6036
Langmuir 1996, 12, 6036-6043
Neutron Reflection from Hexadecyltrimethylammonium Bromide Adsorbed on Smooth and Rough Silicon Surfaces Giovanna Fragneto and Robert K. Thomas* Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, U.K.
Adrian R. Rennie Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, U.K.
Jeffrey Penfold Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0RA, U.K. Received May 13, 1996. In Final Form: September 3, 1996X Neutron reflection has been used to investigate the structure of hexadecyltrimethylammonium bromide (C16TAB) layers adsorbed at the silicon/silicon oxide/aqueous interface. Separate isotopic labeling of groups of four methylene groups at a time made it possible to determine the structure of the layer at a higher resolution than previously possible. The structure of the layer was investigated on smooth and rough surfaces. The roughness of the surface has a significant effect on the properties of the layer. On the rough surface the bilayer is shown to be thicker and to be unsymmetrical in the direction of the surface normal. The surface coverage was also found to be lower on the rough surface. As in previous studies the surface was found not to be completely covered, lending support to the idea that adsorption is in the form of aggregates. The division into smaller isotopically labeled fragments shows, however, that the aggregates strongly resemble bilayer fragments, and their overall thickness, at 32 ( 1 Å, is substantially less than the diameter of a micelle.
Introduction Neutron reflection has been successfully used for studies of surfactant adsorption at hydrophilic solid surfaces,1-4 and a significant fraction of the work done has involved the adsorption of the cationic surfactant hexadecyltrimethylammonium bromide (C16H33N(CH3 )3+Br-, denoted C16TAB) on hydrophilic quartz, silica, and silicon surfaces. The results indicate that at the critical micelle concentration (cmc) bilayers of surfactant are formed on the hydrophilic surface with an overall thickness of 34 Å on the three surfaces.3,4 Other techniques have also been used to study aspects of the adsorption of C16TAB, for example, on silica,5-7 but these provide little or no direct information concerning the structure of the adsorbed layer. More recently Manne and Gaub have shown that C14TAB forms spherical aggregates on silica using atomic force microsopy.8 The AFM experiment is extremely sensitive to lateral inhomogeneity but has poor resolution in the * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, November 15, 1996. (1) McDermott, D. C.; Lu, J. R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R.; Langmuir 1992, 8, 1204. Lee, E. M.; Thomas, R. K.; Cummins, P. G.; Staples, E. J.; Penfold, J.; Rennie, A. R. Chem. Phys. Lett. 1989, 162, 196. Lee, E. M.; Thomas, R. K.; Rennie, A. R. Europhys. Lett. 1990, 13, 135. McDermott, D. C.; Kanelleas, D.; Thomas, R. K.; Rennie, A. R.; Satija, S. K.; Majkrzak, C. F. Langmuir 1993, 9, 2404. (2) Rennie, A. R.; Lee, E. M.; Simister, E. A.; Thomas, R. K.; Langmuir 1990, 6, 1031. (3) McDermott, D. C.; Thesis, D. Phil Oxford, 1993. (4) McDermott, D. C.; McCarney, J.; Thomas, R. K.; Rennie, A. R. J. Colloid Interface Sci. 1994, 162, 304. (5) Bijsterbosch, B. H. J. Colloid Interface Sci. 1974, 47, 186. Gu, T.; Huang, Z. Colloids Surf. 1989, 40, 71. Gu, T.; Huang, Z.; Sun, Z.; Forsling, W. Acta Chem. Scand. 1991, 45, 526. Connor, P.; Ottewill, R. H. J. Colloid Interface Sci. 1971, 37, 643. Ralston, J.; Kitchener, J. A. J. Colloid Interface Sci. 1975, 50, 242. (6) Kekicheff, P.; Christenson, H. K.; Ninham, B. W. Colloids and Surf. 1989, 40, 31. (7) Pashley, R. M.; McGuiggan, P. M.; Horn, R. G.; Ninham, B. W. J. Colloid Interface Sci. 1988, 126, 569. (8) Manne, S.; Gaub, H. E. Science 1995, 270, 1480.
S0743-7463(96)00464-7 CCC: $12.00
direction normal to the surface. Neutron reflection has exactly the opposite characteristics, and the results of the two techniques should be complimentary. One of the main difficulties in working with the surface of hydrophilic silica is that the surface is very variable in terms of its adsorption of surfactants, and the origins of this behavior are not understood. In this paper we extend the earlier neutron reflection work by examining the effect of the roughness of the underlying surface on the structure of the layer and by using more selective isotopic labeling of the surfactant in order to obtain higher resolution information about the bilayer structure in the direction normal to the interface. In previous work on surfactants adsorbed at the solid/liquid interface we have used simple labeling schemes, for example, labeling the hydrophobic or hydrophilic groups as a whole. In recent experiments on the air/water interface we have shown that a considerable improvement in the resolution may be obtained by using partially labeled chains, and we now extend that method to the solid/liquid interface. The C16TAB chain was labeled in blocks of four carbon atoms, and we used the set of isotopic species given in Table 1. The notation ‘0’Cm in Table 1 means that the material is a mixture of two surfactant isotopes, one having m carbons bonded to protons and the other having all the carbons deuteriated, mixed in such a proportion that the scattering length density of that part of the molecule is zero.
Theory of Neutron Reflection In a neutron reflection experiment, the specular reflection, R, is measured as a function of the wave vector (9) Lu, J. R.; Li, Z. X.; Smallwood, J.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1995, 99, 8233.
© 1996 American Chemical Society
Hexadecyltrimethylammonium Bromide on Silicon
Langmuir, Vol. 12, No. 25, 1996 6037
Table 1. Surfactants Used and Water Contrasts for Reflectivity Measurements of Adsorption on the Two Blocks at the Surfactant cmc surfactant species
rough block (A)
smooth block (B)
dC16dTAB ‘0’C12dC4dTAB ‘0’C8dC8dTAB ‘0’C4dC12dTAB hC16hTAB dC4‘0’C12hTAB dC16hTAB
D2O, H2O, and water2.07 D2O and water2.07 D2O and water2.07 D2O and water2.07 D2O, H2O, and water2.07 D2O and water2.07 D2O and water2.07
D2O, H2O, and water2.07 D2O and water2.07 D2O and water2.07 D2O and water2.07 D2O and water2.07
transfer, κ, perpendicular to the reflecting surface, where
κ)
4π sin θ λ
Table 2. Properties of Materials Used material
densitya (g cm-3)
volumeb (Å3 )
Si SiO2 H2O D2O water2.07 water3.41 C16H33 C16D33 N(CH3 )3+BrN(CD3 )3+Br-
2.32 2.16 0.9975 1.105 1.038 1.059 0.82 0.94 1.59 1.77
20 47 30 30 30 30 456 456 145 145
lengthc (Å)
bd (10-4 Å)
22 22 4 4
0.415 1.585 -0.168 1.905 0.621 1.023 -1.733 33.11 0.240 9.566
a Reference 18. b Calculated from densities. c Fully extended chains.19 d Scattering lengths from ref 20.
(1)
θ is the glancing angle of incidence, and λ is the wavelength of the incident neutron beam. R(κ) is related to the scattering length density across the interface, F(z), by the approximate relation
lengths and scattering length densities of the species used in the present study are given in Table 2. Experimental Details
F(z)being a function of the distance perpendicular to the interface. These relations show that there is a direct relationship between a reflectivity profile and the scattering length density profile, and hence the composition profile through 1 normal to the interface. In practice the only satisfactory route to the composition profile is by fitting models and using the more exact optical matrix formulation of the reflectivity. Using this second method of analysis, the measured data are compared with a reflectivity profile calculated using the optical matrix method applied to different model density profiles. The model typically consists of a series of layers, each with a scattering length density F and thickness t, into which the interfacial roughness between any two consecutive layers, σ, may be incorporated if necessary. The calculated profile is compared with the measured profile, and F and t for each layer are varied until the optimum fit to the data is found. Although any one profile is not necessarily a unique solution, different isotopic contrasts usually supply sufficient additional information to ensure the uniqueness of the interpretation. Once each layer has been characterized by its thickness and scattering length density, then the area per molecule or the coverage is easily determined, as described in ref 11. Because different nuclei may scatter neutrons with different amplitudes and, in the case of protons and deuterons, with opposite phases, the use of a combination of protonated and deuteriated materials can substantially change the reflectivity profile of a system while maintaining the same chemical structure. It is also possible, by adjustment of the H/D ratio, to prepare solvents which are matched to the surface so that the contrast between surface and solvent is zero, giving a reflectivity profile arising only from the interfacial material. The scattering
In the following description the two blocks will be indicated as rough or block A and smooth or block B. Reflectivity measurements on the rough block were made on the CRISP neutron reflectometer while measurements on the smooth one were made on the SURF reflectometer, both located at the Rutherford Appleton Laboratory. Details of the two instruments are given in refs 12 and 13. The substrates were single crystals of silicon with dimensions 12.7 × 5.08 × 2.54 cm3, purchased from Semiconductor Processing Inc., Massachusetts. Block A (rough) was Syton polished on one large face (111) on a Logitech polisher in the Cavendish Laboratory, Cambridge, U.K. This was our first attempt to polish a silicon block in-house, and the quality of the surface was not as high as expected from a professionally polished block. For example, some scratches were clearly visible. Block B (smooth) was as polished by Engis, U.K., on one large face (111). The silicon blocks were initially soaked for 10 min in nitric acid in order to eliminate impurities which might have assembled on the surfaces during storage. In the case of block A the oxide already present was removed by etching for 10 min in a pH buffered mixture of NH4F (40%)/HF (10%) 7:1 by volume. The surface formed was fairly hydrophobic because of the presence of siloxane groups. In order to make it hydrophilic the block was soaked after rinsing in water in an RCA2 solution (NH4OH/ H2O2/H2O 1:1:5 v/v) for 5 min at 70 °C.14 Upon removal from this solution the block was hydrophilic because of the silanol groups on the surface. Block B was not etched and was made hydrophilic by using an RCA1 solution (HCl/H2O2/H2O 1:1:6 v/v) for 5 min at 70 °C.14 The different surfactant isotopes (see Table 1) were synthesized in our laboratories.9 Surface tension measurements at the air/ solution interface of the isotopic species of C16TAB in H2O gave no indication of either isotope effects or impurities. The cmc was found to be 9.0 × 10-4 M.9 High-purity water was used for the preparation of the surfactant solutions and the cleaning of the substrate. D2O (>99.8%) was purchased from CDN, Canada, from which water having scattering length densities of 2.07 × 10-6 Å-2 (water2.07) and 3.41 × 10-6 Å-2 (water3.41) was prepared by mixing H2O and D2O in the mass ratios 0.520:0.480 and 0.401: 0.599, respectively. Other chemicals used in the cleaning of the solid substrates were the highest purity reagents available from Aldrich. The coherent scattering length densities and some physical dimensions of materials used in this work are given in Table 2. Once the surface had been characterized, the various C16TAB isotopes were adsorbed from aqueous solutions of different isotopic compositions at the cmc. The temperature of the CRISP and SURF sample areas was raised to 28 °C when the surfactant solutions were used, since the C16TAB Krafft point is at about 25 °C. The measurements made are summarized in Table 1.
(10) Heavens, O. S. Optical Properties of Thin Films; Butterworths: London, 1955. (11) Fragneto, G.; McDermott, D. C.; Lu, J. R.; Thomas, R. K.; Rennie, A. R.; Satija, S. K.; Gallagher, P. D. Langmuir 1996, 12, 477.
(12) Penfold, J.; Ward, R. C.; Williams, W. G. Rutherford Appleton Report RAL-87-014-1987. (13) ISIS 1995, Annual Report, p 104. (14) Kern, W. J. Electrochem. Soc. 1990, 137, 1887.
R(κ) )
16π2 |Fˆ (κ)|2 κ2
(2)
where Fˆ (κ) is the one-dimensional Fourier transform of F(z)
Fˆ (κ) )
∫-∞+∞exp(-iκz)F(z) dz
(3)
6038 Langmuir, Vol. 12, No. 25, 1996
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Table 3. Parameters Used To Fit the Oxide Layer on Rough (A) and Smooth (B) Blocks (Gi Is the Scattering Length Density in Water of Isotopic Composition i) block
thickness (Å)
roughness (Å)
FD2O (×106 Å-2)
FH2O (×106 Å-2)
F2.07 (×106 Å-2)
F3.41 (×106 Å-2)
A B
23 ( 2 10 ( 1
14 ( 2 6(1
4.08 ( 0.05 3.41 ( 0.05
1.40 ( 0.05 3.41 ( 0.05
2.42 ( 0.05 3.41 ( 0.05
2.94 ( 0.05 3.41 ( 0.05
Figure 1. Neutron reflectivity profiles and fitted curves at the Si/SiO2/water interface of (a) block A and (b) block B. The points are D2O (b), H2O (9), water2.07 (]), and water3.41 (2). The fitted lines were calculated using the parameters given in Table 3.
Results Bare Surface. Before the structure of a surfactant layer on the hydrophilic silicon can be determined, it is necessary to know the structure of the underlying surface. This was done for each block by measuring four reflectivity profiles for the bare silicon surface in contact with D2O, H2O, water2.07, and water3.41. The observed and fitted reflectivity profiles for both blocks are shown in Figure 1 while the structural parameters of the interface obtained by fitting the same model structure to all the data are summarized in Table 3. For block A the parameters in Table 3 are consistent with the presence of an oxide layer 23 ( 2 Å thick consisting of about 60% of Si2O3 (F ) 2.65 × 10-6 Å-2) and 39% water. This is consistent with the high value of the roughness (14 Å) necessary to fit the data and is a plausible description for this type of solid surface. We have found in previous studies that the oxide on a silicon block can be porous and incorporate water,1 although this is the first time that an overall composition corresponding to an incomplete oxide has been found using neutron reflection. For block B the parameters in Table 3 show that the oxide layer on this smooth block is thinner than that on block A and has a relatively small value of the roughness at the interface. Only a narrow range of compositions and thicknesses could be made to fit the data, varying from a 10 Å thick layer consisting of 100% SiO2 to a 15 Å layer consisting of 78.8% SiO2 and 21.2% water. The latter may be interpreted as either a uniform layer or, as is more probable, a layer showing some roughness with
the water located more toward the side of the liquid phase. The values are in good agreement with past results, and a more detailed description of a similar layer is found in ref 11. Surfactant Layer. The surfactant species used are listed in Table 1. Apart from the fully protonated and fully deuteriated species, we also used isotopes with partially labeled chains in an attempt to highlight parts of the adsorbed layer and obtain a higher resolution picture of the structure at the interface than obtained previously. In the present case the use of the isotopic species was particularly useful in showing the effect of the surface roughness on the adsorption. The more detailed labeling, however, did make it more difficult to analyze the data from the solid/liquid interface in comparison with the air/ liquid experiment. This was partly because it was not possible to eliminate the signal from all parts of the interface except the chosen fragment and hence there were always contributions to the reflectivity from at least two regions of the interface, and partly because of the larger number of layers necessary to describe the structure adequately. The neutron reflectivity profiles and the fitted curves for the adsorption of the fully protonated (hC16hTAB) and fully deuteriated (dC16dTAB) surfactants are shown in Figure 2, and the parameters used to fit the data are summarized in Table 4. A single-layer model was found to fit all but one (the fully deuteriated C16TAB on the rough block in D2O) of the sets of measured data on the fully substituted surfactants. The first difference between the layers adsorbed on the two surfaces is that the layer on the rough block is thicker (37 ( 1 Å) than the one on the smooth block (32 ( 1 Å) by about 5 Å . Moreover, to fit the data satisfactorily for the rough block, a greater roughness is needed not only at the oxide layer (∼14 ( 2 Å) but also at the adsorbed layer interface (5-8 Å). The value of the overall thickness of the layer on both blocks is consistent with the formation of a bilayer with a nearly complete overlap of the alkyl chains, the length of the fully extended molecule being about 25 Å. On the rough block the fully deuteriated surfactant could not be described by a one-layer model and the logical next choice was a three-layer model with the head-group regions distinguished from the chain region and the bilayer being symmetrical. However, this model also failed to fit the data, and only when the inner and outer layers of the bilayer were allowed to be different, as shown schematically in Figure 5a, did we succeed in obtaining a good fit. This result is further supported by the data from the partially labeled surfactants discussed below. From Table 5, where the properties of the model derived from Table 4 are summarized, it can be seen that the coverage of surfactant is significantly higher on the smooth block (B) and that within the layer the surfactant is more tightly packed. There are also smaller differences between the adsorbed amounts of the fully protonated and fully deuteriated surfactants. This should not be regarded as a definite isotope effect because it is more probable that it results from deficiencies in the models used to fit the data. The values of the surface excess are similar to those determined by McDermott et al.3,4 and Bijsterbosch, Connor, and Ralston5 while the values of the thickness are in good agreement with those determined by McDermott (34 Å),3 Rennie (40 Å),2 Kekicheff (32 Å),6 and Pashley
Hexadecyltrimethylammonium Bromide on Silicon
Langmuir, Vol. 12, No. 25, 1996 6039
Table 4. Parameters Used To Fit the Fully Protonated and Deuteriated Surfactant Layer on Rough (A) and Smooth (B) Blocks block
layer
thickness (Å)
roughness (Å)
FD2O (×106 Å-2)
FH2O (×106 Å-2)
F2.07 (×106 Å-2)
A
oxide hC16hTAB oxide hC16hTAB oxide dC16dTAB dC16dTABa dC16dTABb oxide dC16dTAB
23 ( 2 37 ( 2 10 ( 1 32 ( 1 23 ( 2 37 ( 2 14 ( 1 23 ( 1 10 ( 1 32 ( 1
14 ( 1 5(1 6(1 0 14 ( 1 8(1 5(1 8(1 6(1 5(1
2.55 ( 0.05 2.83 ( 0.05 3.41 ( 0.05 1.79( 0.05 4.08 ( 0.05
1.67 ( 0.05 -0.38 ( 0.05 2.50 ( 0.05 2.86 ( 0.05
1.34 ( 0.05 0.95 ( 0.05 3.41 ( 0.05 0.47 ( 0.05 3.10 ( 0.05 4.08 ( 0.05
3.41 ( 0.05 4.04 ( 0.05
3.41 ( 0.05 4.87 ( 0.05
B A
B a
4.85 ( 0.05 6.08 ( 0.05 3.41 ( 0.05 6.18 ( 0.05
Disordered. b Ordered. Table 5. Properties of the Surfactant Layer on the Two Blocks block surfactant Aa (Å2) Γa (µmol m-2) hC16hTAB dC16dTAB average hC16hTAB dC16dTAB average
A B
a
63 ( 1 69 ( 1 66 ( 4 56 ( 1 63 ( 1 60 ( 5
2.64 ( 0.04 2.41 ( 0.03 2.52 ( 0.16 2.97 ( 0.05 2.64 ( 0.04 2.8 ( 0.2
φC16TAB
n
0.53 ( 0.01 0.54 ( 0.01 0.53 ( 0.02 0.69 ( 0.01 0.70 ( 0.01 0.69 ( 0.02
18.0 ( 0.5 20.0 ( 0.5 19.0 ( 1.5 9.0 ( 0.5 10.5 ( 0.5 10 ( 1.5
The area and surface excess refer to two molecules of the bilayer.
Table 6. Parameters Used To Fit Partially Labeled Surfactant Layers on Rough (A) and Smooth (B) Blocks
B
A
B
(33 Å).7 It is worth noting that the last two measurements were made on mica, which is known to have a very smooth surface. Of the partially labeled surfactants listed in Table 1 only the ‘0’C16-mdCmdTAB with m ) 4, 8, and 12 were
oxide inner central outer oxide inner central outer
(a) ‘0’C12dC4dTAB 23 ( 2 14 ( 1 3.21 ( 0.05 7(1 0 4.85 ( 0.05 21 ( 1 0 3.73 ( 0.05 8(1 5(1 6.08 ( 0.05 10 ( 1 6(1 3.41 ( 0.05 10 ( 1 0 5.77 ( 0.05 12 ( 1 3(1 0.60 ( 0.05 10 ( 1 5(1 5.77 ( 0.05
2.46 ( 0.05 3.19 ( 0.05 1.95 ( 0.05 3.66 ( 0.05 3.41 ( 0.05 3.70 ( 0.05 0.06 ( 0.05 3.70 ( 0.05
oxide inner central outer oxide inner central outer
23 ( 2 12 ( 1 12 ( 1 12 ( 1 10 ( 1 8(1 16 ( 1 8(1
(b) ‘0’C8dC8dTAB 14 ( 1 3.86 ( 0.05 0 4.85 ( 0.05 0 3.73 ( 0.05 5(1 6.08 ( 0.05 6 (1 3.41 ( 0.05 6(1 6.67 ( 0.05 6(1 4.27 ( 0.05 6(1 6.67 ( 0.05
oxide inner central outer oxide inner central outer
23 ( 2 12 ( 1 12 ( 1 12 ( 1 10 ( 1 10 ( 1 12 ( 1 10 ( 1
(c) ‘0’C4dC12dTAB 14 ( 1 3.40 ( 0.05 0 5.37 ( 0.05 4.83 ( 0.05 6(1 6.09 ( 0.05 6(1 0 0 6(1
A
A
Figure 2. Neutron reflectivity profiles and fitted curves at the Si/SiO2/C16TAB/water interface of (a) hC16hTAB on block A, (b) dC16dTAB on block A with D2O (b), H2O (]), and water2.07 (9), and (c) block B with hC16hTAB/D2O (b), dC16dTAB/D2O (O), dC16dTAB/H2O (]), hC16hTAB/water2.07 (9), and dC16dTAB/ water2.07 (0). The continuous lines are profiles calculated using the parameters in Table 4.
F2.07 (×106 Å-2)
layer
B
thickness roughness (Å) (Å)
F D 2O (×106 Å-2)
block
2.55 ( 0.05 3.19 ( 0.05 1.95 ( 0.05 3.66 ( 0.05
3.30 ( 0.05 3.48 ( 0.05 2.91 ( 0.05 4.04 ( 0.05 3.41 ( 0.05 5.29 ( 0.05 3.65 ( 0.05 5.29 ( 0.05
adsorbed on the smooth surface. Figures 3 and 4 show the measured and fitted reflectivity profiles from the partially labeled surfactants on both blocks, and the parameters used to fit the data are summarized in Tables 6 and 7. ‘0’C12dC4dTAB was studied on both rough and smooth surfaces from solutions in D2O and water2.07. Neither a single nor a two-layer model would give a satisfactory fit to the observed reflectivities. The model finally chosen, which gave a good fit to both sets of data, consisted of three layers, the inner and outer layers being the headgroups and the central layer being the chain region, and the overall thickness was constrained to be approximately the same as for the fully substituted surfactants. Sche-
6040 Langmuir, Vol. 12, No. 25, 1996
Fragneto et al.
Figure 4. Neutron reflectivity profiles and fitted curves at the Si/SiO2/C16TAB/water interface of block A from dC“0”C12hTAB/ D2O (b), dC“0”C12hTAB/water2.07 (9), dC16hTAB/D2O (O), and dC16hTAB/water2.07 (0). The continuous lines are profiles calculated using the parameters in Table 7. Table 7. Parameters Used To Fit Partially Labeled Surfactant Layers on Rough (A) Block
Figure 3. Neutron reflectivity profiles and fitted curves at the Si/SiO2/C16TAB/D2O (a) and water2.07 (b) interface of block A and from the Si/SiO2/C16TAB/water interface of block B (c). In parts a and b the isotopic species of C16TAB are “0”C12dC4dTAB (b), “0”C8dC8dTAB (9), and “0”C4dC12dTAB (]). In part c the isotopic species of C16TAB are “0”C12dC4dTAB/D2O (b), “0”C12dC4dTAB/water2.07 (9), “0”C8dC8dTAB/D2O (O), and “0”C4dC12dTAB/water2.07 (0). The continuous lines are profiles calculated using the parameters in Table 6.
matic diagrams of the division of the layers for this isotope on rough and smooth blocks are given in Figures 5b and 6b, respectively. Note that the division into the three layers is both different on the two types of block and different from the division for other isotopes. The composition of the different layers can be derived from the fitted parameters of Table 7, and the values are given in Table 8. In the case of block A the inner and outer layers contain different fractions of the various components, as deduced from the dC16dTAB/D2O contrast above, but on block B the two layers have the same composition, and the central layer is more compact than on the rough block. Note that this isotopic species on block B, which defines the central hydrophobic region of the layer more closely, shows that the water is not distributed uniformly
layer
thickness (Å)
oxide inner central outer oxide inner outer
roughness (Å)
F D 2O (×106 Å-2)
F2.07 (×10& Å-2)
23 ( 2 12 ( 1 12 ( 1 12 ( 1
(a) dC4‘0’C12hTAB 14 ( 1 3.31 ( 0.05 0 2.83 ( 0.05 0 4.83 ( 0.05 6(1 3.04 ( 0.05
2.55 ( 0.05 0.95 ( 0.05 2.95 ( 0.05 0.99 ( 0.05
23 ( 2 31 ( 1 5(1
(b) dC16hTAB 14 ( 1 2.71 ( 0.05 0 5.30 ( 0.05 0 3.80 ( 0.05
2.55 ( 0.05 3.61 ( 0.05 1.10 ( 0.05
in the layer, although the total amount is the same as found for the fully substituted isotopes. The labeling in this surfactant species was particularly useful for discriminating between monolayer and bilayer formation. This was especially the case for the profile with water2.07, since a monolayer would have the effect of creating two approximately equal layers of quite different scattering length densities (about 12 Å thick), which would give a reflectivity very different from that observed. For this reason we are able to discard any structure resembling a monolayer of surfactant on either of the two blocks. ‘0’C8dC8dTAB was also studied using the two contrasts of D2O and water2.07. The model found to describe the adsorbed surfactant most successfully again consisted of three layers, the inner and outer layers being the headgroups and the central layer being the chain region (see Figures 5c and 6c). The composition of the layers, derived from the parameters of Table 6, is given in Table 8. As in the previous case, the inner and outer layers on block A are different but on block B they are the same. On block B the inner and outer layers consist only of the headgroups while the central layer is formed by the chains. The deuteriated part of the chains and the ‘0’ scattering length fragments in this layer are present in the same amount. The overall thickness is ∼36 Å on block A and ∼32 Å on block B. Again the water is not distributed uniformly in the layer, although the relatively wide central region gives a less nonuniform distribution than for the previous isotope. The total amount of water is the same as in the case of the fully substituted isotopes. This was also a useful contrast for demonstrating that there was not a monolayer of surfactant molecules on either surface. ‘0’C4dC12dTAB was the last surfactant to be adsorbed on both surfaces from solutions in D2O and water2.07. It is more difficult in the case of a surfactant with such a
Hexadecyltrimethylammonium Bromide on Silicon
Figure 5. Schematic representation of the model employed to fit the data from C16TAB, at the different isotopic compositions, adsorbed on block A. Note that the surfactant chains are not drawn as a realistic representation of the structure but in order to identify the key observations that the inner and outer layers are different and that the inner layer is less densely packed.
small isotopically substituted part to localize the position of the labeled fragment in the layer, and indeed in the present analysis the determination of the exact position of the fragment ‘0’C4 proved to be impossible and one of the profiles (D2O on the smooth block) could not be fitted well. The remaining profiles were fitted with a threelayer model (see Figures 5d and 6d). The composition of the layers, from the parameters of Table 6, is given in Table 8. It is possible to deduce that the ‘0’ groups are localized more toward the solid surface on block A, confirming the nonuniformity of the bilayer. On block B they could not be localized. This was not completely unexpected because if the system is as shown in Figure 6, it would imply that a six-layer model (including the oxide) would be needed to fit the data. Again the water is not distributed uniformly in the layer, although the total amount is the same as in the case of the fully substituted isotopes. dC4‘0’C12hTAB was adsorbed on block A from solutions in D2O and water2.07. This isotopic species is complementary to ‘0’C4dC12dTAB in that the labeled parts are reversed, and a similar model of the interface is therefore expected to be appropriate. A three-layer model for the surfactant was found to fit both profiles. All layers have a thickness of 12 ( 1 Å, so that the overall thickness is ∼36 Å. As in the previous case for ‘0’C4, it was not possible to define the exact position of the deuteriated fragment. From the parameters of Table 7 the compositions of the layers given in Table 8 were derived. The values above are in good agreement with values from ‘0’C4dC12dTAB, although not in perfect agreement with all the others. It has already been mentioned that it is difficult to localize small fragments (four methylene groups) at an interface as complicated as the rough silicon/water interface. dC16hTAB was adsorbed on block A from solutions in D2O and water2.07. A two-layer model for the surfactant
Langmuir, Vol. 12, No. 25, 1996 6041
Figure 6. Schematic representation of the model employed to fit the data from C16TAB, at the different isotopic compositions, adsorbed on block B. Note that the surfactant chains are not drawn as a realistic representation of the structure but in order to identify how the labeling of the surfactant translates into the division of layers for the modelling and to show that the inner and outer layers are identical. Table 8. Compositions of the Sublayers for Partially Labeled Surfactants inner layer central layer outer layer inner layer central layer outer layer
(a) ‘0’C12dC4dTAB 39% water 26% ‘0’ 42% water 41% ‘0’ 57% water 6% ‘0’ 48% water 12% ‘0’ 9.5% water 90.5% ‘0’ 48% water 12% ‘0’
inner layer central layer outer layer inner layer central layer outer layer
(b) ‘0’C8dC8dTAB 39% water 26% ‘0’ 42% water 41% ‘0’ 57% water 6% ‘0’ 30.5% water 35% water 31.5.5% ‘0’ 30.5% water
35% dTAB 17% dC12 37% dTAB 69.5% dTAB 33.5% dC12 69.5% dTAB
inner layer central layer outer layer inner layer central layer outer layer
(c) ‘0’C4dC12dTAB 44% water 12% ‘0’ 44% water 21% ‘0’ 48% water 30.5% water 25% water 19% ‘0’ 30.5% water
44% dC12dTAB 35% dC12 52% dC12dTAB 69.5% dC12dTAB 56% dC12 69.5% dC12dTAB
block A
inner layer central layer outer layer
(d) dC4’0’C12hTAB 44% water 44% ‘0’ 44% water 21% ‘0’ 48% water 52% ‘0’
block A
inner layer outer layer
(e) dC16hTAB 40% water 48%dC16 63% water
block A block B
block A block B
block A block B
35% dTAB 17% dC12 37% dTAB 40% dTAB 40% dTAB
12% dC4 35% dC4
12%hTAB 37% hTAB
was found to fit both profiles. The layer closer to the surface is 31 Å thick and consists mainly of the deuteriated chain region while the outer layer, 5 Å thick, represents the protonated heads. The thickness of the inner layer is higher than the fully extended chain length of the surfactant molecule, indicating again the formation of a bilayer. A three-layer model did not fit the data, and therefore it was not possible to locate exactly the position
6042 Langmuir, Vol. 12, No. 25, 1996
Fragneto et al. Table 9. Parameters Used for Fitting thickroughness (Å) ness (Å)
layer
FD2O (×106 Å-2)
F2.07 (×106Å-2)
volume fraction
oxide surfactant
(a) Fully Protonated Surfactant Layer at cmc/30 on the Rough Block 23 ( 2 14 ( 1 2.55 ( 0.05 1.67 ( 05 33 (1 4 ( 1 4.43 ( 0.05 1.75 ( 0.05 0.37 ( 0.01
oxide surfactant
(b) Fully Deuteriated Surfactant Layer at cmc/30 on the Rough Block 23 ( 2 14 2.55 ( 0.05 3.10 ( 0.05 33 ( 1 0 1.21 ( 0.05 2.80 ( 0.05 0.39 ( 0.01
(c) Fully Protonated Surfactant Layer at cmc/10, cmc/30, and cmc/100 on the Smooth Block
Figure 7. Neutron reflectivity profiles and fitted curves at the Si/SiO2/C16TAB/water interface of block A from surfactants at a concentration of cmc/30: hC16hTAB/D2O (b), hC16hTAB/ water2.07 (9), dC16dTAB/H2O (O), and dC16dTAB/water2.07 (0). The continuous lines are profiles calculated using the parameters in Table 9.
Figure 8. Neutron reflectivity profiles and fitted curves at the Si/SiO2/hC16hTAB/D2O interface of block B from hC16hTAB/ D2O at concentrations of cmc (b), cmc/10 (9), cmc/30 ([), and cmc/100 (4). The continuous lines are profiles calculated using the parameters in Table 9.
of the head-group region adjacent to the surface, although the low values of the SLD of the oxide indicate that the head-groups have penetrated the oxide layer. The composition of the layers is given in Table 8. Measurements were made at a concentration of cmc/30 on the rough block from solutions of both the fully deuteriated surfactant in H2O and water2.07 and the fully protonated one in D2O and water2.07, and at cmc/10, cmc/ 30, and cmc/100 on the smooth block from solutions of the fully protonated surfactant in D2O. The measured data and the fitted curves are shown in Figures 7 and 8, the fitting parameters being listed in Table 9. In all cases the adsorbed surfactant volume fraction decreases substantially below the cmc, and now the adsorbed amount and the volume fractions of the surfactant on the two blocks are similar. The thickness is less at this lower concentration, and the layer on the smooth block is slightly thinner than that on the rough block, just as found at the cmc. Note that on the smooth block measurements were made only in one water contrast so that more assumptions are necessary to obtain the parameters of the layer. Conclusions There is general agreement in the literature that at concentrations around the cmc C16TAB adsorbs onto silica in some sort of bilayer structure. These conclusions are
conc
thickness (Å)
FD2O (×106 Å-2)
volume fraction
cmc/10 cmc/30 cmc/100
30 ( 1 28 ( 1 28 ( 1
3.73 ( 0.05 3.99 (0.05 3.68 ( 0.05
0.40 ( 0.01 0.36 ( 0.01 0.41 ( 0.01
based mainly on the amount adsorbed5 and the force distance curves in force balance measurements.6,7 Surface forces13 and more recent results from FTIR and NMR14,15 also indicate that the bilayer is not a continuous structure and is probably better regarded as a hemimicelle structure. Very recently noncontact atomic force microscopy has been used to image C14TAB on the hydrophilic surfaces of mica and amorphous silica and has revealed aggregates on silica that are circular in their projected shape on the surface.8 This is the clearest evidence that the CTABs do not adsorb uniformly on the hydrophilic silica surface. All the neutron results on C16TAB on silica have consistently shown that the surface is far from completely covered, which could be explained either by some sort of defective bilayer or by the adsorption of micelle-like aggregates.3 Although neutron specular reflection gives the most direct measurement of the overall thickness of these aggregates, it gives no information about the inplane structure and so it is not possible to make a quantitative statement about either the lateral dimension of the aggregates or the defect free length in the case of a defective bilayer. The evidence from neutron reflection that the surfactant does not cover the surface completely rests on the resolution of the conflict between the two observations. The first is that C16TAB adsorbs as a bilayer, and the present results show this unambiguously for the first time. The second is that there is always apparently a significant fraction of water in the central part of the bilayer. Thus the measurement that most closely determines the amount of water in the central hydrophobic region is the isotopic species in Table 8a (see also Figure 5b for the definition of the layers), and this shows that there is about 10% water in the central region. Because neutron specular reflection does not give any direct information about the lateral structure, it cannot distinguish between lateral segregation of the bilayer and water or uniform incorporation of the water into the bilayer. The former is physically much more probable and the sensitivity of the adsorbed amount to the exact treatment of the surface is also more consistent with segregation. A rather openly packed bilayer containing a significant fraction of water would surely be much less responsive to what must be only minor changes in surface structure. The present results go further than our previous results toward confirming the “bilayer” structure in that isotopic (15) Kung, K. H. S.; Hayes, K. F. Langmuir 1993, 9, 263. (16) Soderlind, E.; Stilbs, P. Langmuir 1993, 9, 2024. (17) Rutland, M. W.; Parker, J. L. Langmuir 1994, 10, 1110. (18) Weast, R. C., Ed. Handbook of Chemistry and Physics, 54th ed.; Chemical Rubber Co.: Cleveland, OH, 1973. (19) Tanford, C. J. J. Phys. Chem. 1972, 76, 3020. (20) Sears, V. F. Neutron News 1993, 3, 26.
Hexadecyltrimethylammonium Bromide on Silicon
labeling clearly shows that there are surfactant head groups on the inside and outside of the layer. If these aggregates were simply rather weakly adsorbed micelles, their thickness would be expected to be of the order of twice the fully extended length of a molecule. However, the measurement of the layer thickness from the present results indicates that on a smooth surface the chains are either extensively interdigitated or strongly tilted away from the surface normal, neither of which is typical of an unperturbed micelle structure. We conclude that if the adsorbed aggregates are micelles they must be quite strongly flattened. This result is not inconsistent with the atomic force microscopy observations of Manne and Gaub8 but adds resolution in the vertical direction to their observations of circular symmetry in the plane of the aggregates. However, it may be premature to compare the results too closely until all the questions concerning the variation of the adsorption with surface treatment of the silica have been resolved. This is the first time to the authors’ knowledge that the effects of roughness have been explicitly examined. There are a number of possible effects that could be associated with extra roughness. For example, it will increase the effective surface area, which should increase the adsorption. On the other hand, if isolated surfactant species are being adsorbed, the environment at the surface might expose the hydrophobic part of the surfactant to more of
Langmuir, Vol. 12, No. 25, 1996 6043
the hydrophilic silica surface. We have observed three clear results. The first is that the amount adsorbed decreases as the surface becomes rougher. This could be because of the effect above on isolated surfactant molecules, or it could be because the rough surface is less well able to match the curvature of the adsorbed aggregates. The second effect is that the thickness increases on the rough surface. This could be because the effect of the surface on an adsorbed aggregate is weaker or simply that the roughness on average adds to the thickness of the layer by causing variations in the position of the center of the aggregate along the surface normal. The third effect we have observed is that the bilayer is no longer symmetrical when the surface is rough and there are then fewer head-groups in the layer adjacent to the silica than in the one projecting into the aqueous solution. Since part of the roughened surface is covered with the chain groups of surfactant molecules adjacent to it, the hydrophobic area available for the second layer of surfactant is effectively larger than the hydrophilic area available for the first layer. Acknowledgment. G.F. thanks Unilever for support and the Rutherford Appleton Laboratory for a CASE Award. LA9604644
