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Displacement Reactions of Covalently Attached Organosilicon Monolayers on Si Joseph W. Krumpfer and Alexander Y. Fadeev* Department of Chemistry and Biochemistry, Seton Hall UniVersity, South Orange, New Jersey 07079 ReceiVed April 10, 2006. In Final Form: July 20, 2006 The covalently attached monolayers of alkylsilanes (R(CH3)2SiX) on Si undergo complete displacement by the solutions of different organosilanes (R′(CH3)2SiX). By varying the reaction time, the degree of displacement can be controlled offering a convenient method for the preparation of surfaces with mixed functionalities (R and R′).
Organosilicon monolayers chemically grafted onto solids show good thermal, chemical, and hydrolytic stability and find numerous applications in adsorption, separation, adhesion control, water-proofing, and so forth.1 Hydrophobic monolayers derived from alkyl- and fluoroalkylsilanes show no reactive functionalities, and they are typically considered to be inert, permanent modifications of solid surfaces. In this work, we describe an example of the unusual reactivity of such monolayers: the reactions of displacement by organosilanes at a solidsolution interface. According to ellipsometry, XPS, and contact angles, the covalently attached monolayers (CAM) of monofunctional silanes (R(CH3)2SiX) supported on Si wafers undergo complete displacement by solutions of different organosilanes (R′(CH3)2SiX). By varying the reaction time, the degree of displacement can be controlled offering a convenient method for the preparation of surfaces with mixed functionalities (R and R′). The initial surfaces for this study were prepared by the reactions of (CH3)3SiN(CH3)2, C18H37Si(CH3)2N(CH3)2, and C6F13(CH2)2Si(CH3)2N(CH3)2 with Si wafers yielding CH3-Si, C18H37-Si, and C6F13-Si surfaces, respectively. (See Supporting Information for experimental details.) The thickness and contact angles of the Si-supported CAMs indicated full surface modification with the formation of hydrophobic CAMs of alkyl- and fluoroalkyl groups2,3 (Table). Displacement of CAMs by Silanes in Solution. The reactions were carried out by soaking the Si-supported CAMs in toluene solutions (5 vol %) of different silanes, including (CH3)3SiX, C18H37Si(CH3)2X (X ) Cl and N(CH3)2), CH2dCHSi(CH3)2Cl, BrCH2Si(CH3)2Cl, and NH2(CH2)3Si(CH3)2OCH3. The reactions were carried out at 70 °C. The change in the monolayer thickness (ellipsometry) was the most notable for the displacement reactions involving C18H37- and CH3-Si surfaces. The reaction between CH3-Si CAM and C18H37Si(CH3)2N(CH3)2 was characterized by an increase in thickness, and that of C18H37-Si CAM with (CH3)3SiN(CH3)2, by a decrease in thickness (Figure 1). The observed change in thickness suggested complete displacement of the initial CAMs by CAMs of the silanes presented in solution. We point out that the reactions were rather slow yet the displacement rates (by orders of magnitude) were comparable to those of the formation of CAMs through the reaction of trialkylsilanes with bare Si wafers. The reactions start upon * Corresponding author. E-mail:
[email protected]. (1) Plueddemann, E. W. Silane Coupling Agents, 2nd ed.; Plenum: New York, 1991. (2) Fadeev, A. Y.; McCarthy, T. J. Langmuir 1999, 15, 3759. (3) Fadeev, A. Y. Hydrophobic Monolayer Surfaces: Synthesis and Wettability. In Encyclopedia of Surface and Colloid Science, 2nd ed.; Taylor & Francis, Boca Raton, FL, 2006.
Figure 1. Monolayer thickness vs time of contact with solutions of different silanes. Table 1. Characteristics of the Monolayers Supported on Si Wafers CA (adv/rec), deg surface group
thickness, nm
H2O
C16H34
-Si(CH3)3 -Si(CH3)2C18H37 -Si(CH3)2C2H4C6F13
0.44 1.33 1.44
100/92 104/93 109/90
39/32 25/13 68/43
contact; however, it takes ∼24-48 h for the reaction to go to completion.2 The ellipsometry data was further supported by XPS and contact angles. For example, for the reaction of C18H37Si CAM with (CH3)3SiN(CH3)2, the XPS carbon percent (15° takeoff angle) decreased from 51 atom % (initial) to 40% (1 h), 37% (2 h), and, after 32 h, leveled off at ∼33%, which was characteristic of the CH3-Si CAM. The hexadecane contact angles increased (because of an increase in the CH3 vs CH2 groups) as follows: 25/13° (initial), 35/31° (1 h), 40/35° (2 h), and 40/36° (16 h), indicating the CH3-Si CAM. Water contact angles for the C18H37- and CH3-CAMs were rather similar (Table 1) and thus showed little change upon displacement. We found that the chemistry of the surface groups in CAMs and the silane in the solution have very little (if any) effect on the reactions. Results similar to those shown in Figure 1 were also observed for a range of systems, including the displacement of C6F13-Si CAMs by (CH3)3SiCl and C18H37Si(CH3)2Cl and the displacement of C18H37-CAMs by CH2dCHSi(CH3)2Cl, BrCH2Si(CH3)2Cl, and NH2(CH2)3Si(CH3)2OCH3 (Supporting Information). Also, we found no significant difference in the displacement kinetics for trialkylsilanes with different leaving groups (X ) N(CH3)2 vs Cl). The reaction temperature and the concentration of silane, however, were found to be important.
10.1021/la060969m CCC: $33.50 © 2006 American Chemical Society Published on Web 08/23/2006
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At room temperature, the displacements were extremely slow (weeks), and no reactions (assessed by ellipsometry and contact angles) were observed for CAMs in pure solvents. We speculate that the displacement reaction involves the hydrolysis of the SiS-O-Si bonds in the CAMs. Most likely, the hydrolysis of CAMs is catalyzed by traces of HCl (chlorosilanes) or HN(CH3)2 (N,N-dimethylaminosilanes) that are inevitably present in the solutions of silanes as a result of their reaction with moisture.1 (For the reaction of NH2(CH2)3Si(CH3)2OCH3, the amino group of the silane can act as a catalyst for the hydrolysis of the SiSO-Si bonds.) Once the hydrolysis of the “old” CAM begins, the silane from the solution (present in large excess) can react with the available surface sites, which ultimately leads to the formation of a “new” CAM. We thus concluded that the displacement reactions were common for CAMs in contact with solutions of different silanes. The only exception that we found was for the silane with perfluorinated groups (C6F13). Whereas the C6F13Si CAMs could be easily displaced by the other silanes, the C6F13 silane showed no activity in the displacement of the other CAMs. We attribute this due to the higher reactivity of fluorinated silanes versus alkylsilanes, specifically, in their reaction with moisture.4 In solution, fluorinated silanes quickly hydrolyze to silanols (Rf (CH3)2SiOH), which can further react either with the surface silanols, yielding CAM, or condense, giving siloxanes ([Rf (CH3)2Si]2O). We surmise that the majority of the fluorinated silane in solution is used up to form the nonreactive siloxane prior to any appreciable hydrolysis of the existing CAM. We also found that surfaces derived from alkyltrichlorosilanes (selfassembled monolayers, SAMs) showed no reactivity in the displacement reactions. This was attributed to the significantly greater grafting density of SAMs (∼4.5 nm-2) as compared to that of CAMs (∼2.4 nm-2). Wettability of Mixed Surfaces. We found that the contact angles of the binary mixed surfaces monotonically change with the degree of displacement of one groups by the others. The (4) Tripp, C. P.; Veregin, R. P. N.; Hair, M. L. Langmuir 1993, 9, 3518.
Letters
Figure 2. Contact angle hysteresis (deg) by water (blocks) and hexadecane (diamonds) and monolayer thickness (Å) (open symbols) for the first few hours of displacement of C18H37-Si CAM by (CH3)3SiCl.
hysteresis of contact angles (θA-θR), however, changed in a complex fashion, suggesting changes in the monolayer homogeneity upon displacement. Figure 2 shows the results for a series of C18H37-CAM with increasing fraction of CH3 groups. The most notable feature in Figure 2 is a significant increase in the contact angle hysteresis after 1 h of reaction time. We attribute this to increased “molecular roughness”3 of the mixed C18H37/ CH3 surfaces with an intermediate mixing ratio. It is notable that the data for the two probe fluids agreed well and correlated with changes in the monolayer thickness. After a longer reacting time, the monolayer thickness and contact angle hysteresis began to achieve steady values, suggesting the approximate completion of displacement and the formation of molecularly smooth surfaces. Acknowledgment. We acknowledge support from the NSF (CMS-0304098). Supporting Information Available: Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org. LA060969M
