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Adsorption of Mixed Cationic and Nonionic Surfactants at the Hydrophilic Silicon Surface from Aqueous Solution: The Effect of Solution Composition and Concentration† J. Penfold,*,‡ E. J. Staples,§ I. Tucker,§ and R. K. Thomas| ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, U.K., Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, U.K., and Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, U.K. Received February 24, 2000. In Final Form: May 23, 2000 The adsorption of the mixed cationic and nonionic surfactants of hexadecyltrimethylammonium bromide (C16TAB) and hexaethylene glycol monododecyl ether (C12EO6) at the hydrophilic silicon-solution interface has been measured by specular neutron reflection. The effect of solution composition and concentration on the adsorbed amount, surface structure, and composition has been investigated at pH 2.4. Specular neutron reflection, in combination with H/D isotopic substitution of both the solvent and surfactant, enables detailed structural and compositional information about the adsorbed layer to be obtained. The results obtained are compared with those reported elsewhere for the adsorption of the same surfactant mixture at the air-water interface and for surfactant mixing in micelles. Consistent with other studies, the structure of the adsorbed layer is described as a “defective” bilayer or “flattened” micelles. The variation in the adsorbed amount with solution composition is a maximum at an equimolar solution composition. For solutions richer in C16TAB, the surface and solution compositions are identical (consistent with ideal mixing). Whereas for solutions richer in C12EO6 there is a departure from ideality, and the surface is richer in the cationic surfactant.
Introduction In the widespread domestic, industrial, and technological uses of surfactants, mixtures are mostly used to provide optimal performance and effect.1 Despite their extensive use, the behavior of mixed surfactants at interfaces remains poorly understood at a molecular level. Recent theoretical developments,2,3 and the application of new experimental techniques, such as pulsed field gradient NMR,4 specular neutron reflection,5,6 and the optical techniques of sum frequency and second harmonic generation,7 to different interfaces have provided a new impetus in the study of mixed surfactants at interfaces and in micelles. In a number of recent papers8-12 we have shown how the use of specular neutron reflection and small-angle * To whom all correspondence should be addressed. † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millennum”. ‡ ISIS Facility, Rutherford Appleton Laboratory. § Unilever Research, Port Sunlight Laboratory. | Physical and Theoretical Chemistry Laboratory, Oxford University. (1) Phenomena in mixed surfactant systems; Scamehorn, J. F., Ed.; ACS Symposium Series 311; American Chemical Society: Washington, DC, 1986. (2) Nikas, Y. F.; Puvvada, S.; Blankschtein, D. Langmuir 1992, 8, 2680. (3) Franses, E. I.; Siddiqui, F. A.; Abu, D. J.; Chang, C. H.; Wang, N. H. L. Langmuir 1997, 11, 3177. (4) Griffiths, P. C.; Stilbs, P.; Paulsen, K.; Howe, A. M.; Pitt, A. R. J. Phys. Chem. B 1997, 101, 915. (5) Penfold, J.; Thomas, R. K. J. Phys.: Condens. Matter 1990, 2, 1369. (6) 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. (7) Bain, C. D.; Davies, P. B.; Ward, R. N. Langmuir 1994, 10, 2000. (8) Staples, E. J.; Thompson, L.; Tucker, I.; Penfold, J.; Thomas, R. K.; Lu, J. R. Langmuir 1993, 9, 1651.
neutron scattering, SANS, can provide information about structure and composition of mixed surfactants at the air-water interface and in mixed micelles. More recently it has also been shown that specular neutron reflection can be used to obtain detailed information about the adsorption of surfactants13-19 and mixed surfactants20,21 at both hydrophilic and hydrophobic solid-solution interfaces. Specular neutron reflection provides information primarily in the direction perpendicular to the surface or interface and, in combination with H/D isotopic substitution, enables detailed structural and compositional information to be obtained. A complementary and parallel development has been the exploitation of atomic force microscopy, AFM, to study primarily the lateral organization of surfactants at solid surfaces.22-24 Another (9) Penfold, J.; Staples, E. J.; Thompson, L.; Tucker, I.; Hines, J.; Thomas, R. K.; Lu, J. R. Langmuir 1995, 11, 2496. (10) Penfold, J.; Staples, E. J.; Thompson, L.; Tucker, I. Colloids Surf., A 1995, 102, 127. (11) Penfold, J.; Staples, E. J.; Cummins, P. G.; Tucker, I.; Thompson, L.; Thomas, R. K.; Simister, E. A.; Lu, J. R. J. Chem. Soc., Faraday Trans. 1996, 92, 1773 and 1347. (12) Penfold, J.; Staples, E. J.; Tucker, I.; Thomas, R. K. J. Colloid Interface Sci. 1998, 201, 223. (13) Lee, E. M.; Thomas, R. K.; Cummins, P. G.; Staples, E. J.; Penfold, J.; Rennie, A. R. Chem. Phys. Lett. 1989, 162, 196. (14) McDermott, D. C.; McCarney, J.; Thomas, R. K.; Rennie, A. R. J. Colloid Interface Sci. 1994, 167, 304. (15) McDermott, D. C.; Lu, J. R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R. Langmuir 1992, 8, 1204. (16) Fragneto, G.; Lu, J. R.; McDermott, D. C.; Thomas, R. K.; Rennie, A. R.; Gallagher, P. D.; Satija, S. K. Langmuir 1996, 12, 477. (17) Fragneto, G.; Thomas, R. K.; Rennie, A. R.; Penfold, J. Langmuir 1996, 12, 6036. (18) McDermott, D. C.; Kanellas, D.; Thomas, R. K.; Rennie, A. R.; Satija, S. K.; Majkrzak, C. F. Langmuir 1993, 9, 2404. (19) Fragneto, G.; Thomas, R. K.; Rennie, A. R.; Penfold, J. Science 1995, 267, 657. (20) Penfold, J.; Staples, E. J.; Tucker, I.; Thompson, L. Langmuir 1997, 13, 6638. (21) Penfold, J.; Staples, E. J.; Tucker, I.; Thompson, L. J.; Thomas, R. K. Int. J. Thermophys. 1999, 20, 19. (22) Manne, S.; Gaub, H. E. Science 1995, 270, 1480.
10.1021/la0002637 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/21/2000
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complementary experimental approach is ellipsometry, and it has been shown to powerfully provide equilibrium and especially kinetic information on surfactant adsorption at solid surfaces.25 For mixtures, this approach, as recently shown by Brinck and Tiberg26 on nonionic mixtures, provides information primarily on total adsorbed amounts (and only indirectly on composition) and kinetics. In this paper we describe the use of specular neutron reflectivity to investigate the effect of solution composition and concentration on the adsorption of the surfactant mixture of C16TAB and C12EO6 at the hydrophilic siliconsolution interface. The results obtained are compared with results obtained for the adsorption of the same mixed surfactants at the air-water interface and from mixing in micelles. The measurements described in the paper use the approaches developed in SANS and neutron reflectivity studies on surfactant mixing in micelles and at interfaces. The same detailed experimental approach, described in an earlier paper20 on the effect of pH on the adsorption of the same surfactant mixture, C16TAB and C12EO6, at the hydrophilic silicon surface, is used here to determine the structure of the adsorbed layer and its composition.
such that particular components or fragments can be highlighted. It is this selectivity that makes the neutron reflectivity method so powerful. This relies on there being no isotopic dependence on structure or adsorption, and this has been well established and discussed elsewhere.27 We have shown elsewhere20,21 that in evaluating data for mixed surfactant adsorption at the liquid-solid interface, 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 but different isotopic compositions. Currently the way of analyzing such data for the liquid-solid interface is to assume a structural model and calculate the reflectivity using the exact optical matrix method.28,29 Providing sufficient uniquely different isotopic combinations are measured, detailed structural information can be obtained with some confidence.21 Experimental Details
This can be applied to both the adsorbate and solvent
The specular neutron reflection measurements were made on the SURF reflectometer6 at the ISIS pulsed neutron source, using the “white beam time-of-flight” method. Measurements were made in the Q range 0.012 to ∼0.4 Å-1 using a wavelength band of 1.0-7.0 Å and three different glancing angles of incidence of 0.35, 0.8, and 1.8°. A sample geometry, which is now well established for studies of adsorption at the liquid-solid interface, was used.13 That is, the neutron beam is incident at glancing angles at the liquid-solid interface by transmission through the crystalline silicon upper phase. The sample cell was similar to that used for Poiseuille flow measurements at grazing incidence.28 The different isotopically labeled solutions were deliverd from stock solutions using a Merck LaChrome HPLC pump, model L-7100. The polished silicon single crystal (〈111〉 face) was obtained from Semiconductor Processing, Boston, MA, and was used without additional surface treatment. The illuminated area was 30 × 60 mm2, and the resolution in Q, ∆Q/Q, was ∼4%. The data were normalized for the incident beam spectral distribution, detector efficiency, and silicon transmission using standard procedures and established on an absolute reflectivity scale by reference to the direct beam intensity.17 The reflectivity measurements were made at two different surfactant concentrations, 10-4 and 10-3 M, in 0.1 M NaBr and at a pH of 2.4 (adjusted by the addition of HCl). Measurements were made in 0.1 M NaBr to ensure that deviations from ideal mixing were not extreme and to ensure a direct comparison with previous work on surfactant mixing in micelles and at the airwater interface.11,31 We have previously shown20 that pH has a profound effect on the adsorption (both composition and structure of the adsorbed layer) of the mixed surfactants C16TAB and C12EO6 at the hydrophilic silicon surface, and the measurements reported here were all made at a pH of 2.4. At 10-4 M, five different solution compositions (mol % C16TAB/C12EO6), 70/30, 60/40, 50/ 50, 40/60, and 30/70, were measured. At 10-3 M, four different solution compositions, 80/20, 60/40, 40/60, and 20/80, were measured. In all cases the isotopic combinations of h-C16TAB/ h-C12EO6, h-C16TAB/d-C12EO6, and d-C16TAB/h-C12EO6 in D2O and d-C16TAB/d-C12EO6, h-C16TAB/d-C12EO6, and d-C16TAB/hC12EO6 in H2O were measured. The contribution to the overall reflectivity at the silicon/solution interface from the thin oxide layer on the silicon surface is significant and must be included in any model calculations and analysis. The oxide layer was characterized by making measurements of the silicon/water interface (in the absence of surfactant) with different water contrasts (D2O, H2O, and water index matched to silicon, cmSi). A 10 Å layer with a density close to
(23) Patrick, H. N.; Warr, G. S.; Manne, S.; Aksay, I. A. Langmuir 1997, 13, 4349. (24) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160. (25) Tiberg, F.; Jonsson, B.; Tang, J.; Lindman, B. Langmuir 1994, 10, 2294. (26) Brinck, J.; Tiberg, F. Langmuir 1996, 12, 5042. (27) Crowley, T. L.; Lee, E. M.; Simister, E. A.; Thomas, R. K. Physica B 1991, 173, 143.
(28) Heavens, O. S. Optical properties in thin films; Butterworth: London, 1953. (29) Penfold, J. In Neutron, X-ray and light scattering; Lindner, P., Zemb, T., Eds.; Elsevier: New York, 1991. (30) Penfold, J.; Staples, E. J.; Tucker, I.; Fragneto, G. Physica B 1996, 221, 325. (31) Penfold, J.; Staples, E.; Thompson, L.; Tucker, I.; Hines, J.; Thomas, R. K.; Lu, J. R.; Warren, N. J. Phys. Chem. B 1999, 103, 5204.
Specular Neutron Reflection Specular neutron reflection gives information about the concentration and composition profile perpendicular to a surface or interface on a molecular scale, and its theory is described in detail elsewhere.5 The specular reflectivity, R(Q) (where Q is the wavevector transfer normal the surface and is define as Q ) 4π/(λ sin θ), θ is the grazing angle of incidence and λ the neutron wavelength) can be expressed in the kinematic approximation27 as
R(Q) )
16π2 |F(Q)|2 Q2
(1)
where F(Q) is the one-dimensional Fourier transform of F(z), the average neutron scattering length density profile in the direction normal to the interface
F(Q) )
∫-∞+∞F(z) exp(iQz) dz
(2)
∑i Ni(z)bi
(3)
and where
F(z) )
Ni is the number density profile of species i and bi is its neutron scattering length. In the context of using specular neutron reflection to study surfactant adsorption at interfaces, H and D have vastly different scattering powers for neutrons. H/D isotopic substitution can be used to manipulate the neutron refractive index profile at the interface, related to the scattering length density profile by
n(z) ) 1 -
π2 F(z) 2π
(4)
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SiO2 was obtained, consistent with other recent measurements.16,17,20,21 The nature of the Si/SiO2 interface was also established by measurements of C12EO6 and separately at the solid-solution interface at concentrations greater than the critical micelle concentration (cmc). Measurements similar to those previously reported were made, and adsorbed amounts and absorbed layer structures consistent with these previous observations were obtained.13-17,20,21 The h-C12EO6 was obtained from Nikkol. The h,d-C16TAB and d-C12EO6 were synthesized by R. K. Thomas at Oxford; details of the preparation, purification, and characterization are given elsewhere.32 High-purity water was used throughout (Elga Ultrapure), and the D2O was obtained from Fluorochem. The sample cell and tubing were all cleaned using alkaline detergent (Decon 90), followed by copious washing in ultrapure water.
Results and Discussion. Although neutron reflectivity measurements were made for the differently isotopically labeled combinations of the two surfactants in D2O and in H2O (see Experimental Section for full details), it is instructive to discuss qualitatively the data in D2O. Figure 1 shows the reflectivity profiles for the C16TAB/C12EO6 mixture in D2O at 10-4 M and at two different solution compositions, 70 and 30 mol % C16TAB. Each figure shows three different reflectivity profiles for the three different isotopically labeled combinations of the two surfactants. For the combination h-C16TAB/h-C12EO6 there is a large contrast between the adsorbed layer and the silicon and D2O bulk phases, and this provides an estimate of the total adsorption. The deuterated surfactant (d-C16TAB or d-C12EO6) will be closely matched to the D2O, and so the combinations h-C16TAB/d-C12EO6 and d-C16TAB/h-C12EO6 provide an estimate of the amount of C16TAB and C12EO6 adsorbed at the interface, respectively. The differences in reflectivity for the different isotopic combinations and the two different solution compositions in Figure 1 are an indication of the sensitivity of the measurements with the different labeling to the surface composition. For both solution compositions the reflectivity for h-C16TAB/hC12EO6 is similar, indicating that the total adsorbed amount is relatively constant (and this is borne out in the more detailed analysis presented later, see, for example, Figure 2 and later discussion). For the solution composition 70 mol % C16TAB the reflectivities for h-C16TAB/hC12EO6 and h-C16TAB/d-C12EO6 are similar, consistent with a surface composition rich in C16TAB. In contrast, for the solution composition 30 mol % C16TAB the reflectivity for h-C16TAB/d-C12EO6 and d-C16TAB/hC12EO6 are similar, and this is consistent with the surface composition being closer to equimolar (see Figure 3 and subsequent analysis). A similar qualitative analysis and description can be applied to the data measured in H2O, and a complete set of data (in D2O and H2O) for one solution composition and concentration is shown in Figure 2. In H2O the reflectivity is sensitive to the deuterated surfactant at the interface, and the hydrogeneous surfactant will be closely matched to the H2O. Hence measurements with both surfactants deuterated will give an estimate of the total amount at the interface, and measurements when only one surfactant is deuterated will give an estimate of the amount of that component at the interface. To provide an enhanced sensitivity to the surface structure and composition, measurements and model refinement of a range of different isotopically labeled combinations are necessary, and based on previous optimizations20 we have used the following combinations: (32) Lu, J. R.; Lee, E. M.; Thomas, R. K.; Penfold, J.; Fitsch, S. L. Langmuir 1998, 9, 1352.
Figure 1. (a) Specular reflectivity for 10-4 M 70 mol % C16TAB/30 mol % C12EO6 at the silicon/D2O interface: (b) h-C16TAB/h-C12EO6; (O) h-C16TAB/d-C12EO6; (2) d-C16TAB/h-C12EO6. (b) As in part a but for 30 mol % C16TAB/70 mol % C12EO6.
h-C16TAB/h-C12EO6, h-C16TAB/d-C12EO6, and d-C16TAB/ h-C12EO6 in D2O and d-C16TAB/d-C12EO6, h-C16TAB/dC12EO6, and d-C16TAB/h-C12EO6 in H2O. An analysis of the reflectivity profiles in terms of a single layer of uniform density does not provide a set of parameters for the different contrasts which represents a consistent description of the adsorbed mixed surfactant layer, and a more complex model for the structure of the adsorbed layer is required. The simplest model that is consistent with all the data is a fragmented interpenetrating bilayer of flattened micelles, as described previously in detail elsewhere.20 The main features of the model are as follows: The adsorbed layer can be 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 an additional parameter is required to account for the inter-mixing or overlap between headgroups and alkyl chains. This is described by a parmeter fc, which is the fraction of alkyl chains in the headgroup region. The model can be described by three thicknesses, d1, d2, and d3, the area/molecule in the bilayer of each surfactant, Ac and An (for the C16TAB and C12EO6), the fractional coverage, f (the fraction of surface covered by bilayer patches or flattened micelles), and fc. From known
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previous observations for C16TAB/C12EO620 and previous values reported for pure C12EO613,14,16,20 and C16TAB15,17,20 adsorbed layers. The extent of the hydrocarbon chain region (d2) is less than the fully extended chain length for either C12 or C16 but can be accounted for by the fraction of alkyl chains in the headgroup regions (fc ∼ 0.2). The inter-mixing of the headgroups and alkyl chains is also seen in the pure adsorbed layers and at the air-water interface, but the requirement here is to pack two headgroups of significantly different sizes and the two different alkyl chain lengths are additional driving forces. The area per molecule of C16TAB and C12EO6 (Ac, An) in the mixed surface layer reflects the surface composition and total amount adsorbed. The total adsorbed amount is given by
Γ)
Figure 2. (a) As in Figure 1a. (b) As in Figure 1a except at the silicon/H2O interface and (2) d-C16TAB/d-C12EO6. Solid lines are model calculations as described in the text.
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, Ac, An, f, and fc) for each profile on a least-squares basis to give the minimum spread in the model parameters from the reflectivities for all the differently isotopically labeled combinations that were measured. The parameters for the oxide layer at the hydrophilic silicon surface were included and determined from separate measurements (see Experimental Section). The results of such an analysis procedure are shown in Figure 2 for 10-4 M 70 mol % C16TAB/C12EO6, and the fitted parameters for all the measurements are listed in Table 1. The parameters listed in Table 1 are the mean values obtained from the analysis described above, and the quoted errors are estimated from the spread in values obtained from the fits to the different profiles. Measurements were made at two different surfactant concentrations, 10-4 and 10-3 M, and for a range of solution compositions from 80 to 20 mol % C16TAB, and the mean values for all these combinations are summarized in Table 1. The structural parameters summarized in Table 1 are similar to those previously reported by us for C16TAB/ C12EO6 at the hydrophilic silicon/solution interface.20 There is no obvious systematic variation with surfactant concentration or composition of d1, d2, d3, and f; the mean values from the measurements are d1 ) 11.6 ( 1.2, d2 ) 13.4 ( 1.7, d3 ) 14.3 ( 2.0, and f ) 0.71 ( 0.05. The fractional coverage is ∼0.7 and is consistent with our
(
)
2f 1 1 + Na Ac An
(5)
where Na is Avogadro’s number The variation of adsorbed amount with solution composition for both concentrations (10-4, 10-3 M) is shown in Figure 3. The total adsorbed amount at both concentrations shows a similar trend with composition, and the adsorbed amount is a maximum at an equimolar solution composition. Compared to the pure C12EO6 or C16TAB adsorbed layers, the mixture shows an enhanced adsorption, evidence of the synergy of mixing. The maximum adsorbed amount is ∼9 × 10-10 mol cm-2, whereas for C16TAB or C12EO6 alone the maximum adsorption is ∼6 × 10-10 mol cm-2. Thomas and Hines33 have reported that the adsorbed amount for the anionic/zwitterionic mixture of sodium dodecyl sulfate, SDS, and dodecyldimethylamino actetate, Betaine, at the hydrophilic silicon/solution interface goes through a maximum with concentration at a concentration greater than the cmc. For nonionic mixtures with large differences in cmc Brinck and Tiberg26 also report a maximum in the adsorption with concentration at low total solution concentrations. The data reported here are consistent with those observations, although the variation with concentration has not been systematically studied in sufficient detail to confirm that observation. The variation of surface composition with solution composition for both surfactant concentrations (10-4, 10-3 M) is shown in Figure 4. For solution compositions rich in C16TAB, the solution and surface compositions are in close agreement, consistent with ideal mixing as observed at the air-water interface11 and in mixed micelles.31 From SANS measurements the C16TAB/C12EO6 mixed micelles in 0.1 M NaBr31 show ideal mixing, with similar micelle and solution compositions over a wide range of solution compositions and concentrations. Specular neutron reflection measurements for C16TAB/C12EO6 mixtures in 0.1 M NaBr11 at the air-water interface and for concentrations similar to those used in this study also indicate ideal mixing. However, in the measurements reported here for both surfactant concentrations and for solution compositions
