1204
Langmuir 1992,8, 1204-1210
Study of the Adsorption from Aqueous Solution of Hexaethylene Glycol Monododecyl Ether on Silica Substrates Using the Technique of Neutron Reflection D. C. McDermott, J. R. Lu, E. M. Lee, and R. K. Thomas* Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3Q2, United Kingdom
A. R. Rennie School of Chemistry, Bristol University, Cantock's Close, Bristol BS8 I T S , United Kingdom Received November 5, 1991.I n Final Form: January 29, 1992 The adsorbed amount and the structure of layers of hexaethylene glycol monododecyl ether (C1&6) adsorbed on three different silica substrates have been determined by neutron reflectivity. The substrates studied were amorphous silica, crystal quartz, and the oxide layer on a silicon crystal. The characteristics of the substrates, in particular surface roughness, were determined prior to adsorption. The adsorbed surfactant was found to form a bilayer of overall thickness 49 f 4 A, with each bilayer unit occupying an area of 44 f 4 A2, at and above the critical micelle concentration of ClzE6 (8.7 X 10-5 M). The identical model was applied to all substrates, although the coverage varied between 40% and 60% and may be correlated with the surface roughness of the solid substrates.
Introduction Neutron reflection as a technique for determining structures of adsorbed layers has been reviewed in detail by Penfold and Thomas.' Much of this work has been concerned with the liquid-air interface, although a number of experiments have involved solid-liquid interfaces, including studies of surfactants at an amorphous silicawater i n t e r f a ~ e . ~ , ~ Neutron reflection experiments require macroscopic, planar substrates. These have a number of advantages over the colloidal systems frequently employed for adsorption studies a t the solid-liquid interface. Such colloids, which rely on the adsorbed layer for steric stabilization, are susceptible to flocculation, so limiting the range of pH and ionic strength for which measurements can be made. The surface roughness and the porosity of the particles are often difficult to characterize and differences in surface curvature may invalidate comparisons between samples. However, much useful data has been derived from silica-water colloidal systems, using small angle neutron scattering (SANS)? fluorescence decay,5dynamic light scattering,6 and solution depletion techniques.' This paper describes the adsorption of the nonionic surfactant hexaethylene glycol monododecyl ether (C12Ed on a range of planar silica substrates. The substrates studied were amorphous silica, crystalline quartz, and a silica layer formed at the surface of silicon. From data measured at the solid-pure water interface, we gain insight into the form and magnitude of the surface roughness of the solid substrate. We have then determined the amount of material adsorbed and the structure of the adsorbed (1) Penfold, J.; Thomas, R. K. J. Phys.: Condens. Matter 1990, 2, 1369. (2) Lee, E. M.; Thomas, R. K.; Cummins, P. G.; Staples, E.; Penfold, J.; Rennie, A. R. Chem. Phys. Lett. 1989, 162, 196. (3) Rennie, A. R.; Lee, E. M.; Simister, E. A,; Thomas, R. K. Langmuir 1990, 6, 1031. (4) Cummins, P. G.; Staples, E.; Penfold, J. J. Phys. Chem. 1990,94, 3740. -.
( 5 ) Levitz, P.; Van Damme, H. J. Phys. Chem. 1986, 90, 1302. (6) Bohmer, M. R.; Koopal, L. K.; Janssen, R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R. Langmuir, in press. (7) Bijsterbosch, B. J. J. Colloid Interface Sci. 1974, 47, 186.
0743-746319212400-1204$03.0010
layer and compared the structural parameters of the adsorbed surfactant layer on each of the solid substrates. The effects of roughness on the amount of adsorbed surfactant are discussed. The data obtained on the amorphous silica block are compared with data previously obtained on a silica substrate by Lee et aL2 The nature of the silica surface has been reviewed by Iler.8 The two main factors which might affect the nature of the silicaiwater interface are the structure of the underlying substrate and the method of surface preparation. In the present work the variations in the structure of the underlying material are large. For example, crystalline and amorphous quartz have quite different adsorptive properties in adsorption from the vapor phasewe On the other hand our surface preparation was identical for the three materials used. It is thought that any roughness produced in the surface treatment will tend to mask differences in the underlying substrate.8 Much attention has been given to a third influence on the adsorptive properties, the dehydrationirehydration process. However, this seems to be less important when adsorption is from aqueous solution provided that the surface has had sufficient time to equilibrate with the water. The adsorption of an amphiphile at the silica/water interface will be determined partly by any specific interaction of the head group of the amphiphile with groups on the surface of the silica and partly by hydrophobic bonding. In the case of cationic surfactants where there is the added attractive interaction between unlike charges on amphiphile and surface (above pH about 2) the hydrophobic bonding is generally considered to dominate. In the case of nonionic amphiphiles such as C1&&hydrogen bonding to the surface may be important and its influence will vary according to the number of ethylene oxide groups in the headgroup and possibly according to the nature of the silica surface. Differences between the nature of the adsorbed layer on each of the three substrates would indicate both that hydrogen bonding is important and that it is affected by the structure of the underlying material. ( 8 ) Iler, R. K. The Chemistry of Silica; Wiley-Interscience:
1979.
0 1992 American Chemical Society
New York,
ClZEs Adsorbed on Silica
Neutron Reflection The reflection of neutrons at interfaces can be described by the same equations as light polarized perpendicular to the plane of reflection.9 These equations depend only on the refractive index profile normal to the surface, which is related to the coherent scattering length density. In turn, the scattering length density profile derives from the chemical composition and density of scattering species. Hence detailed information about the interfacial region may be determined. The reflectivity is measured as a function of momentum sin 6/A, transfer normal to the interface, Q, where Q = 47~ with 6 being the glancing angle of incidence and A the wavelength of the incident neutron beam. In a typical analysis, the measured data are compared with a calculated reflectivity profile, which can be calculated exactly for any model interface using the optical matrix method.1° The model consists of a series of layers each with a scattering length density, p, and thickness, t. An additional parameter, u, which describes the interfacial roughness between any two consecutive layers is included in the matrix calculation. The matrix elements are multiplied by an exponential term which has been shown to be equivalent" to a random density distribution between consecutive layers i and i + 1
for small values of u, the root mean square roughness. By variation of p and t for each layer, the calculated profile may be compared with the measured profile until the optimum fit to the data is found. Although any one profile may not provide a unique solution, by using different isotopic contrasts one can ensure that an accurate model of the interface is determined.'* Contrast variation relies on the fact that different nuclei may scatter neutrons with quite different amplitude and, in the case of protons and deuterons, with opposite phase. Thus by use of a combination of protonated and deuterated materials the reflectivity profile of a system can be substantially changed while retaining the same chemical structure at the interfacial region. Furthermore one can, by adjustment of the H/Dratio, prepare solvents which are contrast matched to the surface. The contrast between the surface and the solvent is then zero, giving a reflectivity profile arising only from the adsorbed material.
Experimental Details The reflectivity profiles in this work were measured using the neutron diffractometer, D17, a t the Institut Laue-Langevin, Grenoble, France.l3 A schematic diagram is shown in Figure la. The range of momentum transfer was achieved by varying the angle of incidence and the neutron wavelength. Using wavelengths from 8.7 to 30 8, and angles from 0.4 to 7', it is possible t o measure the range of momentum transfer from 0.003 to 0.18 A-1. The sample cell is shown in Figure Ib. It consists of a solid block of the chosen substrate of dimensions 100 mm X 50 mm X 10 mm which is sealed against a P T F E trough (-40-mL capacity) by anitrile rubber "0"ring. The sample cell is mounted with the large faces vertical in the neutron beam. It is fully enclosed by an acrylic plastic box, fitted with mica windows, (9)Lekner, J. TheoryofReflection;Martinua Nijhoff: Dordrecht, 1987. (10)Heavens, 0. S. Optical Properties of Thin Films; Butterworth: London, 1955. (11)Penfold, J.The Adaption of Methods in Multilayer Optics for the Calculation of Specular Neutron Reflection. RAL-88-0088; (Didcot: Rutherford Appleton Laboratory: Didcot, 1988. (12)Crowley, T. L.; Lee, E. M.; Simister, E. A.; Thomas, R. K. Physica
B 1991,174. (13)Neutron Facilities at the High Flux Reactor, Institut LaueLangevin, Grenoble, 1990.
Langmuir, Vol. 8, No. 4, 1992 1205
*
which is thermostated a t 298 0.2 K by circulating water through the body of the metal securing plates. The collimated neutron beam, defined by a 15 mm circular diaphragm and a 0.2-mm vertical slit a t 2.4 m separation, enters the substrate block through the end face and is specularly reflected on striking the solidliquid interface a t a glancing angle. The reflected beam emerges through the opposite end face. The faces through which the beam enters and exits the block are almost perpendicular to the beam, avoiding further reflection, and it is this constraint that limits the largest angle that can be measured with a block of given geometry. Limits for the lowest angle are given by the incident collimation and the requirement that the length of substrate illuminated by the incident beam is less than the length of the block. A perfect parallel beam of width 0.2 mm inclined at 0.4O will illuminate a length of =30 mm. This is somewhat increased on projection of the divergent beam from the final slit onto the surface. A precise alignment of the sample surface in the beam is required if the center of the beam is to strike the center of the block. This is achieved by optimization of the signal a t very low angles using a linear translation. In practice it is useful to vary the collimation and particularly the final slit to increase the flux for measurements a t larger angles for which the reflectivity is low and the problem of illumination is no longer significant. The neutron beam is attenuated on passing through the substrate blocks primarily by wide angle scattering, but with someabsorption. For example, the transmission of 12-Aneutrons through 100 mm of amorphous silica is approximately 25%. Although the transmission through the crystalline substrates is higher, calculation of the absolute reflectivity must allow for this effect. Our procedure is to measure the intensity of the attenuated direct beam, I,,, for each wavelength and collimation by translating the block to allow passage of the beam, through the full length of the block, without reflection. The variation in path length of the reflected beam, I,, introduces negligible errors a t the low angles used during the experiment. All reflectivity values, R , are thus reported as ratios
R = IJI0 A further consequence of this scattering is a background intensity that is easily measured on the multidetector. This is significantly larger than that arising from the transmitted beam in the liquid and can amount to about of the incident intensity. Our procedure for estimating the true reflected intensity is to estimate the background from an area of cells adjacent to the reflected beam on the position-sensitive detector. An amount is then subtracted which is proportional to the solid angle used for integration of the reflected peak. In practice the signal to background ratio limits measurements a t high momentum transfer before other constraints become important. The substrate blocks used in these experiments were singlecrystal quartz (supplied by Gooch and Housego),amorphous silica (Suprasil, supplied by Hellma, U.K.), and single crystal silicon (supplied by Specac). The blocks were polished on all faces using polishing rouges and supplied as specified to a flatness of h/10 (632 nm) and a smoothness of 40/20 on the large faces. The blocks were then plasma etched (with a Bio-Rad PT7100 R F Plasma barrel etcher) for 15 min in oxygen and then for 45 min in argon. The face used for adsorption on the crystal quartz was (1010). The P T F E troughs were soaked in a 4 % concentrated HFIconcentrated HN03 mixture for 24 h, rinsed thoroughly, and then soaked in ultrapure water (Elgastat UHQ) for a similar period of time. The nitrile rubber "0"ring was cleaned in ethanol and then rinsed and soaked in ultrapure water. The CI2E6 was used as supplied (by Nikko chemicals) without further purification. The coherent scattering length densities and physical dimensions of the materials used in this work are shown in Table I. (14)Willis, B.T. M., Ed. Chemical Applications of Thermal Neutron Scattering; Oxford University Press: Oxford, 1973. (15)Weast, R. C., Ed. Handbook of Chemistry and Physics; Chemical Rubber Co.: Cleveland, OH, 1973. (16)Tanford, C. J. Phys. Chem. 1972,76, 3020. (17)Tanford, C.; Nosaki, Y.; Rohde, M. F. J. Phys. Chem. 1977,81, 1555.
1206 Langmuir, Vol. 8, No. 4, 1992
McDermott et al.
a
2.4m
b i
4 Evacuated Flight Path
Velocity Selector n
1u
4
15mm
Diaphragm
Q
b
d
6
Figure 1. (a) The scheme of a typical reflection experiment using the D17 instrument. (b) The sample cell used for the reflection experiments. Table I. Properties and Dimensions of Materials Used in This Study P !a
lo*
material amorphous silica crystal quartz silicon H20
A-2
3.41 4.17 2.07 -0.56 6.35
density,* g cm-3 2.155 f 0.014 2.631 f 0.018 2.320 f 0.016
volume,'
A3
extended length,A
30 1.108 30 -0.39 0.802 350 16.7d (E0h 0.64 1.138 385 16.6-21.0' a Scattering lengths used to calculate p are taken from ref 14. * Densities measured (silica substrates) or taken from ref 15. Calculated from density. Taken from ref 16. e Taken from ref 17. Length depends on chain configuration.
Dz0 C12H25
0.997
Results and Discussion Characterization of Substrates. Initially, neutron reflectivity profiles were measured from the solid-water interface to characterize the substrate. In the case of amorphous silica, three different substrate/water profiles were measured, using H20, D20, and water contrast matched to amorphous silica. We denote this by water3.41, the scattering length density being 3.41 X lo+ A-2. The water3.41 is prepared by mixing H20 and D20 in the mass ratioof 1:1.5. The reflected signal at this contrast was too
small to be measured a t Q = 0.005 A-l. This demonstrates the cleanliness of the amorphous block and its surface purity, but gives no information about the surface roughness which is obtained from the other two profiles. To characterize the quartz crystal, profiles were measured from the H20 and DlO/quartz interface. No measurable reflection could be seen when water index matched to crystal quartz (waterr.17) was placed in the cell. For the silicon block, additional profiles were measured in order to determine as accurately as possible the density and thickness of the oxide layer formed on the silicon surface. In addition to the standard HzO and D20 profiles, we measured profiles in which the water was matched to silicon (waterz.07)and to amorphous silica (water3.41).From these four profiles the nature of the Si02 layer could be deduced. The complete set of eight substrate/water profiles is shown in Figures 2-4. A noticeable feature of the model fits to the substrate/ DzO profiles is the discrepancy at small values of momentum transfer. Although one would expect unit reflectivity below a critical value Qc, the observed reflectivity does not exceed 0.78 f 0.07. This can be explained either by departures from planarity of the surface or by angular divergence of the incident beam, or a combination of both. The half-width of the angular divergence is estimated to
C12E6 Adsorbed
Langmuir, Vol. 8,No. 4, 1992 1207
on Silica
Figure 2. Neutron reflectivity profiles measured at the amorphous silica/purewater igterface,shown with the calculated "best fit" profiles: (a) silica/pure Hz0;(b) silica/pure DzO.
Figure 4. Neutron reflectivity profiles measured at the Si/SiO$ pure water interface,shown with the calculated'best fit"profiles: (a)Si/SiO$pure HzO; (b) Si/SiOdpure D20;(c) Si/SiO$waterZ,o,; (d) Si/SiO$water~,~l. Table 11. Parameters Used To Fit the Solid Substrate/ Pure Water Data oxide layer
F>
solid substrate P . 10-6A-* t,A amorphous silica amorphous silica" crystal quartz silicon layer 1 2.4 f 0.1 12.0 f 2.0 layer 2 3.41 f 0.01 32.0 f 1.5
-'!
"
roughness reflectivity u, A below Q, 14.5 f 1.0 0.78 f 0.07 8.0 f 2.0 8.0 f 1.0 0.78 f 0.07
12.0 f 1.0 0.78 f 0.07
Data from ref 2.
002
DO'
0,06 008 0.D
0 Ik' Figure 3. Neutron reflectivity profiles measured at the crystal quartz/purewater interface, shown with the calculated "best fit" profiles: (a) quartz/pure HzO/ (b) quartz/pure DzO. be 0 . 2 O from the dimensions of the diaphragm and slits and is sufficient to explain the entire loss in the intensity a t low momentum transfer. These measurements were not made at sufficiently high resolution to distinguish any large-scale features of the surface. A polished surface is not expected to be smooth over dimensions from the neutron wavelength up to the size of the polishing particles (tens of micrometers). The large momentum transfer of the neutron experiment is sensitive to roughness up to about 50 A and this is discussed below. The present data give no direct information about larger features.
As one moves to larger angles, beam divergence becomes less significant and appears only as a small decrease in the resolution of the experiment, which is then dominated by the spread in wavelength, AX/X. At large momentum transfers the features of surface topography that will be most important will correspond to local roughness on a scale of a few angstroms or tens of angstroms. This will give a reduction in the reflectivity compared with a sharp interface which increases exponentially with momentum transfer. It is features of these dimensions that one would expect to have most effect on the adsorption of surfactants. This effect is modeled by the roughness parameter described earlier. The values determined for the interfacial roughness are listed in Table 11, along with the parameters used to fit the corresponding profiles. An attempt was made to fit a single layer model, representing the oxide formed a t the surface of the silicon, to the data measured from the Si/SiOz/water interface. Although reasonable fits were obtained for a uniform la er of scattering length density in the range of 3.4 X IO4 an improved fit was found when a two-layer model was employed. The first layer represents the sub-oxide, SiO, ( x < 2), which exists between the bulk silicon and the second layer, which has the same scattering length density as amorphous SiOn. This is in accordance with previous work that has shown that the 'native" oxide layer is not entirely s t o i c h i o m e t r i ~ . ~Although ~J~ the true distribution may be smooth, this two block model describes
&,
1208 Langmuir, Vol. 8, No. 4, 1992
McDermott et al.
Table 111. Parameters Used To Characterize the Adsorbed Surfactant Layer
a
surface coverageo mg m-*
solid substrate
%
amorphous silica amorphous silicaC crystal quartz siliconiSiO2
40 f 2 75fl 60 f 5 60f3
1.36 2.44 2.04 2.04
areal headaroup,bA-z
layer thickness, A
surfactant concn, M
44 f 4 46 f 6 44 f 4 44 f 4
49 4 49 f 6 49 f 4 49 f 4
2.0 x 10-4 8.7 X 8.7 x 10-5 8.7 x 10-5
Coverages quoted refer to the average coverage over the whole surface. Areaiheadgroup
the oxide layer within the limits of resolution of this experiment. The outermost silica layer has a thickness of 32 f 2 8, and a scattering length density of 3.41 x lo4 k2. A roughness of 12 A between this silica layer and bulk water was also required to fit the profiles. Adsorbed Surfactant Layers. With the surfaces shown to be clean and of known roughness, reflectivity profiles were measured from the solid/surfactant solution interfaces. Two profiles were measured on amorphous M, both in Dz0 silica at a C12E6 concentration of 2.0 X and water3.41. Three profiles were measured on crystal quartz at a concentration of 8.7 X M (the critical micelle concentration, cmc, of C12E6), one each in DzO, water4.17, and water2.50. For the silicon substrate two profiles were measured, also at the cmc, one each in D2O and water4.17. Isotopic effects on the thermodynamic properties of surfactants seem generally to be negligible except, possibly, close to phase boundaries such as cloud points. Since we are well below the cloud point of C12E6, we assume that the structure of the interface is not affected by isotopic substitution. Attempts were made to fit the data to a monolayer model. The first consisted of a single layer which represents the surfactant molecules lying flat on the surface. A second attempt consisted of two layers, one for the ethylene oxide headgroups and the other for the hydrocarbon chain region orientated perpendicular and adjacent to the surface. The scattering length densities and layer thicknesses were varied, but no model was found to be consistent with all of the measured data. The model that was found to fit the data is that of a bilayer consisting of three layers. The parameters used to devise this model were constrained in a number of ways. The scattering length densities used to fit each profile had to be consistent with a single chemical composition in all of the different solvent contrasts. Furthermore the model had to be chemically and physically realistic. The area/two molecules, A (Eareaiheadgroup, see Table 111) multiplied by the thickness of the chain region, t,, must equal the volume of two chains, 2 V,, which is approximately 700 A3
A t , = 2V, For a given area the apparent volume of the headgroup region, v h , can be calculated from the thickness of the headgroup region, th V , = At, The (EO)6group occupies a volume V,, which is estimated from the bulk density to be 385 A3, leaving an excess volume, V,, which is assumed to be occupied by water V , = A t h - V,, A first region of 16 f 2 A thickness contains headgroups and water only. This region is mixed in with the silica (18) AI-Bayati, A. H.; Orrman-Rossiter, K. G . ; van den Berg, J . A,; Armour, D. G. Surf. Sci. 1991, 241, 91. (19) Finster, J.; Schulze, D.; Bechstedt, F.; Meisel, A. Surf. Sci. 1984, 153, 1063.
*
= areaitwo molecules.
Data from ref 2.
using the same roughness parameter that was required to fit the water profiles. The second region of 16 f 2 A is taken to be pure hydrocarbon chain with no water present. This assumption has some subtle consequences which are discussed below. The third region is similar to the first region with a mixture of headgroups and water only but has a thickness of 17 f 2 A. The density of the chain ~ is slightly more region is taken to be 0.802 g ~ m -which dense than liquid but less dense than crystalline C12H26. The parameters used to fit the data are shown in Table 111. The overall thickness of the adsorbed bilayer is thus 49 f 4 A. This is thicker than the value of 39 f 5 A determined by small angle neutron scattering? although it is exactly the value obtained previously by Lee et al.? Ottewill and Walkerz0found a thickness of 50 f 10 A for an adsorbed layer of C12E6 on polystyrene latex, while Gellan and Rochester21determined a layer thickness of 44 A for ClzEB on colloidal silica. A bilayer leaflet occupies an area of 44 f 4 A2 on the silica surface, which compares well with the values determined by Lee et al. (46 A2), Ottewill and Walker (40 Az, assuming a bilayer) and Gellan and Rochester (44-50 A2). The number of water molecules/(E0)6 headgroup is calculated to be 10.5 f 1.5. This is slightly lower than the value of 12 f 3 that was obtained previously by Lee et ala2 This type of bilayer model is seen to fit all of the data, on each of the silica substrates studied. The measured profiles are shown, along with the corresponding fits, in Figures 5-7. The maximum coverage was 60% on both the Si/SiOn and crystal quartz substrates. This is lower than the value of 75 76 found by Lee et al. on a different amorphous silica block. This may be related to the roughness of the substrate. The block used by Lee et al. had a measured roughness of 8 A which is less than the 14.5-A roughness that was required to fit the present data. It is possible that a larger roughness, which is on a length scale comparable to th, may “break up” the bilayer and limit the adsorption. We have noticed in a number of other experiments at the solid/liquid interface that, in general, the rougher the surface the lower the adsorbed amount. The model we have fitted to the data is an average consisting of a mixture of bilayer regions and water. In assuming that the hydrocarbon part of the bilayer contains no water, we obtain the result that 40% of the surface area has no adsorbed surfactant. The total surface excess is independent of this assumption. Our interpretation would also be consistent with 100 % coverage of the surface with a bilayer whose hydrocarbon region contained 40 % water. Thus the system may range from the unlikely model of a uniform wet bilayer to that of a highly defective bilayer, which could alternatively be described as flattened micelles. Unfortunately, the lateral resolution of the experimental only excludes heterogeneities larger than thousands of angstroms. (20) Ottewill, R. H.; Walker, T. Kolloid-2. 1968, 227, 108. (21) Gellan, A.; Rochester, C. H. J . Chem. Soc., Faraday Trans. 1985
81 2235, 3109.
C I ~ Adsorbed E~ on Silica
Langmuir, Vol. 8,No. 4, 1992 1209
"
002 004 005 0.08 010
002
DOL 006
Figure 5. Neutron reflectivity profiles measured at the amorphous silica/aqueous C l ~ &solution interface, shown with the calculated "best fit" profiles: (a) SiliCa/c1~&in D20; (b) silica/ in water.^,^^.
008 0.10
Q /A'
Q /A-1
Figure 7. Neutron reflectivity profiles measured at the Si/SiOz/ aqueousClzE6solution interface,shown with the calculated'best fit" profiles: (a) Si/SiOz/ClZE6 in D2O; (b) Si/SiOz/ClzE6 in
lh HEXAOXYETXYLCNk. m d WAT6.R
50Ai
IC HYDROCARBON PURE CHAIN
Ih HEXAOXYElHYLENt and WATER
Figure 8. Schematic representation of the adsorbed layer of C12E6 at the silica/aqueoussolution interface.
Figure 6. Neurton reflectivity profiles measured at the crystal quartz/aqueous C& solution interface, shown with the calculated "best fit" profiles: (a) quartz/ClzE6 in D20; (b) quartz/ C12E6 in water4,l-i;(c) quartz/ClzE6 in water2,N.
Conclusions Our results show that the nonionic surfactant, C12E6, adsorbs onto each of the three different silica substrates as a bilayer. A schematic "cartoon" of the structure is
shown in Figure 8. On all of the silica substrates studied, the adsorbed bilayer has an overall thickness of 49 f 4 A and an arealmolecule of 44 f 4 A2. Arealmolecule here means the area occupied by two molecules in the bilayer under the assumption that the hydrocarbon region is dry. The SiISiO2 substrate has certain advantages for determining the structure because of the interference fringes in the profiles which result from the contrast provided by the layer of SiOz. The position and depth of these fringes is very sensitive to the thickness and scattering length density of the surfactant layer, facilitating accurate and precise fitting of the data. This effect has been discussed by Sivia et a1.22and termed 'speckle holography". The silicon substrate is ideal for such experiments because contrasting layers of different thickness can readily be formed at the surface by controlled thermal oxidation. However, the need to model the oxide layer requires additional measurements which introduce further errors (22) Sivia, D. S.; Hamilton, W. A.; Smith, G.S.; Rieker, T. P.; Pynn,
R.J. Appl. Phys. 1991, 70,732.
McDermott et al.
1210 Langmuir, Vol. 8,No. 4, 1992
to the final structure of the adsorbed layer. These factors result in similar errors in the structural parameters determined on each of the silica substrates. The structure of the adsorbed bilayer is remarkably similar on each of the substrates investigated here, with identical thicknesses, within the quoted errors. Furthermore, it is similar to the structure of ClzEs adsorbed on silica particles, determined by small angle neutron scattering.4 This could indicate one of two things. Either the surface treatment and equilibration has made the hydrogen bonding properties of the different surfaces identical, or hydrogen bonding only plays a minor role in the adsorption, the binding being determined by hydrophobic bonding. The thickness of the first region shows that the headgroup is approximately fully extended. The values for thearea/headgroup implies that 10.5 f 1.5 water molecules are associated with each headgroup. The length of the chain region is considerably shorter than two fully extended 4 which implies that there is hydrocarbon chains ( ~ 3 A) considerable overlap of the hydrocarbon chains in this region. The thickness of the third region is, within errors, the same as the first, demonstrating the symmetry of the bilayer structure. The structure does not change with concentration above the critical micelle concentration.
The coverages quoted in Table I11 refer to the percentage of the observable surface that is covered by the bilayer. The coverages obtained are all considerably less than 100% which would imply that any bilayer only exists in regions of the surface and that water covers the remainder of the surface. This is consistent with the effects of roughness which appear to reduce the surfactant coverage of the solid substrates. The model that correctly fits all of the data has the bilayer mixed in with the rough surface and it is this tendency to mix that may break the uniformity of the bilayer, resulting in less than full coverage. Clearly a better understanding of both microscopic and macroscopic roughness is required in order to understand fully the adsorption properties of small surfactant molecules at the solid-solution interface.
Acknowledgment. We thank J. Penfold, E. J. Staples, and P. G. Cummins for useful discussions. We thank the Institut Laue-Langevin and the Science and Engineering Research Council for provision of neutron facilities. D.C.M. thanks the SERC and Unilever plc for a CASE studentship. Registry No. SiOz, 7631-86-9; Si, 7440-21-3; quartz, 1480860-7.
