Coating of Anionic Surfactants onto Poly(ethylene) Surfaces Studied

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Langmuir 1998, 14, 3214-3222

Coating of Anionic Surfactants onto Poly(ethylene) Surfaces Studied with X-ray Photoelectron Spectroscopy J. P. Lens, J. G. A. Terlingen, G. H. M. Engbers, and J. Feijen* Department of Chemical Engineering, Section Polymer Technology and Biomaterials and Institute for Biomedical Technology (BMTI), University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Received September 10, 1997. In Final Form: March 16, 1998 Poly(ethylene) (PE) samples were immersed in aqueous solutions of different anionic surfactants, viz. sodium dodecane sulfate (SDS), sodium 10-undecene sulfate (S11(:)), and sodium 10-undecenoate (C11(:)). When the PE samples were removed from the solutions, dried, and analyzed with X-ray photoelectron spectroscopy (XPS), surfactant was detected at the surface only if the PE samples were wetted by the surfactant solution. Wetting was accomplished by pretreating the polymer with an argon plasma for 5 s. In this way, a relatively hydrophilic surface was obtained with advancing and receding water contact angles of 89° and 31°, respectively. In a second approach, 1 vol % of hexanol was added to the surfactant solutions, thereby decreasing the surface tension of the solutions to a value below 31 mN/m, allowing direct coating of PE with the surfactants. XPS was performed to estimate the orientation of the molecules in the surfactant layer at the surface. Surfactant molecules were randomly oriented in a homogeneous layer. The thickness of this layer varied from 10 to 60 Å and increased with increasing surfactant concentration of the immersion solution.

Introduction Polymeric surfaces have been modified to improve their performance in many applications. A wide variety of surface modification techniques are available. Gas discharge techniques are currently extensively used to functionalize polymeric surfaces. Due to the complexity of the reactive phase, it is difficult to control the surface chemistry using these techniques. Previously, a method was developed, called plasma immobilization, by which it is possible to introduce specific functional groups at polymeric surfaces.1 With this method, a thin layer of a compound, generally a surfactant, is coated from an aqueous solution onto the surface of a polymeric substrate. Subsequently the substrate is treated with an inert gas plasma by which part of the precoated surfactant is covalently coupled to the polymeric surface. A substantial part of the functional groups of the surfactant remains present at the surface during this process. A wide variety of functional groups can be introduced at a polymeric surface by choosing the appropriate surfactant. The first step in the plasma immobilization process is to obtain a polymeric substrate with a thin layer of surfactant at its surface. It is important to realize that these samples have to be in a dry state. This means that, although adsorption of surfactants at the solid/liquid interface in solution might occur, the surfactants must also be present when the polymeric substrate is removed from the surfactant solution and dried. The adsorption of surfactants at the solid/liquid interface of hydrophobic surfaces is a thermodynamically driven process and has been extensively described.2-4 The adsorption is dominated by London-van der Waals dis* To whom all correspondence should be addressed. (1) Terlingen, J. G. A.; Feijen, J.; Hoffman, A. S. J. Biomater. Sci., Polym. Ed. 1992, 4, 31. (2) Meyers, D. Surfactant Science and Technology; VCH Publishers: New York, 1988. (3) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley and Sons: New York, 1988. (4) Chen, S. H.; Frank, C. W. Langmuir 1989, 5, 978.

persion force interactions and proceeds according to a Langmuir isotherm. At low concentrations, the surfactant molecules may be oriented parallel to the substrate surface, slightly tilted, or L-shaped. Upon increasing the surfactant concentration, the amount of adsorbed material also increases. The molecules gradually become oriented more perpendicular to the surface with their hydrophilic heads oriented toward the aqueous phase. Finally, surface saturation is attained at or near the critical micelle concentration (cmc) of the surfactant, where the surfactant molecules are closely packed in the form of a monolayer. Much less is known about the state of a surfactant layer at the surface after the polymer has been removed from the surfactant solution. It has been demonstrated that when polymeric samples were dipped in an aqueous surfactant solution and subsequently dried, polymeric samples were obtained containing a thin layer of surfactant at their surface.5,6 Although the process was called adsorption, it is more appropriate to call this process coating in order to discriminate it from real adsorption at a solid/liquid interface. It was not clear which mechanisms were responsible for the retention of the surfactants at the polymeric surfaces nor how the molecules were oriented in the coated surfactant layer. It was assumed that when coating was performed well above the cmc of the surfactants, a monolayer was obtained with surfactant molecules oriented perpendicular to the surface, analogous to adsorption at a solid/liquid interface. However, X-ray photoelectron spectroscopy (XPS) indicated that the surfactant molecules were randomly distributed in a homogeneous layer.5 In this paper, coating of anionic surfactants onto polymeric substrates is further studied using a surfacesensitive technique like XPS. With XPS, it is possible to perform depth-profiling studies in which the composition (5) Terlingen, J. G. A.; Feijen, J.; Hoffman, A. S. J. Colloid Interface Sci. 1993, 155, 55. (6) Terlingen, J. G. A.; Brenneisen, L. M.; Super, H. T. J.; Pijpers, A. P.; Hoffman, A. S.; Feijen, J. J. Biomater. Sci., Polym. Ed. 1993, 4, 165.

S0743-7463(97)01025-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/15/1998

Anionic Surfactants on Poly(ethylene) Surfaces

of thin surface layers is analyzed. The analysis can be obtained either by destructive or nondestructive techniques. Destructive techniques, using for example ion sputtering as a “sectioning” method, are hardly used in the analysis of polymeric samples. The nondestructive profiling methods are based on either the energy or the emission angle dependence of the escape depth of the emitted electrons. The escape depth of electrons of a given kinetic energy (KE) varies between its full value perpendicular to the surface and a minimum at glancing emission. Depth profiling by variation of the emission angle requires a very flat surface. In other cases it is more convenient to use the dependence of the escape depth of the electrons on their KE at a constant overall take-off angle.7 Sodium dodecane sulfate (SDS), sodium 10-undecene sulfate (S11(:)), and sodium 10-undecenoate (C11(:)) were coated onto poly(ethylene) (PE). These particular surfactants are used in further immobilization experiments in order to improve the blood compatibility of polymeric materials.8,9 PE was used as a model substrate because, due to its chemical structure, coating of surfactants can easily be evaluated with XPS. The influence of wetting of the PE surface by the surfactant solutions on the coating process was investigated. Different possibilities for orientation of molecules in coated surfactant layers were evaluated by comparing XPS results with theoretical models. The XPS results were further used to calculate the thickness of coated surfactant layers. Additionally, coating of radiolabeled SDS onto PE was carried out to verify the results of the thickness calculations based on the XPS analyses. Finally, the influence of the surfactant concentration on the thickness of the coated surfactant layers was investigated. Experimental Section Materials. Poly(ethylene) (PE) foil without additives (LDPE, type 2300, thickness 0.2 mm) was obtained from DSM (Geleen, The Netherlands). Sodium 10-undecenoate (C11(:)) was purchased from Sigma Chem. Corp. (St. Louis, MO). Sodium 10undecene sulfate (S11(:)) was synthesized by sulfating 10undecene-1-ol with a pyridine-SO3 complex.9 Radioactive -labeled sodium dodecane sulfate (SD35S) was purchased from Amersham International Plc (Aylesbury, U.K.). All other chemicals were purchased from Merck (Darmstadt, Germany). Solvents were of analytical grade. All chemicals were used as received. Cleaning of Glassware. All glassware was cleaned by rinsing three times with toluene, three times with acetone, three times with deionized water, and finally three times with acetone and then drying in air. Cleaning of PE Samples. PE samples (13 × 25 mm2) were ultrasonically cleaned (10 min, four times in each liquid) successively in dichloromethane, acetone, and deionized water and then dried at room temperature (RT) in vacuo. Determination of the Critical Micelle Concentration (cmc) of Surfactants. The cmc of surfactants was determined by measuring the surface tension and the conductivity of a range of aqueous solutions of different surfactant concentrations. Surface tension was measured with the Wilhelmy plate technique using an annealed platinum plate and a Processor Tensiometer K12, Kru¨ss GmbH (Hamburg, Germany). The conductivity was determined using a PW9526 Digital Conductivity Meter and a platinized electrode with a current source of approximately 25 mA (Philips, Eindhoven, The Netherlands). All measurements were performed at RT. (7) Miller, D. R.; Peppas, N. A. J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1986, C26, 33. (8) Lens, J. P.; Terlingen, J. G. A.; Engbers, G. H. M.; Feijen, J. Langmuir 1997, 13, 7052. (9) Lens, J. P.; Terlingen, J. G. A.; Engbers, G. H. M.; Feijen, J. Polymer, in press.

Langmuir, Vol. 14, No. 12, 1998 3215 Coating of Surfactants onto PE. Clean PE samples were put in test tubes, and an aqueous solution of C11(:), S11(:), or SDS was added. Optionally, the solution contained 1 vol % of hexanol. After 15 min the solution was removed from the test tube and the samples were dried at RT in vacuo. The resulting samples are coded as PE/C11(:), PE/S11(:), and PE/SDS, respectively. Plasma Treatment of PE Samples. Plasma treatment of PE is extensively described elsewhere.8,9 Briefly, PE samples were placed in a glass reactor and evacuated to a pressure of 1 × 10-5 mbar. An argon flow was established through the reactor, and the samples were treated with an argon plasma (5 s, 45 W, 0.07 mbar). Subsequently, the reactor was brought to atmospheric pressure with argon. The samples were removed from the reactor and turned, and the other side of the samples was plasma-treated according to the same procedure. Washing of Plasma-Treated PE Samples. Polymeric samples that were treated with an argon plasma were immersed in an aqueous solution of 0.1 mM HCl for 1 h at RT, rinsed twice with 0.1 mM HCl, and then dried at RT in vacuo. X-ray Photoelectron Spectroscopy (XPS). A Kratos XSAM-800 apparatus equipped with a Mg KR X-ray source (15 kV, 10 mA) was used to analyze the surface composition of the samples. The analyzer was placed perpendicular to the sample surface. Survey scans (0-1100 eV) and detail scans were recorded at an analyzer pass energy of 40 eV (fwhm Ag 3d5/2: 1.2 eV) and an X-ray spot size with a diameter of 3 mm. Survey scans were used to qualitatively determine the elemental composition of the samples. No charge neutralization was applied and the reported values of the binding energies were referenced to the C 1s peak for aliphatic carbon, which was assigned a value of 284.8 eV.10 The relative peak areas for the different elements were calculated by numerical integration of the detail scans, considering empirically derived sensitivity factors. After normalization, an elemental composition in atomic percentages was obtained. All calculations were performed using the multiuser DS 800 software system (Kratos, Manchester, England). Contact Angle Determinations. Advancing and receding water contact angles were determined using the Wilhelmy plate technique with the aid of an Electrobalance, Model RM-2, Cahn/ Ventron (Paramount, CA).11 The interfacial velocity was 4 mm/ min. Adsorption of SD35S onto Polymeric Samples. PE samples were immersed in 5 mL of an aqueous solution of 0.020 or 0.20 M SDS containing 1 mol % of SD35S and 1 vol % of hexanol. The activity of the solutions was (1.31 ( 0.01) × 103 and (8.18 ( 0.03) × 103 cpm/µL, respectively. After 15 min the samples were removed from the SDS solution. The samples were dried in air for 5 min, after which air was carefully blown over the samples in order to remove all of the liquid. When small droplets, which have a very high activity (vide supra), would remain on the surface, erroneous results would be obtained. Circular disks with a diameter of 10 mm were cut from each PE sample. The samples were put in 20 mL of aqualuma, and the activity was determined using a 1219 Rackbeta Liquid Scintillation Counter (LKB, Finland). The activity of the samples was converted to the amount of surfactant coated onto the PE surface and to the thickness of the coated surfactant layer.

Results and Discussion Coating of Surfactants onto PE. The cmc of the studied surfactants was determined by conductivity and surface tension measurements (Table 1). The values obtained with these two methods correspond well with each other and with values reported in the literature.3,12-15 (10) Wagner, C. D. In Practical Surface Analysis. Volume 1: Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; John Wiley & Sons, Ltd: New York, 1990; p 595. (11) Damme, van H.; Hogt, A. H.; Feijen, J. J. Colloid Interface Sci. 1990, 114, 167. (12) Kale, K. M.; Zana, R. J. Colloid Interface Sci. 1977, 61, 312. (13) Woolley, E. M.; Burchfield, T. E. J. Phys. Chem. 1985, 89, 714722. (14) Sprague, E. D.; Duecker, D. C.; Larrabee, C. E. J. Colloid Interface Sci. 1983, 92, 416.

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Table 1. Values of the cmc of Different Surfactants Obtained with Conductivity (cmcΛ) and Surface Tension (cmcγ) Measurements and Values of the Surface Tension of Aqueous Solutions of the Surfactants at Concentrations of Three Times Their cmc (n ) 3, (sd) surfactant C11(:) S11(:) SDS

cmcΛ (mol/L)

cmcγ (mol/L)

10-2

10-2

(9.7 ( 0.2) × (9.4 ( 0.5) × (3.0 ( 0.1) × 10-2 (2.8 ( 0.2) × 10-2 (7.8 ( 0.1) × 10-3 (8.1 ( 0.1) × 10-3

γ3×cmc (mN/m) 35.5 ( 0.2 37.9 ( 2.2 37.8 ( 0.3

Table 2. XPS Analysis of PE/C11(:) and PE/SDS Samples That Were Precoated from Aqueous Solutions of 0.30 M C11(:) and 0.020 M SDS, Respectively (n g 3, (sd) sample

atom % O

atom % Na

atom % S

PE/C11(:) PE/SDS

2.3 ( 1.1 1.3 ( 0.4

0.8 ( 0.2 0.3 ( 0.1

0.15 ( 0.06

Initial coating experiments were performed with aqueous surfactant solutions with concentrations of three times their cmc. If coating would be related to adsorption at the solid/liquid interface in solution, maximal coverage of the surface by surfactant is obtained above the cmc. XPS analysis of coated PE/C11(:) and PE/SDS samples showed small amounts of oxygen, sodium, and sulfur at their surfaces (Table 2). In another study much larger amounts of oxygen, sodium, and sulfur were detected at poly(propylene) (PP) surfaces coated with SDS.5 Thus, the low atomic percentages of oxygen, sodium, and sulfur on the PE/C11(:) and PE/SDS samples indicate that hardly any surfactant is present. It was observed that the PE samples were hardly wetted by the surfactant solutions. The polymeric samples were almost dry after removal from the solutions except for some small droplets at the surface. Very small white spots were observed on the precoated samples after drying. Obviously, the water of the small droplets evaporates upon drying and the surfactants are deposited onto these parts of the surface. At this point it is hypothesized that a successful coating of surfactants onto a polymeric substrate only occurs if the polymeric surface is wetted by the coating solution. Thus, although adsorption at the solid/liquid interface in solution might occur, it will be impossible to obtain dry polymeric samples containing substantial amounts of coated surfactant at their surface if the surfactant solution does not wet the polymeric surface. The extent of wetting or more precisely spreading wetting of a solid substrate by a liquid can be described by the spreading coefficient SL/S which is defined as3,16

SL/S ) γSA - (γSL + γLA)

(1)

where γSA, γSL, and γLA refer to the solid/air, solid/liquid, and liquid/air interfacial tensions, respectively. If SL/S is negative, the liquid will not spread spontaneously on the substrate. Complete wetting occurs if SL/S is positive and thus if γLA e (γSA - γSL). For nonpolar (low-energy) surfaces (γSA - γSL) may be replaced by a critical value γc characteristic of the substrate. The value of γc for PE is about 31 mN/m.3,16,17 Thus the surfactant solutions that were used for the coating experiments did not wet the PE surfaces because the surface tensions of the solutions were larger than the value of γc for PE (Table 1). To improve wetting, there are two possibilities. First, the value of (15) Durairaj, B.; Blum, F. D. Polym. Prepr. 1985, 26, 239. (16) Blake, T. D. In Surfactants; Tadros, T. F., Ed.; Academic Press: London, 1984; p 221. (17) Baszkin, A.; Ter-Minassian-Saraga, L. J. Polym. Sci., Part C 1971, 34, 243.

Table 3. XPS Analysis of Plasma Treated PE (PE-Ar 5) Samples That Were Precoated from an Aqueous Solution of 0.30 M C11(:) (PE-Ar 5/C11(:)) and of PE Samples That Were Precoated from an Aqueous Solution of 0.30 M C11(:) Containing 1 vol % Hexanol (PE/C11(:)-hexOH) (n g 3, (sd) sample

atom % O

atom % Na

PE-Ar 5 PE-Ar 5/C11(:) PE/C11(:)-hexOH

9.0 ( 1.4 13.4 ( 0.2 10.9 ( 1.2

6.2 ( 0.3 6.2 ( 0.4

(γSA - γSL) can be increased, and second the value of the liquid/air interfacial tension γLA (surface tension of the liquid), can be reduced. Both possibilities were investigated. To improve wetting of the PE surface by the aqueous surfactant solutions, the value of (γSA - γSL) can be increased or in other words the hydrophilicity of the polymeric surface has to be increased. This can be accomplished by introducing polar functional groups at the surface by means of for example a gas discharge treatment.18-20 When PE samples are treated with an argon plasma for 5 s (PE-Ar 5 samples), oxygen-containing groups are introduced at the surface of the polymer (Table 3). Due to the presence of oxygen-containing groups, the value of γSA is increased and γSL is decreased.21 This was reflected in relatively low advancing and receding water contact angles of PE-Ar 5 samples (θa ) (89 ( 2)°; θr ) (31 ( 7)°) compared to those of untreated PE (θa ) (109 ( 1)°; θr ) (83 ( 2)°). An aqueous solution of 0.30 M C11(:) wetted the PE-Ar 5 samples. When the samples were removed from the surfactant solution, a liquid film remained at the surface which slowly retracted. No white spots were observed at the surface of the dried samples (PE-Ar 5/C11(:) samples). Substantial amounts of oxygen and sodium were detected at the surface of PE-Ar 5/C11(:) samples (Table 3). The additional shoulder on the high-binding-energy side in the C 1s spectrum of PE-Ar 5/C11(:) samples (288.3 eV) indicates the presence of carbon atoms with an oxidation state typical for carboxylate groups (Figure 1).22,23 Therefore it was concluded that C11(:) was coated onto the PEAr 5 samples. This confirms the hypothesis that coating of surfactants on PE is only possible if the polymeric surface is wetted by the surfactant solution. The second possibility to improve wetting of PE by the surfactant solution is to decrease the surface tension of the solution. Generally, the surface tension of aqueous surfactant solutions decreases with increasing surfactant concentration. As the cmc is reached, the surface tension reaches a plateau. When a nonpolar (hydrophobic) substrate is present in the surfactant solution, the value of γSL is decreased in a similar way.2 Apparently, the effectiveness of the studied surfactants in reducing the values of γLA and γSL is too low to induce spreading wetting of PE. The surface tension of a surfactant solution can be decreased even more when a third component is added. (18) Yasuda, H. Plasma Polymerization; Academic Press: Orlando, FL, 1985. (19) Behnisch, J.; Holla¨nder, A.; Zimmerman, H. J. Appl. Polym. Sci. 1993, 49, 117. (20) Holla¨nder, A.; Behnisch, J.; Zimmerman, H. J. Polym. Sci., Polym. Chem. Ed. 1994, 32, 699. (21) Kuznetsov, A. Y.; Bagryansky, V. A.; Petrov, A. K. J. Appl. Polym. Sci. 1993, 47, 1175. (22) Dennis, A. M.; Howard, R. A.; Kadish, K. M.; Bear, J. L. Inorg. Chim. Acta 1980, 44, L139. (23) Hammond, J. S.; Holubka, J. W.; deVries, J. E.; Dickie, R. A. Corros. Sci. 1981, 21, 239.

Anionic Surfactants on Poly(ethylene) Surfaces

Figure 1. XPS C 1s spectra of PE-Ar 5 and PE-Ar 5/C11(:) samples. The arrow indicates the shoulder caused by the carboxylate groups. The PE-Ar 5/C11(:) samples were obtained by immersing PE-Ar 5 samples in an aqueous solution of 0.30 M C11(:).

Medium-chain alcohols such as butanol, pentanol, and hexanol can be used as cosurfactants. These alcohols are partially soluble in water. When medium-chain alcohols are added to solutions of surfactants with concentrations above the cmc, they are partitioned between the micellar pseudophase and the micellar solution.24-26 At low alcohol concentrations, the alcohol molecules in the micellar pseudophase penetrate the micelles and are solubilized in the micelle palisade layer, which contains the surfactant head groups, water, and counterions. The addition of medium-chain alcohols to surfactant solutions changes properties such as cmc, micelle ionization degree, micelle size and shape, and surface tension of the solutions.27-29 Most changes are due to shielding of the electrostatic repulsions between the charged head groups of the surfactant molecules. Consequently, they pack more densely at a liquid/air or liquid/solid interface, which leads to lower values of γLA and γSL.30 Addition of 1 vol % of hexanol to an aqueous solution of 0.30 M C11(:) decreased the surface tension from 35.5 to 29.2 mN/m. Simultaneously, γSL may be decreased. The resulting solution wetted PE, and when the PE samples were removed from the surfactant solution, again a liquid film remained at the surface which slowly retracted. The dried samples (PE/C11(:)-hexOH) contained substantial amounts of oxygen and sodium at their surfaces (Table 3). The difference with PE/C11(:) samples that were coated from an aqueous solution without hexanol is clear (Table 2). Besides carboxylate-group-containing surfactants, also sulfate-group-containing surfactants were coated onto PE surfaces with this method. XPS analysis of PE/S11(:)hexOH samples showed, besides oxygen and sodium, substantial amounts of sulfur (see below). These results indicate that the method of adding hexanol to the (24) Manabe, M.; Shirahama, K.; Koda, M. Bull. Chem. Soc. Jpn. 1976, 49, 2904. (25) Backlund, S.; Bakken, J.; Blokhus, A. M.; Høiland, H.; Vikholm, I. Acta Chem. Scand. 1986, A40, 241. (26) Zana, R. Adv. Colloid Interface Sci. 1995, 57, 1. (27) Zana, R.; Yiv, S.; Strazielle, C.; Llanos, P. J. Colloid Interface Sci. 1981, 80, 208. (28) Overbeek, J. T. G.; Bruyn, P. L. d.; Verhoeckx, F. In Surfactants; Tadros, T. F., Ed.; Academic Press: London, 1984; p 111. (29) Varela, A. S.; Macho, M. I. S.; Gonza´lez, A. G. Colloid Polym. Sci. 1995, 273, 876. (30) Valiente, M.; Thunig, C.; Munkert, U.; Lenz, U.; Hoffmann, H. J. Colloid Interface Sci. 1993, 160, 39.

Langmuir, Vol. 14, No. 12, 1998 3217

Figure 2. Schematic representation of coating of surfactant onto PE. The amount of coated material will be dependent on the surfactant concentration, the velocity by which the liquid film retracts, and the rate of evaporation of the liquid.

surfactant solution to improve wetting of PE samples and consequently to induce coating of the surfactants onto PE seems to be generally applicable to anionic surfactants. From the examples given above it is clear that, as was hypothesized, surfactant could be coated onto PE if the polymer was wetted by the surfactant solution. At this point it is not yet clear which processes are responsible for the observed phenomena. It seems that it is insufficient to extend the mechanisms of adsorption of surfactant at the PE solid/liquid interface to coating of surfactant onto the polymeric surface. In the case when PE samples are immersed in surfactant solutions with or without hexanol, adsorption at the solid/liquid interface will occur by dispersion interactions between the surface and the hydrophobic tail of the surfactant. Theoretically, the amount of adsorbed material will be somewhat larger when hexanol is present, but this cannot account for the large differences between the atomic percentages of oxygen and sodium in the PE/C11(:) and PE/C11(:)-hexOH samples. When PE samples are pretreated with an argon plasma, the surface becomes polar and adsorption at the solid/liquid interface can also occur through hydrogen bonding and acid-base interactions. However, it seems also unlikely that this promotes coating of C11(:) onto PE. It is likely that, instead of the amount of surfactant adsorbed at the solid/liquid interface, the thin liquid film that remains initially present at the PE surface after removal of the coating solution plays a predominant role (Figure 2). Due to gravity, this liquid film retracts. Simultaneously, water and possibly hexanol evaporate. Consequently, a thin layer of surfactant may be deposited onto the polymeric surface. In this process, the amount of deposited material will mainly be dependent on the surfactant concentration, the velocity by which the liquid film retracts, and the evaporation rate of the liquid instead of being dependent on the amount of surfactant adsorbed at the solid/liquid interface. When this process is actually occurring, it is also not expected that there will be orientation of the surfactant molecules in the deposited layer because this would be a direct consequence of the adsorption at the solid/liquid interface. Furthermore, the cmc is then also not expected to have a substantial influence on the thickness of the coated surfactant layer as a function of the concentration of surfactant in the coating solution.

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Figure 3. Modeled monolayer (A), bilayer (B), and homogeneous overlayer (C) structures of a surfactant on a substrate. The atomic composition of the different layers for SDS coated onto PE is given in parentheses.

In the next sections the orientation of the surfactants (if any) at the PE surface and the influence of the surfactant concentrations on the thickness of the coated surfactant layer are discussed. We choose to add hexanol to the coating solution to obtain PE samples with a thin layer of surfactant at their surfaces after drying because this was less laborious than pretreating the polymeric samples with an argon plasma. Estimation of the Orientation and the Layer Thickness of Coated Surfactant Layers on PE. XPS analysis can be used for (semi) quantitative elucidation of the structure and the thickness of the coated layer by comparison of the XPS results with surface structure models.31 On the basis of the discussion in the previous section, it seems unlikely that there will be any orientation of molecules in the coated surfactant layers. However, if a coated surfactant layer on a polymeric substrate would have a certain structure, the ones that are most probable are depicted in Figure 3. In Figure 3A a schematic drawing is presented of a surfactant monolayer in which the molecules are oriented perpendicular to the surface with their ionic head groups (layer 1) oriented toward air. The carbon chains of the surfactant molecules are assumed to be fully extended and are present next to the surface in layer 2. The adsorption of surfactants at the solid/liquid interface of nonpolar surfaces such as PE is actually occurring as depicted in Figure 3A.2-4 However, it is not likely that this conformation is formed in air. The bilayer model (Figure 3B) is an extension of the monolayer model with an additional surfactant layer on the first one in such a way that the ionic head groups of the two layers are opposite each other. This conformation is unlikely to be formed in solution because this would mean that the hydrophobic tails are exposed to the aqueous environment. However, in air, a hydrophobic medium, this situation is less unlikely than the monolayer model. Finally, a schematic drawing of a homogeneous layer without orientation of the surfactant molecules is given in Figure 3C. For SDS coated onto PE the atomic compositions of the different layers for the three models are given in parentheses. The values of d1-d5 for the surfactants used in this study were estimated assuming CsCsC and CdCsC bond angles of 109.5° and 120° and CsC and CdC bond (31) Herder, P. C.; Claesson, P. M.; Herder, C. E. J. Colloid Interface Sci. 1987, 119, 155.

lengths of 1.54 and 1.34 Å, respectively.32 The sum of the size of the ionic head group and a sodium ion was taken as 5.3 and 3.7 Å for sulfate-group- and carboxylate-group containing surfactants, respectively.33 Thus, for example for a bilayer structure of SDS coated onto PE (Figure 3B) values of d3 ) 11 (1.54 × sin 54.75°) ) 13.8 Å and d4 ) d3 + 2 × 5.3 ) 24.4 Å were calculated. With XPS, the surface of a sample is radiated by an X-ray source having photons with an energy of hν eV. Electrons of all elements present, except hydrogen and helium, can be emitted from their subshells provided that the energy of the X-ray source is higher than the binding energy (BE) of the bound electrons. The kinetic energy (KE) of the emitted electrons is given by

KE ) hν - BE - W

(2)

where W is a work function term.34 The measured intensity Ii of an electron originating from an element i present at a concentration Xi in the substrate can be described by a function of the electron escape depth35,36

Ii )

dz ∫0∞Xi(z) exp(-z λi )

I∞i λi

(3)

where I∞i is the intensity for an elemental bulk standard, λi is the effective escape depth of the photoelectrons perpendicular to the surface in nanometers, and Xi(z) is the local concentration of element i at depth z. I∞i can be calculated, but it is customary to use published sets of measured I∞i data from standard known reference compounds. Actually intensity ratios are used where the values of I∞i are referred to the F 1s peak with I∞F defined as unity. The escape depth is given by (32) The Handbook of Chemistry and Physics, 66th ed.; Weast, R. C., Ed.; CRC Press: Cleveland, OH, 1985-1986. (33) Klevens, H. B. J. Am. Oil Chem. Soc. 1953, 30, 74. (34) Briggs, D.; Rivie`re, J. C. In Practical Surface Analysis. Volume 1: Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; John Wiley & Sons, Ltd: New York, 1990; Chapter 3. (35) Hofmann, S. In Practical Surface Analysis. Volume 1: Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; John Wiley & Sons, Ltd: New York, 1990; Chapter 4. (36) Seah, M. P. In Practical Surface Analysis. Volume 1: Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; John Wiley & Sons, Ltd: New York, 1990; Chapter 5.

Anionic Surfactants on Poly(ethylene) Surfaces

λi ) λi° cos φ

Langmuir, Vol. 14, No. 12, 1998 3219

(4)

where λ°i is the inelastic mean free path (IMFP) of the electrons and φ is the angle of emission of the detected electrons with respect to the normal to the sample surface. In this study, φ was always 0° and therefore λi in eq 3 can be replaced by λ°i. The IMFPs of electrons are dependent on their energy and the material (elements, inorganic compounds, or organic compounds) through which they pass and can be expressed in nanometers (λ°n), in monolayers (λ°m), or in milligrams per square meter (λ°d). IMFPs can experimentally be determined or can be obtained from theoretical predictions. Presently, there is still a discussion about which values should be used for the IMFP of different elements. Seah and Dench made a compilation of IMFPs and provided a set of relations for different classes of materials which show a reasonably good correlation with experimental data although the scatter in the latter is large.37,38 For organic compounds the value of the electrons’ IMFP can best be described by the relations

λ°d ) 49E-2 + 0.11E1/2

(5)

λ°d × 103 F

λ°n )

(6)

where F is the bulk density in kg/m3 and E is the energy of the emitted electrons above the Fermi level ()KE + W) in eV. When electrons have to pass through an inorganic compound, the value of the IMFP can be calculated according to -2

λ°m ) 2170E

1/2

+ 0.72(aE)

(7)

λ°n ) aλ°m

(8)

The monolayer thickness a (nm), is given by

a3 )

[ (

(9)

( )) ( ( ))]

-d1 ) Xi,1 1 - exp ∞ λ°i,1 Ii

+ Xi,b exp

BE (eV) E (eV)

-d1 λ°i,1

(10)

where Xi,o and Xi,b are the local concentrations of element i in layer 1 and the bulk of the substrate, respectively, and λ°i,1 is the IMFP of the electrons of element i through layer 1. Layer 1 contains only the ionic head groups of the surfactant, and therefore this layer is assumed to be of an inorganic nature. Consequently, the IMFP of electrons through this layer is approximated by the IMFP (37) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2. (38) Cave, N. G.; Cayless, R. A.; Hazell, L. B.; Kinloch, A. J. Langmuir 1990, 6, 529.

C 1s

O 1s

S 2p

Na 1s

284.8 968.8

532.0 ( 0.5 721.6 ( 0.5

168.7 ( 0.1 1084.0 ( 0.1

1072.6 ( 0.3 181.0 ( 0.3

Table 5. Values of the IMFPs of Electrons from Different Subshells of Some Elements through Several Organic and Inorganic Compounds compound

F (×103 λ°n (C 1s) λ°n (O 1s) λ°n (S 2p) λ°n (Na 1s) (Å) (Å) (Å) (Å) kg/m3)

PE SDS, S11(:), C11(:) HSO4Na HCO2Na

0.93 0.99

36.8 34.5

31.7 29.8

38.9 36.6

15.9 14.9

2.44 1.92

24.2 24.3

20.9 21.0

25.7 25.7

10.7 10.7

through HSO4Na or HCO2Na for the sulfate- and carboxylate-group-containing surfactants, respectively. Since layer 2 (the alkyl chains of the surfactants) has approximately the same chemical nature as the substrate PE, they are taken together as a continuous bulk layer comprised of only carbon. For the bilayer model (Figure 3B) the intensity of the electrons from element i is given by39

Ii I∞i

)

[ (

( )) (( ( )) ( )) ( ( ) ( ))]

Xi,3 1 - exp exp

-d3 λ°i,3

-d3 λ°i,3

+ Xi,4

1 - exp -

d4 - d3 λ°i,4

d4 - d3 -d3 exp λ°i,4 λ°i,3

+ Xi,b exp -

× (11)

Finally for the homogeneous overlayer (Figure 3C) it can be derived that

Ii

A × 1024 FnN

where A is the atomic or molecular weight in g/mol, n is the number of atoms in the molecule, N is Avogadro’s number, and F is the bulk density in kg/m3. The values of the energies above the Fermi level of electrons of different elements were calculated from their binding energies using eq 2 (Table 4). The values of the IMFPs of the electrons through some organic and inorganic materials were calculated using eqs 5-9 (Table 5). Equation 3 can be integrated for the three proposed models in Figure 3. For the monolayer model (Figure 3A) the intensity of the electrons from element i is given by

Ii

Table 4. Average Binding Energies and Energies above the Fermi Level of Electrons from Different Subshells Excited by Mg Kr Radiation of 1253.6 eV (n g 10, (sd)

I∞i

[ (

) Xi,o 1 - exp

( )) -d λ°i,0

( ( ))]

+ Xi,b exp

-d λ°i,0

(12)

where Xi,o and Xi,b are the local concentrations of element i in the overlayer and the bulk of the substrate, respectively, and λ°i,0 the IMFP of the electrons of element i through the overlayer. Equation 12 is comparable to eq 10. When a surfactant is coated onto a polymeric substrate, as depicted in Figure 3A and B, it is thus possible to calculate from eq 10 and 11, respectively, which atomic composition will be measured with XPS. This is accomplished using the relation

() ∑( ) Ii

fi )

I∞i

n

Ii

i)1

I∞i

(13)

where fi is the fractional atomic content of element i. The results of the calculations together with the XPS data of PE samples onto which C11(:), S11(:), or SDS was coated from solutions of two different concentrations are given in Table 6. If a monolayer of C11(:) is coated onto (39) Hazell, L. B.; Brown, I. S.; Freisinger, F. Surf. Interface Anal. 1986, 8, 25.

3220 Langmuir, Vol. 14, No. 12, 1998

Lens et al.

Figure 4. Theoretical results of the XPS analysis of a PE sample containing a SDS layer on its surface. The atomic percentages of carbon, oxygen, sulfur, and sodium are given as a function of the thickness of the surfactant layer. Table 6. XPS Analysis of PE/C11(:), PE/S11(:), and PE/SDS Samples Prepared by Coating from Aqueous Solutions Containing 1 vol % Hexanol of Two Concentrations and Theoretical Estimations of the Atomic Composition of a Monolayer and a Bilayer of the Surfactants on PE (n g 3, (sd) atom % O PE/C11(:)

PE/S11(:)

PE/SDS

monolayer bilayer 0.10 M 0.30 M monolayer bilayer 0.05 M 0.20 M monolayer bilayer 0.020 M 0.20 M

8.0 10.1 6.4 ( 0.3 10.9 ( 1.2 14.2 17.5 7.9 ( 0.4 19.0 ( 2.0 14.2 17.0 5.9 ( 0.7 18.2 ( 1.2

atom % S

atom % Na

3.0 4.0 0.9 ( 0.1 3.0 ( 0.6 3.0 3.9 1.5 ( 0.3 4.5 ( 0.3

7.2 6.0 5.5 ( 0.1 6.2 ( 0.4 6.2 4.7 1.9 ( 0.2 5.2 ( 0.6 6.2 4.4 2.4 ( 0.4 5.4 ( 0.4

PE according to Figure 3A, the measured atomic percentages of oxygen and sodium should be about the same. For the PE/S11(:) and PE/SDS samples the atomic percentages should be twice as high for sodium as for sulfur. This is a result of the large difference between the values of the IMFPs of sodium and the other elements and because the monolayer is thinner than the sampling depth (≈3λ° cos φ). Consequently, a relatively larger atomic percentage of sodium is measured than would have been expected on the basis of the atomic compositions of these layers. Comparing the calculated data with the measured values of the atomic percentages of oxygen, sulfur, and sodium, it must be concluded that the monolayer model is not a realistic picture of the empirical coated surfactant layers. Also no conformity is found between the theoretical data for the bilayer model and the measured data of polymeric samples precoated from solutions of a low surfactant concentration. At higher concentrations, the measured atomic percentages are closer to the theoretical expected values for a bilayer model. However, especially for PE/S11(:) and PE/SDS samples, the deviations are too large to conclude that the coated surfactants are oriented

Table 7. Layer Thicknesses of SDS Coated onto PE Estimated from XPS Analyses and Labeling Experiments with SD35S as a Function of the SDS Concentration in the Coating Solution (n g 3, (sd) d(XPS) (Å) 0.020 M 0.20 M

11 ( 1 62 ( 14

SD35S coated (mol/m2) 10-6

(6.0 ( 1.2) × (2.9 ( 0.2) × 10-5

d(labeling) (Å) 17 ( 3 85 ( 6

according to the bilayer model. Therefore it is concluded that there is no orientation of the surfactant molecules coated on PE according to Figure 3A or B and it is assumed that the coated layer can be depicted as a homogeneous overlayer on a PE substrate (Figure 3C). The conclusions drawn from the orientation analyses are in agreement with the observations of Terlingen et al., who studied the coating of SDS onto PP.5 In an XPS study on PP/SDS samples no effect of varying the take-off angle φ on the detected amount of sulfur was found. On the basis of these results it was postulated that the sulfur was homogeneously distributed throughout the sampling depth. When for example an oriented monolayer had been coated, the atomic percentage of sulfur should have increased with increasing φ. Assuming a homogeneous layer of a coated surfactant on a polymeric substrate, eqs 12 and 13 can be used to calculate the expected atomic percentages of carbon, oxygen, sulfur, and sodium as a function of the thickness of the surfactant layer, as is shown for SDS (Figure 4). With these data it is possible to estimate the thickness d of the overlayer of the surfactants on PE. This is done by an iterative process in which the value of d is varied and the resulting calculated atomic composition is compared to the measured composition of the precoated polymeric samples. The value of d giving the best fit according to a least-squares method was taken as the thickness of the overlayer. The average values of d for different PE/SDS samples precoated from 0.020 and 0.20 M SDS solutions are given in Table 7. When the concentration is increased, the thickness of the coated surfactant layer is also increased.

Anionic Surfactants on Poly(ethylene) Surfaces

Langmuir, Vol. 14, No. 12, 1998 3221

Figure 5. XPS analysis of PE/C11(:) samples as a function of the C11(:) concentration in the coating solution (A) and the thickness of the coated layer calculated from the XPS data and eqs 12 and 13 (B) (n g 3, (sd).

Figure 6. XPS analysis of PE/S11(:) samples. The atomic percentages of oxygen, sodium, and sulfur are given as a function of the S11(:) concentration in the coating solution (n g 3, (sd).

This was expected because it was hypothesized that at a higher concentration more surfactant is deposited at the polymeric surface. The results of the estimation of the layer thickness from XPS data were compared with the results from coating experiments with radiolabeled SDS (Table 7). The orders of magnitude of the thicknesses calculated from the XPS measurements were in reasonably good agreement with the data obtained from the coating experiments with SD35S. Therefore, eqs 9 and 12 can be applied to get an indication of the thickness of a coated surfactant layer on a polymeric sample. Influence of the Surfactant Concentration on the Thickness of the Coated Layer. Coating of SDS onto PE was performed at concentrations of 2.5 and 25 times the cmc of the pure surfactant (Table 7). Since the hexanol in the coating solution decreased the cmc, these values will be even larger for the studied system. The amounts of coated surfactant strongly differed, indicating that there was no plateau value of amount of coated material above the cmc. The influence of the surfactant concentration of the coating solution on the amount of coated material at the surface of precoated PE samples was also investigated for C11(:) (Figure 5) and S11(:) (Figures 6 and 7). The amounts of sodium and oxygen on the PE/C11(:) samples increased upon increasing C11(:) concentrations. Analogous to the profiles calculated for PE/SDS samples (Figure 4), the data for sodium and oxygen diverged with increasing C11(:) concentration. The sodium signal saturated at lower depth than the oxygen signal. No plateau value of coated material was obtained for the PE/ C11(:) samples when the concentration of C11(:) in the coating solution was increased above the cmc. The cmc

Figure 7. Thickness of a coated S11(:) layer on PE as a function of the S11(:) concentration in the coating solution calculated from the XPS data of Figure 6 and eqs 12 and 13 (n g 3, (sd).

of C11(:) in the presence of 1 vol % hexanol was determined to be (7.9 ( 0.1) × 10-2 M. The thickness of the coated surfactant layer calculated from these XPS data increased with increasing C11(:) concentrations below and above the cmc. The same trends were found for the coating of S11(:) onto PE. Increasing amounts of oxygen, sodium, and sulfur were detected at the surface of the PE/S11(:) samples with increasing S11(:) concentrations of the coating solution. Again the thickness of the surfactant layer increased with surfactant concentration above the cmc. These observations again confirm the hypothesis that coating of surfactants onto PE is not dominated by the mechanisms of adsorption of the surfactants at the solid/ liquid interface in solution.

3222 Langmuir, Vol. 14, No. 12, 1998

Conclusions Dry PE samples with a thin layer of an anionic surfactant on their surface can be obtained. For this purpose it is necessary that the polymeric surface is wetted by the surfactant solution. If this is not the case, there are two possibilities to achieve this. First, the hydrophilicity of the polymer can be increased. This may be done by introducing polar groups at the polymeric surface with the use of an argon plasma treatment. Second, the surface tension of the coating solution can be decreased

Lens et al.

by adding hexanol. The surfactants coated onto the PE surface are present as a homogeneous overlayer without orientation of the surfactant molecules. The thickness of the layer can be increased by increasing the surfactant concentration in the coating solution. Acknowledgment. This study was financed by Cordis Europa N.V., Roden, The Netherlands. LA971025N