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Determination of the Ordering of Alkylhalogenosilanes on a Silicon Substrate Using FTIR-ATR Spectroscopy Reena Banga and Jack Yanvood" Materials Research Institute, Shefield Hallam University, City Campus, Pond Street, Shefield, S I 1WB, United Kingdom
Anthony M. Morgan Pilkington plc, Hall Lane, Lathom, Lancashire, LAO 5UF, United Kingdom Received July 12, 1994. I n Final Form: November 21, 1994@ Infrared linear dichroism has been employed to explore the orientational "ordering" of n-octadecyltrichlorosilane (OTS) on a silicon attenuated total reflection crystal as a function of coverage. Although absolute measurements of the alkyl chain orientation are difficult, changes of orientation can be detected as the coverage increases (with time), from submonolayerto monolayer. It is shown that the orientational ordering, as judged by the dichroicratios of vas(CHz)and vs(CH2)bands, increases significantly as monolayer coverage is reached. 1. Introduction Adsorption of amphiphilic molecules onto a polar substrate leads to the formation of oleophobic, closely packed, monomolecular films. Many silanes exhibit amphiphilic properties. Silanes are also used in HPLC to reduce the polarity of the stationary, silica phase and in anion chromatography.' Alkylhalogenosilanes are especially effective hydrophobizing agents, since the head group, CH3 or CF3, is hydrophobic. For example, OTS, n-octadecyltrichlorosilane, CH3(CH2)17SiC13,is a well-known and often used hydrophobizing agenta2-I4This molecule is known to form well-ordered monolayers on silicon and other substrates, and as expected, the infrared spectra of such monolayers have been studied before. In previous attenuated total reflection (ATR)work on s i l i ~ o n ~ sit~was , ~ Jimpossible ~ to get well-defined information on the degree of orientational order of the OTS molecules since no account was taken of the effect of the nature of the silicon oxide layer on the ATR crystal. This work explores the effect of taking this extra layer into account in the electric field calculations. It also explores the result of preparing a rather different substrate surface, more in line with the procedures employed in the glass industry. Although we still are not able to calculate precise angles of orientation, we are able to demonstrate good agreement between calculated and ~~
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Abstract published in Advance ACS Abstracts, J a n u a r y 15,
1995. (1)Ito, K.; Ariyoshi,Y.;Tarabiki, T.; Surahara, H.Anal. Chem. 1991, 63, 273-276. (2) Angst, D. L.; Simmons, G. W. Langmuir, 1991, 7, 2236-2242. (3) Silberzan, P. Langmuir, 1991, 7, 1647. (4) Tripp, C. P.; Hair, M. L. Langmuir, 1992, 8, 1120-1126. (51 Kallury, K. M.; Thompson, M.; Tripp, C. M.;Hair, M. L.Langmuir, 1992. 8. 947-954. (Si Wasserman, S. R.; Yu, T. Tao; Whiteside, G. M. Langmuir 1989, 5 f4), 1074-1087. ( 7 )Gun, J.; Sagiv, J. J . Colloid. Interface Sci. 1986, 112 (21, 457471 ._ -.
(8) Cheng, S. S.; Scherson, D. A,; Sukenik, D. A. J . A m . Chem. SOC. 1992,114, 5436-5437. (91 Maoz, R.; Sagiv, J. J . Colloid. Interface Sci. 1984, 100 (21, 465-
observed dichroic ratios (for the v,(CHz) bands). Furthermore, we are able to explore the ways in which the collective average orientational order of the OTS molecules changes with coverage on a "model" industrial glass surface. In this way we are able to provide additional physical insight into the behavior of OTS on silicodsilicon oxide, especially at less than monolayer coverage, a regime of industrial importance. 2. Theory The linear dichroic ratio of a particular infrared band of a sample is defined as the ratio of the integrated intensities of the perpendicular and parallel polarized spectra for that band. Thus:
A,/cm-l - ATE linear dichroic ratio = -- A,Jcm-' ATM whereA is the integrated intensity (band area), TE is the transverse electric (s) or perpendicular field (=Ey),and TM is the transverse magnetic ( p ) or parallel field (=E, +E,) E,, Ey,and E, are the components of the evanescent wave in the x , y , and z directions, respectively, in a n ATR experiment. Ey is perpendicular to the plane of incidence and parallel to the surface. E, is parallel to the plane of incidence and parallel to the surface. E, is parallel to the plane of incidence and perpendicular to the surface.15 Figure 1 shows the directions of the electric field amplitude components relative to a surfactant molecule on a substrate and the angle, 8,the surfactant molecule chain director makes with the z axis. The linear dichroic ratio can be writtenI6 as follows:
where 8 = the angle the chain director of the sample molecule makes with the z axis (see Figure 1). For perfect orientation (8 = 01, eq 1 becomes:
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(10) Tripp, C. P.; Hair, M. L. Langmuir, 1992, 8, 1961-1967. (11)Cohen, S. R.; Naaman, R.; Sagiv, J. J . Phys. Chem. 1986, 90 (14), 3054-3056. (12) Ulman, A,: Schildkraut, J. S.: Penner, T. L. J . Am. Chem. SOC. 1988, 110, 6136-6144. (13)Sagiv, J. J . A m . Chem. Soc. 1980, 102, 92. (14) Kiselev, A. V.; Lisichkin, G. V.; Nikilin, Y. S. Russ. J . Phys. Chem. 1983,57 (71, 1113.
0743-7463/95/2411-0618$09.00/0
(15) Harrick, J. N. Internal Reflection Spectroscopy;Harrick Scientific Corp.: New York, 1967. (16) Sperline, R. P.; Song, Y. P.; Freiser, H. Langmuir, 1992,8 (91, 2183-2191.
0 1995 American Chemical Society
Langmuir, Vol. 11, No. 2, 1995 619
Ordering of Alkylhalogenosilanes
‘. ‘.
3.0-‘’..
__._.. E*X
_ - - E2Y
- E22
-
2.5
2.0-
............... .............
........ ........
1
1.0-
>
0.5-
-0.04 -0.02
Figure 1. Directions of the electric field components of the evanescent wave relative to a surfactant species on a substrate. The angle of orientation, 8,the surfactant alkyl chain director
makes with the z axis is shown. Table 1
orientation
linear dichroic range
random perfect
0.897 1.094
(3) Where the transition moment vectors are the squared dipole changes of the considered vibrations. Measurements of the linear dichroic ratio by FTIRATR make it possible to determine the change in orientation of samples deposited onto a silicon ATR internal reflection element (IRE). Other groups9J5have calculated the linear dichroic ratio for thin films, (t
