Langmuir 1998, 14, 5913-5917
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Two-Stage Growth of Octadecyltrimethylammonium Bromide Monolayers at Mica from Aqueous Solution below the Krafft Point William A. Hayes and Daniel K. Schwartz* Department of Chemistry, Tulane University, New Orleans, Louisiana 70118 Received June 8, 1998. In Final Form: July 22, 1998 The time-dependent growth of octadecyltrimethylammonium bromide (C18TAB) monolayers at mica surfaces from aqueous solution, at a temperature below the Krafft point, is investigated with transmission IR spectroscopy, wetting measurements, and atomic force microscopy (AFM). Under these deposition conditions, C18TAB adsorbs predominately as the monomer rather than as an aggregated solution species resulting in monolayer films. Formation of the C18TAB monolayer is characterized by two distinct growth regimes. A rapid initial adsorption is observed which results in a 2-D liquid phase comprised of randomly oriented alkyl chains. When the films are allowed to assemble for times longer than 24 h, distinct, densely packed, monolayer high islands of C18TA+ molecules are observed (2-D solid). As the assembly time is increased further, the relative surface coverage of the semicrystalline islands increases substantially. In this growth regime, the mica surface is covered by a two-phase film comprised of the 2-D liquid and solid phases. Comparison of these results to those previously obtained for the adsorption of octadecylphosphonic acid (OPA) on mica clearly shows a distinctly different growth mechanism for the two systems.
Introduction The adsorption behavior of single and double long-chain alkylammonium ions from aqueous solution has been studied at a variety of solid surfaces, including graphite,1,2 silica,3-5 and mica.6-15 The behavior is complicated by the presence of solution aggregate forms, dimers, n-mers, and micelles, near and above the critical micelle concentration (cmc). Several recent studies have focused on the adsorption behavior of these species. Atomic force microscope (AFM) images have indicated the presence of bilayers, long and short cylinders, and spherical surface aggregates. The size, shape, and proclivity of formation of these different surface structures have been shown to depend on a variety of factors including the hydrophilicity of the substrate, the concentration of the organic ions in solution, the ionic strength and pH of the solution, and the counterion present. A smaller body of work has been * To whom correspondence should be addressed. Fax: (504) 8655596. Telephone: (504) 862-3562. E-mail:
[email protected]. tulane.edu. (1) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (2) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (3) Gu, T.; Huang, Z. Colloids Surf. 1989, 40, 71-76. (4) Wangnerud, P.; Jonsson, B. Langmuir 1994, 10, 3268-3278. (5) Rutland, M. W.; Parker, J. L. Langmuir 1994, 10, 1110-1121. (6) Chen, Y. L.; Chen, S.; Frank, C.; Israelachvili, J. J. Colloid Interface Sci. 1992, 153, 244-265. (7) Eriksson, L. G. T.; Claesson, P. M.; Erikkson, J. C.; Yaminsky, V. V. J. Colloid Interface Sci. 1996, 181, 476-489. (8) Johnson, S. B.; Drummond, C. J.; Scales, P. J.; Nishimura, S. Langmuir 1995, 11, 2367-2375. (9) Kekicheff, P.; Christenson, H. K.; Ninham, B. W. Colloids Surf. 1989, 40, 31-41. (10) Liu, Y.; Wu, T.; Evans, D. F. Langmuir 1994, 10, 2241-2245. (11) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169187. (12) Pashley, R. M.; McGuiggan, P. M.; Horn, R. G.; Ninham, B. W. J. Colloid Interface Sci. 1988, 126, 569. (13) Tsao, Y.-H.; Yang, S. X.; Evans, D. F.; Wennerstrom, H. Langmuir 1991, 7, 3154-3159. (14) Tsao, Y.-H.; Yang, S. X.; Evans, D. F. Langmuir 1992, 8, 11881194. (15) Sharma, B. G.; Basu, S.; Sharma, M. M. Langmuir 1996, 12, 6506-6512.
directed at the adsorption behavior of these organic ions under conditions where no aggregates are present in solution, such as at temperatures below the Krafft point or at concentration values below the cmc (below the Krafft point, no micelles are formed in solution at any concentration). The adsorption of hexadecyltrimethylammonium bromide (C16TAB) on mica has been studied both above and below the cmc by wetting and surface force apparatus (SFA) measurements7,9 and AFM.15 Both SFA and wetting measurements provide convenient methods for in-situ analysis of the monolayer growth process and have been employed with a good deal of success. A rapid initial adsorption of C16TA+ at the mica surface, which is driven by an electrostatic attraction between the solvated C16TA+ and negative lattice sites in the mica, is observed. Subsequent organization of the film is thermodynamically driven by the so-called “hydrophobic effect”. Chargecharge repulsion and surface diffusion effects may also play an important role in the ordering process. In contrast to the variety of surface structures observed when the adsorption takes place from solutions near or above the cmc, at concentrations well below the cmc (or at temperatures below the Krafft point) the adsorption proceeds predominately through the random adsorption of monomeric C16TA+ followed by rearrangement on the mica surface. Chen et al. have quantified the adsorption kinetics of C16TA+ monolayers using X-ray photoelectron spectroscopy (XPS) and wetting measurements on films prepared under a variety of conditions.6 Below the cmc, the normalized adsorption density as measured by XPS rose rapidly and reached a plateau (fractional coverage of about 0.6) after about 10 s in solution. The surface coverage then remained constant up to about 1 day in solution. For exposure times longer than 1 day the surface coverage again rose reaching a maximum fractional coverage value of ≈0.9 after about 1 month of exposure. In this paper we use AFM in conjunction with IR spectroscopy and contact angle measurements to investigate the processes involved in the formation of monolayer
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Hayes and Schwartz
Figure 1. Series of ex-situ, contact-mode AFM images showing the surface topography of the coated mica surface as a function of the duration of exposure to a 0.10 mM C18TAB solution. Key: (A) 1 h, (B) 24 h, (C) 48 h, (D) 72 h, (E) 120 h, (F) 168 h, and (G) 336 h. The final panel (H) shows a representative line scan through image (E). The line scan corresponds to the black line through image (E).
films of octadecyltrimethylammonium bromide (C18TAB) at a temperature below the Krafft point. These complementary techniques allow for a thorough analysis of both the surface micro- and macroscale structures at various stages of the adsorption process. The data presented indicate that following an initial induction period in which a loosely packed layer of adsorbed C18TA+ is present on the surface, discrete, densely packed, monolayer high islands begin to form. Subsequently, these islands grow and coalesce, forming a layer in which distinct regions of well-ordered alkyl chains are present.
statistical noise. A more detailed description of experimental considerations is given elsewhere.17
Results and Discussion
Octadecyltrimethylammonium bromide (Aldrich, Milwaukee, WI) was used as received. Water from a Milli-Q UV+ purification system (Millipore, Bedford, MA) was used in the preparation of the 0.10 mM C18TAB solution and for contact angle measurements. Sample preparation consisted of placing freshly cleaved, 0.5 in. diameter mica sheets into a holder fashioned from Teflon tubing. The substrates were then immersed in C18TAB solution for specific times at room temperature. Upon removal, the samples were rinsed with Millipore H2O and blown dry with a dry nitrogen stream. The samples were imaged with a Nanoscope III atomic force microscope (Digital Instruments, Santa Barbara, CA). All images were acquired in contact mode using silicon nitride tips. Surface coverage values were estimated using the height histogram of each image and averaged over several trials. Contact angle measurements were made using a custom-built contact angle goniometer. The “static” contact angle was obtained by following a procedure described by Bain et al. in which a 1 µL drop was formed at the end of a needle and brought into contact with the surface.16 The needle was then removed and the contact angle measured. Infrared absorption data were obtained using a Mattson Cygnus 100 spectrometer in transmission mode with a 6.4 mm pinhole defining the incident beam. Because of the low signal from the submonolayer samples, data were averaged over 3200 scans for both the film and the bare substrate in order to minimize
Atomic force microscope (AFM) images clearly show that upon extended exposure (g24 h) that the C18TAB molecules aggregate to form islands on the mica surface with a measured height of 1.7 ( 0.3 nm. Figure 1 shows images of samples that were in solution for 1, 24, 48, 72, 120, 168, and 336 h (Figure 1A-G). These images are representative of images obtained from at least three macroscopically separated regions on several different samples and exhibit the typical behavior of coverage vs time observed on over 30 samples. From this series it is evident that as the time of exposure of the mica substrate to the assembly solution is increased, the surface coverage of islands increases. Also worth noting is the time required for the onset of monolayer island formation. After 24 h of exposure to the assembly solution a fractional coverage of only 0.01 is observed by AFM. For times less than 24 h, little or no coverage of the mica surface by islands is observed. The subsequent AFM images are featureless and exhibit subnanometer roughness (Figure 1A). As the exposure time is increased to 48 h (Figure 1B) and subsequently to 72 h (Figure 1C) the fractional surface coverage increases to 0.03 and 0.10. As the duration of exposure is extended to 120 (Figure 1D) and 168 h (Figure 1E) a further increase in the coverage is observed. After 336 h of exposure to the assembly solution, a semicontiguous network of monolayer high islands is seen (Figure 1G). The increased island coverage is a direct result of both an increase in the average diameter of the individual islands and an increase in the number density of islands. The series of images is consistent with a nucleation and growth mechanism followed by coalescence of the monolayer islands. For longer exposure times, multilayer deposition occurs,
(16) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-355.
(17) Woodward, J. T.; Doudevski, I.; Sikes, H. D.; Schwartz, D. K. J. Phys. Chem. B 1997, 101, 7535-7541.
Experimental Section
Octadecyltrimethylammonium Bromide Monolayers
Figure 2. Plot of the fraction of the mica surface covered by C18TAB islands as a function of the duration of exposure to a 0.10 mM C18TAB solution. The error bars indicate the standard deviation of the measured coverage from randomly sampled regions of each monolayer.
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Figure 3. Series of transmission IR spectra showing the spectral evolution of the -CH stretching modes as the duration of exposure to a 0.10 mM C18TAB solution is varied. The lines drawn through the data represent the best fit to Lorentzian line shapes. The dashed line corresponds to the expected band position of the asymmetric methylene stretching mode for a liquidlike alkane monolayer.
making further analysis problematic. The regions of the initial monolayer which are still visible show that the first layer remains intact and has a high surface coverage (>0.9). Since this multilayer adsorption is observed only for long exposure times and only on densely covered regions of monolayer, we speculate that it is a later stage of adsorption which occurs on the (now) hydrophobic surface. A representative line scan corresponding to the dark line in Figure 1E is shown in Figure 1H. The island heights are 1.7 ( 0.3 nm high for all images acquired. This height value is lower than what is expected for a fully extended chain (2.8 nm) and is also lower than experimentally determined heights for octadecyltrichlorosilane (OTS) islands on partially formed films (2.5 nm).18 This discrepancy may be due to two major factorss(i) the regions between the islands of the C18TAB films may be filled with molecules in which the alkyl chains are not fully extended, and (ii) the alkyl chains tilt significantly from surface normal to maximize energetically favorable van der Waals interactionssor a combination of both factors. From AFM analysis alone, determination of which factor predominates is not possible. The surface coverage data are summarized in Figure 2, which is a plot of the amount of the mica surface covered by monolayer islands as a function of the duration of exposure to the assembly solution. The coverage values are obtained from three independent trials from freshly prepared C18TAB/H2O solutions. This plot shows that there is a lengthy induction period before the onset of monolayer island formation. Also, the growth of the monolayer film takes place on a rather long time scale. The existence of this lengthy induction period before the appearance of any features on the mica surface indicates that solution aggregates are not the primary method of adsorption. If this were the case, one would expect to observe the presence of features at very short times. The AFM images alone are not sufficient for determination of the total coverage of C18TAB present on the mica surface. Using AFM alone we cannot be certain whether
the lower regions of the images (areas between the islands) correspond to the bare mica surface, or if these regions are covered by a thin layer of C18TAB molecules. In the former scenario, the island coverage derived from the AFM images would be an accurate representation of the total surface coverage. However, in the latter case, the coverage estimate will be significantly lower than the true total coverage. To address this issue, transmission IR and contact angle measurements were performed. Figure 3 shows a series of transmission IR spectra which were obtained on samples that were in solution for 1, 24, 72, 168, and 336 h. The IR spectrum of the 1 h sample shows two broad absorption bands centered at 2924 cm-1 and 2852 cm-1, which correspond to the asymmetric methylene (νa, CH2) and the symmetric methylene (νs,CH2) stretching modes. The positions of these bands are consistent with the presence of a disordered layer of C18TAB molecules present at the mica surface.19 The second spectrum from the top is that of a 24 h sample. An increase in the peak intensity and a peak position shift to lower energy (2922 cm-1) of the asymmetric mode are observed. The spectral changes become even more pronounced as the exposure is increased further. The spectrum of the 72 h sample shows a further increase in the intensity of the methylene stretching modes as well as a shift of the asymmetric stretch band to 2920 cm-1. After 168 h in solution this band is shifted to 2919 cm-1. By using the vertical dashed line, which is at 2924 cm-1, as a guide to the eye, one may clearly see the spectral evolution. The peak position of the asymmetric methylene stretch can be used as a measure of the degree of order present in a surface-bound, long-chain hydrocarbon species. As the exposure time is increased, the average degree of order, as indicated by the peak position of the asymmetric
(18) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354-3357.
(19) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.
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Figure 4. Plot of the integrated peak intensity of the asymmetric methylene stretching band as a function of the fraction of the mica surface covered by C18TAB islands. The error bars indicate the uncertainty of the accuracy of the peak intensity as determined by a nonlinear least-squares fitting procedure.
methylene mode, is increased. IR spectra obtained after 1 h of immersion, suggest the presence of adsorbed C18TAB molecules even at this relatively short immersion time. This IR spectrum also indicates that there is a low degree of order in the adsorbed alkyl chains (νa,CH2 ) 2924 cm-1). This observation is consistent with the flat, featureless AFM images recorded after 1 h of assembly. With no significant degree of order present in the adsorbed molecules, it is not possible using AFM to directly detect submonolayer coverage. Figure 4 is a plot of the integrated spectral intensity of the νa,CH2 mode as a function of the amount of the mica surface covered by C18TAB islands. The spectral intensity of the νa,CH2 increases as the fractional island coverage increases. This observation indicates that the adsorbed monomers become more densely packed on the mica surface as the surface coverage increases. This trend is consistent with the AFM images which show the evolution of the monolayer film to a higher average degree of order with increased exposure to the assembly solution. To further determine the extent to which the mica surface is covered with adsorbate, the static contact angle was measured as a function of immersion time. Figure 5 shows a sharp initial decrease in the cosine of the water contact angle (cos θ). For short times (up to 8 min in the assembly solution) cos θ quickly drops to about 0.35. This value remains constant for exposures up to 24 h. This initial decrease in cos θ is consistent with the formation of an adsorbed hydrocarbon layer at the mica surface. Taken together, the transmission IR results and the contact angle data indicate the initial presence of an adsorbed layer with minimal alkyl chain order. As the exposure time is increased further, cos θ is seen to reach a plateau between the period of initial adsorption and the onset of formation of islands. With the onset of island formation (>24 h) cos θ decreases as the average degree of order of the film increases. As the submonolayer film develops a higher average degree of order, the adsorbed film becomes more hydrophobic. This observation is consistent with the conversion from a methylene to a methyl-terminated surface. In a previous study, we looked at the adsorption of octadecylphosphonic acid (OPA) on mica from dilute THF solution. In this case, formation of distinct, small islands
Hayes and Schwartz
Figure 5. Plot of the cosine of the “static” contact angle as a function of the duration of exposure to a 0.10 mM C18TAB solution. The error bars indicate the degree of reproducibility of the measurement.
Figure 6. Comparison of the cosine of the “static” contact angle to the fractional coverage of partially formed films of C18TAB and OPA.
with little adsorbed material between the islands was apparent even at very short exposure times.17,20,21 A comparison of partially formed films of OPA and C18TAB indicates that the resultant surface structures vary considerably. Figure 6 is a plot of cos θ as a function of the amount of the mica surface covered by islands for the two adsorbates. This plot shows that for comparable coverage values (as determined by AFM) the contact angles of the C18TAB films are much larger than those of the OPA films. The fact that cos θ extrapolates to approximately unity for OPA partial monolayers is consistent with negligible adsorbate coverage between islands while the extrapolation of cos θ to 0.37 for C18TAB at zero (20) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Langmuir 1996, 12, 3626-3629. (21) Woodward, J. T.; Schwartz, D. K. Langmuir 1997, 13, 68736876.
Octadecyltrimethylammonium Bromide Monolayers
coverage is consistent with a disordered monolayer between islands.22-24 The slow kinetics of the island nucleation and growth is a matter of considerable interest. Several causes are possible, including a significant barrier to island nucleation (large critical nucleus), slow surface transport of adsorbed molecules, or the possibility that the island growth is contingent on charge migration or some other structural change within the substrate itself. Continuing work, including temperature-dependent and in situ studies, will address these issues. Conclusion The formation of distinct, densely packed, monolayer islands of C18TA+ adsorbed on mica from a dilute aqueous solution has been characterized by AFM. The presence of a two-phase system containing regions of high packing density and regions of loosely packed C18TA+ is demon(22) Israelachvili, J. N.; Gee, M. L. Langmuir 1989, 5, 288. (23) Lamb, R. N.; Furlong, D. N. J. Chem. Soc., Faraday Trans. 1 1982, 78, 61. (24) Cassie, A. B. D. Discuss. Faraday Soc. 1952, 75, 5041.
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strated by the use of AFM, IR, contact angle, and comparison to previous experiments. For the concentration studied (0.10 mM CTAB/H2O), the onset of island formation is shown to occur after ca. 24 h of exposure of the mica substrate to the assembly solution, before which time there is no significant ordering of the alkyl chains observed. This is followed by further growth of the islands and nucleation of new islands. These islands eventually begin to coalesce and form a semicontiguous, monolayerhigh film at the mica surface. Acknowledgment. This work was supported by the National Science Foundation (Grant no. CHE-9614200), the donors of the Petroleum Research Fund, administered by the American Chemical Society, the Center of Photoinduced Processes (funded by the National Science Foundation and the Louisiana Board of Regents), and the Louisiana Education Quality Support Fund Contract LEQSF(1996-99)-RD-B-12. LA980664A
