Langmuir 2002, 18, 5755-5760
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Adsorption of Mixed Anionic and Nonionic Surfactants at the Hydrophilic Silicon Surface J. Penfold,*,† E. Staples,‡ I. Tucker,‡ and R. K. Thomas§ ISIS Facility, CLRC, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, United Kingdom, Port Sunlight Laboratory, Unilever Research, Quarry Road East, Bebington, Wirral, United Kingdom, and Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, United Kingdom Received October 12, 2001. In Final Form: February 13, 2002
Specular neutron reflectivity has been used to determine the structure and composition of the mixed anionic and nonionic surfactants adsorbed at the hydrophilic silica solid-solution interface. Measurements show that the anionic surfactant sodium dodecyl sulfate (SDS) is adsorbed at the hydrophilic silica surface in the presence of the nonionic surfactant hexaethylene monododecyl ether but has no affinity for the surface in the absence of the nonionic surfactant. The variations of adsorbed amount, composition, and structure of the adsorbed layer are shown to reflect the different affinities of the two surfactants for the hydrophilic surface. At a solution concentration greater than the critical micellar concentration, the adsorbed amount decreases for solutions increasingly rich in SDS, and the surface composition is not consistent with the pseudo phase approximation.
Introduction The study of the adsorption of mixed surfactants at interfaces is of considerable current interest because the mechanisms that determine the synergistic behavior frequently exploited in the widespread domestic, technological, and industrial applications are not well understood at a molecular level. These widespread applications include detergency, fabric conditioning, dyeing, mineral flotation, colloid stability, and surface modification, and so adsorption at the air-solution, solution-solid, and liquid-liquid interfaces are all of relevence. The pseudo phase approximation or regular solution theory (RST)1 has provided most of the thermodynamic and theoretical basis of our understanding of the behavior of mixed surfactants, both in self-assembly in solution and in adsorption at interfaces. Despite its widespread applicability, there is increasing experimental evidence (see, for example, ref 2) of the limitations of this approach. This has stimulated the development of the molecular thermodynamic theories of Blankschtein et al.3 and Hines,4 in which molecular details and interactions are incorporated within a thermodynamical framework. This implies a need to correlate information about structure and composition in mixed surfactants, and techniques such as small-angle neutron scattering (SANS) and neutron reflectivity are now addressing such issues. In a number of recent papers we have shown how the use of specular neutron reflectivity and SANS can provide information about the structure and composition of mixed surfactants at the air-water interface,5 the liquid-solid †
ISIS Facility. Unilever Research. § Oxford University. ‡
(1) Holland, P. M. Colloids Surf. 1986, 19, 171. (2) Hines, J. D.; Thomas, R. K.; Garrett, P. R.; Rennie, G. K.; Penfold, J. J. Phys. Chem. B 1997, 101, 9215. (3) Puuvada, D.; Blankstein, D. J. Phys. Chem. 1990, 92, 3710. (4) Hines, J. D. Langmuir 2000, 16, 7575. (5) Thomas, R. K.; Lu, J. R.; Penfold, J. Adv. Colloid Interface Sci. 2000, 84, 143.
interface,6,7 and the oil-water interface8 and in mixed micelles.9,10 In the use of specular neutron reflectivity to study adsorption at interfaces, through measurements in combination with H/D isotopic substitution, information primarily in the direction perpendicular to the surface or interface is obtained, and this enables structural and compositional information about the adsorbed layer to be deduced.5 Other complementary techniques provide additional information. Atomic force microscopy (AFM) provides relevant information about the lateral organization of adsorbed layers of surfactants at solid surfaces.11,12 In contrast, ellipsometry can powerfully provide both equilibrium and kinetic information on surfactant adsorption at solid surfaces.13,14 It provides information primarily on total adsorption, only indirectly on composition, and is relatively insensitive to the detailed structure of the surface layers. This increasing pool of new experimental data, where information about both structure and composition are obtained, is providing a stimulus to theoretical developments. The central assumption of RST, that the excess entropy of mixing is zero, and the role of changing hydration on mixing are now being considered.15 At the liquid-solid interface an additional factor needs to be taken into account: the role of the specific interaction of the surfactants with the surface. It is this aspect of (6) Penfold, J.; Staples, E. J.; Tucker, I.; Thompson, L. J. Langmuir 1997, 13, 6638. (7) Penfold, J.; Staples, E. J.; Tucker, I.; Thomas, R. K. Langmuir 2000, 16, 8879. (8) Staples, E. J.; Tucker, I.; Penfold, J. J. Phys. Chem. B 2000, 104, 607. (9) Penfold, J.; Staples, E. J.; Thompson, L. J.; Tucker, I.; Hines, J.; Thomas, R. K.; Lu, J. R. Langmuir 1995, 11, 2496. (10) Penfold, J.; Staples, E. J.; Thompson, L. J.; Tucker, I.; Hines, J.; Thomas, R. K.; Lu, J. R.; Warren, N. J. Phys. Chem. B 1999, 103, 5204. (11) Manne, S.; Gaub, H. E. Science 1995, 220, 1480. (12) Patrick, H. N; Warr, G. S; Manne, S; Aksay, I. A. Langmuir 1997, 13, 4349. (13) Tiberg, F.; Johnsson, B.; Tang, J.; Lindman, B. Langmuir 1994, 10, 2294. (14) Brink, J.; Tiberg, F. Langmuir 1996, 12, 5042. (15) Thomas, R. K.; Hines, J. D. In preparation.
10.1021/la011546h CCC: $22.00 © 2002 American Chemical Society Published on Web 06/21/2002
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adsorption of mixed surfactants at the liquid-solid interface that is the thrust of this study. We have started to address this particular issue. In previous studies we have reported the use of specular neutron reflectivity to investigate the adsorption of mixed cationic and nonionic surfactants of hexadecyltrimethylammonium bromide (CTAB) and hexaethylene glycol monododecyl ether (C12E6) at the hydrophilic silicasolution interface, where the effects of solution composition concentration and pH on the adsorption have been reported.6,7 In particular, we have shown that modifying the surface interface by changing the solution pH demonstrates the role of the specific interaction of the surfactants with the surface in determining the composition and structure of the adsorbed layer. In this study we address a different aspect of this issue. We have characterized the adsorption of the SDS/C12E6 mixture at the hydrophilic silica surface, where SDS does not adsorb in the absence of the nonionic cosurfactant. The formation of silanol groups (SiOH), when the interface of silicon is exposed in water, makes the surface hydrophilic, and for pH g2.0 the surface is negatively charged.16 Nonionic surfactants, such as C12E6, interact with this hydrophilic surface due to hydrogen bonding of the ether oxygens of the ethylene oxide group with the surface OH groups. Anionic surfactants, such as SDS, show no affinity for such negatively charged hydrophilic surfaces. We demonstrate that the surface compositions for C12E6/SDS mixtures are significantly different from the bulk compositions, the composition at the air-water interface9 and the micellar composition,5,6 previously obtained from neutron reflectivity and SANS measurements. Furthermore, the nature of the interaction with the surface is reflected in the structure of the surface layer. Experimental Details Specular neutron reflectivity measurements were made on the SURF reflectometer17 at the ISIS pulsed neutron source, using the “white beam time-of-flight” method. Measurements were made in the Q range of 0.012-0.4 Å-1, using a wavelength band of 1-7 Å and three different glancing angles of incidence, 0.35, 0.8, and 1.8°. A sample geometry, which is now well established for studies at the liquid-solid interface, was used,18 where the neutron beam is incident at the liquid-solid interface by transmission through the crystalline silicon upper phase. A sample cell similar to that used for Poiseulle flow measurements at grazing incidence19 was used, and the different isotropically labeled solutions were delivered from such solutions using a Merck Lachrome HPLC pump. The polished silicon single crystal (〈111〉 face) was obtained for semiconductor processing (Boston, MA) and was used without further surface treatment. The illuminated area was 60 × 30 mm2, and the resolution in Q, ∆Q/Q, was ∼4%. The data were normalized for the incident beam spectral distribution and detector efficiency and established on an absolute reflectivity scale by reference to the direct beam intensity.20 Specular neutron reflection gives information about the concentration on composition profile in a direct perpendicular to the surface or interface on molecular scale, and details of its (16) Iler, R. K. The Chemistry of Silica; Wiley-Interscience: New York, 1979. (17) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E.; Roser, S. J.; McLure, I. A.; Hillman, A. R.; Richards, R. W.; Staples, E. J.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899. (18) Lee, E. M.; Thomas, R. K.; Cummins, P. G.; Staples, E. J.; Penfold, J.; Rennie, A. R. Chem. Phys. Lett. 1989, 162, 196. (19) Penfold, J.; Staples, E.; Tucker, I.; Fragnetto, G. Physica B 1996, 221, 325. (20) Fragnetto, G.; Thomas, R. K.; Rennie, A. R.; Penfold, J. Langmuir 1996, 12, 6036.
Penfold et al. theory are described elsewhere.21 The specular reflectivity, R(Q) (where Q is the wave-vector transfer normal to the surface or interface and defined as Q ) 4π sin θ/λ, where θ is the grazing angle of incidence and λ the neutron wavelength), is given by the kinematic approximation22 as
16π2 |F(Q)|2 Q2
R(Q) )
(1)
where F(Q) is the one-dimensional Fourier transform of F(z), the average neutron scattering length density distribution in the direction normal to the interface
F(Q) )
∫
+∞
-∞
F(z) exp(iQz) dz
(2)
∑N (z)b
(3)
and
F(Q) )
i
i
i
Ni is the number density distribution of species i, and bi is its neutron scattering length. Different isotopes have different neutron scattering lengths, and D and H have a particularly large difference. Hence, D/H isotropic substitution can be used to manipulate the neutron refractive index profile at the interface, where the neutron refractive index is
n(z) ) 1 -
λ2 F(z) 2π
(4)
and is related to the reflectivity through eq 1. It is this selectivity that makes the neutron reflectivity method so powerful, and the H/D isotopic substitution can be applied to both the adsorbate and solvent. It relies on there being no isotopic dependence on the structure or adsorbed amount, and this is now well established for these types of systems.23 The neutron reflectivity measurements were made at a solution concentration of 10-3 M [greater than the mixture critical micellar concentration (cmc)] for different solution compositions and for mole ratio mixtures of SDS/C12E6 in the range 10/90 to 50/50. The measurements were made in 0.1 M NaCl and at a pH of 2.4 (adjusted by the addition of HCl). The measurements were made for the isotopic concentration of h-SDS/h-C12E6 and d-SDS/ h-C12E6 in D2O and of h-SDS/d-C12E6 and d-SDS/h-C12E6 in H2O (where h,d-SDS and h,d-C12E6 refer to the alkyl chain of the surfactant being hydrogenous or deuterated, respectively). Addi tional measurements were made for the bare interface (no added surfactant) in D2O, H2O, and water index matched to silicon (cmSi) to characterize the nature of the oxide layer at the silicon surface. The h-C12E6 was obtained for Nikkol. The d-C12E6 and h,d-SDS were synthesized by R. K. Thomas, Oxford, U.K., and details of the preparation, purification, and characterization are given elsewhere.23 High-purity water was used throughout (Elga Ultrapure), and D2O was obtained from Fluorochem. The sample cell and associated tubing were cleaned in alkaline detergent (Decon 90), followed by copious washing in ultrapure water. The gap between the cell and the silicon surface was ∼100 µm and the total sample volume, ∼1 mL. Sample changing using the HPLC pump replaced the sample volume ∼10 times between measurements and showed no evidence of cross-contamination. Typical measurement times for a single reflectivity profile, over the entire Q range measured, was ∼60 min per sample. We have shown elsewhere6,24 that in evaluating reflectivity data for mixed surfactant adsorption at the liquid-solid interface, (21) Thomas, R. K.; Penfold, J. J. Phys.: Condens. Matter 1990, 2, 1369. (22) Crowley, T. L.; Lee, Simister, E. A.; E. M.; Thomas, R. K. Physica B 1991, 173, 142. (23) Lu, J. R.; Lee, E. M.; Thomas, R. K.; Penfold, J.; Flitsch, S. L. Langmuir 1993, 9, 1352. (24) Penfold, J.; Staples, E. J.; Tucker, I.; Thompson, L. J.; Thomas, R. K. Int. J. Thermophys. 1999, 20, 19.
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Figure 1. Specular reflectivity, R(Q), as a function of wave-vector transfer, Q, for 10-3 M 20/80 SDS/C12E6/0.1 M NaCl/pH 2.4: (b) h-SDS/C12E6 in D2O; (O) d-SDS/h-C12E6 in D2O; (2) d-SDS/h-C12E6 in H2O; (4) h-SDS/d-C12E6 in H2O. The solid lines are calculated curves using the model described in the text. Table 1. Model Parameters for 10-3 M 20/80 SDS/C12E6/0.1 M NaCl/pH 2.4
the surface composition and structure are closely linked. The determination of the surface structure and composition relies on being able to combine reflectivity profiles for a range of solutions of the same chemical composition but different isotopic compositions. Currently the way of analyzing such data for the liquidsolid interface is to assume a structural model and calculate the reflectivity using the exact optical matrix method.25,26 It has been well established by reflectivity and SANS on nonionic surfactants,18 cationic surfactants,20 and mixtures6,7 that the nature of the adsorbed layer is a “fragmented bilayer” or “flattened micelles”. This is consistent with the cooperative nature of the adsorption isotherms and of the observations of lateral structure from AFM.11,12 The adsorbed layer is then described by three layers: a layer of thickness d1 adjacent to the solid surface containing both surfactant headgroups, and associated hydration; a layer of thickness d2 containing hydrocarbon chains interpenetrating or overlapping from both sides of the bilayer; and a layer of thickness d3 adjacent to the fluid phase and again containing both surfactant headgroups and hydration. The structure of the bilayer contains some disorder and edge effects, and an additional parameter is required to account for the intermixing and an overlap between headgroups and alkyl chains. This parameter, fc, is the fraction of alkyl chains in the headgroup region. The model is then described by three thicknesses, d1, d2, and d3, the area/molecule in the bilayer of each surfactant, Aa and An (for SDS and C12E6), the fractional coverage, f (the fraction of surface covered by the bilayer or flattened micelle patches), and fc. We have previously shown that for surfaces with a poor affinity for one of the surfactants the distribution across the bilayer is nonuniform, and this included by a model parameter, fd, which described the distribution. fd can be thought of as a weighting function which described that asymmetry, where for fd of unity the surfactant composition is the same in both halves of the bilayer, and for fd of zero all of the SDS will be in the outer layer. From known molecular volumes and scattering lengths, the scattering length density of each of the layers can be estimated. The procedure adopted was to refine the individual model parameters (d1, d2, d3, Aa, An, f, fc, and fd) for each profile on a least-squares basis to give the minimum spread in the model parameters for all of the differently isotopically labeled combinations that were measured. The surface of the silicon has a thin native oxide layer, and the thickness, density, and composition of that layer will depend on the surface treatment. The oxide layer at the silicon surface
Figure 1 shows the reflectivity data for a solution of SDS/C12E6 at a concentration of 10-3 M in 0.1 M NaCl and pH 2.4 and for a solution composition of 20/80 mol % SDS/ C12E6. The different isotopically labeled combinations illustrate the sensitivity to both the structure and composition of the adsorbed layer. For the measurements in D2O, the combination h-SDS/h-C12E6 provides an estimate of total amount adsorbed and the combination d-SDS/h-C12E6 (where the SDS is now effectively “contrast matched” to the D2O) an indication of the amount of C12E6 at the interface. The two combinations, d-SDS/h-C12E6 and h-SDS/d-C12E6, in H2O highlight the amounts of SDS and C12E6, respectively, at the interface. The main model parameters for the data in Figure 1 and their variation for the different isotopic combinations measured are summarized in Table 1. Typical errors in the fitted parameters are estimated and quoted in Table 2, along with the mean values obtained for different solution compositions. The area/molecule for SDS and C12E6 reflect the surface composition and total adsorbed amount, and
(25) Heavens, O. S. Optical Properties of Thin Films; Butterworth: London, U.K., 1953.
(26) Penfold, J. In Neutron, X-ray and Light Scattering; Lindner, P., Zemb, T., Eds.; Elsevier: New York, 1991.
Aa (Å2) An
contrast h-SDS/h-C12E6/D2O d-SDS/h-C12E6/D2O h-SDS/d-C12E6/H2O d-SDS/h-C12E6/H2O
96 86 94 100
36 33 28 36
f
fc
fd
0.35 0.38 0.42 0.40
0.1 0.1 0.1 0.1
0.47 0.51 0.49 0.50
d1 (Å) d2 d3 14 13 16 15
29 31 24 27
15 20 14 15
makes a small but important contribution to the reflectivity, and it is essential to characterize the nature of that layer. Measurements in D2O, H2O, and cmSi are used to determine the thickness and density of the oxide layer. For the block used in this study the oxide layer corresponds to a layer of 10 Å and a scattering length density close to that of SiO2 (0.35 × 10-5 Å2 ( 0.01). These parameters are typical of the range of parameters reported for the oxide layer on silicon.2,3,14 These parameters are included in the modeling of the reflectivity profiles measured in the presence of surfactant.
Results and Discussion
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Penfold et al.
Figure 2. Specular reflectivity, R(Q), as a function of wave-vector transfer, Q, for 10-3 M h-SDS/h-C12E6/0.1 M NaCl/pH 2.4 in D2O for (b) 20/80 SDS/C12E6 and (O) 40/60 SDS/C12E6. The solid line is for D2O. Table 2. Mean Model Parameters for 10-3 M SDS/C12E6/0.1 M NaCl/pH 2.4 solution composition
Aa (Å2)
An
f
fc
fd
d1 (Å)
d2
d3
Γ total (×10-10 mol cm-2)
mol % SDS
10/90 20/80 30/70 40/60 50/50
95 94 ( 5 161 202 200
36 33 ( 3 35 39 38
0.52 0.39 ( 0.03 0.29 0.19 0.12
0.1 0.1 0.1 0.1 0.1
0.52 0.49 ( 0.02 0.41 0.53 0.55
12.0 14 ( 1 12 11 12
31.0 27 ( 3 32 27 28
16 16 ( 2 16 14 14
6.8 5.3 ( 0.5 3.3 2.0 1.3
0.27 0.26 ( 0.03 0.18 0.16 0.15
the total adsorbed amount is given by
Γ)
2f (1/Aa + 1/An) NA
(5)
where NA is Avogadro’s number. The total adsorbed amount and composition of the surface layer (mole percent SDS) decreases markedly for solutions richer in SDS. This is illustrated clearly in Figure 2, where the reflectivity for h-SDS/h-C12E6 in D2O is shown for two different solution compositions (20/80 and 40/60). For the solution composition of 20/80 the pronounced fringe is indicative of a high adsorption, whereas the data for 40/60 are much closer to the profile for D2O and consistent with a much lower level of adsorption. For solution compositions richer in SDS than 50/50, the reflectivity is hardly distinguishable for D2O and is indicative of very little adsorption. Figure 3 shows the variation of adsorbed amount and composition of the surface with solution composition. From the values of fd in Tables 1 and 2 it is evident that there is more SDS in the outer layer than in the layer adjacent to the solid surface, and Table 3 shows the mean composition and the composition in each half of the bilayer as a function of solution composition. Figure 3 shows the variation in surface composition with solution composition. At low SDS solution compositions the surface is richer in SDS. For solution compositions of 30/70 and greater the surface is depleted of SDS compared to the solution composition. Furthermore, the total adsorption, as well as the amount of SDS at the interface, decreases markedly for solutions increasingly rich in SDS. The surface adsorption (or surface activity) of surfactants stems predominantly from the hydrophobicity of the alkyl chain. In surfactant mixtures the synergistic
Figure 3. Variation of (O) surface composition (mol % SDS) and (b) adsorbed amount (×10-10 mol cm-2) as a function of solution composition for 10-3 M SDS/C12E6/0.1 M NaCl/pH 2.4.
adsorption arises from favorable alkyl chain packing and from headgroup interactions. The headgroup interactions are particularly important in ionic-nonionic mixtures, where mixing reduces electrostatic interactions. At the air-water interface and in micelles the surface and micelles at concentrations close to the cmc are initially rich in the most surface active component (C12E6 in this case), and the surface and micelle composition evolves
Adsorption of Surfactants at the Hydrophilic Silicon Surface Table 3. Variation of Surface Composition with Solution Composition for SDS/C12E6/0.1 M NaCl/pH 2.4 solution composition (mol % SDS)
mean surface composition
composition of layer - adjacent to solid
composition of layer adjacent to solid
10/90 20/80 30/70 40/60 50/50
0.27 ( 0.03 0.26 0.18 0.16 0.15
0.17 ( 0.05 0.15 0.07 0.08 0.08
0.40 ( 0.05 0.36 0.25 0.22 0.22
toward the solution composition with increasing concentration. It is related to the free monomer concentration of each of the components in solution. For the concentration used here, in 0.1 M NaCl the micelle and surface composition will be close to the solution composition.9,10 At the liquid-solid interface the situation is more complex. There is little or no adsorption below the cmc, and at the cmc the adsorption increases markedly. This is consistent with cooperative adsorption, and the adsorbed layer is now of the form of surface aggregates (see earlier discussion). In the absence of a suitable theoretical treatment it might be assumed that the pseudo phase approximation is applicable and that the surface composition closely mirrors the micelle composition. This is observed in many cases,7 but it ignores the added complication of a specific interaction or affinity with the solid surface. We have previously demonstrated this with the variation in solution pH for the mixture C12E6/C16TAB.6 For the negatively charged silica surface, SDS alone would not absorb, and this has already been shown elsewhere.24 For the solutions rich in C12E6, the SDS is adsorbed by virtue of its synergistic coadsorption with the C12E6. The structural details, furthermore, show that the SDS is predominantly in the outer part of the surface layer and not immediately adjacent to the solid surface. The total amount of adsorption, as well as the amount of SDS at the interface, decreases markedly for solutions increasingly rich in SDS. This has also been observed elsewhere for the different mixture SDS/C10E6.27 It is attributed to the desorption of the C10E6 due to the preferential function of mixed SDS/C10E6 micelles in solution. The structural model, derived from previous and similar studies on different surfactants and surfactant mixtures,6,7,18,24 provides a good description of the surface structure and is consistent with the range of isotopic combinations measured. Apart for the solution richest in SDS (10/90) the adsorbed amounts and fractional coverage of surface surface are lower than previously reported for C12E6, C16TAB, and C12E6/C16TAB mixtures.6,7 This is due to the poor affinity of the SDS for the surface, and this also has an impact of the surface structure. The predominantly headgroup regions adjacent to both the solid surface and solution are slightly broader and closer to that obtained for pure C12E6 layers.6 Similarly, the alkyl chain region is broader than observed for the C12E6/C16TAB mixtures6,7 and comparable to that obtained for pure C12E6 layers,6 consistent with less interdigitation of the alkyl chains. A number of related studies have been reported,27-31 and a comparison with the result reported here is made.
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Thibaut et al.27 have studied the adsorption of C10E6/SDS mixtures at the water-silicon interface, with measurements of the adsorption isotherm and flow microcalorimetry. At pH 7 they observed adsorption of the C10E6 but not of the SDS. In the presence of SDS, they observed a restriction of the adsorption of C10E6 due to the formation of mixed micelles in solution, and this was confirmed by microcalorimetry measurements. The addition of SDS/ C10E6 mixtures to a surface precoated with C10E6 resulted in a desorption of the C10E6 and was attributed to the formation of mixed micelles in solution. This is entirely consistent with our observation that the total adsorption, as well as the amount of SDS, decreases for solutions increasingly rich in SDS. The results are qualitatively similar to those reported here, given the different nonionic surfactant and the absence of electrolyte in their study. From their study they were unable to draw any conclusions about the nature of the adsorbed layer and its composition. Colombie et al.28 have studied competitive adsorption of the anionic surfactant, SDS, and the nonionic surfactant, Triton X-403, on polystyrene latex particles by a variety of experimental techniques, including filtration, desorption via serum replacement, and 1H NMR spectroscopy. Both SDS and Triton X-403 were found to adsorb at the polystyrene solution interface. Below a concentration of 2.5 × 10-3 M the nonionic surfactant adsorbed preferentially, and at higher concentrations competitive adsorption took place, qualitatively consistent with the pseudo phase approximation, although no quantitative comparison was made. Triton X-403 was found to adsorb to a surface precoated in SDS and vice versa. Detailed comparison with our results is difficult, as the polystyrene surface shows an affinity for both surfactants, and the situation is more comparable with our previous measurements on C12E6/C16TAB.6,7 Kronberg et al.30 have also studied the competitive adsorption of SDS and the nonionic surfactant, nonylphenol deca(oxyethene glycol) monoether, onto polystyrene latex with UV spectroscopy and refractive index increment measurements. Their results were shown to be consistent with the pseudo phase approximation and showed that the surface in that case has no additional effect on the competitive adsorption. Turner et al.29 have also shown that SDS adsorbs strongly to the polystyrene-water interface by neutron reflection and attenuated total reflection infrared spectroscopy. Xu et al.31 have studied the adsorption of anionic-nonionic surfactant mixtures of SDS and C12E8 at a sodium kaolinite surface. Kaolinite possesses both positive and negative sites, and so both anionic and cationic surfactants, as well as the nonionic surfactants, will adsorb at its surface. Their data showed an enhanced adsorption of both anionic and cationic surfactants (SDS in the case of the anionic surfactant) in the presence of the nonionic surfactant C12E8 in the pre-cmc regime. This was attributed to chain-chain interactions, associated with the formation of mixed surfactant “surface micelles”. In the region of the cmc SDS adsorption decreased with increasing C12E8 concentration due to the formation of mixed micelles in solution, a process discussed by Thibaut et al.27 in their study of SDS/C10E6 adsorption at the silica-water interface. Summary
(27) Thibaut, A.; Misselyn-Bauduin, A. M.; Grandjean, J.; Broze, G.; Jerome, R. Langmuir 2000, 16, 9192. (28) Colombie, D.; Landfester, K.; Sudol, E. D.; El-Aasser, M. S. Langmuir 2000, 16, 7905. (29) Turner, S. F.; Clarke, S. M.; Rennie, A. R.; Thirtle, P. N.; Cooke, D. J.; Li, Z. X.; Thomas, R. K. Langmuir 1999, 15, 1017. (30) Kronberg, B.; Lindstrom, M.; Stenius, P. In Phenomenon in Mixed Surfactant Systems; ACS Symposium Series 311; Scamehorn, J. F., Ed.; American Chemical Society: Washington, DC, 1986; p 225.
We have shown that although SDS alone does not adsorb at the hydrophilic silica-solution interface, in the presence of the nonionic surfactant C12E6 there is coadsorption for solutions rich in C12E6. The adsorbed amount and mole (31) Xu, Q.; Vauuderan, T. V.; Somasundaran, P. J. Colloid Interface Sci. 1991, 142, 528.
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fraction of SDS at the interface decreases markedly with increasing mole fraction of SDS in solution, and for solutions richer than equimolar there is little detectable surfactant adsorption at the interface. The lack of affinity of the SDS for the surface is reflected in the structure of the adsorbed layer, and the distribution of SDS in the surface layer is nonuniform. The adsorption of SDS at the interface in the presence of C12E6 is due to the synergistic
Penfold et al.
interaction between the two surfactants. Quantitative predictions of this adsorption behavior would require modification to the pseudo phase approximation to include the unfavorable interaction of the SDS with the similarly charged surface, and the data presented here would provideanappropriatetestofsuchtheoreticaldevelopments. LA011546H
