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Langmuir 2005, 21, 1424-1427
Chain-Length Dependence of the Conformational Order in Self-Assembled Dialkylammonium Monolayers on Mica Studied with Soft X-ray Absorption Georg Ha¨hner,*,† Mathias Zwahlen,†,‡ and Walter Caseri‡ School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9ST, U.K., and Department of Materials, ETH-Ho¨ nggerberg, Wolfgang-Pauli-Strasse 10, CH-8093 Zu¨ rich, Switzerland Received August 30, 2004. In Final Form: November 11, 2004 Self-assembled alkyl chain based monolayers on mica are important for industrial and technological processes since they can be employed for an organic modification of the inorganic substrate. The conformational structure and orientational order of the films determine the interaction of the modified substrate with the environment and the chemical character and stability of its surface. We have studied the conformational order in ion exchanged dialkylammonium monolayers adsorbed on mica depending on the length of the alkyl chains systematically with near-edge X-ray absorption fine structure spectroscopy (NEXAFS). In addition, films were characterized by water contact angle measurements. The experimentally determined average tilt angles of the chains are discussed in terms of the degree of order. It was found that the absolute number of gauche defects in the films increases with decreasing chain length.
Introduction Monolayer films of alkyl-based surfactants play an important role for the organic modification of inorganic substrates. In particular, self-assembled monolayers (SAMs) have attracted significant interest over the past 2 decades due to their ease of preparation and long-term stability. Parameters such as the packing density of the molecules in the monolayer films and their orientational order are important for potential applications. They determine, for example, the wettability of the modified surfaces and are essential for creating uniform planes of functionality in the case of terminating functional groups. The majority of work with SAMs has been performed with alkanethiols on gold. The orientation of alkanethiols is determined by the bonding mechanism of the molecules to the gold surface and the van der Waals interaction between neighboring molecules, which depends on the packing density.1 The intermolecular interaction increases with increasing chain length. As a result, alkanethiol SAMs with molecules containing more than 12 carbon atoms show a higher degree of conformational order than shorter ones.1,2 Alkanes with less than 12 carbon atoms have occasionally been described as disordered or “liquid”.1 With a view to potential application, films on substrates other than noble metals can be of equal or even higher interest. Micas and clays play a crucial role in many industrial processes. Films on these insulating substrates are employed in connection with modern materials, for instance in composites made of polymers and mica particles.3 The embedded particles have to be modified in order to avoid pronounced agglomeration in common polymer matrixes. A prerequisite is the match of the * To whom correspondence should be addressed. E-mail: gh23@ st-andrews.ac.uk. † University of St. Andrews. ‡ ETH-Ho ¨ nggerberg. (1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (2) Ha¨hner, G.; Kinzler, M.; Thu¨mmler, C.; Wo¨ll, C.; Grunze, M. J. Vac. Sci. Technol., A 1992, 10(4), 2758. (3) Theng, B. K. G. Formation and Properties of Clay-Polymer Complexes; Elsevier: Amsterdam, 1979.
surface energies of the particles and the polymer, which depends on the order in layers of surface-bound alkane moieties. Self-assembled dialkylammonium films with chain lengths ranging from 12 to 18 carbon atoms adsorbed on mica and prepared via ion exchange have been reported to show a significant degree of order.4-7 It is not a priori obvious, however, how the conformational structure and the orientational order in monolayer films on mica changes with the alkyl chain length. Results from the thiol gold system cannot simply be transferred to mica, since the bonding mechanism and molecular arrangements are different. In the case of ion exchanged films on mica the packing density in the resulting film is determined by the surface cation density of the layered silicate surface. In addition, for dialkylammonium films two alkyl chains are connected to one surface (ion) site. As a result, there must be gauche conformations present in at least one of the chains close to the ammonium group if both chains are stretched and are oriented parallel.4,6,8 In consequence, the two chains of a surfactant molecule will have different heights, leaving the end group of the chain without gauche defects close to the ammonium group more exposed. At room temperature the outermost groups are also present in gauche conformation.6,9 Therefore, an increasing amount of disorder is expected with decreasing chain length even for a constant number of gauche defects present at room temperature. The modification of the mica surface with alkylammonium compounds via ion exchange has been studied with different analytical techniques.4-6,10-15 Densely packed (4) Brovelli, D.; Caseri, W. R.; Ha¨hner, G. J. Colloid Interface Sci. 1999, 216, 418. (5) Fujii, M.; Li, B.; Fukada, K.; Kato, T.; Seimiya, T. Langmuir 2001, 17, 1138. (6) Osman, M. A.; Ernst, M.; Meier, B. H.; Suter, U. W. J. Phys. Chem. B 2002, 106, 653. (7) Ha¨hner, G.; Zwahlen, M.; Caseri, W. R. J. Colloid Interface Sci., submitted for publication. (8) Okuyama, K.; Soboi, Y.; Iijima, N.; Hirabayashi, K. Bull. Chem. Soc. Jpn. 1988, 61, 1485. (9) Heinz, H.; Castelijns, H. J.; Suter, U. W. J. Am. Chem. Soc. 2003, 125, 9500.
10.1021/la047841u CCC: $30.25 © 2005 American Chemical Society Published on Web 01/19/2005
Self-Assembled Dialkylammonium Monolayers on Mica
films with a high degree of order and orientation can be prepared. The influence of the length of the alkyl chain on the established molecular orientation has been studied theoretically9 or has been deduced from indirect experimental evidence,5 but a direct determination of this parameter and a quantification has not been reported yet. In the present study we describe a systematic investigation of the conformational order in quaternary dialkyldimethylammonium films prepared via ion exchange on mica substrates. In another study we reported that adsorption from a mixture of equal volumes of water and methanol results in better ordered films than adsorption from other solvents.7 The molecules investigated here are of the composition Me2NR2+, where R denotes an alkyl group with 8-18 carbon atoms. The synchrotron-based technique of near-edge X-ray absorption fine structure spectroscopy (NEXAFS) was employed to determine the molecular orientation in the films. This technique allows a quantification of the established order in terms of the number of gauche defects present, thus revealing a detailed picture on the molecular arrangement in ion exchanged films. A low density of molecules on the surface is not a problem for the technique employed in contrast to other techniques such as infrared spectroscopy.6 A sufficient signal-to-noise ratio is achievable even with a severe charging of the insulating substrate.16 Experimental Section Substrates. Muscovite mica was obtained in sheets from Provac (Balzers, Liechtenstein) and cut into pieces of approximately 1 cm2. Immediately before immersion of the substrates into solutions of LiCl or alkylammonium salts for ion exchange the samples were cleaved with adhesive tape. Chemicals. Lithium chloride (g98%) was purchased from Fluka (Buchs, Switzerland) and used without further purification. A large excess of Li+ ions is well-known to exchange the K+ ions on the surface plane of muscovite completely and can also increase the maximum surface coverage in a subsequent exchange with other cationic surfactants.17 Ion exchange necessitates desorption of the surface ions simultaneously to the adsorption of the replacing surfactant in contrast to covalent adsorption. In densely packed films this two-particle process can be limited by the steric restrictions imposed by the entering and leaving species, so that ions with a smaller ionic radius will be replaced faster. Hence Li+ ions with their rather small ionic radius are superior to other ions with bigger radii. The dialkyldimethylammonium species employed, didodecyldimethylammonium iodide (2C12), ditetradecyldimethylammonium bromide (2C14), dihexadecyldimethylammonium bromide (2C16), and dioctadecyldimethylammonium bromide (2C18) were all purchased from Fluka, (Buchs, Switzerland) in pro analysi quality (p.a. g 98%). Dioctyldimethylammonium iodide (2C8) was synthesized according to a procedure described in the literature.18 Its purity was comparable to that of the commercial surfactants. Solvents. Fresh ultrapure water (Easypure, Barnstead, g18.2 MΩ cm) was used for preparation of aqueous solutions. Methanol was purchased from Fluka (Buchs, Switzerland) in pro analysi quality (p.a. g 98%). (10) Osman, M. A.; Caseri, W. R.; Suter U. W. J. Colloid Interface Sci. 1998, 198, 157. (11) Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P. Chem. Mater. 1994, 6, 1017. (12) Jasmund, K.; Lagaly, G.; Eds. Tonminerale und Tone; Steinkopf: Darmstadt, Germany, 1993. (13) Weiss, A. Angew. Chem., Int. Ed. Engl. 1963, 2, 134. (14) Lagaly, G. Solid State Ionics 1986, 22, 43. (15) Lagaly, G.; Benecke, K. Colloid Polym. Sci. 1991, 269, 1198. (16) Ha¨hner, G.; Marti, A.; Caseri, W. R.; Spencer, N. D. J. Chem. Phys. 1996, 104, 7749. (17) Herzog, E. Ph.D. Thesis (No. 11363), ETH Zurich, Switzerland, 1996. (18) Ralston, A. W.; Reck, R. A.; Harwood, H. J.; DuBrow, P. L. J. Org. Chem. 1948, 13, 186.
Langmuir, Vol. 21, No. 4, 2005 1425 Sample Preparation. Pieces of muscovite mica were immersed into 0.5-1 M aqueous LiCl solutions for at least 3 days. After removal of the substrates from the solutions, they were rinsed with the solvent. This procedure results in an exchange of the surface potassium cations with lithium ions. X-ray photoelectron spectroscopy (XPS) measurements show that, after immersion of the mica in the LiCl solution for more than 3 days, the recorded potassium signal does not decrease further, which confirms a complete exchange of the surface K+ with Li+ ions.17 The exchange with Li+ ions ensures that a subsequent exchange with the surfactant ions is complete, as lithium ions can be exchanged easier than potassium ions.10 Dialkyldimethylammonium films were prepared by immersing the mica substrates into solutions of the dialkyldimethylammonium ions. The substrates were transferred into solutions of 1 mM 2C8, 0.4 mM 2C12, 0.2 mM 2C14, 0.2 mM 2C16, or 0.1 mM 2C18. These values were described as “optimal” in another study.5 All concentrations are below the critical micelle concentration (cmc) in water. The surfactants can be completely dissolved at these concentrations in a 1:1 mixture of water and methanol at room temperature. The samples remained in the solutions for approximately 2 days and were then rinsed with the solvent to remove any possible excess material on the surface. All samples were introduced into the ultrahigh-vacuum chamber immediately after preparation. Contact Angle Measurements. Advancing water contact angle measurements were performed with a commercial NRL C. A. goniometer (Rame-Hart, Inc., Mountain Lakes, NJ). Sessile drops of 3 µL volume were deposited onto the surface. NEXAFS. NEXAFS experiments were carried out on Exxon beam-line U1A at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (New York). The base pressure in the vacuum end chamber was
