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Adsorption of Polyelectrolyte/Surfactant Mixtures at the Air-Solution Interface: Poly(ethyleneimine)/Sodium Dodecyl Sulfate† J. Penfold,*,‡ I. Tucker,§ R. K. Thomas,| and J. Zhang| ISIS, Rutherford Appleton Laboratory, CCLRC, Chilton, Didcot, OXON OX11 0QX, United Kingdom, Unilever Research and Development Laboratory, Port Sunlight, Quarry Road East, Bebington, WIRRAL CH63 3JW, United Kingdom, and Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, OXON OX1 2JD, United Kingdom Received February 24, 2005. In Final Form: April 12, 2005 Neutron reflectivity and surface tension have been used to characterize the adsorption of the polyelectrolyte/ionic surfactant mixture of poly(ethyleneimine) (PEI) and sodium dodecyl sulfate (SDS) at the air-water interface. The surface tension behavior and adsorption patterns show a strong dependence upon the solution pH. However, the SDS adsorption at the interface is unexpectedly most pronounced when the pH is high (when the polymer is essentially a neutral polymer) and when the polymer architecture is branched rather than linear. For both the branched and the linear PEI polymer/surfactant complex formation results in a significant enhancement of the amount of SDS at the interface, down to surfactant concentrations ∼10-6 M. For the branched PEI a transition from a monolayer to a multilayer adsorption is observed, which depends on surfactant concentration and pH. In contrast, for the linear polymer, only monolayer adsorption is observed. This substantial increase in the surface activity of SDS by complexation with PEI results in spontaneous emulsification of hexadecane in water and the efficient wetting of hydrophobic substrates such as Teflon. In regions close to charge neutralization the multilayer adsorption is accentuated, and more extensively ordered structures, giving rise to Bragg peaks in the reflectivity data, are evident.
Introduction Polymer-surfactant mixtures in aqueous solution are extensively used in a wide range of domestic, industrial, and technological applications. In applications such as fabric and hair conditioners, hair shampoos, paints, coatings, cosmetics, foodstuffs, and drug delivery systems, the polymer is included as a viscosity modifier, stabilizing aid, and deposition aid. Given this broad range of applications, polymer-surfactant mixtures have been extensively studied; there is now a substantial body of information and understanding of many aspects of their bulk properties.1-3 In contrast, the surface properties of polymer-surfactant mixtures have been much less extensively studied and, consequently, are less well characterized and understood. This has in part been due to a lack of suitable surface techniques and due to the difficulties associated with the interpretation of measurements such as surface tension. The behavior of weakly interacting nonionic polymerionic surfactant mixtures, such as poly(ethylene oxide)) (PEO)/sodium dodecyl sulfate (SDS)4 and poly(vinylpyrrolidone) (PVP)/SDS5, is relatively well understood and characterized. The classical polymer-surfactant surface tension behavior, as initially described by Jones,4 with two break points at surfactant concentrations T1 and T2, †
Part of the Bob Rowell Festschrift special issue. Rutherford Appleton Laboratory. § Unilever Research and Development Laboratory. | Oxford University. ‡
(1) Interaction of surfactants with polymers and proteins; Goddard, E. D., Ananthapandmanbhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Polymer-surfactant systems; Kwak, J. C. T., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1998; Vol. 77. (3) Robb, I. D. Anionic surfactants. Surf. Sci. Ser. 1981, 11, 109. (4) Jones, M. N. J. Colloid Interface Sci. 1967, 73, 36. (5) Chari, K.; Hossain, T. Z. J. Phys. Chem. 1991, 95, 3307.
corresponding to the onset of micellization on the polymer (critical aggregation concentration) and to the concentration at which free micelles form (critical micellar concentration, cmc), respectively, is now well established and understood in such systems. A range of techniques, including small angle neutron scattering,6 viscosity,7 light scattering,8 surfactant electrode, and isothermal titration calorimetry/differential scanning calorimetry measurements,9 have established the nature of the corresponding solution complexes. More recent measurements, using surface-sensitive techniques such as neutron reflectivity10-12 and radiotracers,5 have demonstrated how it can be difficult, even in these relatively straightforward cases, to relate surface and solution behavior and, hence, explain the surface tension behavior. The more strongly interacting polyelectrolyte-ionic surfactant mixtures give rise to a yet more complex solution and surface behavior, and both aspects are relatively less well understood compared to the weakly interacting nonionic polymer-surfactant mixtures. The presence of a strong surface interaction and the formation of surface polymer-surfactant complexes are responsible for the more complex range of surface behaviors, as demonstrated by surface tension;13 more complex patterns of surface tension and adsorption are observed. These are (6) Cabane, B.; Duplessix, R.; Zemb, T. J. Phys. 1985, 46, 2161. (7) Saito, S. In Nonionic surfactants: Physical Chemistry; Schick, M. J., Ed.; Marcel Dekker: New York, 1987. (8) Dubin, P. L.; Oteri, R. J. Colloid Interface Sci. 1983, 95, 453. (9) Li, Y.; Xu, R.; Bloor, D. M.; Penfold, J.; Holzwarth, J. F.; WynJones, E. Langmuir 2000, 16, 8673. (10) Cooke, D. J.; Dong, C. C.; Lu, J. R.; Thomas, R. K.; Simister, E. A.; Penfold, J. J. Phys. Chem. B 1998, 102, 4912. (11) Cooke, D. J.; Blondel, J. A. K.; Lu, J. R.; Thomas, R. K.; Wang, Y.; Han, B.; Yan, H.; Penfold, J. Langmuir 1998, 14, 1990. (12) Purcell, I. P.; Lu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold, J. Langmuir 1998, 14, 1637. (13) Goddard, E. D. J. Colloid Interface Sci. 2002, 256, 228.
10.1021/la0505014 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/10/2005
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exemplified by the different ranges of behavior observed in the anionic polyelectrolyte-cationic surfactant mixture of poly(styrene suphonate) (PSS)/alkyl trimethylammonium bromide (CnTAB), recently studied by surface tension and neutron reflectivity,14-16 PSS/CnTAB studied by surface tension, ellipsometry, and film drainage measurements,17-19 and PSS, poly(acrylamide-methyl propane sulfonate), DNA, and Xanthan with C12TAB studied by surface tension and X-ray reflectivity.20 Different and more dramatic changes in surface tension are observed in the cationic polyelectrolyte/anionic surfactant mixtures of poly(dimethyl diallylammonium chloride) (polyDMDAAC)/ SDS21-23 and cationic starch (cellulose)/anionic surfactant mixtures.24,25 In the PSS/CnTAB mixtures, at relatively low polymer concentrations ( 14, the surface tension behavior changes and a peak in the surface tension appears before T2. The occurrence of a peak in the surface tension is demonstrated in its most extreme form for the polyDMDAAC/SDS mixture.21,22 The sharp increase in the surface tension between T1 and T2 is associated with a partial depletion of the polymer/surfactant complex from the surface and the subsequent decrease in surface tension due to increased SDS adsorption as the free SDS monomer concentration increases. This contrasting surface behavior between CnTAB/PSS and SDS/polyDMDAAC has been rationalized as arising from the competition between the formation of surface and solution polymer-surfactant complexes. The CnTAB/PSS mixture is a case where the formation of surface polymer-surfactant complexes is more energetically favorable compared with the formation of solution complexes, and this gives rise to the multilayer adsorption. In contrast, in the case of the SDS/polyDMDAAC mixture, the surface and solution complexes are expected to have free energies that are more comparable. This paper continues from the earlier studies on PSS/ CnTAB and SDS/polyDMDAAC with a focus on the exploration of the effect of moderating or manipulating the strength of the electrostatic interaction between the polyelectrolyte and the ionic surfactant and of the role of the polymer architecture. The moderation of the interaction between the polymer and the surfactant by variation of charge density and structure has been studied to a (14) Taylor, D. J. F.; Thomas, R. K.; Hines, J. D.; Humphreys, K.; Penfold, J. Langmuir 2002, 18, 9783. (15) Taylor, D. J. F.; Thomas, R. K.; Penfold, J. Langmuir 2002, 18, 4748. (16) Taylor, D. J. F.; Thomas, R. K.; Li, P. X.; Penfold, J. Langmuir 2003, 19, 3712. (17) Monteux, C.; Williams, C. E.; Meunier, J.; Anthony, O.; Bergeron, V. Langmuir 2004, 20, 57. (18) Monteux, C.; Williams, C. E.; Bergeron, V. Langmuir 2004, 20, 5367. (19) Monteux, C.; Llauro, M. F.; Baigli, D.; Williams, C. E.; Anthony, O.; Bergeron, V. Langmuir 2004, 20, 5358. (20) Stubenrauch, C.; Albouy, P.-A.; v. Klitzing, R.; Langevin, D. Langmuir 2000, 16, 3206. (21) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K.; Taylor, D. J. F. Langmuir 2002, 18, 5147. (22) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K. Langmuir 2002, 18, 5139. (23) Vaknin, D.; Dahlke, S.; Travesset, A.; Nizri, G.; Maagdassi, S. Phys. Rev. Lett. 2004, 93, 218302. (24) Merta, J.; Stenius, P. Colloids Surf., A 1999, 149, 367: 1997, 122, 2431. (25) Tarade, E.; Samoshina, Y.; Nylander, T.; Lindman, B. Langmuir 2004, 20, 1753.
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limited extent for SDS/polyDMDAAC, where the polyDMDAAC homopolymer was replaced by a random copolymer26 or a block copolymer27 based on polyDMDAAC, and in this way the effects of charge density have been explored. In an alternative approach Staples et al.22 have investigated the surface adsorption of the polymer-surfactant mixture of polyDMDAAC/SDS/C12E6. Here the nonionic cosurfactant, C12E6, moderates the interaction, and significant changes are observed with varying surfactant composition. We adopt here a different approach with the polymer-surfactant mixture of poly(ethyleneimine) (PEI)/ SDS, where the strength of the electrostatic interaction can be manipulated by pH. At low pH the polymer is a highly charged polyelectrolyte, whereas at high pH it is essentially a neutral polymer. Furthermore, PEI exists in both linear and branched forms and the role of the polymer architecture can also be investigated. There have been a number of studies on PEI/SDS mixtures reported in the recent literature, but the emphasis has been on the bulk solution behavior rather than interfacial properties. Zhou et al.28 have reported the formation of supramolecular ordered structures from PEI/sodium alkyl sulfate complexes. Different liquid crystalline-like phases were observed, which depended upon alkyl chain length, pH, molecular weight, and architecture. Maszaros et al.29 reported the observation of complex formation of PEI/SDS, which was attributed to noncooperative binding. Winnik et al.30,31 investigated the formation of polymer/surfactant complexes between branched PEI and SDS using fluorescence labeling studies and microcalorimetry measurements. They identified both monomeric and micellar binding of SDS to PEI and provided evidence for both electrostatic and hydrophobic interactions. Li et al.32,33 also reported monomer and micelle binding of SDS to PEI and ethoxylated PEI, with a strong affinity of the SDS for PEI at high and low pH. We have previously reported the adsorption of an ethoxylated PEI/SDS mixture at the air-solution interface by surface tension and neutron reflectivity,34 where the surface tension and adsorption were shown to be strongly dependent upon the solution pH and the behavior was largely dominated by the ethoxylation of PEI. Davies et al.35,36 have investigated the adsorption of SDS and PEI mixtures at the hydrophobic solid surface by sum frequency spectroscopy. From a higher degree of SDS ordering at the interface they inferred that there was a strong enhancement of the adsorption of SDS on addition of PEI. The effect was most pronounced at low pH, presumably because of the protonation of the PEI amino (26) Glinal, K.; Moussa, A.; Jonas, A. M.; Laschewsky, A. Langmuir 2002, 18, 1408. Voigt, U.; Khrenov, V.; Tauer, K.; Haln, M.; Jaeger, W.; Klitzing, R V. J. Phys.: Condens. Matter 2003, 15, 5213. (27) Creeth, A. M.; Cummins, P. G.; Staples, E. J.; Thompson, L.; Tucker, I.; Penfold, J.; Thomas, R. K.; Warren, N. Faraday Discuss. 1997, 104, 245. (28) Zhou, S.; Burger, C.; Chu, B. J. Phys. Chem. B 2004, 108, 10819. (29) Meszaros, R.; Thompson, L.; Bosi, M.; Vargo, I.; Gilanyi, T. Langmuir 2003, 19, 609. (30) Winnik, M. A.; Bystryak, S. M.; Siddiqui, J. Macromolecules 1999, 32, 624. (31) Winnik, M. A.; Bystryak, S. M.; Chassenieux, C.; Strashko, V.; Macdonald, P. M.; Siddiqui, J. Langmuir 2000, 16, 4495. (32) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Warr, J.; Penfold, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 3093. (33) Li, Y.; Ghoreshi, S. M.; Warr, J.; Bloor, D. M.; Penfold, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2001, 17, 5657. (34) Penfold, J.; Taylor, D. J. F.; Thomas, R. K.; Tucker, I.; Thompson, L. J. Langmuir 2003, 19, 7740. (35) Windsor, R.; Neivandt, D. J.; Davies, P. B. Langmuir 2001, 17, 7366. (36) Windsor, R.; Neivandt, D. J.; Davies, P. B. Langmuir 2002, 18, 2199.
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groups. In contrast, Meszaros et al.37 used optical reflectometry to study the adsorption of PEI and PEI/SDS mixtures at the hydrophilic silica-solution interface. The increased PEI adsorption was attributed to a strong nonelectrostatic affinity, and the SDS/PEI adsorption was associated with the adsorption of SDS/PEI bulk complexes. Much of the focus of recent publications on PEI has been on its complexation with oppositely charged polyelectrolytes, such as in the potential pharmaceutical applications of complexes with DNA,38 in the formation of multilayers on solid surfaces with other polyelectrolytes and surfactants,39-41 and in meso-structured film growth.42 Here we report neutron reflectivity and surface tension measurements that investigate the effects of pH and polymer architecture on the adsorption behavior at the air-solution interface of the polymer-surfactant mixture of PEI/ SDS. Measurements were made for both linear and branched PEI, for different polymer molecular weights, at different solution pHs in the range 3.0-10.0, and for a wide range of SDS concentrations (from ∼10-6 to ∼0.1 M). Experimental Section (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 an error of 0.05 mN/m, and short equilibration times were observed. The surface tension readings were corrected using the Harkins and Jordan method.43 (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.44 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,45 it is just related to the square of the Fourier transform of the scattering length density profile, F(z),
R(Q) )
|∫
16π2 Q2
F(z)e-iQz dz
|
2
(1)
where F(z) ) ∑ini(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). This has now been powerfully demonstrated for the study of surfactant adsorption (for the determination of adsorbed amounts and surface structure) in a wide range of surfactants, surfactant mixtures, and polymer-surfactant mixtures.46 (37) Meszaros, R.; Thompson, L.; Vargo, I.; Gilany, I. Langmuir 2003, 19, 9977; Langmuir 2004, 20, 5026. (38) Choi, J. H.; Choi, J. S.; Suh, H.; Park, J. S. Bull. Korean Chem. Soc. 2001, 22, 46. (39) El Khont, R. J.; Johal, M. S. Langmuir 2003, 19, 4880. (40) Johal, M. S.; Ozer, B. H.; Carson, J. L.; St. John, A.; Robinson, J. M.; Wang, H. L. Langmuir 2004, 20, 2792. (41) Halthur, T. J.; Elosfsson, U. M. Langmuir 2004, 20, 1793. (42) Edler, K. J.; Goldar, A.; Brennan, T.; Roser, S. J. Chem. Commun. 2004, 14, 1724. (43) Harkins, W. D.; Jordan, H. F. J. Am. Chem. Soc. 1930, 52, 1751. (44) Thomas, R. K.; Penfold, J. J. Phys.: Condens. Matter 1990, 2, 1369. (45) Lekner, J. Theory of reflection; Martinus Nijhoff: Dordrecht, 1987. (46) Lu, J. R.; Thomas, R. K.; Penfold, J. Adv. Colloid Interface Sci. 2000, 84, 143.
The specular neutron reflectivity measurements were made on the SURF reflectometer47 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 > 0.25 Å-1) is dominated by the 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 provided 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.48 The reflectivity data presented in the paper is plotted from 0.048 to 0.3 Å-1, with the background subtracted. For a deuterated surfactant in null reflecting water, nrw (92 mol % H2O/8 mol % D2O with a scattering length of zero, the same as 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.49 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 given by
A)
∑b /Fτ i
(2)
i
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 Å2.50 The simplest model, a single layer of homogeneous thickness, describes most of the reflectivity data with sufficient accuracy to obtain adsorbed amounts. The data are not measured 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, can be sensitive to the choice of model, the adsorbed amount depends on the product Fτ and so is largely independent of the detailed model.50 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 not synthesized the deuterium-labeled polymer and do not obtain a direct 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 (47) 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.; McLure, I. A.; Hillman, A. R.; Richards, R. W.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899. (48) Lee, E. M.; Thomas, R. K.; Penfold, J.; Ward, R. C. J. Phys. Chem. 1989, 93, 381. (49) Penfold, J. In Neutron, x-ray and light scattering; Lindner, P., Zemb, T., Eds.; Elsevier: New York, 1991. (50) 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.
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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. For many regions of the adsorption conditions explored (surfactant, polymer concentration, solution pH) the surface adsorbed layer is well-described as a thin monolayer, and eqs 2 and 3 can be applied to an analysis in terms of a single layer of uniform composition. However, there are regions where the adsorption is stronger, and the structure of the adsorbed layer is more complex, and within the Q range measured a simple uniform layer is not an adequate description. In this case the simple single layer model is extended to two or more layers, and the pragmatic choice of the simplest model required to describe both complementary data sets (d-SDS/PEI/nrw, d-SDS/PEI/D2O) simultaneously is applied. Indeed, for all the neutron reflectivity data the leastsquares model fitting, in which both data sets are analyzed simultaneously, is applied,51 and the layer thicknesses are constrained to be identical for the two different “contrast” data. (iii) Materials and Measurements Made. The protonated SDS, h-SDS, was obtained from BDH. The deuterated SDS (dSDS, CD3(CD2)11SO4Na) was synthesized and purified by methods previously described.52 The chemical purity of the surfactants was assessed by surface tension. The branched PEI, molecular weights of 2K and 25K, was obtained from Aldrich, and the linear PEI, molecular weight of 25K, was obtained from PolySciences; the polymers were used as supplied. Deuterium oxide (D2O) was supplied by Aldrich, and high-purity water (Elga Ultrapure) was used throughout. The glassware and poly(tetrafluoroethylene), Teflon, troughs used for the neutron measurements were cleaned using alkaline detergent (Decon 90), followed by copious washing in high-purity water. The neutron reflectivity and surface tension measurements were made at 25 °C, for a polymer concentration of 20 ppm and at pHs 3, 7, and 10. The solution pH was adjusted by the addition of either HCl or NaOH. The measurements were made in the SDS concentration range ∼10-6 to 0.1 M. The surface tension measurements were made for h-SDS in H2O, and limited measurements in D2O and with d-SDS showed no significant isotope effects on the surface tension data. The neutron reflectivity measurements were made for d-SDS/PEI/nrw and d-SDS/PEI/D2O.
Results and Discussion (i) 25K Molecular Weight Linear PEI. The surface tension data for 20 ppm 25K molecular weight linear PEI and SDS, in the SDS concentration range 2 × 10-6 to 3 × 10-2 M and at solution pHs of 3.0, 7.0, and 10.0, are shown in Figure 1. The surface tension shows a marked dependence on the solution pH and is broadly similar to data obtained for both CnTAB/PSS14-16 and polyDMDAAC/SDS.21,22 For pHs 7 and 10 the surface tension at low surfactant concentrations decreases rapidly with increasingly surfactant concentration to a plateau value which is comparable to that obtained for polyDMDAAC/SDS,21 but at a lower surface tension than that observed for CnTAB/PSS.14-16 At pH 10 there is a modest increase in the surface tension at the higher SDS concentrations, ∼5 × 10-3 M, before an eventual decrease. This is very similar to the behavior observed for CnTAB/PSS.16 The surface tension data at pH 7.0 are reminiscent of those obtained for polyDMDAAC/ SDS,21 where following the initial decrease in the surface tension at low surfactant concentrations, a sharp increase which gives rise to a peak in the surface tension is observed (51) Brown, A. S. Langmuir 1998, 14, 5532. (52) Lu, J. R.; Morrocco, A.; Su, T. J.; Thomas, R. K.; Penfold, J. J. Colloid Interface Sci. 1993, 158, 303.
Figure 1. Surface tension as a function of SDS concentration for SDS/25K molecular weight linear PEI (20 ppm): (O) pH 3.0, (b) pH 7.0, and (4) pH 10.0.
at concentrations between 10-3 and 10-2 M. The surface tension behavior at the lowest pH, pH 3.0, is broadly similar to that at pH 7, except that at intermediate concentrations the plateau region of low surface tension is hardly present. The patterns of surface tension observed are markedly different from the classical behavior of surface tension for nonionic polymer/ionic surfactant mixtures, with well-defined discontinuities at T1 and T2, as described originally by Jones.4 Neutron reflectivity measurements at the air-solution interface have been made for the linear 25K molecular weight PEI (20 ppm)/SDS mixture, in a similar SDS concentration range and at the same pH values as the surface tension. Measurements were made for the isotopic combination d-SDS/PEI/nrw and for a more limited range of SDS concentrations for d-SDS/PEI/D2O. Figure 2 shows the reflectivity for a subset of the data at pH 7 and for SDS concentrations of 3 × 10-6, 7 × 10-5, and 3 × 10-2 M. The reflectivity data for the isotopic combination d-SDS/ PEI/nrw provide a direct measure of the amount of SDS and its distribution (thickness of the interfacial layer) at the interface because the PEI and solvent are matched to the air phase (see Figure 2a). In contrast, the reflectivity data for the isotopic combination d-SDS/PEI/D2O (see Figure 2b) provide a measure of the amount of PEI at the interface. In this case the SDS and solvent are matched and give sufficient contrast that the PEI becomes visible. Over the entire range of SDS concentrations measured the reflectivity data can be described adequately as a single layer of uniform composition, for which we can obtain a thickness, τ, and a scattering length density, F. For the contrast d-SDS/PEI/nrw this provides a direct estimate of the amount of surfactant at the interface (using eq 2). Where both contrasts were measured (d-SDS/PEI/nrw and d-SDS/PEI/D2O) the data were modeled simultaneously with the thickness constrained to be the same for both contrasts, and eqs 2 and 3 provide an estimate of the amount of surfactant and polymer in the surface layer. The solid lines in Figure 2a,b are model fits using that approach, and the key parameters are summarized in Table 1. In the Q range of Figure 2a, the slope of the reflectivity curve is related to the thickness of the adsorbed layer and the magnitude of the reflectivity to the adsorbed amount. Hence, it is clear for the data presented in Figure 2a that the adsorbed layer thickness is more or less constant and that there is a greater amount of SDS at the interface at SDS concentrations of 7 × 10-5 and 3 × 10-2 M than at
PEI/SDS Adsorption at the Air-Solution Interface
Langmuir, Vol. 21, No. 22, 2005 10065 Table 1. Model Parameters for 25K Molecular Weight Linear PEI/SDS SDS concn (M)
Figure 2. Neutron reflectivity for SDS/25K molecular weight linear PEI (20 ppm) at pH 7.0: (a) d-SDS/PEI/nrw, (b) d-SDS/ PEI/D2O, (O) 3 × 10-6 M SDS, (b) 7 × 10-5 M SDS, and (4) 3 × 10-2 M SDS.
3 × 10-6 M. For the data in Figure 2b, with the same isotopic combination of polymer and surfactant but replacing nrw with D2O, the situation is somewhat different. The greater the departure of the reflectivity from that for pure D2O, the more PEI there is at the interface. Hence, there is less PEI at the interface for 3 × 10-2 M SDS than at the lower SDS concentrations (3 × 10-6 and 7 × 10-5 M). The variation in the adsorbed layer thickness and the amount of SDS and PEI at the interface as a function of SDS concentration are shown in Figure 3a-c. The thickness of the adsorbed layer is ∼20 Å and is broadly independent of pH and surfactant concentration. This is similar to what has been observed in other related systems, for example, SDS/polyDMDAAC.21 The thickness is also similar to that expected from a C12 alkyl chain surfactant alone.46 This implies that the surfactant is adsorbed at the interface as monomer and not micelles and that the polymer is in a relatively extended conformation within the surface layer. The amount of SDS at the interface is also broadly independent of SDS concentration and solution pH, except at the lowest SDS concentrations (10-3 M) the amount of polymer at the interface decreases with increasing surfactant concentration, similar to that observed in SDS/ polyDMDAAC.21,22 The evaluation of the reflectivity data in terms of the thickness and composition of the adsorbed layer indicates the adsorption at low surfactant concentrations of polymer/surfactant complexes at the interface. These are partially displaced at higher surfactant concentrations by SDS. In marked contrast to the surface tension behavior, the adsorption is not strongly dependent upon the solution pH. (ii) 25K Molecular Weight Branched PEI. The surface tension and neutron reflectivity were also measured for a branched PEI to look for differences with the linear PEI described in the previous section. Measurements were made for 20 ppm 25K molecular weight branched PEI and SDS in the SDS concentration range of 2 × 10-6 to 5 × 10-3 M and at solution pHs of 3.0, 7.0, and 10.0. The surface tension data are shown in Figure 4, and as was observed for the linear PEI, the branched PEI/SDS surface tension shows a marked dependence on solution pH.
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Figure 4. Surface tension as a function of SDS concentration for SDS/25K molecular weight branched PEI (20 ppm): (O) pH 3.0, (b) pH 7.0, and (4) pH 10.0.
Figure 3. SDS/25K molecular weight linear PEI (20 ppm): (O) pH 3.0, (b) pH 7.0, and (4) pH 10.0; (a) layer thickness, (b) SDS adsorbed amount, and (c) adsorbed amount of PEI.
At pH 3 the surface tension variation with SDS concentration is broadly similar to that expected classically from a nonionic polymer/ionic surfactant mixture.4,13 At low SDS concentrations the surface tension decreases, and at higher concentrations the surface tension has two distinct break points at ∼10-3 M and ∼10-2 M SDS. In nonionic polymer/ionic surfactant mixtures these would be attributed to T1 and T2.4,5,13 There is a marked difference
between the surface tension behavior at pH 3 and that observed for pHs 7 and 10. At pHs 7 and 10 the initial sharp decrease in surface tension occurs at lower surfactant concentrations and is followed by a plateau region with a surface tension value similar to that observed for the linear PEI in Figure 1 and a small peak with increasing surfactant concentration. The form is similar to that reported for C14TAB/PSS.16 The neutron reflectivity measurements at the airsolution interface have been measured for the 25K molecular weight branched PEI (20 ppm)/SDS mixture in a SDS concentration range similar to that used for the surface tension measurements and at pHs 3, 7, and 10. Measurements have been made for the isotopic combinations of d-SDS/PEI/nrw and d-SDS/PEI/D2O. Figure 5 shows the reflectivity data for both isotopic combinations at pH 10 and surfactant concentrations of 5 × 10-3, 7 × 10-4, and 3 × 10-4 M. The data in Figure 5 are different to the equivalent data shown in Figure 2 for the linear PEI and indicate a more complex surface structure. The reflectivity for the highest SDS concentration (5 × 10-3 M) is consistent with a simple monolayer ∼18 Å thick, whereas the data for 3 × 10-4 and 7 × 10-4 M are consistent with a more complex surface structure. Indeed the reflectivity for the lowest concentration in Figure 5 (3 × 10-4 M) shows a pronounced interference fringe, consistent with a thicker layer, ∼60 Å. At surfactant concentrations where the surface layer is no longer a simple thin monolayer, a thick layer of uniform composition is also not an adequate description. This was demonstrated for CnTAB/PSS,15 and the structure of the surface layer in this work is broadly similar to that reported for CnTAB/PSS. The reflectivity was analyzed by a simultaneous least-squares fit to the two different contrast data, d-SDS/PEI/nrw and d-SDS/PEI/ D2O. The minimum number of layers to provide a consistent model, in which the thicknesses were constrained to be the same for each contrast, was used. The key model parameters are listed in Table 2, for the measurements at pHs 3, 7, and 10. The most pronounced variation with surfactant concentration was observed for pH 10, and this is shown plotted in Figure 6 as scattering length density versus distance from the interface, z, for six different SDS concentrations, from 5 × 10-3 to 2 × 10-4 M. At the highest and lowest concentrations (5 × 10-3 and 2 × 10-6 M) the data are consistent with a single
PEI/SDS Adsorption at the Air-Solution Interface
Langmuir, Vol. 21, No. 22, 2005 10067 Table 2. Model Parameters for 25K Molecular Weight Branched PEI/SDS SDS concn (M) 10-4 3 × 10-4 7 × 10-4 10-3 5 × 10-3 10-5 3 × 10-5 7 × 10-5 10-4 3 × 10-4
7 × 10-4 10-3 5 × 10-3 2 × 10-6 5 × 10-6 10-5 3 × 10-5
Figure 5. Neutron reflectivity for SDS/25K molecular weight branched PEI (20 ppm) at pH 10.0: (a) d-SDS/PEI/nrw, (b) d-SDS/PEI/D2O, (O) 5 × 10-3 M SDS, (b) 7 × 10-4 M SDS, and (4) 3 × 10-4 M SDS.
monolayer. With decreasing and increasing SDS concentration from those extremes it requires two, three, and eventually four layers to fully describe the data. In general the fourth layer corresponds to a very dilute or weak layer. A similar trend is observed at pH 7, except that the regions where a single monolayer is an adequate description cover a wider range of SDS concentrations. In contrast, at pH 3 a simple single monolayer describes the data over most of the SDS concentration range. The variation in the total layer thickness with surfactant concentration is plotted in Figure 7a for the different solution pH values (for the pH and concentration combinations where a very dilute fourth layer is included in the model; it is excluded from the estimates of thickness in Figure 7a and from the estimates of the adsorbed amount in Figure 7b,c). For the isotopic combination d-SDS/PEI/nrw the adsorbed amount in each layer can be estimated from eq 2, and this can be expressed as a surfactant volume fraction, φs, where φs ) F/Fs and Fs is 6.9 × 10-6 Å-2, the scattering length density of the deuterium-labeled surfactant molecule. From the data for the isotopic combination of d-SDS/ PEI/D2O the volume fraction of solvent, φw, in the layer can be estimated from φw ) (F - φsFs)/Fw, where Fw is the scattering length density of D2O, 6.35 × 10-6, and the polymer volume fraction, φp, is, hence, 1.0 - φs - φw. The adsorbed amounts of surfactant, presented in Figure 7b, and the polymer volume fractions in Figure 7c are the
7 × 10-5
10-4
3 × 10-4
7 × 10-4 10-3 5 × 10-3
τ (Å)
F (×10-6 Å-2)
ASDS (Å2)
in nrw
in D2O
18 21 17 12 50 18 17
3.2 3.5 3.8 0.8 -0.2 3.2 3.6
(i) pH 3.0 5.3 47 5.4 37 5.6 43 6.4 280 6.2 5.8 49 5.8 44
21 28 21 36 25 43 26 31 27 16 16 16 20 31 36 20 18
2.8 -0.2 3.1 -0.02 3.3 0.12 3.1 1.1 4.0 1.4 3.0 -0.01 4.0 0.3 -0.14 3.7 3.8
(ii) pH 7.0 4.8 46 6.0 4.8 42 6.1 5.1 33 6.1 530 5.1 34 6.1 82 5.2 25 6.2 178 5.8 57 6.0 5.2 34 6.2 296 6.2 5.4 37 5.6 39
24 31 17 12 36 28 10 18 18 7 32 27 25 15 14 22 21 7 30 21 30 13 14 18 21 33 33 20 30 18
2.45 -0.08 2.27 0.91 0.16 3.02 -0.06 1.47 3.4 0.2 0.7 -0.01 3.2 0.7 1.9 -0.08 3.1 0.5 1.5 -0.06 3.9 1.8 3.5 0.2 4.3 1.0 -0.09 3.6 0.01 3.7
(iii) pH 10.0 4.6 47 6.04 4.9 56 5.4 241 6.2 480 5.0 32 6.4 6.0 102 4.8 44 5.0 5.9 127 6.2 5.0 34 6.0 205 6.0 100 6.1 5.1 41 5.3 6.0 64 6.1 5.1 24 6.2 121 5.7 56 6.1 5.3 30 6.1 80 6.2 5.3 38 6.1 5.7 41
φs 0.46 0.51 0.54 0.11 0.46 0.52 0.41 0.49 0.48 0.02 0.45 0.16 0.58 0.2 0.43 0.58 0.04 0.53 0.55 0.36 0.42 0.13 0.02 0.44 0.21 0.49 0.14 0.1 0.46 0.1 0.28 0.2 0.46 0.07 0.21 0.56 0.26 0.51 0.63 0.15 0.53 0.01 0.54
φw
φp
0.34 0.29 0.28 0.88 0.98 0.41 0.35
0.20 0.20 0.18 0.01 0.02 0.13 0.13
0.32 0.95 0.27 0.96 0.31 0.94 0.340.31 0.8 0.18 0.75 0.45 0.95 0.18 0.92 0.99 0.28 0.29
0.27 0.05 0.27 0.04 0.21 0.04 0.24 0.04 0.24 0.05 0.12 0.05 0.24 0.04 0.01 0.19 0.16
0.34 0.95 0.33 0.71 0.95 0.31 1.0 0.71 0.22 0.76 0.83 0.98 0.28 0.84 0.65 0.96 0.31 0.75 0.71 0.97 0.24 0.69 0.34 0.96 0.16 0.8 0.97 0.26 0.96 0.32
0.3 0.05 0.25 0.16 0.03 0.25 0.08 0.29 0.10 0.07 0.02 0.26 0.06 0.07 0.04 0.23 0.18 0.07 0.03 0.2 0.05 0.15 0.04 0.21 0.05 0.03 0.21 0.03 0.14
values summed over each of the layers in the model of the surface structure (except the final dilute layer as discussed earlier). The results show pronounced surface layering at the higher pH values of 7 and 10, and this layering also corresponds to a strong surface adsorption at the interface. The results presented in Figure 7c for the variation in the polymer volume fraction with SDS concentration are somewhat different from that observed for the linear PEI/ SDS and are now strongly pH-dependent. At pH 3 the variation is similar to that in Figure 3c for the linear PEI/SDS mixture. For pHs 7 and 10 the polymer volume fraction increases from ∼20% at low SDS concentrations to a maximum ∼35% (where multilayering occurs), and
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Penfold et al.
Figure 6. Scattering length density profiles for SDS/25K molecular weight branched PEI (20 ppm) at pH 10 for model fitting (see text for description), for SDS concentrations in the range 2 × 10-6 to 5 × 10-3 M (from top to bottom and left to right 5 × 10-3, 7 × 10-4, 3 × 10-4, 1 × 10-5, 5 × 10-6, and 3 × 10-6 M): (solid line) d-SDS/PEI/nrw and (dotted line) d-SDS/PEI/D2O.
at higher SDS concentrations it falls to a value ∼10%, similar to that obtained for the linear PEI/SDS and for SDS/polyDMDAAC.21 (iii) 2K Molecular Weight Branched PEI. The surface tension and neutron reflectivity were measured for another branched PEI of lower molecular weight, (2K compared to the 25K in the previous section). Measurements were made for 20 ppm 2K molecular weight branched PEI and SDS, in the SDS concentration range 10-5 to 2 × 10-2 M, and at solution pHs of 3, 7, and 10.0. The surface tension data are shown in Figure 8, and in contrast to the surface tension data for the 25K molecular weight linear and branched PEI/SDS mixtures there is no marked variation with solution pH. The surface tension data for pHs 3 and 7 are almost identical, showing a sharp decrease at low surfactant concentrations (
