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MATERIALS AND INTERFACES Self-Assembled Monolayer Films from Liquid and Supercritical Carbon Dioxide Randy D. Weinstein*,† Department of Chemical Engineering, Villanova University, Villanova, Pennsylvania 19085
Dong Yan and G. Kane Jennings*,‡ Department of Chemical Engineering, Vanderbilt University, Nashville, Tennessee 37235
We report the formation of self-assembled monolayers (SAMs) on gold substrates by exposure to n-alkanethiols [CH3(CH2)n-1SH; n ) 8, 10, 12, 16, and 18] in liquid and supercritical carbon dioxide. The results of this novel study show that an environmentally friendly solvent can be used to form highly crystalline SAMs with few gauche defects and that pressure as well as exposure time can be used to affect the structural and barrier properties of the monolayer film. Reflectance infrared spectroscopy, electrochemical impedance spectroscopy, and wetting measurements were used to characterize the SAMs. The effects of pressure (76-300 bar) and adsorption time (3-90 min) on the formation of the SAMs were explored. The overall chain density of these SAMs was greater than that for SAMs formed in common organic solvents such as ethanol. The properties of the SAMs were slightly affected by the pressure during formation. At 35 °C, as the carbon dioxide pressure increased (from 76 to about 140 bar), the packing density and resistance of the SAM increased. SAMs prepared at higher pressures ranging from about 140 to 300 bar exhibited similar resistances, capacitances, and canted structures. There was also no significant difference in using liquid (25 °C and 103 bar) or supercritical (35 °C and 103 bar) carbon dioxide for SAM formation. Supercritical carbon dioxide also enabled the formation of SAMs using polar adsorbates (-OH- and -CO2H-terminated thiols) to prepare high-energy surfaces that are wet by water. Introduction Self-assembled monolayers (SAMs) are molecular films that form spontaneously on metal substrates from solution.1 SAMs have many practical applications including inhibiting corrosion and oxidation2 and facilitating or preventing wetting.3 The most commonly studied system of SAMs is that of n-alkanethiols [CH3(CH2)n-1SH] on gold.3a Because of the synthetic flexibility of thiols and the ease of preparing monolayers on gold,4 these SAMs have enabled the straightforward formation of tailored organic interfaces for studying biocompatibility,5 immobilization of proteins,6 friction,7 and electron transfer.8 During the formation of alkanethiolate SAMs on gold, the assembling thiol molecules must displace solvent molecules at or near the metal surface and form a densely packed monolayer film. The choice of solvent is important and can greatly affect the kinetics of SAM formation9 and the properties10 of the SAM. Traditionally, typical organic solvents such as ethanol, hexane, and chloroform have been used for SAM formation because of their availability and ease of use in the * Authors to whom correspondence should be addressed. † E-mail:
[email protected]. Fax: (610) 5197354. ‡ E-mail:
[email protected]. Fax: (615) 3437951.
laboratory. Ideally, the selected solvent, in addition to being able to solvate millimolar concentrations of the potential adsorbate, should have little or no interaction with the metal surface and minimal interaction with the assembling adsorbates.9a Solvents with smaller molecular sizes that have weaker interactions with either the metal surface or the adsorbate have been shown to produce monolayers that exhibit superior properties10 and/or rapid kinetics.9a Another advantageous feature would be the ability tune the solvent to affect the properties of the SAMs. Liquid and supercritical (or pressurized) carbon dioxide are potentially superior solvents for SAM formation compared to traditional liquid organic solvents. First, CO2 is a small molecule that has little affinity for metals such as gold. In addition, the self-diffusion coefficient of CO2 is 1-3 orders of magnitude greater than that of traditional solvents,11 which might enable SAMs to form more rapidly in CO2. Material transport in compressed CO2 should also be increased because of the low viscosity of CO2. Typically, the viscosity of compressed CO2 is about 0.5-1 order of magnitude lower than that of traditional liquid solvents.12 With supercritical CO2, there is also the potential for tuning the solvent properties with changes in temperature and/ or pressure. Figure 1 shows how the density, selfdiffusivity, and viscosity of pure CO2 change with pressure at 40 °C. For reference, the critical point of
10.1021/ie000954y CCC: $20.00 © 2001 American Chemical Society Published on Web 03/30/2001
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Figure 1. Density, viscosity, and diffusivity of pure carbon dioxide at 40 °C (adapted from Subramaniam and McHugh13).
CO2 is 31 °C and 74 bar. With changes in pressure, the transport properties of supercritical CO2 will change and, hence, will potentially impact the formation of SAMs. In addition to having attractive tunable physical properties, CO2 has many other advantages over traditional organic solvents. Pressurized CO2 is environmentally friendly; inexpensive; nontoxic; nonflammable; and, with a drop in pressure, can revert back to the vapor phase, causing liquids and solids to drop out of solution and be easily recovered. Supercritical CO2 is such a promising environmentally friendly solvent that it has received much attention as a reaction14 and extraction15 medium in the literature. Furthermore, CO2 and other supercritical solvents have begun to be used for the formation of nanoparticles16 and self-assembled structures of block copolymers,17 as well as for many other colloidal applications.18 Although there is a vast amount of research utilizing CO2 for materials processing and formation on the nanoscale, we are unaware of any previous uses of CO2 to form SAMs. One of the purposes of this study was to verify that CO2 could be used as a solvent in the formation of SAMs. Various n-alkanethiols [CH3(CH2)n-1SH; n ) 8, 10, 12, 16, and 18] were used to form SAMs on gold surfaces. We wished to compare the molecular structure and barrier properties of SAMs formed with liquid and supercritical CO2 to those formed with a traditional liquid solvent, ethanol. We also set out to explore the effects of solvent properties on the formation of SAMs. Supercritical CO2 has the potential to affect the packing density of the SAM through simple changes in temperature and/or pressure. Finally, we wanted to demonstrate the formation of SAMs in CO2 using polar adsorbates [HS(CH2)11OH and HS(CH2)10CO2H] to enable the preparation of high-energy surfaces that are wet by water. Experimental Section Materials. Gold shot (99.99%) and silicon(100) wafers were obtained from J&J Materials (Neptune City, NJ)
and Montco Silicon (Royersford, PA), respectively. Chromium-coated tungsten rods were obtained from R. D. Mathis (Long Beach, CA). All chemicals, including n-alkanethiols (Aldrich), potassium ferricyanide [K3Fe(CN)6; Aldrich], and potassium ferrocyanide trihydrate [K4Fe(CN)6‚3H2O; Aldrich] were used as received. Pure ethanol (AAPER) and grade-5 CO2 (BOC Gases) were also used as received. Deionized water (16.7 MΩ) was purified with a Modu-Pure system. Sample Preparation. Chromium (100 Å) and gold (1500 Å) were evaporated sequentially in a diffusionpumped chamber (operating pressure of 4 × 10-6 Torr) onto silicon [Si(100)] wafers. For SAMs formed in ethanol, the monolayer films were prepared by immersing evaporated gold substrates into solutions containing 1 mM alkanethiol in ethanol at 35 °C for a fixed time. The temperature and adsorption time were selected to provide a direct comparison with films prepared from supercritical CO2. Upon removal, the samples were rinsed with ethanol and dried under a stream of nitrogen. SAM Formation in CO2. SAMs from CO2 were prepared without stirring in a 25-mL 316 stainless steel reactor with a sapphire window similar to the one described previously.19 In a typical experiment, a gold substrate, approximately 3 cm by 1 cm, was sealed in the reactor. The reactor was then preheated while lowpressure CO2 (30 psi) was flushed through the system. Once the desired temperature was reached, an Isco 260D syringe pump was used to pressurize the reactor with CO2. Pressurization took between 20 s (low pressure) and 1 min (highest pressure). The CO2 exiting the pump was cooled in a shell and tube heat exchanger; otherwise, the rapid pressurization would heat the fluid and overshoot the desired temperature. During the pressurization process, a fixed volume of liquid thiol was injected into the reactor via a six-way Valco 6CU valve to produce a concentration of 1 mM in the 25-mL reactor. If a solid thiol was used, it was sealed in the reactor with the gold substrate at the start of the experiment. Pressure let-down proved to be somewhat more difficult. If the pressure of the system was released quickly, thiol would drop out of solution onto the gold substrate and produce a film on the formed monolayer. To prevent this, the pressure was reduced to around 80 bar, and simultaneously, approximately 60 g of CO2 was flushed through the reactor over a 2-min period before the pressure was then reduced to atmospheric. Reflectance Infrared (IR) Spectroscopy. IR spectra were obtained in single-reflection mode with a BioRad Excalibur infrared spectrometer containing a universal reflectance attachment. The p-polarized light was incident at 80° from the surface normal. The reflected light was detected with a narrow-band MCT detector cooled with liquid nitrogen. Spectral resolution was 2 cm-1 after triangular apodization. Spectra were referenced to those of SAMs prepared on gold from octadecanethiol-d37, and 1000 scans of both sample and reference were collected. Simulations of infrared spectra were performed using a computer program developed and provided by Atul Parikh and David L. Allara.20 These simulated spectra were used to estimate the average cant and twist of the SAMs formed from CO2 and ethanol based on the best agreement of the intensities of the methylene modes.
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Figure 2. Schematic illustration of an alkanethiolate adsorbate that is canted 30° (R) from the surface normal and twisted 0° (β) around its molecular axis. The arrows indicate the transition dipole moments for the following stretching modes: νs(CH2), νa(CH2), νs(CH3), and νa(CH3). The intensities of the infrared bands for these transition dipole moments depend on their projection along the surface normal.
The isotropic reference data required for the simulations were obtained by A. Parikh using (C18S)2 in a KBr matrix. Electrochemical Impedance Spectroscopy. Electrochemical impedance measurements of SAM-coated gold samples were performed with a CMS300 electrochemical impedance system (Gamry Instruments) interfaced to a personal computer. A glass cell equipped with a gold-coated silicon counter electrode and a Ag/ AgCl/saturated KCl reference electrode contained an aqueous solution of 0.1 M Na2SO4, 1 mM K3Fe(CN)6, and 1 mM K4Fe(CN)6‚3H2O. The measurements were made at the open circuit potential with a 5 mV ac perturbation that was controlled between 50 mHz and 20 kHz. Film resistance and capacitance values were determined by fitting the EIS data with an equivalent circuit2b consisting of a solution resistance in series with a parallel combination of interfacial capacitance and charge-transfer resistance using software provided by Gamry. Wetting Measurements. Advancing and receding contact angles were measured on static drops of water or hexadecane with a Rame-Hart manual goniometer. Contacting liquids were advanced or retreated (1 µL/s) prior to measurement via an attached syringe supplied by Rame-Hart. The syringe tip remained in the drop during measurement. Both sides of ∼5 µL drops were measured at two different locations on a sample. Results We used reflectance infrared (IR) spectroscopy to assess the molecular structure and crystallinity of SAMs formed on gold from CO2 and ethanol. Figure 2 schematically illustrates the orientations of the transition dipole moments for methylene (-CH2-) and methyl (-CH3) groups in a trans-zigzag extended alkanethiolate adsorbate that is tilted from the surface normal at an angle R and twisted about its molecular axis by an angle of β (here set equal to 0°). During an IR measurement, infrared light polarized parallel to the plane of incidence reflects from the gold surface to
Figure 3. Reflectance infrared spectra of SAMs formed by 1-h exposure of gold to 1 mM solutions of CnSH (n ) 8, 10, 12, 16, and 18) in CO2 (103 bar, 35 °C, solid line) or ethanol (35 °C, dashed line).
generate an electric field gradient normal to the surface.21 The intensity of a given band in the IR spectrum is proportional to the square of the component of its transition dipole moment oriented along the surface normal.20,22 Thus, as a monolayer film becomes more densely packed and the average molecular cant (R) decreases, the intensity of the methylene modes should decrease since their dynamic dipole is oriented less normal to the surface. Figure 3 shows the C-H stretching region of infrared spectra for SAMs prepared by exposure of gold substrates to 1 mM solutions of the alkanethiol (CnSH) in supercritical CO2 (103 bar, 35 °C) or liquid ethanol (1 bar, 35 °C) for 1 h. The major peaks shown in these spectra correspond to the asymmetric methyl [νa(CH3)], asymmetric methylene [νa(CH2)], symmetric methyl [νs(CH3)], and symmetric methylene [νs(CH2)] stretching vibrations. The asymmetric and symmetric methyl modes appear at 2965-2966 cm-1 and at 2879 cm-1, respectively, for SAMs formed both in ethanol and in CO2. The intensities of the methyl modes are also independent of solvent and indicate similar average orientations for the terminal methyl groups in the SAMs formed from these solvents. The asymmetric and symmetric methylene modes appear at 2919-2920 cm-1 and 2851-2852 cm-1, respectively, for SAMs with chain lengths (n) of 10 or greater, and these positions are independent of solvent. These positions indicate that the alkyl chains are primarily trans-zigzag extended with few gauche conformers.22 These positions show that highly crystalline monolayers can be prepared from
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supercritical CO2. The position of the asymmetric methylene (2923 cm-1) mode for n ) 8 indicates a significantly greater density of gauche defects within that SAM because of the inability of the shorter-chained thiolate to pack as efficiently as the longer-chained analogues.23 A remarkable feature of Figure 3 is that the asymmetric methylene modes are significantly less intense for SAMs formed from CO2 compared with those for SAMs formed from ethanol, whereas the symmetric methylene modes exhibit intensities that are independent of solvent. These results suggest that SAMs formed from CO2 exhibit a different average orientation than those from ethanol. Based on the intensities of the methylene modes, we simulated the average cant and twist for C18 SAMs using the methodology of Parikh and Allara.20 The simulations reveal that the C18 SAMs formed from a 1-h exposure of the thiol (1 mM) in CO2 exhibit an average cant (R) of 26° and a twist (β) of 53°, whereas those in ethanol exhibit an average cant of 31° and a twist of 55° after 1 h of adsorption. The smaller average cant angle for SAMs in CO2 suggests an overall greater chain density, which can result from either a closer intermolecular spacing of the alkanethiolates on the surface or the presence of fewer defects. If the effect of chain density is related to kinetics, then a longer adsorption time from ethanol should eventually result in a SAM that achieves the level of packing that occurs as the result of a 1-h adsorption from CO2. To determine whether the denser packing of SAMs from CO2, as shown in Figure 3, is a result of a more rapid assembly, we characterized the properties of the films ex situ as a function of adsorption time (Figure 4). During the formation of SAMs, their average structural properties change continuously until a “complete” monolayer is formed.24 These structural changes affect the barrier properties of the films as measured with electrochemical redox probes in solution.2b-e,10b Figure 4a-c displays the temporal evolution of the methylene [νa(CH2) + νs(CH2)] intensity (from IR spectra), capacitance, and resistance, respectively, for SAMs formed by exposure of gold to 1 mM solutions of C12SH in CO2 (103 bar, 35 °C) or ethanol (35 °C). In general, the methylene intensity decreases with time, signifying the evolution of a less canted, more densely packed film that, as evidenced by the temporal capacitance and resistance measurements, becomes effectively thicker and more electrochemically blocking with time. Although experimental times of less than 3 min were not readily accessible with the current reactor system because of the required times for pressurization and depressurization, the results do provide a time scale at which a partial monolayer with gauche defects transforms into a highly crystalline SAM. During formation, the position of the asymmetric methylene peak decreases from ∼2921 after 3 min to ∼2919 after 90 min of exposure. The collective results in Figure 4a-c suggest that highly crystalline SAMs can be prepared from either solvent in ∼45 min. SAMs formed over 24 h exhibited the same values, within experimental error, for capacitance, resistance, and methylene intensity as those formed in only 1 h. The results in Figure 4 also show that SAMs formed in ethanol, while exhibiting a similar time scale for formation, are not as densely packed and, in general, not as electrochemically blocking as those formed in CO2, although the differences in capacitance (Figure 4b) and resistance (Figure 4c) are slight in many cases.
Figure 4. Time dependence of the (a) methylene intensity [νa(CH2) + νs(CH2)], (b) interfacial capacitance, and (c) chargetransfer resistance for C12 SAMs formed from CO2 (103 bar, 35 °C) or ethanol (35 °C). Electrochemical impedance measurements were performed in an aqueous solution containing 0.1 M Na2SO4, 1 mM K3Fe(CN)6, and 1 mM K4Fe(CN)6‚3H2O.
Varying the pressure of supercritical CO2 provides a means for tuning the density, self-diffusivity, and viscosity of the solvent, as shown in Figure 1. We examined the effect of pressure on the formation of SAMs based on 30-min adsorptions from CO2 with 1 mM CH3(CH2)11SH at 35 °C (Figure 5). The solubility of the thiol was verified visually through the sapphire window in the reactor, and concentrations of 1 mM were attainable under all experimental conditions investigated. The intensities of the methylene modes [νa(CH2) + νs(CH2)] from IR spectroscopy decrease with increas-
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Figure 6. Reflectance infrared spectra for SAMs formed by 30min exposure of gold to 1 mM solutions of C12SH in CO2 at 103 bar and either 25 °C (liquid CO2) or 35 °C (supercritical CO2). The spectra have been offset for clarity. Table 1. Static Wetting Properties of Water and Hexadecane on Films Formed on Gold from CO2 (103 bar, 35 °C) contact angles (advancing, receding; in degrees)a adsorbate
H2O
HD
HSC10CO2H HSC11OH HSC12
21, 15 30, 11 116, 104
-,
