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Langmuir Monolayer Flow across Hydrophobic Surfaces Adam B. Steel,*,† Brady J. Cheek, and Cary J. Miller‡ Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742 Received April 21, 1998. In Final Form: July 10, 1998 Langmuir monolayers are observed to flow into the interface between hydrophobic organothiol monolayer modified gold surfaces and water, forming a bilayer. From time-dependent capacitance measurements, the rate of bilayer formation (e.g., interface penetration) has been quantitatively determined. The capacitance-time profile can be modeled with a single monolayer flow parameter. The value of the flow parameter is sensitive to the Langmuir monolayer, the structure and composition of the hydrophobic surface layer, and the Langmuir film surface pressure. The monolayer flow rate is largely controlled by dynamical interactions occurring within the entire bilayer contact region. We find that the lateral flow of the Langmuir film into the hydrophobic surface/water interface is well described by a simple pressuredriven flow model. Possible applications of lateral flow are discussed including the initial design of a monolayer chromatography system.
Introduction The principles of chromatography hold that the resolving power of a column increases with the number of eluantstationary phase interactions. It follows that the ultimate geometry for a separation would involve continuous contact between the eluant and stationary phase. Such a case is found for a fluid bilayer: a bilayer composed of one stationary layer and one flowing layer would provide the continuous contact necessary for such an “ultimate resolution” chromatography. Bilayer membranes are perhaps the most simple molecular architectural forms. Over and above their tremendous biochemical significance in delineating cellular structures and their organizational role in photosynthesis, respiration, and neuronal function, the bilayer structure offers a simple means of exploiting the intimate contact between molecules.1 While the major impetus for the study of bilayer structures supported on solid surfaces has been as biomembrane mimics,2 other more technological uses can be envisioned.3 Understanding the dynamic nature of bilayer structures is crucial for the development of separation technologies based on this molecular architecture, and lateral diffusion of components in supported bilayers has been well-characterized.4 Sackmann and Groves have used supported bilayers in twodimensional electrophoretic separations.5 However, the controlled, directed lateral movement of one monolayer past another necessary for chromatography has proven more difficult to achieve. We have developed a remarkably simple method to produce a chromatographic bilayer assembly using selfassembled monolayers (SAMs) of alkanethiols on gold and †
Current address: Gene Logic, Inc., Gaithersburg, MD 20878. Current address: i-STAT Corp., Kanata, Ontario K2L 1T9, Canada. ‡
(1) Weissmann, G., Clairborne, R., Eds. Cell Membranes, Biochemistry, Cell Biology and Pathology; HP Publishing Co.: New York, 1975. (2) Plant, A. L. Langmuir 1993, 9, 2764-2767. (3) (a) Allara, D. L. Biosens. Bioelectron. 1995, 10, 771-783. (b) Gopel, W.; Heiduschka, P. Biosens. Bioelectron. 1995, 10, 853-883. (4) (a) Smith, A. B.; McConnell, H. M. Proc. Natl. Acad. Sci. U.S.A. 1978, 75, 2759-2763. (b) Subramaniam, S.; Seul, M.; McConnell, H. M. Proc. Natl. Acad. Sci. U.S.A. 1983, 83, 1169-1173. (5) (a) Groves, J. T.; Boxer, S. G. Biophys. J. 1995, 69, 1972-1975. (b) Stelzle, M.; Miehlich, R.; Sackmann, E. Biophys. J. 1992, 63, 13461354. (c) Stelzle, M.; Sackmann, E. Biochim. Biophys. Acta 1989, 981, 135-142.
fluid Langmuir films at the air/water interface. Bilayer structures are formed by movement of a fluid Langmuir film into a hydrophobic SAM/water interface. In this paper, the measurement of lateral monolayer flow rates and an investigation into the role of Langmuir film and hydrophobic SAM properties are presented in detail. Studies describing the equilibrium partitioning of insoluble surfactants between the air/water and solid/water interfaces have been described previously in the literature.6 The present study is intrinsically different in that the Langmuir film must flow across the air/liquid interface and then penetrate an established solid/liquid interface. Previous studies generally involve trapping film material at the solid/water interface by positioning the substrate after Langmuir film deposition. A cartoon of the lateral flow process is given in Figure 1. A hydrophobic surface, consisting of a SAM-modified, gold-coated glass plate, is positioned at the air/water interface. The perimeter of the hydrophobic surface provides a connection between the air/water and solid/ water interfaces through an air/water/solid line boundary. For monolayer-coated gold surfaces, the capacitance of the metal/water interface is a function of the dielectric medium thickness (i.e., the thickness of the SAM). Application of an excess of an insoluble amphiphile solution (e.g., oleic acid) to the air/water interface results in a uniform monolayer film covering the entire open subphase interface within a few seconds. For certain amphiphiles, the spread film is pressurized to a constant value, the equilibrium spreading pressure (ESP), under these conditions. The pressurized Langmuir monolayer flows across the triple phase boundary into the solid/water interface over a longer time scale (seconds to minutes) until, in the case of a SAM-coated electrode, a complete bilayer structure is formed. We determine the lateral flow rate by observing the change in capacitance of the electrode as the bilayer is formed. We have measured lateral flow rates for a variety of Langmuir films and SAMs to develop an appropriate description of the phenomenon. We find that the lateral (6) (a) Bizzotto, D.; Lipkowski, J. Prog. Surf. Sci. 1995, 50, 237-246. (b) Bizzotto, D.; Noel, J.; Lipkowski, J. Thin Solid Films 1994, 248, 69-77. (c) Noel, J.; Bizzotto, D.; Lipkowski, J. J. Electroanal. Chem. 1993, 344, 343-354.
S0743-7463(98)00451-X CCC: $15.00 © 1998 American Chemical Society Published on Web 08/19/1998
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Figure 1. Depiction of the electrode geometry for observation of lateral flow. A SAM-modified gold on glass substrate is positioned at the air/water interface. In lateral flow, a piston oil Langmuir film is introduced to the solution interface and quickly spreads to cover the entire interface. Subsequently, the Langmuir film moves as a monolayer across the triple phase boundary and into the SAM/water interface, forming a bilayer. Lateral flow is complete when a bilayer is formed at the electrode/solution interface.
flow rate is suprisingly well-described by a simple, pressure-driven flow model. Our observations show that the lateral flow rate of the fluid monolayer film depends on the molecular topology and chemical functionalities present in both the surface and flowing monolayers.7 A strong dependence of the flow rate on molecular features is reasonable given the intimate contact between the mobile and stationary monolayers, much like the role of interlayer coupling between sheets in a bilayer in controlling lateral diffusion.8 The dependence of flow rate on molecular properties and the remarkable simplicity of the method has potential applications in surface characterization, sensor development, and high-resolution separations. Experimental Section General Reagents. High-purity solvents were used in all syntheses and chromatographies. Subphase electrolyte solutions were made with deionized water (Milli-Q system, Millipore). Ethanol (95%; Pharmco) was employed as the self-assembly deposition solvent for all thiols. Hexadecane (99%; Aldrich) was passed through activated, neutral alumina prior to use in contact angle measurements. Thiourea (Aldrich), potassium chloride (Sigma), 11-bromo-1-undecanol (Aldrich), 11-bromo-1-undecanoic acid (Aldrich), 10-undecylenyl alcohol (Aldrich), 2-mercaptoethylamine (Sigma), butyric anhydride (Aldrich), isobutyric anhydride (Fluka), trifluoroacetic anhydride (Baker), and hydrochloric acid (Baker) were used as received. Organothiols. The n-alkanethiols, CH3(CH2)n-1SH, with n ) 3-10, 12, and 16 were purchased from Aldrich in the highest available purity and purified via chromatography on silica gel (7) While it is clear that microscopic substrate roughness could have an influence on the lateral flow rate, we have not considered this effect directly but have attempted to minimize variability due to this factor by carefully controlling substrate preparation and experimental protocol. (a) Creager, S. E.; Hockett, L. A.; Rowe, G. K. Langmuir 1992, 8, 854861. (b) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636-7646. (c) Guo, L.-H.; Facci, J. S.; McLendon, G.; Mosher, R. Langmuir 1994, 10, 4588-4593. (d) Wilbur, J. L.; Biebuyck, H. A.; MacDonald, J. C.; Whitesides, G. M. Langmuir 1995, 11, 825-831. (8) Seul, M.; Subramaniam, S.; McConnell, H. M. J. Phys. Chem. 1985, 89, 3592-3595.
Steel et al. with cyclohexane as the solvent. 1-Octadecanethiol (Aldrich) was recrystallized twice from absolute ethanol. 1,2-Dipalmitoylsn-glycero-3-phosphothioethanol (Avanti Polar Lipids), 2,2dimethyl-1-decanethiol (Aldrich), thiocholesterol (Aldrich), cyclohexyl mercaptan (Aldrich), p-thiocresol (Aldrich), and benzyl mercaptan (Aldrich) were used as received. 1-Eicosanethiol,9 N-mercaptoethylperfluorooctylamide,10 1-mercapto-10-undecene, 11-bromo-1-undecanethiol, 11-methoxy-1-undecanethiol, and methyl 11-mercaptoundecanoate11 were produced following literature procedures. Hexyl dodecyl 1,1′-disulfide (HDD) was produced by titration of an ethanolic solution containing a 4:1 molar ratio of 1-hexanethiol to 1-dodecanethiol with iodine.12 A slight excess of molecular iodine was added as judged by a persistent brown solution color. Purification of HDD was by reverse-phase flash chromatography (C18 silica, absolute ethanol). The purity (>95%) and composition of the mixed disulfide was confirmed by gas chromatography and mass spectrometry. Piston Oils. Oleic acid (Aldrich), oleyl cyanide, methyl oleate, ethyl oleate, oleyl acetate (NuChek Prep), and 1,2-dioleyl-snglycero-3-phosphoethanolamine-Lissamine Rhodamine B (Avanti) were used as received. Oleyl alcohol (Aldrich) was purified by flash chromatography (silica, chloroform). Oleyl butyrate, isobutyrate, and trifluoroacetate were synthesized by refluxing oleyl alcohol with the corresponding anhydride in acetonitrile for 24 h. The acetonitrile solution was extracted with heptane and the heptane fraction washed with water. An oil was collected for all three oleyl esters by rotary evaporation which was purified by flash chromatography on silica gel with chloroform. The structure and purity of the products were assessed by infrared spectroscopy and thin-layer chromatography. The conversion from alcohol to ester was confirmed by complete loss of the O-H stretching peak at 3340 cm-1 and the growth of the ester carbonyl stretch at 1737 cm-1, or 1788 cm-1 for the trifluoroacetate ester. Pressure-area isotherms for all piston oils were collected on a home-built Langmuir trough. Preparation of Gold Electrodes. Gold electrodes were prepared on glass substrates with freshly cleaved smooth edges. Smooth edges were created by scoring a short segment on a plate glass face and carefully snapping the glass to give two freshly cleaved edges. Freshly cleaved edges were preferred to regular microscope slide glass edges because of the large surface roughness on the latter’s edge. The plate glass was then cut into roughly 1 × 3 in. pieces and cleaned in a chromic acid bath at 50 °C. After being rinsed with copious amounts of water, the substrates were placed in a radio-frequency sputtering chamber with the cleaved edges normal to the metal source. Before coating with the metal layers, the substrates were cleaned with a 50 W argon plasma for 30 s. To promote gold adhesion, a chromium underlayer of ca. 50 nm was coated on the substrates prior to the deposition of ca. 200 nm of gold. The gold-coated substrates were placed in SAM deposition solutions (>20 mM thiol in ethanol) immediately upon removal from the sputtering chamber, and the SAM was allowed to form for at least 12 h before use. Lateral Flow Rate Measurement. Substrates were positioned at the air/solution interface in a Teflon trough (20 cm2 surface area) with the smooth SAM-coated edge in contact with the solution surface. The SAM-coated electrode serves as the working electrode in a three-electrode cell. A saturated calomel electrode (SCE) and platinum wire, located in a surface-isolated chamber of the trough, served as the reference and counter electrodes, respectively. The electrolyte was 10 mM potassium chloride, pH 2 (HCl), in all cases. The temperature of the subphase was in the range 20-23 °C. The capacitance of the SAM-coated electrode was calculated from the ac admittance. The admittance magnitude and phase angle were measured using a lock-in amplifier (Stanford Research Systems, model SR530) under computer control at a frequency (9) Miller, C. J.; Cuendet, P.; Gratzel, M. J. Phys. Chem. 1991, 95, 877-886. (10) Hallmark, V.; Hoffmann, C.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610-4617. (11) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (12) An excess of the 1-hexanethiol was used because it simplified the subsequent purification step as the dihexyl disulfide was easily resolved from HDD, whereas didodecyl disulfide was not.
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of 10 Hz with a 10 mV ac excitation. At this frequency the admittance phase angle was typically in the range 88-90° so that the admittance magnitude was converted directly into an equivalent capacitance. The lock-in amplifier was connected to the cell using a potentiostat (EG&G PAR, model 360) that holds the system at 0.0 V versus the SCE reference. Piston oils were applied in excess of monolayer coverage by placing a drop of the neat piston oil on the surface using a small glass rod. Introducing surfactants in this manner results in a monolayer on the entire surface and a lens of material at the glass rod. The glass rod with a reservoir of piston oil was left at the solution interface for the duration of the experiment. Bulk Langmuir film materials that are solid at room temperature (e.g., stearic acid) were spread from chloroform solutions. Lateral flow measurements show no dependence on the Langmuir film source, neat vs solution deposition. The surface pressure was measured by differential weight measurements using a filter paper (Whatman, No. 1) Wilhelmy plate suspended from an analytical balance (Denver Instruments, model 100A). Static contact angles were determined by the sessile drop method on a Rame-Hart model 100 goniometer at room temperature and ambient humidity. The drop size was 2 µL and was formed at the end of a blunt-tip needle attached to a 2 mL syringe fitted with a micrometer. Fluorescence images were obtained using a Zeiss Axiovert 135 TV inverted microscope with a Hg lamp source, a filter set for the fluorescent probe, and a CCD camera with intensifier. The Teflon trough used in the electrode capacitance experiments was replaced by a glass trough. A subphase well was produced in a 1 × 3 in. microscope slide by drilling a hole through the slide using a glass coring bit. The bottom of the well was made by sealing a glass coverslip (0.15 mm thickness) over the hole using epoxy. The well volume was between 1 and 2 mL. Substrates for the fluorescence experiments were produced by the same method as the capacitance substrates using 3-mm-thick glass. In a typical experiment, the subphase well was filled with electrolyte (10 mM KCl, pH 2, HCl) and the surface aspirated with a capillary. The substrate was placed at the solution interface and the edge imaged in the microscope. A mixture of piston oil (oleic acid) and fluorescently labeled probe (1 mol % dipalmitoylphosphatidylethanolamine-Lissamine Rhodamine B; 550 nm excitation, 590 nm emission) was applied to the solution interface as a dilute chloroform solution. The movement of fluorescent probes at the substrate surface was recorded using a VCR for subsequent analysis. For ring electrode experiments sections of glass tubing (20 mm diameter, 1.5 mm wall thickness) were cut using a diamond saw, polished, and then Cr/Au-coated as above. The ring was positioned at the air/water interface with the Wilhelmy plate in its interior. Langmuir film material was introduced exterior to the ring using the glass rod method.
Results The flow of Langmuir monolayers into the solid/solution interface is most easily followed by the simple electrochemical experiment suggested by Figure 1. The solid/ solution interface is created by positioning a SAM-modified gold electrode at the surface of an aqueous electrolyte contained in a Langmuir trough. The capacitance of the solid/solution interface is measured by making the gold electrode the working electrode in a three-electrode cell. Figure 2A shows the electrode capacitance as a function of time in a lateral flow experiment where the electrode is modified by 1-dodecanethiol (M12) and oleic acid is the flowing monolayer (filled circles). The capacitance is constant at the beginning of the experiment before the application of the Langmuir monolayer to the air/water interface. This initial capacitance is characteristic of the alkanethiol SAM.13 At t ) 0 s, an excess of oleic acid is applied to the surface of the electrolyte, resulting in the rapid formation of the Langmuir monolayer pressurized (13) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.
Figure 2. Lateral flow measurements of oleic and stearic acids at a 1-dodecanethiol (M12) SAM-modified gold electrode. (A) Plots of the electrode capacitance versus time for a M12-coated gold electrode prior to and after the application of excess oleic acid (filled circles) and stearic acid (open circles) to the air/ water interface. Inset: The capacitance data for oleic acid is plotted versus the square root of time after the application of the oleic acid monolayer. The solid line is the best fit through the linear portion of the capacitance drop used to calculate the flow parameter. (B) Pressure-time profiles measured by the Wilhelmy plate method at the air/water interface during the lateral flow experiment.
to the equilibrium spreading pressure of this amphiphile, 32 mN/m (see Figure 2B). At this point the capacitance of the electrode decreases with time until stabilizing at a new value characteristic of a M12/oleic acid bilayer. The extent of bilayer formation at any point along the capacitance-time curve is calculated using an equivalent
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circuit model containing two capacitors in parallel, one representing the portion of the electrode coated by bilayer and the other the remaining uncoated SAM area. The final bilayer capacitance obtained by lateral flow is equivalent to the bilayer capacitance observed when the same M12 electrode is positioned at a subphase on which the oleic acid monolayer has already been spread. Also presented in Figure 2 is a lateral flow experiment with a M12 electrode and stearic acid as the flowing monolayer (open circles). The capacitance of the M12 electrode changes only slightly upon the addition of stearic acid to the water interface even though the pressure is significant (Figure 2B). The lack of a significant capacitance change indicates that stearic acid does not flow at an appreciable rate into the SAM/water interface. This highlights an essential requirement for a flowing monolayer, fluidity. From the pressure-area isotherms of oleic and stearic acid (data not presented), oleic acid exists as a liquid-expanded film at its ESP, while stearic acid is a liquid-condensed or solid film for the pressures used here.14 We will expand upon the role of the flowing monolayer phase state in later sections. We find that the electrode capacitance decreases linearly with the square root of time during bilayer formation. The data from Figure 2A is replotted versus the square root of time in the inset. From the slope of the plot one can extract a single parameter, the flow parameter (FP), that is characteristic of the lateral flow rate of the Langmuir monolayer. The value of the flow parameter is defined as
(
FP ) m*
)
w CBi - CSAM
(1)
where m is the slope of the C-t1/2 plot, w is the length of the flow path,15 and CBi and CSAM are the bilayer and monolayer capacitance values, respectively. The length of the flow path and the difference in electrode capacitance terms are present to convert the units to cm/s1/2. The flow parameter can be viewed at present as an empirical construct that is closely related to the experimental data. Lateral flow was also observed using labeled flowing monolayer species via fluorescence microscopy on an inverted microscope. A uniform fluorescent front is observed moving across a M12 SAM-modified gold electrode when imaged during a lateral flow experiment using a mixed flowing monolayer containing oleic acid and 1 mol % of a fluorescently labeled lipid. A chronology of a single flow experiment on the fluorescence microscope is given in Figure 3. The M12 monolayer appears as the dark region in the figure. The electrode has been positioned with the air/water/M12 edge just to the left of the imaged area. Bilayer formation by lateral flow of the mixed Langmuir film is observed as the front of brightness moves from the electrode edge into the middle of the electrode. Flow parameter values calculated from the direct observation of the fluorescent monolayers are identical with those measured by the electrochemical technique. The leading edge of the fluorescence broadens as the extent of bilayer formation increases. While the step-function edge of the fluorescent front observed in the figure is a result of the image analysis software, the fluorescence front in the raw data was observed as a uniform intensity gradient. The broadening of the (14) Mingotaud, A.; Mingotaud, C.; Patterson, L. K. Handbook of Monolayers; Academic Press: San Diego, CA, 1993. (15) Because the monolayer flows into the electrode/solution interface from two sides in our system geometry, the total flow path length w is half the width of the electrode.
Figure 3. Fluorescence imaging of lateral flow. Sequential images from the observation of lateral flow of oleic acid containing 1 mol % of a fluorescently labeled lipid at a 1-dodecanethiol (M12) electrode on an inverted fluorescence microscope. The images are of the same 100-µm-wide swath at the time indicated on the figure. The dark region is the M12 SAM and bright areas are where oleic acid and fluorescently labeled lipid have formed a bilayer. The scale bar gives the distance from the edge of the electrode.
fluorescent front region can be used to calculate a diffusion coefficient for the labeled lipid in the flowing monolayer.16 We calculate a diffusion coefficient of 6 × 10-8 cm2/s from the band broadening, which is in good agreement with diffusion coefficients calculated for the same lipid in supported phospholipid bilayers using other methods.4 The driving forces for lateral flow include the pressure of the flowing monolayer and the differing interfacial energies. We can determine the relative contribution of these forces by measuring monolayer flow rates as a function of the monolayer film pressure and changing the nature of the SAM. First, we consider the role of the Langmuir film pressure for a series of oleic acid derivatives with varying ESPs by measuring lateral flow rates at a common surface, M12. Second, using a common flowing Langmuir film, oleic acid, we investigate the role of surface physical properties by measuring lateral flow rates at a variety of SAMs. Influence of Langmuir Film Pressure. Through small changes in the polar headgroup of the oleyl chain, one can produce a range of similar Langmuir monolayers with different equilibrium spreading pressures. These oleyl derivatives can then be used to probe the pressure dependence of the monolayer flow rate. The ESPs for the oleic acid derivatives used in this study are listed in Table 1. The derivatives cover a surprisingly significant pressure range given the subtle differences in their molecular structure. While displaying significantly different ESPs, all of the derivatives have similarly shaped pressure(16) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980; p 128.
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Table 1. Summary of Oleic Acid Derivative Piston Oil Parameters oleic acid derivative
ESPa (mN/m)
FP (×10-3 cm s-1/2)
oleyl trifluoroacetate oleyl butyrate ethyl oleate oleyl cyanide oleyl acetate methyl oleate oleyl isobutyrate oleic acid oleyl alcohol
6.3 ( 0.9 12.4 ( 0.4 15.9 ( 0.7 16.8 ( 0.3 19.3 ( 0.4 19.8 ( 0.3 23.1 ( 0.5 32.5 ( 0.3 36.4 ( 0.5
2.85 ( 0.26 4.39 ( 0.35 5.25 ( 0.23 4.87 ( 0.20 5.74 ( 0.25 5.78 ( 0.14 5.63 ( 0.23 6.31 ( 0.27 7.27 ( 0.18
a The equilibrium spreading pressure (ESP) and flow parameter (FP) results are the average of at least five separate trials.
Figure 5. Flow parameter as a function of n-alkanethiol chain length. The error bars represent (1 standard deviation of at least six independent determinations.
Figure 4. Flow parameter dependence on piston oil equilibrium spreading pressure. The error bars represent (1 standard deviation of at least four independent determinations. The solid line is the best fit line through the data set. The plot uses data from Table 1.
area isotherms, exhibiting liquid-expanded phase behavior in the region of the ESP. The molecular areas of these oleyl derivatives at their respective equilibrium spreading pressures are approximately the same, allowing closer examination of the influence of the film pressure than would be afforded using oleic acid monolayers at pressures below its ESP. For oleic acid monolayers below 32 mN/m where the oleyl chain density is lower than that in a film at the ESP, we observe significantly slower lateral flow rates than expected. The flow parameter of each of the oleic acid analogues was determined at a M12 surface. The flow parameter is a linear function of the square root of the equilibrium spreading pressure, as seen in Figure 4. The significance of the dependence of the flow parameter on the ESP will be discussed in terms of a lateral flow model presented below. Influence of SAM on Lateral Flow. Self-assembled monolayers of n-alkanethiols on gold provide wellcharacterized model surfaces for the study of lateral flow. Adsorbed monolayers of n-alkanethiols (CH3(CH2)n-1SH) with 3 < n < 20 allow one to prepare a broad range of methyl-terminated surfaces that display varying degrees of crystallinity and hydrophobicity.13,17 With conservation
of the SAM terminus, we propose that differences in the flow rate of oleic acid as a function of the n-alkanethiol chain length would then indicate organizational differences between the monolayers. The measured flow parameter as a function of n-alkanethiol chain length is plotted in Figure 5. The monolayer flow parameters increase with increasing chain length and then level off for alkanethiol monolayer lengths above decanethiol (n ) 10).18 The lateral flow behavior of oleic acid on a variety of organothiol monolayer surfaces was investigated, and the results are listed in Table 2. It should be apparent from the rich variety of adsorbed thiol monolayer substrates present in Table 2 that lateral flow is a quite general method to produce a large number of solvent-free supported bilayers. The flow rate tends to decrease with the water contact angle for the substrate, suggesting that the flow rate for oleic acid across a SAM surface is related to the SAM-water interfacial free energy. The interfacial free energy of a SAM surface can be estimated from contact angle measurements using Young’s equation:19
γSL ) γSV - γLV cos θ
(2)
where γSL denotes the solid-liquid interfacial energy, γSV denotes the interfacial tension between the solid and vapor, γLV denotes the liquid-vapor interfacial tension, and θ represents the contact angle. The SAM-vapor interfacial tension is estimated using a generalization of Young’s equation for systems where London dispersion (17) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (18) For the longest alkanethiol monolayers, we generally observe a slight decrease in the flow parameter which may be due to incomplete monolayer formation. (19) Sinniah, S. K.; Steel, A. B.; Miller, C. J.; Reutt-Robey, J. E. J. Am. Chem. Soc. 1996, 118, 8925-8931.
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Table 2. Flow Parameter and Other Data for Diverse SAMs contact anglesb water HD
SAMa HS(CH2)11CH3 HS(CH2)17CH3 PF11 [-CF3] HS(CH2)5CH3 HDD [-CH3] HS(CH2)9CHdCH2 PC16 [-CH3] HSC(CH3)2(CH2)9CH3 TC [-CH(CH3)2] HS(CH2)2CH3 HS(CH2)10CH2Br HS-C6H11 HS-C6H4-CH3 (para) HSCH2-C6H5 HS(CH2)11OCH3 HS(CH2)10COOCH3
112 115 126 100 112 108 110 103 100 85 89 100 99 85 85 73
47 49 70 25 6 38 32 13 10 10 8
