Langmuir 1996, 12, 477-486
477
Structure of Monolayers of Tetraethylene Glycol Monododecyl Ether Adsorbed on Self-Assembled Monolayers on Silicon: A Neutron Reflectivity Study G. Fragneto, J. R. Lu, D. C. McDermott, and R. K. Thomas* Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, U.K.
A. R. Rennie Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, U.K.
P. D. Gallagher and S. K. Satija Reactor Radiation Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received April 10, 1995. In Final Form: September 28, 1995X We have used neutron reflection to study, for the first time, the structure of a surfactant layer adsorbed at the hydrophobic solid/water interface. Isotopic labeling of the water and of the hydrophobic selfassembled layer of octadecyltrichlorosilane (OTS) was first used to characterize protonated and deuteriated hydrophobic layers on the silicon oxide on the (111) face of silicon. This is the first time the structure and composition of such layers has been examined under water. Some water penetration into both the silicon oxide layer and the hydrophobic layer was observed. In the former case this is attributed to roughness of the oxide layer and in the latter to imperfections in the OTS layer. The thickness of the OTS layer was found to be 24 ( 2 Å, in agreement with other measurements in air. The adsorption isotherm of the surfactant tetraethylene glycol monododecyl ether (C12E4) was measured using two independent measurements (different isotopic compositions) on the deuteriated OTS layer. The surfactant was found to reach a constant excess at the critical micelle concentration (cmc) very similar to that at the air/water interface, i.e., an area per molecule of about 50 Å2. The thickness of the surfactant layer was also found to be similar to that at the air/water interface and decreased rapidly with decreasing coverage. Estimates of the angle of tilt of the surfactant molecules from the surface normal were 53 ( 10° and 75 ( 10° at the highest and lowest coverages, respectively. Isotopic labeling of the two halves of the surfactant molecule was used to show that the molecule is partially oriented with the ethylene glycol groups pointing outward toward the aqueous solution. At the cmc the thicknesses of the two halves of the surfactant molecule were both found to be 10 ( 2 Å, to be compared with fully extended chain lengths of 16.9 (alkyl chain) and 14.2 Å (ethylene glycol chain). Some penetration of the OTS layer by the surfactant was observed at the highest surfactant coverages.
Introduction Neutron reflectivity has recently been proved to be a successful technique for the study of the solid/liquid interface, not only for characterizing the solid surface but also for determining the structure and composition of the adsorbed material.1 Studies have been made of amorphous silica, quartz, and silicon surfaces in contact with aqueous solutions of various surfactants.2 For all of these hydrophilic surfaces the formation of bilayers of surfactant has been observed. The head groups are oriented both toward the surface and toward the aqueous solution forming a sandwich of the hydrophobic core between the hydrocarbon chains. In this paper we examine adsorption of surfactant on to a hydrophobic surface, in which case only a monolayer is expected. Silicon is a convenient substrate for such a study: first because it is transparent to neutrons and the incident * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, December 15, 1995. (1) Penfold, J.; Thomas, R. K. J. Phys. Condens. Matter 1990, 2, 1369. (2) McDermott, D. C.; Lu, J. R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R. Langmuir 1992, 8, 1204. Rennie, A. R.; Lee, E. M.; Simister, E. A.; Thomas, R. K. Langmuir 1990, 6, 1031. 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.; McCarney, J.; Thomas, R. K.; Rennie, A. R. J. Colloid Interface Sci. 1994, 162, 304. McDermott, D. C.; Kanelleas, D.; Thomas, R. K.; Rennie, A. R.; Satija, S. K.; Majkrzak, C. F. Langmuir 1993, 9, 2404.
0743-7463/96/2412-0477$12.00/0
beam can be transmitted through the solid and second because there are well established methods for making a reproducible hydrophobic surface. The simplest procedure is to graft a layer of hydrocarbon chains on to the natural oxide surface by self-assembly, for example, by the reaction of octadecyltrichlorosilane (OTS).3 To take full advantage of the possibilities of the different contrasts that can be created by isotopic substitution in neutron reflection, we have prepared two hydrophobic surfaces, one containing the silane with a protonated alkyl chain and the other having a deuteriated alkyl chain. After characterizing the hydrophobic layers, we have adsorbed the nonionic surfactant tetraethylene glycol monododecyl ether (C12E4) from aqueous solution, using isotopic substitution to highlight the structure of different parts of the molecule. OTS is the most common organochlorosilane used for the formation of self-assembled monolayers on oxidized silicon and glass substrates. Its success arises in part from the ability of the trichlorosilyl head group to react with hydroxyls on the substrate surface to form siloxy bonds. The hydrocarbon film is therefore chemisorbed at the surface with densely packed chains in extended conformation, which are oriented nearly perpendicular to the surface. The film is characterized by a remarkable (3) Maoz, R.; Sagiv, G. Langmuir 1987, 3, 1034. Maoz, R.; Sagiv, G. Langmuir 1987, 3, 1045. Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 99, 235. Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 100, 67. Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105. 674.
© 1996 American Chemical Society
478
Langmuir, Vol. 12, No. 2, 1996
physical and chemical stability. Many techniques have been used in the past to characterize self-assembled OTS layers; contact angle measurements,4-12 infrared absorbance and reflectance,4,8,9,11-14 ellipsometry,4-6,8,10,13-15 X-ray reflectivity,5,10,11,15 atomic force microscopy,16 and X-ray photoelectron spectroscopy.6,12 Neutron reflectivity has a number of features that may make it possible to derive additional information about the layer; it is nondestructive, and its high penetration makes it possible to determine the surface structure under water and in the presence of species adsorbed on to the hydrophobic layer. Experimental Details Protonated OTS (C18H37SiCl3 or h-OTS) of 95% purity was purchased from Aldrich and used as received. Deuteriated OTS (C18D37SiCl3 or d-OTS) was synthesized from C18D37Br obtained from Merck, Sharp and Dohme (98% isotopic purity) using the following reactions17
Fragneto et al. Table 1. Properties of Materials Used in This Study material H2O D2O water CMSi water CMSiO2 Si SiO2 -C18H37 -C18D37 -C12H25 -C12D25 -(OC2H4)4OH -(OC2D4)4OD
densitya volumeb lengthc bd F (g cm-3) (Å3) (Å) (10-4 Å) (10-6 Å-2) 0.9975 1.105 1.038 1.059 2.32 2.16 0.7664 0.7768 0.8034 0.9227 1.188 1.294
30 30 30 30 20 47 542 542 350 350 270 270
24.3 24.3 16.9 16.9 14.2 14.2
-0.168 1.905 0.621 1.023 0.415 1.585 -1.868 36.65 -1.37 24.55 2.236 18.56
-0.56 6.35 2.07 3.41 2.07 3.41 -0.366 6.76 -0.391 7.01 0.828 6.87
a Densities from Handbook of Chemistry and Physics, 54th ed.; Weast, R. C., Ed.; Chemical Rubber Co.: Cleveland, OH, 1973. b Molecular volumes calculated from densities. c Lengths of fully extended chains taken from ref 29 for OTS and Tanford (Tanford, C. J. J. Phys. Chem. 1972, 76, 3020.) for surfactants. d Scattering lengths calculated from Sears, V. F. Neutron News 1993, 3, 26.
C18D37Br + Mg f C18D37MgBr C18D37MgBr + SiCl4 f C18D37SiCl3 The Grignard reagent, prepared using standard procedures,18 was added through a glass wool filter to a flask containing SiCl4 dissolved in dry ether over a period of 2 h. The reaction product was filtered by suction and the magnesium salts formed during the reaction extracted with dry ether. Upon removal of the ether, the crude product was purified by vacuum distillation. Analysis by titration with NaOH indicated a purity >98%. Protonated C12E4 (C12H25(OC2H4)4OH) was obtained from Fluka and purified on a silica column.19 The different isotopic species of the surfactant were synthesized in our laboratories.20 Surface tension at the air/solution interface of the isotopic species of C12E4 in H2O gave no indication of isotope effects or of impurities. The critical micelle concentration (cmc) was found to be 6.9 × 10-5 M. High-purity water was used for the preparation of the surfactant solutions and the cleaning of the substrates. D2O (
