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Letters Structure of a Cationic Surfactant Layer at the Silica-Water Interface A. R. Rennie Institut Laue Langevin, Grenoble, France
E. M . Lee, E. A. Simister, and R. K . Thomas* Physical Chemistry Laboratory, University of Oxford, South Parks Road, Ox, ard, O X 3Qz, England Received October 24, 1989. I n Final Form: March 7, 1990 The technique of specular reflection of neutrons has been used to study the structure of a layer of hexadecyltrimethylammonium bromide (CleTAB) adsorbed at the interface between a plate of amorphous silica and solutions of CMTAB. At concentrations of 1/3 and 2/3 the cmc (9 X 10-4 M), the structure which best fitted the reflectivity data can be described either as a defective bilayer or as flattened micelles. The thicknesses of the alkyl chain and head-group regions of the bilayer were found to be 28 f 4 and 6 f 2 A, repectively. At 6 X 10-4 M,80% of the interface was covered with bilayer, and M. At both coverages, the head-group area was found 'to be 30 f 3 A%. this dropped to 35% at 3 X
Introduction Many commercial and industrial processes are closely connected with adsorption of surfactants at the solidliquid interface, common applications including enhanced oil recovery, detergency, and mineral flotation. It is often important to have information not only about the amount of material adsorbed but also on the structure and homogeneity of the adsorbed layer. The present study was designed to give direct structural information about a cationic surfactant adsorbed on silica. In particular, we have studied the adsorption of hexadecyltrimethylammonium bromide (&TAB) on the surface of an amorphous silica block using the technique of neutron reflection. Previous studies of CMTABon silicalg have been complicated by the variety of different surface treatments of the silica (almost always in the colloidal form) which may leave the surface in a state anywhere between completely hydrophilic and completely hydrophobic. All suggest that above the cmc (9.2 X lo4 M) ClsTAB adsorbs as a bilayer, giving rise to a hydrophilic surface. More detailed investigations of the adsorption of ClsTAB on silicate minerals4~6have given some useful ideas as to what type of structure of the C l e T D layer is possible at charged interfaces. Ralston and Kitchener4 studied adsorption on amosite asbestos and concluded that at low concentrations of 10-5 M there is a layer with an area of about 100 A2/molecule while above 10"' M the area per molecule drops to 26 A2. A lowering of contact angle at higher concentrations is attributed to formation of a bilayer with polar head groups next to the aqueous phase. Cases and Mutaftschiev5 have reported adsorption isotherms for CleTAC (the analogous chloride) on biotite and con(1) Bijsterbcach, B. H. J. Colloid Interface Sci. 1974, 17, 186. (2) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (3) Elton, G. A. 2nd Int. Cong. Surf. Act. ZZZElectrical PhenomenuSolid Liquid Interface; Buttenvorths: London, 1957; p 161. (4) Ralston, R.; Kitchener, J. A. J. Colloid Interface Sci. 1975, 50, 243. (5) Cases, J. M.; Mutaftschiev, B. Surf. Sci. 1968, 9, 57.
clude that a bilayer structure is obtained at concentrations somewhat lower than the cmc. More recently, several papers have reported the results of force balance measurements of CISTAB adsorbed on muscovite mica68 and conclude that there is a compact monolayer of maximum thickness 18 A at concentrations just below the cmc. A large contact angle of 60' was also found.8 These results are somewhat different than those observed in other systems, and it is not clear to what extent the results are affected either by desorption as the mica plates are brought together during surface force measurements or by the particular properties of the surface charges on mica. A further force balance study using different quaternary ammonium surfact a n t gave ~ ~ rather different results, and the authors also questioned the purity of the C16TAB used in the previous The intensity of radiation reflected from an interface as a function of wavelength or incident angle (or more generally of the momentum transfer, K, normal to the interface, where K = 47r sin 6/A and 0 is the angle of grazing incidence) depends on the refractive index profile normal to the surface. In the case of neutrons, the latter is directly related to the average chemical composition of the interfacial layer. For any proposed structure of the interfacial region, the reflection profile may be calculated exactly by using the optical matrix method.1° A particular advantage of neutrons is that different isotopes may scatter neutrons with different amplitudes. Protons and deuterons scatter neutrons with opposite phase with the result that proton- and deuteron-containingmaterials have quite different neutron refractive indices. Thus, H/D substitution can change the reflectivity of a system substantially while leaving the chemical structure (6) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 7, 169. (7) Israelachvili, J. N.; Perez, E.; Tandon, R. K. J. Colloid Interface Sci. 1980, 78, 260. (8)Israelachvili, J. N.; Pashley, R. M. J. Colloid Interface Sci. 1984, 98,500. (9) Claesson, P. M.; Christenson, H. K. J. Phys. Chem. 1988,92,1650.
(10)Heavens, 0. S. Optical Properties of Thin Films; Butterworths: London, 1955.
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1032 Langmuir, Vol. 6,No. 5, 1990 unchanged. Furthermore, by adjustment of the H/D ratio the “contrast” between solvent and solid can be reduced to zero, and the reflectivity is then solely from the adsorbed layer. In the present study, we have used H/D substitution to vary the “contrast” of the solvent in order to obtain extra information about the structure of the adsorbed layer. Further discussion of the use of contrast variation has been described in detail by Lee et a1.l1
Experimental Section Reflectivity measurementswere performed by using the smallangle scattering instrument, D17, at the Institut Laue Langevin, Grenoble.12J3 The sample cell consisted of an amorphous quartz block (100 X 50 X 10 mm) that sealed a PTFE container via an O-ring. The quartz was Suprasil (Hellma, UK) polished optically flat on all faces. The mean roughness on the large faces used for adsorption was measured to be 8 A.13 The procedure for cleaning both the silica block and the PTFE sample cell has been described elsewhere.13 As will become clear in the discussion of the results, the cleanliness of the silica surface can be assessed from the reflectivity measured under suitable contrast conditions. The sample was mounted vertically, the beam entering through the side face of the silica block and then being specularlyreflected at the solid/liquid interface. The absolute reflectivity was determined by ratioing the reflected intensity to that of a beam passing through the silica block without reflection. Measurements were made over a range of values of momentum transfer normal to the surface (0.0095-0.12 A-1) by varying both the wavelength and the incident angle. The CleTAB (Aldrich)was purified by recrystallization from an acetone/methanol mixture and dried under vacuum for 12 h. Measurements were made in D20 and in a H20/D20 mixture (molar ratio 1:1.46) having the same scattering length density as the silica block, 3.52 x 104 A-2;11 this is described below as cms water (contrast matched to silica). The HzO used was high purity from an Elga UHQ water purifier. Results Reflectivity profiles were measured at bulk concentra3X and 6 X lo4 M a t about 300 tions of 1 X K, slightly above the Krafft temperature. The measured profiles for both contrasts of 3 X M solution are shown in Figure 1. The signal in Figure l b is from the adsorbed layer only since the contrast condition is such that there is no reflection a t all at the interface between clean silica and cms water. Model curves were calculated by the optical matrix method.l0 The 8-A roughness of the silica surface was incorporated into these calculations. Two models of the structure were considered, a monolayer and a bilayer. The bilayer type structure consisted of three layers representing respectively a head-group layer at the solid surface, a layer of tail groups, and a further layer of head groups close to the bulk solution. The model also allowed for the coverage of the bilayer being less than one. This can be taken into account in two ways, either by calculating the average reflectivity of the bilayer and the bare surface or by calculating the reflectivity appropriate to the average scattering length density. These do not give the same results, which has implications for the nature of the surface structure, which will be discussed below. The optimized fits of the bilayer model to the data are the solid lines in parts a and b of Figure 1. The parameters in the fit were constrained in several ways. Firstly, (11)Lee, E. M.; Thomas, R. K.; Penfold, J.; Ward, R. C. J. Phys. Chem. 1989.93.381. (12)Neutron Facilities at the High Flux Reactor; Institut Laue Langevin, Grenoble, 1988. (13)Lee, E. M.;Thomas, R. K.; Cummins, P. G.; Staples, E. J.; Penfold, J.; Rennie, A. R. Chem. Phys. Lett. In press.
Letters
1
0.02
0.06 momentum transfer/i-I
0.
Figure 1. Observed (points) and calculated reflectivity profiles of C16TAB (a) in D2O and (b) in water contrast matched to silica (3 X 10-4 M). Solid lines are calculated for the bilayer model whose parameters are given in Table I. The dashed lines
are for a monolayer, the parameters being those that optimize the fit to the D20 data. Statisticalerrors in the data lie approximately within the width of the points. Table I. Parameters Used for the Calculation of the Reflectivity Profiles of the Bilayer Model in Fieures 1 and 2 concn, head-group tail-group fractional mean M thickness, A thickness, A coverage area,’JA 2 3 x 10-4 6f2 28f4 0.35f0.05 43i6 6x 6f2 28f 4 0.80f0.08 19* 2 a The mean area refers to the area per molecule over the whole surface. The area per head group is 30 f 3 Az at both coverages.
the scattering length densities had to correspond to a single chemical composition for both isotopic contrasts. Secondly, the area of the head group a t the interface was constrained such that the thickness of the tail-group region multiplied by the area of the head group was equal to the estimated volume of two CISchains14 (about 900A3). The amount of water associated with the head group was fixed to fill the free volume in the head-group region. The fractional coverage of the surface by the bilayer aggregates was varied to optimize the fit. This model gave good agreement with the experimental data for both contrasts, provided that the coverage was included as the average of the scattering length density of the layer. The parameters of the model are shown in Table I. The mean area given in the table is the mean area per adsorbed surfactant molecule; the actual area of a head group was found to be 30 f 3 A2. An attempt was also made to fit a monolayer structure to the data. This model was characterized by two (14) Tanford, C. J. J . Phys. Chem. 1972,76, 3020.
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Letters
solution
quartz 1
0
4 I
20
LO
60
Distance I A
Figure 3. Schematic diagram of the structure of the adsorbed layer of C16TAB at the silica water interface. The solid circles represent the trimethylammonium head group, the open circles the bromide counterions, and the shaded circles the water molecules. Note that the vertical scale is half the horizontal one, so, strictly speaking, the species should be represented by ellipsoids. The model is characterized by 711,T ~ u,, and the area per molecule, the values of which are given in Table I. The structure shown occurs in patches whose dimensions and whose separation must be less than about 1 Km.
0.02
0.06 0.10 p.1, momentum transfer/AFigure 2. Observed (points) and calculated reflectivity profiles of &TAB in (a) D2O and (b) water contrast matched to silica (6 X 10-4 M). The calculated lines are for the bilayer model whose parameters are given in Table I.
parameters, the layer thickness (constrained to be less than the extended length of the molecule, 27 A1*),and the fractional coverage. The monolayer was taken to be of uniform composition, and the parameters were optimized to give the best fit to the solution in DzO. The result is shown as a dashed line in Figure la. When the same structure is used to calculate the profile in cms, it completely fails to explain the observations (dashed line in Figure lb). This shows that the monolayer structure cannot possibly be correct. The profiles measured for a bulk concentration of 6 X M are shown in Figure 2. The results of the bilayer fit are also shown as a continuous line for both contrasts. The parameters used for this fit are also given in Table I. Good fits to a monolayer model could not be obtained for this data with physically sensible values for the head-group area. In the course of these measurements, we have observed that it is possible to remove approximately all of the C16TAB from the silica surface by rinsing with 0.1 M HC1 (pH l),which then gave no reflection when the cell was refilled with cmq water. The accuracy of this measurement is limited by the level of the background scattering, which is such that coverages less than about 5% would not be easily detected. Since almost any imaginable contaminant of the surface is expected to be protonated, measurements of the reflectivity in cms water also measure the cleanliness of the surface to the same accuracy of 5%.
Discussion and Conclusions Our results show that even for concentrations well below the cmc the adsorbed layer is substantially thicker than that expected for a simple monolayer (see Figure 3).
Because the values of the coverage were found to be less then one and it was necessary to use the average scattering length density to calculate the reflectivity, the layer must consist of aggregates whose mean size must be less than about 1 pm. If the mean size of the aggregates were to exceed 1 pm, the reflectivity profile would be the mean of the profiles from covered and uncovered surfaces rather than being determined by the mean composition over the entire surface. The alternative model of the mean of two profiles did not fit the data satisfactorily. The upper limit of 1 pm for the size of the aggregates still leaves open the question of whether they are best described as flattened micelles or as a defective bilayer. It is interesting that the thickness of the tail region for both concentrations is substantially less than twice the extended chain length, which implies a large degree of intermixing of the chains. This is consistent with models of adsorption that propose that large hydrophobic interactions favor aggregation.6 The area per head group of 30 A 2 is not too dissimilar from the value of 26 A2 obtained for the different surface of amosite a s b e ~ t o s . ~ An additional measurement was made of a 1 X lo4 M solution in cmq water. This showed a measurable reflectivity, and therefore there is significant adsorption at this concentration, but in the absence of the data at a second contrast we are unable to draw any conclusions as to the structure of the surfactant layer. Further experiments will be necessary to elucidate this point. In summary we conclude that adsorption of ClsTAB on silica occurs as a thick layer with head groups at both the quartz surface and adjacent to the bulk solution even at concentrations well below the cmc. The amount of surface covered with this type of aggregate varies with concentration, but we are not in a position to determine the exact size of these areas. At the lowest concentration (1X l o 4 M), further experiments with different contrasts will be necessary to elucidate details of the structure. These results show neutron reflectivity to be of great value in determining details of structure in adsorbed layers at the solid-liquid interface. A particular advantage is that one can employ a macroscopic smooth substrate, thus avoiding many of the problems associated with the
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use of colloidal particles as substrates for adsorption.
Acknowledgment. We thank E. J. Staples, J. Penfold, and P. G. Cummins for useful discussion. We also
thank Unilever p.1.c and the Science and Engineering Council for supporting E.A.S. Registry No. CTAB, 57-09-0; SiOz, 7631-86-9.
Additions and Corrections Kloubek, J.: Evaluation of Surface Free Energy of Polyacetylene from Contact Angles of Liquids. 1989, 5, 1127-1130.
Equation 8 was incorrect as published. The correct form should be as follows:
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0 1990 American Chemical Society
