0 Copyright 1992 American Chemical Society
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The ACS Joumal of
Surfaces and Colloids SEPTEMBER 1992 VOLUME 8, NUMBER 9
Letters Effect of Hydrocarbon Chain Length on Arrangement of Chemically Adsorbed Monolayers Tadashi Ohtake,' Norihisa Mino, and Kazufumi Ogawa Central Research Laboratories, Matsushita Electric Industrial Co., Ltd., 3-15, Yagumonakamachi, Moriguchi, Osaka 570, Japan Received March 9,1992. I n Final Form: June 29, 1992 FTIR spectra and contact angles were measured on some monolayersprepared from one of the silyl-type surfactants having a hydrocarbon chain, n-Alkyltrichlorosilane,by a chemical adsorption (CAI technique. Molecular arrangement in the monolayer made by the CA technique, the CA monolayer, was random or bulklike as long as the carbon number of the hydrocarbon chain was less than 8, and the CA molecules were arranged along the surface of the substrate when the number became 8 and over. A monolayer is considered as the most effective material to prepare some molecular devices in the future. A chemical adsorption (CA) technique was rep~rted,l-~ as a new method to prepare the monolayer, and we have studied the process in d e t ~ i i l . ~ ? ~ To construct a three-dimensional artificial molecular structure, it is necessary that the monolayer has the durability to be processed both chemically and physically. For this purpose, the CA monolayer is the most convenient because the CAmonolayer is fixed strongly to the substrate by covalent bonds and is durable for both chemical and physical treatmenh6 On the other hand, LangmuirBlodgett (LB) films are weak and easily damaged. So it is very important to clarify the molecular arrbgement in the CA monolayer. We wish to report here some results obtained by FTIR spectroscopy and measurements of contact angles on the CA monolayers. The CA monolayers were prepared by the following process. (1)Sagiv, J. J. Am. Chem. SOC.1983, 105, 92. (2) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid F i l m 1983,100, 67. (3)Gun,J.; Sagiv, 3. J. Colloid Interface Sci. 1985, 112, 457. (4)Mino, N.; Nakajima, K.; Ogawa, K. Langmuir 1991, 7, 1468. (5) Ogawa, K.;Mino, N.; Nakajima, K.; Azuma, Y.; Ohmura, T. Langmuir 1991, 7, 1473. (6) Ogawa, K.; Mino, N.; Tamura, H.; Hatada, M. Langmuir 1990,6, 851.
SiOz/Al/glass substrates (made by sputtering Si02 (100
A thickness) on sputtered Al (1000 A thickness) on the
glass) were immersed a t 30 OC for 1 h in nonaqueous solutions containing n-alkyltrichlorosilane (CnHzn+lSiC13, n = 2-24). The n-alkyltrichlorosilanee were purchased from Shinetsu Chemical Industries, Ltd., and used without purification. The solvent was composed of 80 wt 5% n-hexadecane, 12 wt 3'% CCb, and 8 wt 5% CHCl3. The n-hexadecane was purchased from Aldrich Chemical Co., Inc., and used without purification, and the CC4 and CHCl3 were purchased from Kanto Chemical Co., Inc., and also used without purification. And then residual nonadsorbed n-alkyltrichlorosilane on the substrate was removed by washing with chloroform, followed by washing with water for stabilizing a monolayer deposited. The water was purified by ultrafiltration of deionized water to satisfy the ultrapurified grade required in the semiconductor device process. Through above treatments, one chemically adsorbed monolayer was prepared, and the surface of the monolayer was very hydrophobic. Contact angles to four standard solutions were plotted as a function of carbon numbers of the n-alkyltrichlorosilane, as shown in Figure 1. All the solutions were water containing ethylene glycol monoethyl ether and purchased from Nacalai tesque, Inc. The surface tensions of the solutions used to measure the contact angles were 54,48, 42, and 36 mN/m. The amount of the solution drop used at one measurement was about 4 pL, and the experiments 0 1992 American Chemical Society
2082 Langmuir, 8o
Vol.8, No.9,1992
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Letters 3.0 2.5
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- this investigation ---.Levine and
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Carbon Number o f a Hydrocarbon-Chain
Figure 1. Contact angles of CA monolayersand previous work8
as a function of carbon number, measured by using standard solutions for wetting propertiesof different surface tensions for wetting properties. The surfacetensionsof the standard solutions and 36 (0)mN/m. are 54 (O), 48 (A), 42 (13,
were done in the air at 23 f 1OC. The drops of one solution were placed at seven positions for one sample, and the contact angles were measured by using a contact angle meter (Model CA-2, Kyowa KaimenkagakuCo., Ltd.) and averaged after excluding maximum and minimum. Three samples were made for each CA monolayer. For each solution, the contact angles increased with increasing the carbon number up to 8 and leveled off at 8 and over, as shown in Figure 1. This result was greatlydifferent from previousresults7-10 obtained on some LB films prepared from fatty acids or fatty amines, in which the contact angles were reported to become constant when the carbon number was and over or 14e10and over. With the LB technique, it can be thought that molecules stand up when the ratio of the hydrophobic portion to the hydrophilic portion becomes large. At least at these carbon numbers, the hydrophobic portion becomes large enough for molecules to stand up. Although we can just see a macroscopic property by measuring the contact angles, it is evident that the molecular structure at the surface of the monolayer changes when the carbon number becomes 8 and over. We further measured IR RA (reflection absorption) spectra of the CA monolayers by using an FTIR spectrophotometer (Shimadzu Model 4300 FTIR spectrophotometer; resolution, 2 cm-1; detector, MCT). For this measurement, a multiple external reflection (MER) technique was used. The angle of incidence was 73O, and the number of external reflections was seven, The details of the MER technique are reported elsewhere." It was also important in this measurement to detect only the absorption components normal to the monolayer surface by using a polarizer. Relative intensities of the band due to the CH2 antisymmetric stretching vibration (integrated inteneities) were plotted as a function of the carbon numbers, as shown in Figure 2. Between C2 and C7, all plots were on straight line L1. When the line L1 was stretched left, the relative intensity became zero at C1. This is just consistent with the fact that there is no methylene group in methyltrichlorosilane (alkyltrichlorosilaneof Cl). The relative ~
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(7)Hardy, W.B.Collected Scientific Papers; Cambridge Univemity Press: Cambridge, 1936. (8) Levine, 0.; Zisman,W.A. J. Phys. Chem. 1967,61,1068. (9) Akatau, H.;Sameshima, J. Bull. Chem. Soc. Jpn. 1986, 11, 791. (LO)Sameehima, J.; Akatau, H.; b m u r a , T. Rev. Phys. Chem. Jpn. 1940,14,56. (11)Mino, N.;Tamura, H.; Ogawa, K. Langmuir 1991,7,2336.
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Carbon Number o f a Hydrocarbon-Chain
Figure 2. Rslative intensity (integratedintensity) change as a function of carbon number of hydrocarbon chain.
intensity between C2 and C7 also increased in proportion to the increase of the methylene groups. When the carbon number was 8 and over, all plots were in the region between the straight line L2 and L3. Especially when the monolayera were prepared perfectly by depoeiting for ample time, the plots were approximately on the line L2 (plotted by 0). The two lines,L1 and L2, crossed between C7 and C8. On the other hand when the CA was performed for a shorter time in order to decrease the molecular density, the relative intensitiea were in the region between the L2 and L3 (plotted by 0). L1 and L3 were also continuous. The slope of L2 was a half that of L1, and the slope of L3 was equal to that of L1. In fact the relative intensities of the CH2 antisymmetric stretching vibration increase in proportion to the number of the methylene groups. On the other hand, if the hydrocarbon chain is aligned perpendicular to the substrate, the direction of the transition moment of the CH2 antisymmetric atretching vibration is parallel to the subatrate,i.e., thevertical moment of the vibration becomes zero. Thus the band due to the CH2 antisymmetric stretching vibration diaappear when the IR spectrum is measured by the RA technique. Accordingly this result may indicate that the CA molecules over C7 were arranged alongthe substrate when the CA was performed satisfactorily,i.e., the data were on L2, because the plots should have come near the line L3 if the CA moleculeawere random and the m o l w density of the monolayer was similar to that of the monolayer below C8. T h e reason why the slope of the line L2 was half of that of L3 is not ale0 clear at preaent. The transmission IR spectraof the thinf i i (sandwiched with two KBr disks) of the original chlorosilyl-type surfactants and the MER IR spectra of CA monolayers are ahown in Figure 3 for comparing the ratio of the abwrbance of the CH2 antisymmetric stretching vibration around 2920 cm-l with thoee of the CH2 scieeoringvibration band around 1470 cm-1. When the hydrocarbon chain length was short, the intensity ratios of the original surfactant to the CA monolayer were similar to each other but were greatly different when the length was long, as shown in Figure 3. This may also support that molecular arrangements in the CA monolayer prepared from surfactants having a short hydrocarbon chain are similar to that of the original liquid and the molecules having a long hydrocarbon chain are arranged along the substrate in the monolayer. In conclueion, the structure of the CA monolayer prepared from n-alkyltrichloroeilane is probably random
Letters
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Langmuir, Vol. 8,No. 9, 1992 2083 monolayer -substrate
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Figure 4. Schematic representation of a CA monolayer when the carbon number of CA molecules is less than 8 (a), 8 and over (b),and 8 and over and the CA is performed for shorter time (c).
3000 2500\
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Woven u mb er / cm-'
Figure 3. IR spectra of CA monolayersand original surfactants around 2920 and 1470cm-1: (a)transmissionspectrumof original surfactant of C3 (carbonnumber is 3);(b) MER spectrum of CA monolayer of c3; (c) transmissionspectrumof original surfactant of C18;(d) MER spectrum of CA monolayer of C18.
or bulklike as long as the number of carbons is less than 8, because the molecules bend and/or lie down disorderly on the substrate as shown in Figure 4a. When the number
of carbons becomes 8 and over, the CA molecules are probably oriented as shown in Figure 4b, except when the CA monolayer was prepared for unsatisfactory time. Even if surfactants having long hydrocarbon chains over C7 are used, it is also difficult to arrange the CA molecules perpendicular to the substrate when the CA is nat performed for satisfactory time. In this case, the hydrocarbon chains of the CA molecules are fixed randomly, as shown in Figure 4c. The preparation mechanism of the CA monolayer is perhaps fundamentally different from that of the LB film. By use of the CA technique, the monolayer, in which molecules arrange, can be easily prepared with chlorosilyl-type surfactants having a shorter hydrocarbon chain than that used in the LB technique. Acknowledgment. The authors thank Director Dr. T. Nittaof Mataushita Electric Industrial Co., Ltd., Central Research Laboratories, and Director Dr. K. Kanai of Matsushita Electric Industrial Co., Ltd., Components and Material Basic Research Laboratories in Central Research Laboratories. Registry No. C2EI, 110-80-5.
