. C. J. Phys. Chem. 1989, 93, 381. (27) Penfold, J. In Neutron, X-ray and Light Scattering; Lindner, P., Zemb, T., Eds.; Elsevier: New York, 1991. (28) Penfold, J.; Thomas, R. K.; Simister, E. A.; Lee, E. M.; Rennie, A. R. J. Phys.: Condens. Matter 1990, 2, SA4121.

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Figure 1. Surface tension curves of different SDS/C12E6 mixtures in 0.1 M NaCl and 20 ppm poly-dmdaac (from 100:0 to 40:60 SDS/C12E6 and annotated in the figure). Solid lines are a guide to the eye only. where bi and Ai are the scattering lengths and areas/molecule of each of the components in the binary mixture. Making three different reflectivity measurements, with both surfactants deuterated and with either of the two surfactants deuterated, provides a self-consistent estimate of the surface composition. In the evaluation of the adsorbed amounts of the SDS and C12E6 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 date 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 3. 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

(4)

where φp is the polymer volume fraction in the layer. (iii) Materials. The protonated surfactants were obtained from Nikkol (C12E6) and BDH (SDS). The deuterated SDS (dSDS, CD3(CD2)11SO4Na) and deuterated C12E6 (alkyl chain deuterated, d-C12-h-E6, CD3(CD2)11(OCH2CH2)OH) were synthesized and purified by methods previously described.29 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.30 The molecular weight used in this study was 100K and was evaluated from viscometry measurements.31 Deuterium oxide (D2O) was supplied by Fluo(29) Lu, J. R.; Hromodova, M.; Thomas, R. K.; Penfold, J. Langmuir 1993, 9, 2417. (30) Negi, Y.; Harada, S.; Ishizuk, O. J. Polym. Sci. 1967, A5, 1951. (31) Asnacios, A.; Langevin, D.; Argillier, J. F. Macromolecules 1996, 29, 7412.

rochem, 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-C12E6 with poly-dmdaac in 0.1 M NaCl solution at a temperature of 25 °C at the airsolution interface. The measurements were made predominantly in nrw using d-SDS/h-C12E6, h-SDS/d-C12-h-E6, and d-SDS/dC12-h-E6. Measurements to estimate the amount of polymer at the interface were made using D2O and d-SDS/d-C12-h-E6. Measurements were made for 80/20 and 40/60 mol % mixtures of SDS/C12E6 at solution concentrations in the range from 10-5 to 10-3 M in nrw and with an added polymer concentration of 20 ppm. The surface tension measurements were made for SDS/ C12E6 solution compositions of 40/60 to 90/10 in the concentration range from 10-6 to 10-2 M and for a polymer concentration of 20 ppm. At a solution composition of 80/20, measurements were made over the same surfactant concentration range for different polymer concentrations in the range from 10 to 80 ppm.

Results (i) Surface Tension Data. The variation in surface tension for the SDS/C12E6/poly-dmdaac mixture with surfactant concentration is shown in Figure 1, for different SDS/C12E6 solution compositions and for a polymer concentration of 20 ppm. There are four distinct break points in the surface tension plots, which are denoted in Figure 1 as C1 (which is ill-defined), C2, C3, and C4. For solutions rich in SDS, the surface tension variation shows an unexpected behavior. The initial decrease in surface tension, normally observed from surfactant solutions, is shifted to lower concentrations than would be expected for an equivalent SDS/C12E6 mixture in the absence of polymer (see Figure 2) (C1). Following a short plateau region (C2), at a concentration of ∼10-4 M there is an abrupt increase in the surface tension (C3). The concentration at which the abrupt rise occurs is independent of the surfactant composition, but the extent of the increase is strongly dependent on composition. It is most pronounced for solutions rich in

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Figure 2. Surface tension curves for 80:20 SDS/C12E6 in 0.1 M NaCl with different concentrations of poly-dmdaac (from 0 to 80 ppm). Solid lines are a guide to the eye.

SDS, and the data for SDS/poly-dmdaac solutions discussed previously22 are also shown in the figure. At a solution composition of 40/60 and for solutions richer in C12E6, there is no subsequent marked change in the surface tension beyond the initial decrease at lower concentrations. For the solutions rich in SDS, the surface tension reaches a maximum value and then decreases with increasing surfactant concentration with a slope that is steeper than that for SDS/C12E6 mixtures without poly-dmdaac. The surface tension then reaches its final plateau value at higher surfactant concentrations (C4). The final value of the surface tension depends on the solution composition. It is lower for solutions more rich in C12E6, indicative of the mixtures rich in C12E6 being more surface active. Figure 2 shows the variation in surface tension for an 80/20 SDS/C12E6/poly-dmdaac solution as a function of surfactant concentration for different polymer concentrations. The initial decrease in surface tension at low surfactant concentrations is essentially independent of polymer concentration. The sharp increase and subsequent decrease in surface tension at higher surfactant concentrations is strongly dependent upon polymer concentration. The surfactant concentration at which the marked increase in surface tension occurs increases with increasing polymer concentration. For a solution composition of 80/20 SDS/C12E6, the limiting values of surface tension at concentrations above and below the peak in the surface tension are identical. It is however different for other solution compositions. For solutions richer in SDS, the final surface tension is higher than its plateau value at intermediate concentrations, whereas for solutions richer in C12E6 the inverse is true. These differences in surface tension reflect the changes in the surface activity of the surface complex for the different solution compositions. The values of C1 to C4 for different SDS/C12E6 compositions and polymer concentrations are summarized in Tables 1 and 2. C1 is largely independent of surfactant composition and polymer concentration. C2 and C3 are very close and depend strongly on polymer concentration but are independent of surfactant composition. C4, however, depends

Table 1. Values of C1-C4 Derived from the 80:20 SDS/ C12E6/0.1 M NaCl/Poly-dmdaac Surface Tension Data polymer concn (ppm)

C1 (M)

C2 (M)

C3 (M)

C4 (M)

10 20 50 80

6 × 10-5 6 × 10-5 6 × 10-5 6 × 10-5

8.7 × 10-5 1.44 × 10-4 3.0 × 10-4 4.48 × 10-4

1.11 × 10-4 1.74 × 10-4 3.34 × 10-4 5.31 × 10-4

2.7 × 10-4 3.04 × 10-4 5.83 × 10-4 7.4 × 10-4

Table 2. Values of C1-C4 Derived from the SDS/C12E6/0.1 M NaCl/20 ppm Poly-dmdaac Surface Tension Data SDS/C12E6 ratio 65:35 70:30 80:20 90:10 100:0

C1 (M) 10-5

6× 6 × 10-5 6 × 10-5 6 × 10-5 6 × 10-5

C2 (M) 10-4

1.5 × 1.42 × 10-4 1.44 × 10-4 1.45 × 10-4 1.41 × 10-4

C3 (M) 10-4

1.85 × 1.73 × 10-4 1.74 × 10-4 1.7 × 10-4 1.49 × 10-4

C4 (M) 2.45 × 10-4 2.55 × 10-4 3.04 × 10-4 4.9 × 10-4 1.54 × 10-3

strongly on both surfactant composition and polymer concentration. (ii) Neutron Reflectivity Data. Neutron reflectivity measurements at the air-water interface have been used to evaluate the amount of SDS, C12E6, and poly-dmdaac at the interface. Measurements in nrw, with deuteriumlabeled SDS and/or C12E6, provide a self-consistent estimate of the amount of SDS and C12E6 at the interface, using eq 3 as described in the experimental section. The poly-dmdaac has a scattering length sufficiently close to zero that its contribution to the reflectivity is negligible. The amounts of SDS and C12E6 and the total adsorbed amount (in mol cm-2) are shown as a function of surfactant concentration for 80/20 and 40/60 SDS/C12E6 and 20 ppm poly-dmdaac in Figures 3 and 4. The variation in the amount of polymer at the interface (expressed as a volume fraction) is shown for 80/20 SDS/ C12E6/20 ppm poly-dmdaac in Figure 3b. The amount of polymer is estimated using eq 4, from measurements in D2O with the SDS and C12E6 deuterated, such that the deviation of the reflectivity from the reflectivity of pure D2O arises from the polymer at the interface. The equivalent data for the 40/60 SDS/C12E6/20 ppm polydmdaac mixture are not plotted but are constant at a

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Figure 3. (a) Variation in adsorbed amount for 80/20 SDS/C12E6/0.1 M NaCl/D2O 20 ppm poly-dmdaac; (O) SDS, (4) C12E6, and (b) total adsorbed amount. (b) Variation of polymer volume fraction for 80/20 SDS/C12E6/0.1 M NaCl/20 ppm poly-dmdaac.

volume fraction of ∼30% and are independent of the surfactant concentration. In addition to the direct information about adsorbed amounts, the neutron reflectivity measurements provide structural information. From the analysis of the neutron reflectivity data described here, it was sufficient to describe the adsorbed layer with the simplest of models, a single layer of uniform scattering length density. The variation of the layer thickness with surfactant concentration for the 80/20 SDS/C12E6 solution is shown in Figure 5. Plotted separately in the figure are the thicknesses obtained when only the polymer or the SDS are visible, and each give rise to a mean thickness of ∼26 Å, which shows no pronounced variation with surfactant concentration. The thicknesses obtained for 40/60 SDS/C12E6 (not shown) are of a similar magnitude. The polymer thicknesses obtained are similar to those obtained for the SDS/

poly-dmdaac mixtures,22 but the surfactant layer thickness is noticeably larger. This implies that the addition of C12E6 has modified the structure of the adsorbed layer. This is particularly noticeable in the extent of the C12E6 which is obtained from the data, which is also plotted in Figure 5. This gives rise to a mean thickness of ∼35 Å. However, this is intrinsic to the surfactant, as there is no significant change in the layer thickness after the desorption of the polymer. In comparison, for the SDS/poly-dmdaac mixture the surfactant layer has a mean thickness of ∼20 Å and that of the polymer is ∼25 Å. Discussion The surface tension behavior is different to that normally observed for surfactants, surfactant mixtures, and surfactant/neutral polymer mixtures. For surfactants and surfactant mixtures, a single sharp break in the

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Figure 4. Variation in adsorbed amount for 40/60 SDS/C12E6/0.1 M NaCl 20 ppm poly-dmdaac; (O) SDS, (4) C12E6, and (b) total adsorbed amount.

Figure 5. Variation in layer thickness for 80/20 SDS/C12E6/ 0.1 M NaCl 20 ppm poly-dmdaac; (b) SDS, (O) polymer, and (4) C12E6.

variation of surface tension with surfactant concentration occurs at the cmc. For other polymer/surfactant mixtures, neutral polymer/surfactant mixtures, the classical pattern of surface tension variation corresponds to two abrupt changes in surface tension at the cac and cmc. Similar variations in surface tension to that reported here have been observed in other polymer-surfactant mixtures18,22,23,32-34 and were discussed in detail in the context of our earlier results for SDS/poly-dmdaac.22 The nature of the adsorption of polymer/surfactant mixtures at interfaces is determined by the nature and strength of the interaction between the polymer and surfactant. This (32) Richardson, R. M.; Pelton, R.; Cosgrove, T.; Zhang, J. Macromolecules 2000, 23, 6269. (33) Merta, J.; Stenius, P. Colloid Polym. Sci. 1998, 273, 924. (34) Green, R. J.; Su, T. J.; Hoy, J.; Lu, J. R. Langmuir 2000, 16, 5797.

has been discussed in detail in two recent papers on dodecyl trimethylammonium bromide (C12TAB)/sodium poly(styrene sulfonate) (NaPSS)36 and SDS/poly-dmdaac22 mixtures. The formation of bulk PSM polymer-surfactant complexes, where the surfactant is in an aggregated form, is the more standard phenomenon which occurs with surfactants and neutral polymers and is driven by the hydrophobic effect. This gives rise to the standard surface tension plot, with two distinct breaks associated with the cac and the cmc. The formation of polymer-surfactant complexes where the surfactant is in monomeric form, PSB, is due to strong electrostatic interactions between the polymer and surfactant. At some point, sufficient surfactant adsorption will exist such that the complex is surface active, PSγ. The properties of the NaPSS/C12TAB mixture were assumed to result from the strong adsorption of PSγ at the interface and from the domination of PSγ over PSM formation.36 In such cases, C1 is not associated with the cac. For the SDS/poly-dmdaac mixture,22 it was argued that the surface properties could be accounted for as an intermediate case, where PSγ and PSM occur at about the same surfactant concentration. It is further assumed here that the SDS/C12E6/poly-dmdaac mixture also falls into this category, except that the nonionic surfactant adds an additional control on their relative contributions. As such, it is likely that C1 coincides with the cac. Hence, C1 is attributed to the cac, and C4 to the cmc. The cac is the point at which micelles form on the polymer. The rapid decrease in surface tension before C1 and the low surface tension between C1 and C2 are associated with the adsorption of surfactant/polymer complexes at the interface. Beyond C4, the surfactant monomer concentration is sufficiently high for free micelles to form. Although in this region the surface tension is now constant, there will be changes in the SDS and C12E6 monomer concentrations15 and hence changes in the surface composition can be expected. The rapid increase in surface tension between C2 and C3 must be associated with desorption of surfactant and polymer, similar to that already reported for SDS/poly-dmdaac.22 This has been (35) Warren, N. Ph.D. Thesis, University of Oxford, Oxford, U.K., 1999. (36) Taylor, D. J. F.; Thomas, R. K.; Penfold, J. To be published.

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Figure 6. As in Figure 2, but with the phase behavior included: (black) precipitates, (red) visible aggregates, (pink) cloudy, (grey) almost clear, and (white) clear.

confirmed with some neutron reflectivity measurements. The variation in the increase in surface tension between C2 and C4 is dependent on the solution composition. The cmc of the mixture will decrease with increasing C12E6 composition, and increased C12E6 will partially compensate the desorption of SDS-rich polymer-surfactant complexes. The eventual disappearance of the surface tension rise for solutions of 40/60 and richer in C12E6 is probably due to the cmc now being close to C2. The strong interaction between a charged polymer and surfactants of opposite charge gives rise to a complex bulk phase behavior. For SDS/poly-dmdaac and SDS/C12E6/ poly-dmdaac mixtures, there are clear and cloudy regions and the formation of visible aggregates and precipitates.35 Stenius et al.23,33 attributed similar changes in surface tension for the mixtures of SDS and SDS/nonionic surfactants with cationic starch as arising from the effects of precipitation. Although precipitation may be a factor in determining the complex surface tension behavior, it is not alone sufficient to explain the trends that are observed. This is shown in Figure 6, where the bulk phase behavior is plotted with the variation in surface tension for 80/20 SDS/C12E6. Staples et al.22 and Green at al.,34 from neutron reflectivity measurements on SDS/poly-dmdaac and SDS/ lysozyme, respectively, were able to correlate the observed changes in surface tension with changes in the relative adsorption of surfactant and polymer (or protein) at the interface and with changes in the structure of that surface layer. The neutron reflectivity measurements provide the additional information required to rationalize and confirm the assumptions made on the basis of the surface tension data alone. For the solution composition of 40/60 SDS/ C12E6, there are no marked changes in the adsorbed amount or the surface composition with surfactant

concentration. The total amount of surfactant increases gradually with the surfactant concentration. The evolution of the surface composition with concentration is substantially modified by the presence of the polymer. In the absence of polymer, the surface would be initially rich in the most surface-active component (in this case C12E6) and evolve toward a composition close to the solution composition.15 However, in the presence of the polymer, the formation of polymer-surfactant complexes at the surface rich in SDS ensures that at low surfactant concentrations the surface is richer in SDS. With increasing surfactant concentration, the amount of C12E6 at the interface increases. In the concentration range from 10-4 to 10-3 M, where the surface tension is essentially constant, there is a noticeable change in the surface composition and in the total amount adsorbed. For the SDS/C12E6 solution composition of 80/20, the total amount of surfactant at the interface remains essentially constant, except at the lower surfactant concentrations. At the lower surfactant concentrations, the increase in the SDS adsorption is attributed to an increase in the formation of SDS-rich surface surfactant/ polymer complexes. At an intermediate surfactant concentration, ∼2 × 10-4 M, the amount of SDS at the interface decreases markedly, and this is partially compensated by an increase in the amount of C12E6 at the interface. This corresponds to the concentration region where the surface tension increases markedly. Below this concentration, the amount of polymer at the interface does not vary significantly and is ∼25 vol %. However, at ∼2 × 10-4 M the amount of polymer at the interface drops significantly. This is the main driving force for the reduction in the SDS adsorption and is attributed to the SDS-rich polymer-surfactant surface complexes becoming less entropically favorable than bulk complexes. The dramatic reduction in SDS and polymer adsorption is

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partially counteracted by an increase in the C12E6 adsorption and a marked change in the surface composition. Nevertheless, taking both the polymer and surfactant into account there is a marked desorption from the interface, and this is responsible for the marked increase in surface tension at that concentration. As inferred from the surface tension data, the mixture cmc will be lower for solution compositions richer in C12E6. Once that cmc is comparable with C2, then the C12E6 monomer concentration will be such that the C12E6 adsorption can increasingly compensate for the desorption of the SDS-rich polymer-surfactant complexes, such that for solutions richer in C12E6 than 40/60 the dramatic increase in surface tension at C2 no longer exists. From the previous discussions on related systems22,36 and from the interpretation of the surface tension and neutron reflectivity data, it is now clear that the amount of adsorption at the interface and the composition of the adsorbed layer are determined by the adsorption of polymer-surfactant complexes at the interface and the competition between the formation of surface and solution complexes. The structural information presented in the previous section provides evidence that the adsorbed polymer-surfactant complexes are of the form PSγ and do not involve surfactant aggregates. The formation of polymer-surfactant complexes also affects the relative monomer concentrations (and hence activities) of the SDS and C12E6 and so modifies the surfactant monomer adsorption. These competing factors give rise to the complex pattern of adsorption and surface tension behavior. The pattern of adsorption can be summarized by three distinct regimes: (i) For C < cac, SDS and C12E6 adsorb in monomer form to the polymer, PSγ, but with a composition rich in SDS. It is now entropically more favorable for the polymer/ surfactant complex to adsorb at the interface than to form in solution, and the amount of SDS and polymer at the interface increases. (ii) For cac < C < cmc, mixed micelles form on the polymer, initially rich in SDS. It now becomes more entropically favorable for polymer/surfactant complexes, in the form PSM, to form in solution than at the interface. As a result, SDS-rich polymer/surfactant complexes desorb, resulting in reduced adsorption of surfactant and

Staples et al.

polymer at the interface. However, this is partially compensated by increased C12E6 adsorption. For solutions rich in C12E6, this effect is dramatically reduced, as reflected by the changing adsorption and surface tension behavior. (iii) For C > cmc, free mixed surfactant micelles form in solution, and the relative activities of the SDS and C12E6 change, resulting in changes in the surface surfactant composition. Many aspects of the SDS/C12E6/poly-dmdaac and the SDS/poly-dmdaac data22 are similar, in that the dramatic increase in surface tension between C1 and C4 is attributed to a partial desorption of polymer and surfactant. However, although the factors controlling their surface behavior are similar, the presence of C12E6 confers some important differences. Significantly, the presence of C12E6 provides the ability to tune the surface behavior and provides some degree of control of the polymer/surfactant interactions. Summary Specular neutron reflection and surface tension have been used to characterize the adsorption of the surfactant mixture of SDS/C12E6 and the cationic polymer polydmdaac at the air-water interface. The surface tension shows a complex pattern of behavior with surfactant concentration, where after the initial decrease in surface tension a marked increase is observed between the cac and cmc for solutions rich in SDS. The neutron reflectivity data show that this dramatic increase in surface tension is correlated with a decrease in the amount of polymer at the interface and a change in the composition of the surfactant at the interface to one more rich in C12E6. The results can be explained qualitatively by the competition between the formation of surface and solution polymersurfactant complexes and the modification of the relative surface activities of the SDS and C12E6 due to the polymersurfactant complex formation. There are similarities between these data and those reported for SDS/polydmdaac.22 The additional factor here is that the C12E6 confers a degree of control or tunability on the surface behavior. LA011863O