5532
Langmuir 1998, 14, 5532-5538
Growth of Highly Ordered Thin Silicate Films at the Air-Water Interface Anthony S. Brown,† Stephen A. Holt,† Philip A. Reynolds,† Jeffrey Penfold,‡ and John W. White*,† Research School of Chemistry, The Australian National University, GPO Box 414, Canberra, ACT 2601, Australia, and Rutherford-Appleton Laboratory, Chilton, Didcot, Oxon OX11 OQX, U.K. Received April 27, 1998. In Final Form: June 30, 1998 The growth of thin silicate-organic films at the air-water interface of surfactant solutions has been studied in situ by X-ray and neutron reflectivity to a resolution of ca. 5 Å. Surfactant in the solution and the air-water interface itself are involved in directing the growth and final structure of the films. Hexadecyltrimethylammonium chloride (C16TAC) and bromide (C16TAB) have been used as the templating surfactants, and the film structure is independent of the anions under the conditions used. In situ X-ray and neutron reflectivity measurements at an early stage of film growth show a slow development of structure in the top 100 Å of the solution, which is consistent with a monolayer of tilted surfactant molecules at the air-water interface, a layer of partly silicated material, and an interdigitated surfactant bilayer or layer of cylindrical micelles oriented with their long axes parallel to the surface. Following this induction period a rapid crystallization occurs to give a structure with a crystallographic repeat distance of 45 Å perpendicular to the surface and composed of alternating surfactant layers and silicate material. The very narrow observed diffraction peaks indicate that the final silicate film is highly ordered.
Introduction 1
We have recently shown that X-ray reflectivity is an excellent way to observe the growth of highly ordered silicate films at the air-water interface from concentrated surfactant solutions. The process has an induction period during which there is an accumulation and ordering of surfactant and silicate precursor at the interface, followed by the rapid growth of a layered structure that shows strong Bragg-like diffraction indicating a crystallographic repeat unit perpendicular to the surface. Here we have combined X-ray reflectivity data with neutron reflectivity data from two different isotopic combinations to elucidate the film growth processes and structures. The extent to which the new neutron reflectivity results allow alternative models for the film structure which were proposed in our earlier work1 to be distinguished is described. Previous studies2,3 of the structure of C16TAB monolayers at the air-water interface have been conducted at surfactant concentrations close to the critical micelle concentration (cmc) (ca. 0.9 mM). Neutron reflectivity measurements using some 30 isotopically labeled compounds were used to elucidate an extremely detailed description of the structure of C16TAB at the air-water interface. Because the surfactant concentrations used in our preparations are ca. 70 times the cmc we have examined the structure of the solutions in the absence of silicate precursor materials to determine whether any subsurface layering of surfactant molecules occurs beneath the surface monolayer. The isotopic variation used for the neutron reflectivity measurements was achieved using deuterated C16TAB (d33-C16TAB) and the fully hydro† ‡
The Australia National University. Rutherford-Appleton Laboratory.
(1) Brown, A. S.; Holt, S. A.; Thien Dam; Trau, M.; White, J. W. Langmuir 1994, 32, 6363-6365. (2) Lu, J. R.; Hromodova, M.; Simister, E.; Thomas, R. K.; Penfold, J. Physica 1994, B198, 120-126. (3) Lu, J. R.; Li, Z. X.; Smallwood, J.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1995, 99, 8233-8243.
genous chloride analogue (h33-C16TAC). The influence of the counterion on the resulting film structures has therefore also been examined. Experimental Section Although silicate films can be grown using surfactant to silicon molar ratios in the range 0.85-1.57,4 we have prepared all our reaction mixtures in the more restricted range of 0.92-1.05. The reaction mixtures were prepared by stirring an aqueous solution of the surfactant (hexadecyltrimethylammonium chloride(C16TAC) or bromide (C16TAB)) into pure distilled water acidified with hydrochloric acid, to which was then added tetraethoxysilane (TEOS). Final concentrations of surfactant and TEOS were ca. 2.0% (w/w) and 1.2% (w/w), respectively. The resultant mixture was stirred at room temperature until completely clear (ca. 10 min) before being placed in the reflectometer. Three modifications of the basic preparation were used to achieve the desired contrast variation for X-ray and neutron reflectivity measurements. The X-ray reflectivity measurements used a mixture containing h33-C16TAC and H2O, while the neutron reflectivity measurements used either h33-C16TAC and D2O or d33-C16TAB and air-contrast-matched water (ACMW). ACMW is a mixture of H2O and D2O with a neutron scattering length density (SLD) of zero, nearly the same as that of air. The details are shown in Table 1. The H2O added to the mixtures in the surfactant and hydrochloric acid solutions was not accounted for in the preparation of the ACMW, and the SLDs of the subphase materials used in the modeling are therefore -0.04 × 10-6 Å-2 for ACMW and 5.50 × 10-6 Å-2 for D2O (cf. 0.0 × 10-6 Å-2 and 6.43 × 10-6 Å-2 for pure materials). The SLD for the X-ray subphase is 9.43 × 10-6 Å-2. The X-ray reflectivity measurements were made using the reflectometer at the Research School of Chemistry, which has been described elsewhere.1,5,6 The data acquisition system involves a Radicon detector that is linear in count rate up to ca. (4) Yanh, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Nature. 1996, 381, 589-592. (5) Jamie, I. M.; Dowling, T. L.; Holt, S. A.; Creagh, D. C. 3rd Vacuum Society of Australia Congress Proceedings, University of Newcastle, Australia, 1995, pp 25-27. (6) Holt, S. A.; Brown, A. S.; Creagh, D. C.; Leon, R. J. Synchrotron Rad. 1997, 4, 169-174.
S0743-7463(98)00485-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/15/1998
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Table 1. Compositions of the Reaction Mixtures Used for X-ray and Neutron Reflectivity Measurements surfactant/water combination
data type
h33-C16TAC/H2O X-ray h33-C16TAC/D2O neutron d33-C16TAB/ACMW neutron
surfactant: surfactant: surfactant: silicon HCl water mole ratio mole ratio mole ratio 1.01 0.92 1.04
0.12 0.12 0.12
1.22 × 10-3 1.39 × 10-3 1.27 × 10-3
300 000 Hz and a gas proportional counter linear up to ca. 15, 000 Hz to monitor the incident beam. These were operated to maximum count rates of 180 000 and 1500 Hz, respectively. The data were measured and the final specular reflectivity profiles produced as previously described.7 For the measurements of surfactant-only solutions, a background was measured by offsetting the detector 0.5° from the specular direction, and this was then subtracted from the specular signal. For the measurements of silicate-surfactant mixtures it was not possible to measure a background because of the time-dependent development of interfacial structure, and a flat background was therefore included as a model parameter in subsequent modeling. This approximation is valid for our purposes, given the very weak dependence of the background on scattering angle, as determined by measurements of the surfactant-only solution. Data for the surfactant-only solution were collected over scattering vectors, Qz in the range 0.014-0.48 Å-1 and for the silicate mixtures in the range 0.03-0.3 Å-1 (Qz () (4π/λ) sin θ), where θ is the glancing angle of incidence and λ the wavelength). The reflectometer was operated at an angular resolution ∆Qz/Qz of 1-8% (∆Qz is constant). Neutron reflectivity data were measured on the time-of-flight reflectometer SURF at the Rutherford-Appleton Laboratory. Data were collected with an angle of incidence of 1.5° and placed on an absolute scale by calibration against D2O. The instrument was operated at a constant resolution ∆Qz/Qz of 5%, and the data rebinned at a resolution of 3%. The kinetics of the film formation necessitated relatively short counting times, and each data set was therefore acquired in 30 min. A flat background has been shown to be appropriate for studies of this kind8 and was subsequently included as a parameter in the modeling of the data. The observed background is predominantly due to incoherent scattering from the sample.
Data Analysis The reflectivity data were modeled using the optical transfer matrix method of classical optics, as described by Penfold.9 The interfacial region is described as discrete layers of uniform scattering material, each with a particular composition (SLD), thickness, and roughness with the adjoining layer. The parameters of the model were refined until the best least-squares fit to the data was achieved. Because of the loss of phase information and the limited range of reciprocal space accessible in the experiment, it is not possible to find a unique model to fit the data. However, some models can be rejected because they are physically or chemically unreasonable, and further constraints on suitable models can be imposed by use of neutron reflectivity measurements made with appropriately chosen isotopic substitutions. We constrain the layer thicknesses and interface roughnesses to be the same regardless of the isotopic composition of the system and refine the model against all the available data simultaneously to obtain an optimized set of constrained parameters, in the spirit of refs 9 and 10. (7) Brown, A. S.; Holt, S. A.; Creagh, D. C.; Yuan, S. Colloids Surf. A Submitted. (8) Penfold, J.; Lee, E. M.; Thomas, R. K. Mol. Phys. 1989, 68, 33-47. (9) Penfold, J. In Neutron, X.-Ray and Light Scattering; Lindner, P., Zemb, Th., Eds.; 1991; pp 223-36. (10) Lee, E. M.; Milnes, J. E. J. Appl. Crystallogr. 1995, 28, 518526.
Figure 1. Specular reflectivity profiles for fully developed silicate films: (a) h33-C16TAC in H2O X-ray data; (b) d33-C16TAB in ACMW neutron data; (c) h33-C16TAC in D2O neutron data. The profiles are offset for clarity.
The quantity minimized in the least-squares fit is χ2
χ2 )
1 n-p
∑i ∑ Q z
[
]
Robs(i,Qz) - Rcalc(i,Qz) σobs(Robs(i,Qz))
2
(1)
where the summation is over all values of the scattering vector, Qz, and over all data sets i. Robs(i,Qz) and Rcalc(i,Qz) are the observed and calculated reflectivities, respectively, σobs(Robs(i,Qz)) is the experimental uncertainty in the observed reflectivity, n and p are the number of experimental data and independent model parameters, respectively. The component of the specular reflectivity associated with the Fresnel reflectivity of a perfectly smooth, abrupt interface has been removed for display purposes by multiplying the reflectivity profiles by Qz4 after subtraction of the fitted or measured background, to highlight the details of the developing interfacial structure. Model-independent structural information can often be determined directly from specular reflectivity profiles. The appearance of modulations in the reflectivity, known as “Kiessig fringes”,11 is related to layer thicknesses, d
d ) 2π/∆Q
(2)
where ∆Q is the spacing between successive minima in the profile. In addition, the presence of Bragg diffraction peaks immediately indicates a periodicity in the direction normal to the surface. Roughness is often included as a refined parameter in the optical transfer matrix formalism, but Lu et al.12 have pointed out that it only has a physical significance if the data are of sufficiently high resolution or if detailed isotopic labeling has been undertaken. As these conditions are not met in the current study, we have assumed each interface to have a fixed roughness of 4 Å, as determined by Lu et al.3 Results and Discussion Effect of Counterions. In separate experiments we have used surfactant with either bromide or chloride counterions to check for any effect on the film structure resulting from the different counterions. Figure 1 shows the X-ray and neutron reflectivity of films grown using C16TAB and C16TAC. In each case there are Bragg diffraction peaks at Qz ) 0.140 and 0.280 Å-1; first- and (11) Foster, M. Crit. Rev. Anal. Chem. 1993, 24, 179-241. (12) Lu, J. R.; Lee, E. M.; Thomas, R. K. Acta Crystallogr. 1996, A52, 11-41.
5534 Langmuir, Vol. 14, No. 19, 1998
Brown et al.
Figure 2. RQz4 vs Qz (left) and model SLD profiles (right) for (a) surfactant-only solutions; (b), (c), and (d) surfactant-silicate solutions after 25%, 75%, and 90% of the time required for diffraction peaks to appear. Black lines refer to h33-C16TAC in H2O X-ray data, yellow lines to d33-C16TAB in ACMW neutron data, and blue lines to h33-C16TAC in D2O neutron data. The lines in the RQz4 vs Qz plots are the best fits to the data. The neutron data for Qz > 0.16 Å-1 represents sample-dependent background and have been excluded from the figure for clarity.
second-order reflections corresponding to a real space repeat distance of 45 Å. Repeated experiments have produced Bragg diffraction peaks over the range Qz ) 0.138-0.142 Å-1 (repeat unit of 44.2-45.5 Å in real space) regardless of the isotopic composition and regardless as to whether the counterion was bromide or chloride. The constancy of this dimension is evidence that the final film structure is insensitive to these factors. In separate experiments13 with H2O/D2O mixtures we have found that (13) McGilvray, D. Vacation Scholarship Report, Research School of Chemistry, Australian National University, January 1998.
the development of the film as indicated by X-ray reflectivity is also to a good approximation insensitive to deuteration. Surfactant-Only Solutions. X-ray reflectivity measurements were made for a solution of h33-C16TAC in H2O, and neutron reflectivity for d33-C16TAB in ACMW. A twolayer model was required to satisfactorily account for the reflectivity data. The reflectivities and corresponding model are shown in Figure 2a and the model parameters given in Table 2. We can see that a seven-parameter fit to two profiles is reasonable and is not overparametrized,
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Langmuir, Vol. 14, No. 19, 1998 5535
Table 2. Model Parameters for the Constrained Fit to the Combined X-ray (h33-C16TAC/H2O) and Neutron (d33-C16TAB/ACMW) Reflectivity Data for Surfactant-Only Systemsa species
d1 (Å)
h33-C16TAC/H2O 11(1) d33-C16TAB/ACMW “
Fj1 (106 Å-2)
d2 (Å)
8.5(1) 6.6(5)
12(2) ''
Fj2 (106 Å-2)
bgd (106)
χ2
10.1(1) 0.0(f) 0.6(1) 6.1(3)
1.4
di and Fji refer to the thickness and SLD of the ith layer, respectively, with the layers numbered from the air-water interface down; bgd is the background; and χ2 is the quantity defined in eq 1. Parameters constrained in the fit are indicated by '', and those which are fixed at their initial values are indicated by (f). a
given the structure apparent in both reflectivity profiles. The area per molecule estimated from a one-layer fit to the d33-C16TAB/ACMW neutron data was 41 Å2; a more realistic estimate based on the alkyl chain layer of the two-layer fit is 45 Å2. Both estimates agree with the 4147 Å2 range reported by Lu et al.3 for solutions of C16TAB at its cmc. The physical density of the hydrocarbon chain layer calculated from the two-layer model is 0.87 g cm-3, some 15% more dense than a typical liquid hydrocarbon. This contrasts with the results of sum-frequency spectroscopy and ellipsometry by Bell et al.14 who report densities “approaching that of a liquid hydrocarbon”. Our values are in excellent agreement with the value 0.87 g cm-3 calculated from the crystal structure of C16TAB15 (two hydrocarbon chains occupy ca. 860 Å3 in the crystal). The two-layer model can be interpreted as a layer of hydrocarbon chains 11(1) Å thick and an aqueous layer containing surfactant headgroups and counterions 12(2) Å thick. Previous observations by Lu et al.,3 for C16TAB at concentrations close to the cmc, indicate that the structure of the surfactant at the interface consists of a monolayer of surfactant adsorbed at the air-water interface with the hydrophobic tail groups directed out of the water. In this structure the carbon atoms of the hydrocarbon chains are tilted successively away from the surface normal to yield an effective layer thickness of 12.6 Å for C16TAB. In a study of C10TAB Lee et al.16 found evidence for a 15 Å layer containing water and counterions beneath the hydrocarbon tails of the surfactant and for a 25 Å layer beneath that containing micelles. In the C14TAB system at a concentration 50 times the cmc, Lu et al.17 found that the surfactant structure was the usual monolayer of surfactant at the interface, an aqueous layer 55 Å thick from which surfactant is absent, and a further layer of surfactant with a 5% excess over the bulk. Addition of further layers to our model gave no improvement in the fit to the data, indicating that there is no evidence to support the existence of a subsurface layer of micelles in our system, despite the much larger surfactant concentration well above the cmc. Surfactant-Silicate Solutions: Induction Period. Reflectivity profiles were recorded as a function of time for three contrasts of the surfactant-silicate mixtures: an X-ray profile for h33-C16TAC in H2O, and neutron profiles for h33-C16TAC in D2O and d33-C16TAB in ACMW. The formation of interfacial structure occurred at different rates in the three systems, but each displayed an induction time followed by fast crystallization periods, and resulting (14) Bell, G. R.; Manning-Benson, S.; Bain, C. D. J. Phys. Chem. 1998, 102, 218-222. (15) Campanelli, A. R.; Scaramuzza, L. Acta Crystallogr. 1986, C42, 1380-1381. (16) Lee, E. M.; Thomas, R. K.; Penfold, J.; Ward, R. C. J. Phys. Chem. 1989, 93, 381-388. (17) Lu, J. R.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1993, 97, 13907-13913.
crystalline phases with identical repeat spacing. The X-ray and neutron experiments were performed at 25 and 30 °C, respectively. The rate of reaction was faster by a factor of 4 at the higher temperature. The reflectivity data were analyzed by assuming the organization of surfactant and silicate precursors at the air-water interface occurred in the same way but at different rates during the induction period. That is, that the three systems all had the same structure when the time of data collection was the same fraction of the induction time. The assumption of a constant rate of interfacial structure development between solution preparation and the appearance of Bragg peaks yields a self-consistent model of the processes involved. The variations in induction time observed for this system are not yet well understood, and further work is currently under way to investigate which factors, including temperature, are responsible. Figure 2 shows RQz4 vs Qz plots and the corresponding SLD profiles for the system soon after the solutions were prepared (Figure 2b), toward the end of the induction period (Figure 2c), and immediately before the appearance of Bragg diffraction peaks (Figure 2d). The dramatic transition from simple reflectivity profiles, which display broad Kiessig fringes with low amplitude, to those containing narrower and more pronounced Kiessig fringes and Bragg peaks is direct evidence that complex interfacial organization has occurred. The corresponding model SLD profiles quantify the increased complexity of the structure at the interface. The development of the interfacial structure can be followed in more detail in Figure 3 and Tables 3 and 4. The simple two-layer model for the surfactant-only system is shown in Figure 3a for reference while the surfactantsilicate models are shown in Figure 3b-i. The best SLD profile at time t ) 0% (t is the fraction of induction time) (Figure 3b) is a two-layer model which differs very little from that of the surfactant-only system. The model is consistent with a monolayer of hydrocarbon chains from the surfactant at the interface together with an aqueous ionic layer containing headgroups, counterions, and silicate precursors, with the silicate component being probably less than half. At t ) 25% a two-layer model (Figure 3c) gives a reasonable fit to the data, but there is sufficient information in the h33-C16TAC in D2O data to warrant the addition of a third layer (Figure 3d). The SLDs of this third layer at 9.40(2) × 10-6, 5.17(6) × 10-6, and -0.01(4) × 10-6 Å-2 are only slightly changed from those of the bulk phase (9.43 × 10-6, 5.5 × 10-6, and -0.04 × 10-6 Å-2). Again we note that only the constraints of simultaneous use of three structured reflectivity profiles allows such a complicated model to be reliably derived. The third layer is consistent with the formation of an interdigitated and/or tilted bilayer of surfactant hydrocarbon chains associating with the aqueous ionic layer above it. In the analogous single-crystalline forms exhibited by C16TAB,15 the dichloroiodide,18 and salicylate monohydrate19 the surfactant molecules form an almost unchanging interdigitated network. The surfactant molecules are packed in a headto-tail arrangement and are tilted with respect to the axes of the unit cell, giving an effective thickness from headgroup to headgroup of surfactant in the crystal of ca. 25 Å. A similarly stable arrangement of surfactant molecules may be adopted in the growth of a surfactant bilayer from solution. If the interdigitation or tilt were (18) Bandoli, G.; Clemente, D. A.; Nicolini, M. J. Cryst. Mol. Struct. 1978, 8, 279-293. (19) Koh, L. L.; Xu, Y. Gan, L. M.; Chew, C. H.; Lee, K. C. Acta Crystallogr. 1993, C49, 11032-1035.
5536 Langmuir, Vol. 14, No. 19, 1998
Brown et al.
Figure 3. Model SLD profiles as a function of time during the induction period. The surfactant-only system (a) is included for comparison. The profiles for the surfactant-silicate system at various fractions, t, of the induction period are shown in (b)-(i). (b) t ) 0%, two-layer model; (c) t ) 25%, two-layer model; (d) t ) 25%, three-layer model; (e) t ) 38%, three-layer model; (f) t ) 50%, three-layer model; (g) t ) 63%, three-layer model; (h) t ) 75%, three-layer model; (i) t ) 90%, five-layer model. Black lines refer to h33-C16TAC in H2O X-ray data, yellow lines to d33-C16TAB in ACMW neutron data, and blue lines to h33-C16TAC in D2O neutron data. Table 3. Model Parameters for the Constrained Fit to the Reflectivity Data for Surfactant-Silicate Systems as a Function of the Fraction, t, of the Induction Perioda species t ) 0% h33-C16TAC/H2O h33-C16TAC/D2O d33-C16TAB/ACMW t ) 25% h33-C16TAC/H2O h33-C16TAC/D2O d33-C16TAB/ACMW t ) 25% h33-C16TAC/H2O h33-C16TAC/D2O d33-C16TAB/ACMW t ) 38% h33-C16TAC/H2O h33-C16TAC/D2O d33-C16TAB/ACMW t ) 50% h33-C16TAC/H2O h33-C16TAC/D2O d33-C16TAB/ACMW t ) 63% h33-C16TAC/H2O h33-C16TAC/D2O d33-C16TAB/ACMW t ) 75% h33-C16TAC/H2O h33-C16TAC/D2O d33-C16TAB/ACMW a
d1 (Å)
Fj1 (106 Å-2)
d2 (Å)
Fj2 (106 Å-2)
13(5) '' ''
9.2(3) 0(8) 6(2)
16(6) '' ''
9.9(1) 6(1) 0.2(3)
14(4) '' ''
9.1(4) 0.1(4) 6(1)
13(5) '' ''
10.2(2) 5.4(6) 0.0(6)
12(4) '' ''
8.8(6) 4.3(9) 6(2)
16(5) '' ''
10.1(1) 5.8(4) 0.3(1)
29(3) '' ''
9(3) '' ''
8(2) 2(1) 9(3)
19(5) '' ''
10.1(1) 5.6(4) 0.2(3)
9(3) '' ''
7(2) 2.2(7) 8(2)
22(5) '' ''
12.3(5) '' ''
4.0(4) 0.7(2) 6.2(2)
13.9(7) '' ''
5(1) 2.0(3) 5.8(3)
d3 (Å)
Fj3 (106 Å-2)
bgd (106)
χ2
2.6(5) 5(1)
4.8
0.1(2) 3.0(4) 5.4(9)
4.8
9.40(2) 5.17(6) -0.1(1)
0.2(1) 3.1(4) 5.2(9)
3.9
34(1) '' ''
9.26(2) 5.09(5) -0.01(4)
0.4(2) 3.1(4) 4.6(9)
6.2
10.4(1) 5.5(2) 0.1(1)
27(3) '' ''
9.26(2) 4.84(5) -0.1(1)
0.4(2) 3.0(3) 5.1(8)
3.3
20(1) '' ''
10.9(2) 5.3(1) 0.2(2)
34(2) '' ''
9.06(3) 4.62(4) 0.2(1)
0.57(4) 2.7(4) 6.0(8)
4.5
19(2) '' ''
10.9(2) 5.3(1) -0.1(1)
27(2) '' ''
8.38(7) 3.81(8) 0.6(1)
0.55(7) 3.6(6) 5(1)
9.2
0.04(1)
Symbols are as for Table 2. For each t the first entry is the X-ray result and the second and third are neutron results.
slightly less pronounced in this system than in the single crystal, the headgroup to headgroup distance would be ca. 28 Å. When a distance of ca. 2 Å is allowed on either side for the interaction of the headgroups with the adjacent ionic aqueous layers, a total thickness of 32 Å is predicted
for this layer. This is in reasonable agreement with the observed thickness of 27-34 Å and with the 34 Å thickness of a C16TAB bilayer adsorbed at the air-solid interface.20 We should particularly note that the SLDs indicate that while the C16TAB component in the layer is substantial
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Langmuir, Vol. 14, No. 19, 1998 5537
Table 4. Model Parameters for the Five-layer Constrained Fit to the Reflectivity Data for the Surfactant-Silicate System at 90% of the Induction Perioda
parameter
h33CC16TAC/H2O (X-ray)
h33C16TAC/D2O (neutron)
d33C16TAB/ACMW (neutron)
d1 (Å) Fj1 (106 Å-2) d2 (Å) Fj2 (106 Å-2) d3 (Å) Fj3 (106 Å-2) d4 (Å) Fj4 (106 Å-2) d5 (Å) Fj5 (106 Å-2) bgd (106)
14.7(6) 6.3(2) 14(1) 10.4(2) 37(2) 7.94(6) 11(2) 10.4(2) 30(1) 8.62(3) 0.1(1)
'' -0.2(4) '' 5.9(4) '' 3.0(1) '' 5.3(4) '' 5.3(1) 2.7(6)
'' 6.9(2) '' 1.2(3) '' 3.4(2) '' 0.2(3) '' 0.3(2) 6.1(9)
a
Symbols are as for Table 2. χ2 ) 6.0 for this model.
there is still some subphase present. The changes in SLD for this layer with time (Figure 3d-h) suggest that the layer initially contains a significant proportion of subphase, either through inclusion into a complete less-dense layer of surfactant or, more likely, by incomplete assembly of a more-dense surfactant layer. The increasingly large differentiation between the SLD of the third layer and that of the underlying subphase shows this. It is consistent with subphase being successively excluded from the layer as the layer becomes more complete. At t ) 90% even the best three-layer model is no longer adequate, giving a χ2 of 110! A five-layer model gives a much better fit to the data with a χ2 of 6.0, details of which are given in Table 4. The thicknesses and SLDs of the two new layers are consistent with a further ionic aqueous and/or siliceous layer and a mainly surfactant bilayer forming beneath the three previously existing layers. The SLD for the aqueous layers (layers 2 and 4) are essentially identical, and the new surfactant layer has a SLD intermediate between that of the topmost surfactant layer and the subphase, again consistent with subphase incorporation into the layer or incomplete layer growth. However, a 23-parameter model fitting three reflectivity profiles is perhaps overparametrization, and it is surprising that the fit is stable and consistent with simpler fits at earlier times. A five-layer model refinement was carried out in which the parameters for layers one to three were fixed at the values for t ) 75% in an effort to reduce the overparametrization, but an acceptable fit to the data was not possible under these conditions. Although we have interpreted the data in terms of surfactant bilayers, we are not yet able to rule out the possibility that the surfactant layers are composed of cylindrical micelles oriented with their long axes parallel to the surface. The changes in SLD for the hydrocarbon chain layers with time could be explained in terms of successive displacement of subphase by surfactant micelles to produce more complete layers of micelles. Further studies are in progress to determine the structure more definitively. Surfactant-Silicate Solutions: Crystallization Period. The high intensity and narrowness of the Bragg diffraction peaks observed after the induction period are evidence that the 45 Å period of the film has a high coherence length in the direction perpendicular to the surface.21 The development of the film structure after (20) McDermott, D. C.; McCarney, J.; Thomas, R. K.; Rennie, A. R. J. Colloid Interface Sci. 1994, 162, 304-310. (21) Warren, B. E. Phys. Rev. 1941, 59, 693-698.
Figure 4. Simulations of the X-ray reflectivity for 11-layer (dashed line) and 33-layer (solid line) models. The model consists of a top layer with a thickness of 12 Å and SLD of 6.0 × 10-6Å-2, beneath which are alternating layers with thicknesses 12 and 33 Å and SLD 10.0 × 10-6 Å-2 and 8.0 × 10-6 Å-2, respectively. All interface roughnesses are fixed at 4 Å.
the first appearance of diffraction peaks is rapid, and the number of layers present very quickly exceeds our present ability to undertake quantitative fitting of the data. To illustrate that the peak intensity and narrowness is indeed a function of the number of repeat units in the structure, we simulated the reflectivity profiles with successively greater numbers of layers using estimates for the model SLD and thickness parameters based on those obtained from fits at the end of the induction period. Figure 4 shows simulations of the X-ray data with an 11-layer model and a 33-layer model. The X-ray SLDs assumed are consistent with the previous data, i.e., alternating hydrocarbon chain dominated regions and regions of high silica content. Similar simulations were performed for the neutron data with analogous results. It is apparent that a large number of repeat units is required to fully describe the data. We attempted to determine the full width at half-maximum of the diffraction peaks, but even when the resolution of the SURF reflectometer was improved to ∆Qz/Qz ) 1.7% the Bragg peaks were narrower than the angular resolution of the reflectometer. If we estimate the full width at half-maximum of the diffraction peak at Qz ) 0.14 Å-1 for the d33-C16TAB/ACMW neutron data to be one-tenth of the angular resolution, the Scherrer equation21 indicates that there are ca. 1125 repeat units at the end of the experiment, corresponding to a total film thickness of ca. 5 µm. Further work is under way to elucidate the details of the transition from induction period to fully developed crystalline structure by modeling the shape of the scattering more closely. The reflectivity measurements and subsequent modeling do not directly address the question of the structure of the silicate layers, or indeed whether silicate is even present in the film structure. It is obvious, however, that the presence of silicate is a prerequisite for the formation of the film, since the surfactant solutions in the absence of silicate produce only a surface-excess monolayer of surfactant molecules. The addition of silicate precursor to the reaction mixture leads to the formation of macroscopic, physically robust films. As part of a separate study,22 the fully developed films have been harvested for analysis. Preliminary gravimetric and microanalytical results show that the films contain C16TA, silica species, halide, and water. The former three are present in the approximate molar ratio 1:2:1. The SLDs for the aqueous/siliceous layers obtained in our modeling of the reflectivity data are consistent with (22) Ruggles, J. L., 1998.
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layers that are mainly water, with a minority electronrich halide and/or silicate content. For example, the X-ray SLDs of 10-11 × 10-6 Å-2 are raised by the halide/silicate content above the pure water value of 9.43 × 10-6 Å-2. A more detailed discussion of the structure of the silicate component will be presented elsewhere.22 X-ray and Neutron Complementarity. A comparison of the X-ray and neutron reflectivity data for this system highlights some of the advantages and disadvantages of the two techniques and how they can be profitably used in a complementary manner. The relatively short data collection times made necessary by the kinetics of the film formation have reduced the maximum useful Qz in the neutron experiments from 0.20 to 0.16 Å-1. The signal beyond 0.16 Å-1 is essentially only an incoherent background, apart from the Bragg diffraction peaks which eventually become intense enough to exceed the background. Since this is a sample-dependent limit, this is the maximum useful scattering vector accessible in the neutron experiment. By contrast, the sample-dependent scattering background is much less in the X-ray experiment, and the maximum accessible scattering vector is determined by instrumental noise. In the present experiments the maximum scattering vector for the X-ray data was limited by the maximum angle of incidence which could be reached by the reflectometer (3.4°) and by the relatively rapid time evolution of the film structures, which necessitated relatively short total counting times. The resulting X-ray data have a useful scattering vector range about twice that of the neutron experiment, with a corresponding doubling in the resolution of the real space structure. The contrast variations available with neutron reflectivity, however, provide useful constraints for the modeling. In particular the different contrasts highlight different features in the films. For example, the h33-C16TAB in D2O profiles at early stages of the film development appear little different to an air-D2O interface, while the corresponding d33-C16TAB in ACMW data clearly show a surface layer of surfactant (see parts b and c of Figures 3). As the third layer begins to form and more complex
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layering processes occur, the h33-C16TAB in D2O data now begin to show more pronounced changes (see parts d-i of Figure 3). The d33-C16TAB/ACMW data are thus able to provide details about near-surface ordering in the system while the h33-C16TAB/D2O data are better suited to revealing the subsurface multilayering process. Together they provide a consistent picture of the time evolution of the film structure throughout the induction period, impossible to obtain with a single contrast. Conclusions The combination of two isotopically different neutron and one X-ray reflectivity data sets has provided a detailed picture of highly crystalline thin silicate film growth from concentrated surfactant solutions at the air-water interface. Each of the reflectivity profiles analyzed individually provides some insight into the development of film structure with time, but a consistent description of the process only emerges when all data sets are considered together. The development of a moderately complex structure (two to five layers) at the air-water interface during the induction period can be understood as a stepwise ordering of surfactant and silicate, followed by the rapid addition of alternating surfactant and silicate layers during the crystallization phase. Further work is under way to understand why the induction occurs so slowly and to determine whether the surfactant layers are composed of bilayers or micelles. Acknowledgment. The authors wish to acknowledge the Director’s discretionary beam time awarded to us at the Rutherford-Appleton Laboratory, which allowed us to collect the neutron reflectivity data. Travel grants from the Australian Government ISTAC/ANSTO Access to Major Facilities Program are also gratefully acknowledged, as is the financial contribution from the Australian Research Council toward the construction and operation of the X-ray reflectometer at the Research School of Chemistry. Mr. J. L. Ruggles kindly supplied analytical data before publication. LA980485T
