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Langmuir 2007, 23, 3690-3698
The Impact of Electrolyte on the Adsorption of Sodium Dodecyl Sulfate/Polyethyleneimine Complexes at the Air-Solution Interface J. Penfold,*,† I. Tucker,‡ R. K. Thomas,§ D. J. F. Taylor,§ J. Zhang,§ and X. L. Zhang§ ISIS, Rutherford Appleton Laboratory, Chilton, Didcot, OXON, OX11 0QX, U.K., UnileVer Research and DeVelopment Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral, CH63 3JW, U.K., and Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford, OX1 2JB, U.K. ReceiVed October 13, 2006. In Final Form: December 8, 2006 The addition of electrolyte (0.1 M NaCl) is shown to have a significant impact upon the surfactant concentration and solution pH dependence of the adsorption of sodium dodecyl sulfate (SDS)/polyethyleneimine (PEI) complexes at the air-solution interface. Substantial adsorption is observed over a wide surfactant concentration range (from 10-6 to 10-2 M), and over much of that range of concentrations the adsorption is characterized by the formation of surface multilayers. The surface multilayer formation is most pronounced at high pH and for PEI with a lower molecular weight of 2K, compared to the higher molecular weight of 25K. These results, obtained from a combination of neutron reflectivity and surface tension, highlight the substantial enhancement in surfactant adsorption achieved by the addition of a combination of the polyelectrolyte, PEI, and a simple electrolyte. Furthermore the effect of electrolyte on the pH dependence of the adsorption further highlights the importance of the hydrophobic interaction in surface surfactant/ polyelectrolyte complex formation.
Introduction Polymer-surfactant mixtures are extensively used in a wide range of industrial, technological and domestic applications to modify and manipulate both solution and surface properties.1 The properties of solution polymer-surfactant complexes have been extensively studied and are relatively well understood.2 However, increasingly in the applications of such mixtures it is the surface behavior that is of central importance, and this is relatively less understood. Furthermore, as pointed out by Goddard,3 this is particularly true for polyelectrolyte/ionic surfactant (of opposite charge) mixtures where a strong surface interaction results in a more complex surface behavior. In such cases, the classical polymer-surfactant surface tension behavior, applicable to non-ionic polymer/ionic surfactant mixtures and described by Jones4 with its associated critical aggregation concentration (cac; onset of polymer bound micellization) and critical micellar concentration (cmc; formation of free surfactant micelles), is no longer observed. Recently, using a combination of surface tension and neutron reflectivity,5 we have made considerable progress in developing an understanding of the surface behavior of such strongly interacting polymer/surfactant mixtures. Neutron reflectivity has proved to be particularly informative, as it provides direct information on the amount adsorbed and on the structure and composition of the adsorbed surface layer. Two particular and different polymer-surfactant mixtures, poly(sodium 4-styrenesulfonate) (PSS)/alkyltrimethylammonium bromides (CnTAB),6-8 * Corresponding author. † Rutherford Appleton Laboratory. ‡ Unilever Research and Development Laboratory. § Oxford University. (1) Interaction of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Goddard, E. D. Colloids Surf. 1986, 19, 301. (3) Goddard, E. D. J. Colloid Interface Sci. 2002, 256, 228. (4) Jones, M. N. J. Colloid Interface Sci. 1967, 72, 36. (5) Creeth, A.; Staples, E.; Thompson, L.; Tucker, I.; Penfold, J. J. Chem. Soc., Faraday Trans. 1996, 92, 589. (6) Taylor, D. J. F.; Thomas, R. K.; Hines, J. D.; Humphreys, K.; Penfold, J. Langmuir 2002, 18, 9783.
and poly(dimethyl diallyl ammonium chloride) (polyDMDAAC)/ sodium dodecyl sulfate (SDS)9,10 have been extensively studied, and represent the two extremes of a range of surface behaviors encountered. PSS/CnTAB mixtures6-8 are characterized by a surface tension behavior that exhibits a broad plateau between an initial decrease in the surface tension at low surfactant concentrations and an ultimate decrease at the cmc. In that plateau region, the surface structure shows a transition from a monolayer to a multilayer structure. In contrast, the surface tension for SDS/ polyDMDAAC9,10 shows a peak in the surface tension at intermediate surfactant concentrations, and this is associated with a partial depletion of the polymer/surfactant complex from the surface monolayer that exists. We have been able to explain and rationalize these different surface tension and adsorption behaviors as arising from the competition between the formation of surface polymer/surfactant monomer complexes and solution polymer/surfactant micelle complexes. For CnTAB/PSS, the surface complexes are more energetically favorable, whereas, for SDS/polyDMDAAC, the free energy of formation of both types of complex is much closer. Bell et al.11 have now developed a thermodynamic model that incorporates these basic ideas and has been shown to encapsulate the principal experimental features that are observed.12,13 The existence of a strong electrostatic interaction between the polymer and surfactant raises the interesting possibility of developing strategies to adjust the strength of that interaction and hence manipulate the adsorption behavior. Such approaches (7) Taylor, D. J. F.; Thomas, R. K.; Penfold, J. Langmuir 2003, 19, 3712. (8) Taylor, D. J. F.; Thomas, R. K.; Li, P. X.; Penfold, J. Langmuir 2003, 19, 3717. (9) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K.; Taylor, D. J. F. Langmuir 2002, 18, 5147. (10) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K. Langmuir 2002, 18, 5139. (11) Bell, C.; Breward, C.; Howell, P.; Penfold, J.; Thomas, R. K. Langmuir, submitted for publication, 2007. (12) Bell, C.; Breward, C.; Howell, P.; Penfold, J.; Thomas, R. K. To be submitted for publication. (13) Penfold, J.; Tucker, I.; Thomas, R. K.; Taylor, D. J. F.; Zhang, X. L.; Bell, C.; Breward, C.; Howell, P. Langmuir 2006, 22, 7617.
10.1021/la063017p CCC: $37.00 © 2007 American Chemical Society Published on Web 02/13/2007
Electrolyte Effect on the Adsorption of SDS/PEI
have included the addition of electrolyte,6 the use of block or random copolymers,14,15 and the addition of a non-ionic surfactant.10 Alternatively, for a range of polyelectrolytes, such as poly(acrylic acid) (PAA), polyethyleneimine (PEI), and polylysine (PLL), pH can be used to manipulate the charge density on the polymer. We have now used this approach to study C12TAB/ PAA,16 SDS/PLL,17 and SDS/PEI18 mixtures, and especially for SDS/PEI this has produced a particularly rich and unexpectedly complex pattern of behavior. Strong surface adsorption, and surface PEI/SDS complexation is observed down to very low SDS concentrations (∼10-6 M). Furthermore, the adsorption is strongly dependent upon polymer architecture and pH. For the linear polymer, monolayer adsorption is observed, whereas, for the branched PEI over extensive regions of concentration, multilayer adsorption is observed. However, what is most remarkable is that the strongest surface effects are observed at high pH, when the charge density on the polymer is lowest and the polymer is essentially non-ionic in nature. Recent studies on SDS/C12E6/PEI mixtures19 confirm this, and show that the mixed surfactant adsorption is dominated by the SDS/PEI interaction, even at high pH. This suggests that it is the nonelectrostatic nature of the SDS/PEI interaction and not the electrostatic interaction that is dominant, and this has also been implied from a range of other studies.20-26 However, many of the associated details are still relatively uncertain. Recent studies, using a range of alkyl sulfates (with alkyl chain lengths of C10 to C14), in combination with polyDMDAAC,13 have shown how electrolyte not only screens the charge interaction between polyelectrolyte and surfactant, but changes the relative values of the surface cac and the solution cac and cmc. Hence, in order to further understand this unusual pH dependence of the PEI/SDS surface adsorption and how electrolyte can be additionally used to manipulate those surface properties, we have used surface tension and neutron reflectivity to investigate the adsorption of PEI/SDS mixtures at the airsolution interface in the presence of NaCl. Experimental Details (i) Surface Tension. Surface tension measurements were made on a Kruss K10T maximum pull digital tensiometer using the du Nouy ring method with a Pt/Ir ring. Before each measurement, the ring was rinsed in high-purity (Elga Ultrapure) water and flamed with a Bunsen burner, and repeated measurements were made until equilibrium was established. (ii) Neutron Reflectivity. Specular neutron reflectivity measurements were made on a SURF reflectometer27 using neutron (14) Voight, U.; Khrenov, V.; Tauer, K.; Haln, M.; Jaeger, W.; von Klitzing, R. J. Phys.: Condens. Matter 2003, 18, 5213. (15) Steitz, R.; Jaeger, W.; Klitzing, R. V. Langmuir 2001, 17, 4471. (16) Zhang, J.; Thomas, R. K.; Penfold, J. Soft Matter 2005, 1, 310. (17) Penfold, J.; Tucker, I.; Thomas, R. K.; Taylor, D. J. F.; Zhang, X. L. Langmuir 2006, 22, 7617. (18) Penfold, J.; Tucker, I.; Thomas, R. K.; Zhang, J. Langmuir 2005, 21, 10061. (19) Penfold, J.; Tucker, I.; Thomas, R. K.; Taylor, D. J. F.; Zhang, J.; Bell, C. Langmuir 2006, 22, 8840. (20) Wang, C.; Tam, K. C. J. Phys. Chem. B 2005, 109, 5136. (21) Keifer, J. J.; Somasundaran, P.; Ananthapadmanabham, K. P. Langmuir 1993, 9, 1187. (22) Robb, I. D.; Stevenson, P. Langmuir 2000, 16, 710. (23) Yoshida, K.; Dubin, P. L. Colloids Surf., A 1999, 147, 161. (24) Anghel, D. F.; Winnik, F. M.; Galatanu, N. Colloids Surf., A 1999, 149, 46. (25) Moloney, C.; Huber, K. J. Colloid Interface Sci. 2000, 164, 4495. (26) Winnik, M. A.; Bystryak, S. M.; Chassenieux, C.; Strashko, V.; Macdonald, P. M.; Siddiqui, J. Langmuir 2000, 16, 4495. (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, M. 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.
Langmuir, Vol. 23, No. 7, 2007 3691 wavelengths, λ, in the range of 0.5-6.8 Å to provide a Q 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°, in a range of 0.048-0.5 Å-1, using what are now well-established experimental procedures.27,28 The specular reflection of neutrons provides information about inhomogeneities normal to an interface or surface, and the technique is described in detail elsewhere.28 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, 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-iQzdz|2 Q2
∫
(1)
where F(z) ) Σi ni(z)bi, ni(z) is the number density of the ith nucleus at a distance z from the interface, 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 a hydrogen (H)/deuterium (D) isotopic substitution (where H and D have vastly different scattering length values and signs 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 polymersurfactant mixtures.28 For a deuterated surfactant in null reflecting water (nrw; 92 mol % H2O/8 mol % D2O has 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.29 The parameters obtained for such a model 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 Σbi is the scattering length of the adsorbed surfactant molecule. This has been shown to be an appropriate method for surfactants, surfactant mixtures, and polymer/surfactant mixtures.29 In the evaluation of the adsorbed amounts of 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 (∑b for perdeuterated SDS (d-SDS) is 2.763 × 10-3 Å and that for PEI is 0.29 × 10-4 Å/monomer, and the corresponding molecular volumes are 388 and 68 Å3, respectively). For the neutron reflectivity data reported here, there are regions of surfactant concentration where the adsorbed layer is well described as a thin monolayer of uniform composition (and hence density), and regions where the surface structure is more complex. In these cases, the simplest model consistent with the data is used to describe the surface structure. As previously observed in other related systems,16-18 for the more complex structures, where a pronounced interference fringe is observed, two or three layers are mostly sufficient to describe the data. In some cases, a pronounced (28) Lu, J. R.; Thomas, R. K.; Penfold, J. AdV. Colloid Interface Sci. 2000, 84, 143. (29) Penfold, J. In Neutron, X-ray, and Light Scattering; Lindner, P., Zemb, T., Eds.; Elsevier: New York, 1991.
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Bragg peak, indicative of more extensive multilayer formation at the interface, is observed. Here, a different approach is required to evaluate the surface structure. In the kinematic approximation, and following the approach of Tidswell et al.30 and Sinha et al.,31 the specular reflectivity for such a multilayer at the interface can be written as R(Q) )
16π2 N | (Fi - Fi+1) exp(-iQdi) exp(-Q2σi2/2)|2 (3) Q4 i ) 0
∑
where Fi is the scattering length density of the ith layer, i ) 0 represents the subphase, di is the distance of the interface between the ith and i+1 layers from the subphase, di ) ∑i li, li is the thickness of the ith layer, σi is the roughness between the ith and i+1 layers, F(N+1) is the upper bulk phase (air), and N is the number of layers (N/2 is the number of bilayers). The definition of the bilayer structure with increasing depth is modified by an exponential decay constant (that is, the contrast between the two layers in the bilayer decreases with increasing distance from the surface), such that Fi ) FN - ∆F exp(-di/η)
(4)
and ∆F ) (Fi - Fi+1), and η is a damping coefficient. The contribution from the surface monolayer in equilibrium with the multilayer structure is included in the form R(Q) )
16π2 (2F)2 sin(Qd/2)2 Q4
(5)
where d and F are the thickness and scattering length density of the monolayer, respectively, and the scattering length density of the subphase is assumed to be zero (nrw). Equations 3 and 5 are added together to provide the total reflectivity. (iii) Materials. The d-SDS was synthesized as described elsewhere32 and purified before use by recrystallization from an ethanol/acetone mixture. The branched PEI samples, molecular weight 2K and 25K, were obtained from Aldrich and used as supplied. Deuterium oxide was supplied by Sigma-Aldrich, and high-purity water (Elga Ultrapure) was used throughout. The glassware and Teflon troughs used for the measurements and sample preparation were cleaned in alkali detergent (Decon 90), followed by copious washing in high-purity water. The surface tension and neutron reflectivity measurements were made at 25 °C. The polymer concentration was 20 ppm (by mass), and the surfactant concentration was varied in the range of 10-6-10-2 M. Measurements were made for the isotopic combination of d-SDS/PEI/nrw, at pH 3, 7, and 10. The solution pH was adjusted by the addition of HCl or NaOH.
Results and Discussion (i) SDS/PEI (2K Branched). Figure 1 shows the surface tension for the 2K branched PEI/SDS mixtures at pH 3, 7, and 10, with (Figure 1a) and without (Figure 1b) 0.1 M NaCl (reproduced from ref 18). The addition of 0.1 M NaCl changes the surface tension behavior at all pH values compared to what is observed in the absence of electrolyte. In the absence of electrolyte, the surface tension was rather similar over the pH range of 3-10. In 0.1 M NaCl, although the surface tension at pH 3 and 7 is similar, it is markedly different at pH 10. At all pH values, the addition of electrolyte has shifted the cmc to lower SDS concentrations as expected, indicated by the change in the slope of the surface tension data at ∼1 mM. Furthermore, at pH 3 and 7, the surface (30) Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Wasserman, S. R.; Whitesides, G. M.; Axe, J. D. Phys. ReV. B 1990, 41, 1111. (31) Sinha, S. K.; Sanyal, M. K.; Satija, S. K.; Majkrzak, C. F.; Neumann, D. A.; Homma, H.; Szpala, S.; Gibaud, A.; Morkov, H. Physica B 1994, 198, 72. (32) 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 2K branched PEI/SDS at pH 3, 7, and 10 (a) in 0.1 M NaCl and (b) in the absence of electrolyte.
tension at low SDS concentrations is considerably reduced compared to what is measured in the absence of electrolyte. Between 10-5 M and the cmc (∼10-3 M), there is a broad plateau in the surface tension, similar to that observed in PSS/CnTAB mixtures.6-8 At pH 7, there is evidence of a small peak in the surface tension at ∼2 × 10-4 M SDS. At pH 10, between 10-6 and 10-3 M SDS concentrations, the surface tension is systematically higher and closer to that measured in the absence of electrolyte. At high SDS concentrations (>10-3 M), the surface tension is systematically lower (
