Kinetics of Octadecyltrimethylammonium Bromide Self-Assembled

Feb 6, 2004 - Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, Department of Chemical and Biological Engineering, University ...
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Langmuir 2004, 20, 2341-2348

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Kinetics of Octadecyltrimethylammonium Bromide Self-Assembled Monolayer Growth at Mica from an Aqueous Solution James M. Mellott,†,‡ William A. Hayes,§ and Daniel K. Schwartz*,‡ Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309, and Crompton Corporation, 1231 Pope Street, Memphis, Tennessee 38108 Received October 16, 2003. In Final Form: January 5, 2004 We have studied the growth kinetics of self-assembled monolayers (SAMs) of octadecyltrimethylammonium bromide (C18TAB) on mica below the critical micelle concentration at 22, 30, 40, and 50 °C. A combination of atomic force microscopy, contact angle goniometry, and transmission infrared spectroscopy was used to follow the growth processes to determine the rates involved in the growth of a C18TAB SAM on mica. The growth of a SAM consisted of four distinct processes: deposition of adsorbate molecules, growth of a disordered 2D liquid phase, nucleation of islands of an ordered 2D solid phase, and subsequent growth of the solid phase. The rates of these various processes are determined, and the activation energies for several processes were calculated including those for the adsorption onto a bare substrate (20 kJ/mol), adsorption into the saturated liquid phase (100 kJ/mol), and nucleation of islands (0.3 kJ/mol). Despite the small activation barrier to island nucleation, the nucleation rate is qualitatively slow, suggesting that entropic effects dominate the nucleation rate.

Introduction A wide variety of amphiphilic molecules have been studied because of their ability to form well-ordered monolayers by self-assembly.1,2 One class of surfactant molecules studied is the alkylammonium salts. An interesting aspect of this class of surfactant molecules is their ability to form aggregates in solution beyond the critical micelle concentration (cmc).3 Many previous papers on alkylammonium ion adsorption have focused on systems where bulk aggregates are present in the solution, and the formation of interfacial bilayers,4-9 hemimicelles, and micelles10 was observed at the interface, while systems where bulk aggregates are not present have not been widely studied. Such aggregate-free systems are either below the Krafft temperature, cmc, or both.9,11-13 In these latter systems, there are several factors that play a role in the formation of a self-assembled monolayer (SAM). One factor is the electrostatic attraction between the ammonium cation and negatively charged sites on the * Author to whom correspondence should be addressed. E-mail: [email protected]. Phone: 303-735-0240. † Tulane University. ‡ University of Colorado. § Crompton Corporation. (1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991. (2) Schwartz, D. K. Annu. Rev. Phys. Chem. 2001, 52, 107. (3) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; Wiley: New York, 1997. (4) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409. (5) Rutland, M. W.; Parker, J. L. Langmuir 1994, 10, 1110. (6) Wangnerud, P.; Jonsson, B. Langmuir 1994, 10, 3268. (7) Johnson, S. B.; Drummond, C. J.; Scales, P. J.; Nishimura, S. Langmuir 1995, 11, 2367. (8) Pashley, R. M.; Israelachvili, J. N. Colloids Surf. 1981, 2, 169. (9) Tsao, Y. H.; Yang, S. X.; Evans, D. F. Langmuir 1992, 8, 1188. (10) Manne, S.; Gaub, H. E. Science 1995, 270, 1480. (11) Kekicheff, P.; Christenson, H. K.; Ninham, B. W. Colloids Surf. 1989, 40, 31. (12) Hayes, W. A.; Schwartz, D. K. Langmuir 1998, 14, 5913. (13) Fuji, M.; Li, B.; Fukada, K.; Kato, T.; Seimiya, T. Langmuir 1999, 15, 3689.

mica surface. Another important factor is the poor solubility of the hydrophobic tail groups in the aqueous solution. It is also likely that the surface diffusion of the monomers and possible electrostatic repulsion between the ionic headgroups could affect the rate of growth of a SAM. Various alkylammonium salts were used in these previous papers, but the most common was hexadecyltrimethylammonium chloride. Octadecyltrimethylammonium bromide (C18TAB) is also frequently investigated, along with double-chained quaternary ammonium salts. It has been shown that the properties of both the surfactant and substrate play a role in the structure of the adsorbed films. For example, Manne and Gaub10 observed spherical surface micelles of C14TAB (tetradecyltrimethylammonium bromide) on silica from a 7 mM (twice the cmc) aqueous solution. When the substrate was changed to mica, aggregates in the form of meandering stripes with a constant spacing were produced but without long-range orientational order. The aggregates formed on mica were proposed to be cylindrical micelles. On graphite (a hydrophobic substrate), parallel half-cylinder aggregates were observed on the surface with C14TAB. However, when a (C12)2DAB (a double-chained quaternary salt, didodecyldimethylammonium bromide) film was formed on mica, a featureless flat image was observed with atomic force microscopy (AFM). The proposed structure was a planar bilayer. These experiments were performed at twice the cmc for the respective surfactants. It has also been demonstrated that these micellar structures can form on a surface at concentrations slightly lower than that required for bulk aggregate formation. Four papers have addressed the formation mechanism or kinetics of the adsorbed alkylammonium layers.12-15 Hayes and Schwartz12 investigated the growth of a SAM of C18TAB from an aqueous solution on mica below the (14) Osman, M. A.; Seyfang, G.; Suter, U. W. J. Phys. Chem. B 2000, 104, 4433. (15) Heinz, H.; Castelijns, H. J.; Suter, U. W. J. Am. Chem. Soc. 2003, 125, 9500.

10.1021/la035935i CCC: $27.50 © 2004 American Chemical Society Published on Web 02/06/2004

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Krafft temperature. AFM in conjunction with the contact angle measurements and IR spectra was used to follow the growth of the SAM at a single temperature and concentration. They found that there was an initial waiting period before island growth began, yet IR and contactangle measurements demonstrated that there was a substantial quantity of C18TAB adsorbed on the mica surface during this period. They proposed that a 2D liquidlike monolayer film formed during this initial induction period. Following the formation of a saturated liquid layer, islands of an ordered 2D solid phase nucleated and grew within the matrix of the liquid film. The kinetics involved in SAM growth have been of interest for years,2 and several groups have studied the kinetics of alkanethiol SAM growth on gold.16 These groups have used a variety of techniques including surface plasmon resonance spectroscopy,17,18 second harmonic generation,19 and quartz crystal microbalances.20 A limitation of these laterally averaging methods is that they sample a large area and thus cannot separate the many processes involved in SAM growth, which include the adsorption of the amphiphile on the surface, nucleation of islands, and growth and aggregation of the islands. Another limitation of these techniques is that it is not possible to distinguish between the various coexisting 2D phases that may exist on the surface. In the current paper, AFM was used to study films that were removed from the solution before the growth of the SAM was complete. Transmission FTIR and contact angle goniometry were used to complement the AFM observations. By using these techniques in concert, it was possible to study the growth of C18TAB SAM on mica over a range of temperatures and to separate the rates of adsorption and nucleation. It was observed that the rates of all of the growth processes increased with temperature; however, nucleation rates increased very little, while adsorption and island growth rates increased significantly. The island growth rate was equal to the adsorption rate over a range of temperatures, suggesting that the island growth was adsorption-limited. Experimental Details C18TAB (99% purity) and hexadecane (99% purity) were purchased from Aldrich (Milwaukee, WI) and used as received. Water from a Milli-Q UV+ purification system (Millipore, Bedford, MA) was used for the preparation of the 0.10 mM C18TAB solution. The concentration chosen was ∼1/3 of the cmc at 40 °C; the Krafft temperature for this system is 38 °C.21 Samples were prepared by immersing freshly cleaved 12.7 mm diameter mica disks into solutions thermostated at the desired temperature for the specified time. Upon removal from the solution, the samples were rinsed with Millipore water to remove the excess solution and then blown dry with dry nitrogen. These samples are referred to as “quenched”. Samples were imaged using either a Nanoscope III MMAFM or AFM-E (Digital Instruments, Santa Barbara, CA). All AFM images were collected in contact mode with silicon nitride tips (k ) 0.12 N/m). At least three regions per sample were imaged. Several samples were prepared and imaged for each immersion time. Island coverage was determined using a histogram of the height profile of each image with the Nanoscope software.22 The static contact angles were measured with a custom-made contactangle goniometer. Contact angles were obtained by using the (16) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (17) Jung, L. S.; Campbell, C. T. Phys. Rev. Lett. 2000, 84, 5164. (18) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731. (19) Dannenberger, O.; Buck, M.; Grunze, M. J. Phys. Chem. B 1999, 103, 2202. (20) Pan, W.; Durning, C. J.; Turro, N. J. Langmuir 1996, 12, 4469. (21) Swanson-Vethamuthu, M.; Feitosa, E.; Brown, W. Langmuir 1998, 14, 1590. (22) Woodward, J. T.; Ulman, A.; Schwartz, D. K. Langmuir 1996, 12, 3626.

Mellott et al.

Figure 1. (a-d) Series of ex situ, height-mode contact AFM images shown at various immersion times in 0.10 mM C18TAB/H2O at 40 °C. (e) The cross section corresponds to the white line drawn through the middle of the 18 h image. procedure described by Bain et al., in which a 1 µL drop on the end of a needle was brought into contact with the surface, and then the needle was removed to leave the drop.23 Transmission infrared spectra were obtained using a Mattson Galaxy or Thermo Nicolet Nexus 470 FTIR spectrometer equipped with a narrow-band MCT/A detector with an incident IR beam of 6.4 mm (defined by the sample holder). Because of the low signal level, IR data were averaged over 2400 scans (at 2 cm-1 resolution) for the sample and then ratioed to a 2400 scan background to enhance the signal-to-noise ratio. For a background sample, the original sample was placed in a Boekel UV Clean for 30 min to remove the monolayer. The spectra were subsequently filtered with a fast Fourier transform and notch filter in IgorPro v.4.0 (Wavemetrics, Inc., Lake Oswego, OR) to remove the interference pattern caused by the mica, and boxcar averaging was used to smooth the data.

Results and Analysis Quenched films of C18TAB were prepared at 22, 30, 40, and 50 °C; the images in Figure 1 are representative of the samples prepared at 40 °C. Samples prepared at other temperatures displayed features that were qualitatively similar, although detailed heights, island sizes, rates of growth, etc. varied as described below. In these images, shades of gray distinguish different heights (with the white features being the tallest), and the irregular raised features are termed islands. In several images, there are more than two relative heights. The tallest features are usually observed in the center of the lower height features (23) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Eval, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.

Kinetics of Octadecyltrimethylammonium Bromide

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Figure 2. Panel a is representative of the extended force curve in water on mica. Panel b is a representative force curve taken in a 0.10 mM C18TAB solution at 40 °C.

Figure 4. Plot of the fractional island coverage (XS) versus time for 22, 30, 40, and 50 °C. The error bars indicate the standard deviation of all of the measurements.

Figure 3. Cartoon demonstrating the proposed model for how the in situ bilayer structure is modified upon removal from an aqueous solution. The relative island height H measured by AFM is also defined.

and are on the order of 1.8-2.0 nm taller. We believe that these are the remnants of the bilayers that are present in solution. Evidence for such bilayers is present in force versus distance data. Part a of Figure 2 is the force curve of the approach between a Si3N4 AFM tip and bare mica in pure water. It demonstrates the expected hard repulsive contact between the tip and substrate, without evidence for the long-range repulsion or adhesion. Part b of Figure 2 is a force curve for the approach between a Si3N4 AFM tip and mica in 0.10 mM C18TAB at 40 °C. The upward curve in Figure 2b is a result of a repulsive electrical double-layer24 force between the AFM tip and surface (this feature has been previously interpreted as evidence for micellar structures in alkylammonium systems).25-27 Upon closer approach, there is a sharp dip in the deflectiondistance curve where the tip pushes through the double layer; at this force, it is possible to image the first layer on the surface. Ducker et al. observed this “jump-in” feature in force curves that were obtained in 0.2 mM C16TACl/H2O solutions when the cylindrical rods were observed in AFM images.25 In their experiment, the liquid cell was purged with pure water, and the repulsion in the force curve was replaced by an attractive force. This was observed in conjunction with the disappearance of the micellar cylindrical rods on the surface.25 In the current system, the in situ structure is suspected to be a bilayer that is stable in solution but is not stable when removed from contact with water. This concept of a stable bilayer that breaks is displayed in the cartoon in Figure 3. The top part of Figure 3 represents a sample in solution where the ordered and disordered monolayer phases are covered (24) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991. (25) Ducker, W. A.; Liu, J.; Min, G. Langmuir 2001, 17, 4895. (26) Ducker, W. A.; Liu, J. J. Phys. Chem. B 1999, 103, 8558. (27) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160.

by a disordered (inverted) second layer. Upon removal from the solution, most of the outer layer is removed; however, remnants are apparently left on the surface. Similar phenomena have been observed with LangmuirBlodgett films when a multilayer is removed through a bare water/air interface.28-30 The relative coverage of these multilayer features does not increase significantly with extended exposure to the solution, and these taller features do not interfere with the AFM measurements of island coverage and height of the first layer; they do, of course, influence the FTIR and contact-angle measurements. In this paper, we focus on the lower (submonolayer) features. The height H of these features is actually the relative height of the monolayer measured by the AFM. Therefore, H is the difference between the thickness of a monolayer island and thickness of the lower-lying phase surrounding the island (this is illustrated in Figure 3). The series of images in Figure 1 clearly demonstrates a progressive increase in the fractional island coverage (XS) with increased immersion time, and the images are consistent with the accepted qualitative model of nucleation and aggregation of the adsorbate molecules to form islands and the eventual coalescence of the islands to form a complete monolayer. The increase in XS over time for 30, 40, and 50 °C can be seen in Figure 4 along with results observed in the previous room temperature (22 °C) experiment.12 At each of the four temperatures, there is an early induction period during which no islands are observed with AFM. The apparent island heights H are shown in Figure 5 as a function of the immersion time. The average relative island height was measured by AFM and was found to decrease with increased immersion time. This decrease in the relative height is most likely associated with an increase in the thickness of the liquid film. The apparent island height generally decreases with increasing temperature, suggesting that the liquid-phase film is thicker at higher temperatures. As the temperature is increased, the island shape and size changes significantly. Figure 6 is a series of representative AFM images for 22, 30, 40, and 50 °C and demonstrates the difference in typical island size and shape. The images in Figure 5 are taken from the sample times when the average coverage is between 5 and 20% (28) Honig, E. P.; Hengst, J. H. T.; den Engelsen, D. J. Colloid Interface Sci. 1973, 45, 92. (29) Peng, J. B.; Ketterson, J. B.; Dutta, P. Langmuir 1988, 4, 1198. (30) Momose, A.; Hirai, Y.; Waki, I.; Imazeki, S.; Tomioka, Y.; Hayakawa, K.; Naito, M. Thin Solid Films 1989, 178, 519.

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Figure 5. Plot of the average island height versus time for 22, 30, 40, and 50 °C showing the decrease in the relative island height with immersion time.

Figure 7. FTIR spectra for films prepared at 40 °C. Samples showing the shift of the C-H stretch modes to lower wavenumbers as the immersion time increases. The vertical lines correspond to νa,CH2 ) 2924 cm-1 and νs,CH2 ) 2855 cm-1, which are representative of a liquid-like film.32-34

Figure 6. AFM images of quenched films prepared at (a) 22, (b) 30, (c) 40, and (d) 50 °C with approximately the same fractional island coverage. Islands become progressively larger and more branched/porous at higher temperatures.

coverage, which is well before coalescence occurs. At 22 °C, the islands are less than 500 nm in diameter, relatively circular in shape, and nonporous, but as the temperature is increased, the diameter of the islands becomes larger for the same coverage and the islands become more porous and branched. Part d of Figure 6 is a 1 h, 50 °C image where the island diameter is greater than 2.5 µm at its largest and the smallest diameter is still greater than 800 nm. The AFM data were complemented in these experiments by IR spectroscopy and contact angle goniometry. Figure 7 is a representative series of the transmission IR spectra of the quenched C18TAB films prepared at 40 °C. In the IR spectra of saturated alkanes, there are two distinct peaks observed for the methylene stretching modes (asymmetric, νa,CH2 ) 2926 cm-1, and symmetric, νs,CH2 ) 2853 cm-1) for a disordered unconstrained chain as observed in a disordered film. As the alkyl chain becomes more constrained/ordered, the peak positions shift to lower wavenumbers.31 The IR spectrum obtained after the 1 h immersion (prior to island formation) displays an asymmetric methylene stretch at 2924 cm-1 and a symmetric

methylene stretch at 2855 cm-1. These peak positions are characteristic of methylene stretches in a disordered alkane.32-35 As the immersion time increases, there is a gradual blue shift of the asymmetric methylene stretch toward 2918 cm-1 and an increase in the peak intensity. The blue shift and increase in intensity are indicative of an increase of the order and density of the adsorbate molecules on the surface.12,36 As discussed below, the shift is due to a combination of increased island coverage as well as gradual ordering within the liquid phase. The IR spectra for samples at longer immersion times are a superposition of the liquid phase, solid phase, and remnant bilayer. In particular, the presence of the remnant bilayer introduces uncertainties, which precludes a detailed quantitative analysis to determine the coverage. However, because AFM images show that the remnant bilayer typically covers only about 10-15% of the surface and does not increase systematically with coverage, we feel that the qualitative trends are representative of the underlying monolayer. The contact angle measurements can be used to further support the conclusion from the IR data that a disordered liquid phase is present before islands begin to form. The Cassie equation37 predicts cos(θ) ) f1 cos(θ1) + fCH3 cos(θCH3), where f1 is the fraction of area between the islands, θ1 is the theoretical contact angle of the area between the islands (θ1 ) 0 if the surface is bare), fCH3 is the fractional coverage of the monolayer, and θCH3 is the theoretical contact angle for the liquid on a tightly packed methylterminated surface (θCH3 ) 45° for hexadecane).3 If the (31) Lin-Vein, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: San Diego, CA, 1991. (32) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (33) Whitesides, G. M.; Nuzzo, R. G.; Laibinis, P. E.; Allara, D. L. J. Am. Chem. Soc. 1992, 114, 9022. (34) Tao, Y.-T.; Chang, S.-C.; Chao, C. J. J. Am. Chem. Soc. 1994, 116, 6792. (35) Porter, M. D.; Widrig, C. A.; Chung, C. J. J. Electroanal. Chem. 1991, 310, 335. (36) Woodward, J. T.; Doudevski, I.; Sikes, H. D.; Schwartz, D. K. J. Phys. Chem. B 1997, 101, 7535. (37) Cassie, A. B. D. Discuss. Faraday Soc. 1952, 75, 5041.

Kinetics of Octadecyltrimethylammonium Bromide

Figure 8. Plot comparing the cosine of the “static” contact angle of submonolayer films at 30, 40, and 50 °C.

density of the disordered film regions was very low, then the extrapolation of cos(θ) to time zero would approach unity, which was observed in earlier experiments;38-40 this is consistent with the presence of only a low-density, disordered 2D vapor between the islands. On the other hand, if the area between the islands was a disordered 2D liquid film, then a greater density of hydrophobic moieties would be exposed to the sessile drop and the contact angle would be expected to be greater than zero but less than the theoretical contact angle for an ordered methylterminated surface. The cosine of the static contact angle of hexadecane, cos(θ), is plotted versus the immersion time in Figure 8, with linear fits of cos(θ) for 30, 40, and 50 °C. For the 30 °C data, cos(θ) extrapolates to a value of 0.917 at t ) 0. The corresponding values are 0.843 and 0.804 for 40 and 50 °C, respectively. The fact that cos(θ) decreases at time zero as the temperature is increased implies an increase in the order or density/thickness of the liquid layer as the temperature increases. This is consistent with AFM observations described above. There are various rates involved in the growth of C18TAB SAMs from an aqueous solution. First, the molecules must adsorb onto the surface from the solution, and this is defined as the deposition rate F. After the molecules adsorb onto the surface, they will aggregate into different phases. In this system, the first phase present is a disordered liquid phase. The rate of liquidphase growth kL is initially close to F but decreases as the liquid phase becomes more dense. Under quasi-equilibrium conditions, the liquid phase would reach a maximum density, at which a second more ordered phase will begin to nucleate. At this point, islands of the second phase will begin to appear at a rate knuc. Typically, the island number reaches a maximum value, and further SAM growth in the aggregation regime is due to the island growth at a rate kS. The island coverage kinetics in the aggregation regime measured from the AFM images can be modeled using Langmuirian kinetics, XS ) 1 - e-kSt [or ln(1 - XS) ) -kSt], to determine the rate of island growth kS for each temperature. A plot of ln(1 - XS) versus time can be found in Figure 9. In Figure 9, there are two distinct slopes for each temperature. This change in slope or rate of growth is evidence for the two distinct regimes of growth. In Figure 10, ln(1 - XS) versus time and the apparent island height H versus time are displayed on the same graph for 40 °C. (38) Woodward, J. T.; Schwartz, D. K. Langmuir 1997, 13, 6873. (39) Israelachvili, J. N.; Gee, M. L. Langmuir 1989, 5, 288. (40) Lamb, R. N.; Furlong, D. N. J. Chem. Soc., Faraday Trans. 1 1982, 78, 61.

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Figure 9. Plot of ln(1 - XS) versus time displaying Langmuirian-type kinetics after an initial induction period.

Figure 10. Island coverage kinetics and relative island height versus time for 40 °C. This plot shows that the change in the growth rates is related to the time at which the liquid phase reaches its maximum density tL,max. The vertical dashed line is at tL,max as described in the text.

The island height initially decreases and eventually reaches a plateau under steady-state conditions. The decrease in the island height is actually due to a gradual increase in the thickness of the liquid phase. At the time the height reaches a minimum value, the rate of island growth accelerates; at this point, the liquid phase has reached its steady-state density and additional adsorbed molecules result primarily in island growth. The time is designated tL,max, and the density of the liquid phase at tL,max is at its maximum and is termed dL,max. After tL,max, the growth of the solid phase is dependent on the growth of the existing islands and not the nucleation of new islands. The fractional area of these islands follows Langmuirian kinetics after the liquid phase reaches its maximum density as shown in Figure 10. The rates kS were determined by a least-squares fit, and the values are shown in Table 1. The data in Figure 10 also permit a calculation of the adsorption rate from the solution into the liquid layer kL. We define a reduced dimensionless density of the liquid phase dL as the thickness of the liquid layer divided by the thickness of a completely ordered well-packed film. The approximate thickness of a complete film is based on the fully extended length of a C18TAB molecule, which is ∼2.5 nm. The thickness of the liquid phase at a given time is calculated by subtracting the relative island height H at a given time from the fully extended length of C18TAB. The density is, therefore, defined as

dL ) (2.5 - H)/2.5

(1)

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Table 1. Calculated Deposition Constants F, Rates of Adsorption in the Liquid Phase kL, and Rate of Island Growth kS for 22, 30, 40, and 50 °C temp (°C)

F (h-1)

kL (h-1)

kS (h-1)

knuc (h-1)

22 30 40 50

0.0161 ( 0.0011 0.0934 ( 0.0208 0.263 ( 0.016 0.749 ( 0.013

0.009 14 ( 0.000 27 0.0449 ( 0.0089 0.0668 ( 0.0059 0.241 ( 0.001

0.008 16 ( 0.001 76 0.0400 ( 0.0028 0.204 ( 0.007 0.200 ( 0.059

2.05 × 10-4 4.47 × 10-4 3.95 × 10-3 4.82 × 10-4

Table 2. Measured Induction Time tind, Island Height at tind Hind, and Island Height When the Liquid Phase Reaches Its Maximum Density Hsat and theCalculated Values for the Liquid Density at tind dL,ind and the Saturated Liquid Density dL,sat for 22, 30, 40, and 50 °C temp tind (°C) (h) 22 30 40 50

24 3 3 1

dL,ind

Hind (nm)

dL,sat

Hsat (nm)

0.321 ( 0.008 0.244 ( 0.015 0.546 ( 0.025 0.527 ( 0.007

1.69 ( 0.02 1.89 ( 0.04 1.13 ( 0.06 1.18 ( 0.02

0.434 ( 0.021 0.520 ( 0.012 0.746 ( 0.037 0.678 ( 0.007

1.42 ( 0.05 1.20 ( 0.03 0.63 ( 0.09 0.80 ( 0.02

where H is measured in nanometers. Assuming that the liquid phase follows Langmuirian kinetics, the adsorption constant F can be estimated with the equation

1 - dL,ind ) e-Ftind

(2)

where tind is the time when islands first appear, and the height at this time is used to calculate dL,ind. To determine the rate of adsorption into the liquid phase at a later time kL, the Langmuirian expression

kL ) F(1 - dL,sat)

Figure 11. Plot of N versus time shown with linear fits in the nucleation regime. These fits were used to calculate knuc.

(3)

is used, where dL,sat is the density of the liquid phase after it becomes saturated. dL,sat is calculated by using the relative island height after it levels off and reaches a plateau. The values for dL,ind, Hind, tind, dL,sat, and Hsat can be found in Table 2. The values for F and kL can be found in Table 1. The final rate that can be extracted from these experiments is the rate of island nucleation knuc. To determine the nucleation rates, it is necessary to plot the island number density per site N versus time (Figure 11). The shareware program ImageJ was used to count the number of islands within a region, and the number of islands was multiplied by the area per site (the cross-sectional area of an adsorbate molecule) and then divided by the area of the image. For C18TAB, the theoretical area per site is 45 Å2.9 The rate of nucleation is then obtained by a leastsquares fit of the early growth region, where nucleation is the dominant process. Values for knuc can be found in Table 1. Discussion SAM growth is a multiple-step process. In this system, first a disordered/low-density film adsorbs onto the surface. In the present case, the film has an appreciable density, more akin to a 2D liquid than a 2D vapor. Once the liquid film reaches a critical density, there is a rapid nucleation of the islands in competition with the continuing growth of the liquid layer. Following this rapid nucleation/liquidlayer growth stage is a steady-state aggregation regime, where nucleation is negligible. In an equilibrium situation, the liquid-phase density would remain constant during the conversion to the solid phase. However, our observations suggest that the nucleation and growth of the solid islands are so slow that the liquid layer continues to thicken, becoming “supercompressed”. Both IR and contact angle measurements support the observation of growth of a disordered film before the

Figure 12. Langmuirian kinetics and island density for 30 °C plotted versus time displaying the liquid growth, nucleation, and island growth regimes. The plot is separated into two regions: regime I and regime II by the dotted line at tL,max (when the liquid reaches the maximum density). Growth of the liquid phase and island nucleation occur in regime I. Regime I begins at t ) 0 and ends at tL,max. Island nucleation begins at tind and ends at Nmax. The 2D solid phase grows in regime II after tL,max and Nmax are reached.

islands begin to appear. In both the IR and contact angle measurements, there is evidence for adsorbate molecules on the surface before the islands are observed with the use of AFM. AFM and contact-angle measurements suggest that the liquid film is thicker at higher temperatures. Evidence for the growth of the liquid phase can be found in all of the forms of data collected in this experiment. Relatively high contact angles at early immersion times (before islands are observed by AFM) are evidence for a high concentration of adsorbate molecules on the surface. In addition, the presence of IR peaks characteristic of disordered adsorbate molecules, combined with the relative island height measured with the AFM, identifies the induction regime and makes it possible to track the increase of the thickness of the disordered layer. This thickness increases with increasing temperature. In Figure 12, the growth of the disordered liquid phase corresponds to regime I and starts at t ) 0 before tind, and

Kinetics of Octadecyltrimethylammonium Bromide

it continues to grow after Nmax is reached. The end of regime I is at tL,max when there is a sharp change in the slope of ln(1 - XS) and the growth is predominantly in the solid phase. Once islands are observed with AFM, the relative island height is observed to decrease as the immersion time increases. This decrease in the relative island height is accompanied by an overall increase in the molecular order as indicated by IR and contact-angle measurements. The increase in order of the IR and contact-angle measurements is greater than can be attributed to the growth of the islands alone, and when the height measurements are considered as well, it can be deduced that the overall increase in order is due to a combination of island growth and the increase of density within the liquid film surrounding the islands. The beginning of the nucleation regime is signaled by the appearance of islands in the AFM images, and the regime ends when the island density reaches a maximum (Nmax). In Figure 11, as the temperature increases, Nmax decreases and is reached at earlier times. At each temperature, Nmax is reached before dL,max is reached. For example, in Figure 12 at 30 °C, Nmax is reached after 20 h, and dL,max is not reached until tL,max, which is at 30 h. This demonstrates that island nucleation occurs during the same time period as the continued monomer adsorption into the liquid phase. Once the disordered liquid phase reaches dL,max and Nmax is reached, the growth process enters the aggregation regime, where the increase in island coverage is dominated by island growth. In Figure 10, the transition from the first growth regime, where the liquid density is increasing, to the beginning of the aggregation regime is easily identifiable. As the relative island height reaches a minimum and stays constant, the island coverage is observed to accelerate. This is the beginning of the aggregation regime, regime II in Figure 12, where the slope of ln(1 - XS) changes by an order of magnitude and the growth of the solid phase is dominant. The final growth regime observed in SAM growth is the coalescence regime. In this paper, it was not possible to study this regime because of the limitations of an ex situ experiment. There are several rates that could be involved in each of these regimes. During the early stages when the disordered film is adsorbing onto the surface bulk diffusion in solution, the surface adsorption and desorption could be factors. In this paper, the surface adsorption and desorption cannot be separated, and the net process is described in terms of the rate of growth of the liquid phase kL. Island nucleation can be described in terms of a combination of the rate for two or more amphiphiles to stand up in solution and form an island and the reverse process of island dissolution. The net nucleation rates presented are a combination of these two rates and are referred to as the nucleation rate knuc. The final regime in this system, in which rates can be extracted, is the aggregation regime. The rates involved in the aggregation regime are the rates for an amphiphile to adsorb to an existing island and the reverse process of desorption from an island. The net rates of aggregation kS are determined and presented in Table 1. In each of these cases, the rate of adsorption must be greater than the rate of desorption; otherwise, a SAM would never form. Each of these rates can be found in Table 1 for each temperature. We note that, except for 40 °C, where systematic errors in the coverage measurements made an accurate calculation of kS difficult, kL and kS are approximately equal. This is consistent with our conjecture that island growth occurs

Langmuir, Vol. 20, No. 6, 2004 2347 Table 3. Calculated Activation Energies for the Deposition of Molecules on the Surface Ea,F, the Adsorption into the Liquid Ea,L and Solid Phase Ea,S Observed during Growth, and Island Nucleation Ea,N Ea,F Ea,L

∼20 kJ/mol ∼100 kJ/mol

Ea,S Ea,N

∼100 kJ/mol ∼0.3 kJ/mol

under approximately steady-state conditions, i.e., molecules are added to the liquid phase from the solution at the same rate as molecules are transferred from the liquid phase to the islands. Assuming Arrhenius behavior, one can calculate the activation energies from these rate constants. The activation energies are presented in Table 3. The activation energy of adsorption Ea,F is the activation barrier for a single adsorbate molecule to adsorb onto the bare mica. This value involves only the transfer of a C18TA+ ion from the solution onto the surface and the subsequent adsorption of the ion onto a site on the mica substrate. The activation barrier for adsorption into the liquid phase Ea,L likely involves the insertion of the charged headgroup into the hydrophobic region of the adsorbed layer. We can think of the free energy of this transition state as being comprised of two components: the creation of a hydrophobic/hydrophilic interface surrounding the headgroup and the elastic energy due to local distortion of the tails in the adsorbed layer. The interfacial energy component can be estimated by modeling the headgroup as a singleended cylinder with a radius of 2.82 Å and height of 3.0 Å and an interfacial free energy per unit area typical of a hydrocarbon/water interface, ∼50 mJ/m2. This gives a value of 24 kJ/mol, which serves as a lower limit for the true activation energy and is, in fact, somewhat less than the measured Ea,L. This suggests that other energetic considerations are also important, e.g., the energy costs associated with alkyl tail deformation near the insertion point. The activation energy for island nucleation Ea,N is several orders of magnitude less than either Ea,L or Ea,S. This nearly activationless nucleation rate is intuitively reasonable because the C18TA+ ions are already adsorbed into a densely packed liquid phase, which is shielded from the hydrophilic medium by the bilayer. Thus, for island nucleation to occur, two or more alkyl chains must adhere and stand up within the liquid phase. During this cooperative process, the chains lose some orientational freedom but achieve a lower energy state by optimizing the intermolecular van der Waals interactions. The actual nucleation rates, however, are qualitatively slow, suggesting that the loss of entropy is a dominant factor. The small value of Ea,N means that the nucleation rates do not increase significantly at high temperatures in comparison with the adsorption and molecular mobility. This results in the formation of large and increasingly branched islands as the temperature increases. Even though Ea,F is significantly smaller than Ea,S and Ea,L, the deposition rates are greater than the rate of the liquidand solid-phase growth, which explains the supersaturation of the liquid phase at longer times. The activation energy for growth of islands Ea,S is of the same order as the activation energy for the adsorption into the liquid phase Ea,L, which is consistent with the fact that the corresponding rates are quite similar. Given the very small activation barrier for islands to nucleate and monomers to stand up in the liquid phase, it is evident that this system is adsorption-limited. Previous groups have attempted to extract energies from past experiments, and the majority have focused on

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alkanethiols. Papers by Blanchard et al.41 and Harris et al.4242 probed the free energy of adsorption ∆Gads for alkanethiols on gold and silver. The experiments performed by Blanchard et al. have determined the free energy of adsorption, enthalpy of adsorption ∆Hads, and entropy of adsorption ∆Sads for octadecanethiol on gold from n-hexane.41 Harris et al. found ∆Gads for decanethiol and ethanethiol on silver from an aqueous solution.42 These previous papers were performed at constant temperatures (the preexponential for the Arrhenius fit was estimated), changed the alkyl chain length and solvents, and showed the difference that substrate properties have on the stability of the monolayer. Also, a simple model was assumed where there was only one process involved in the SAM formation, whereas this paper employs a more complicated model to describe the SAM growth by breaking it into several processes; the experiments described in this paper were performed at several temperatures so that the preexponential for the Arrhenius fit did not have to be estimated, and the data demonstrate that there are several kinetic processes involved in the adsorption of a monolayer. The slow kinetics of C18TAB SAM growth precludes in situ AFM experiments for this system; however, this series of ex situ experiments demonstrates that in situ kinetic studies of other amphiphilic systems could be enlightening in the future. With in situ experiments, it will be possible to break the growth process into more well-defined parts and to better understand the mechanisms involved in SAM growth. (41) Blanchard, G. J.; Schessler, H. M.; Karpovich, D. S. J. Am. Chem. Soc. 1996, 111, 9645. (42) Harris, J. M.; White, H. S.; Hatchett, D. W.; Uibel, R. H.; Stevenson, K. J. J. Am. Chem. Soc. 1998, 120, 1062.

Mellott et al.

Conclusions C18TAB SAM growth was studied using a variety of methods and shown to occur via a multistep process. Initially, adsorbate molecules form a homogeneous disordered 2D liquid phase. Once the liquid phase reaches a critical density, the islands of the 2D solid were observed to nucleate. However, the growth of these islands was so slow that the coexisting 2D liquid phase continued to densify during island growth, thereby slowing the adsorption into that phase until a steady state was reached. The various growth processes (i.e., adsorption, island nucleation, and island growth) were observed over a range of temperatures from 22 to 50 °C. The rate of each of the respective processes increased with temperature; however, the nucleation rates increased very little, while the adsorption and island growth rates increased significantly. The island growth rate was equal to the adsorption rate over a range of temperatures, suggesting that island growth was adsorption-limited. Activation energies for various processes were calculated, including those for the adsorption onto a bare substrate (20 kJ/mol), adsorption into the saturated liquid phase (100 kJ/mol), and nucleation of the islands (0.3 kJ/mol). The extremely small activation barrier to the island nucleation suggested that slow nucleation rates were primarily due to entropic effects. A consequence of the small Ea,N was the formation of increasingly large and branched islands at higher temperatures. Acknowledgment. J.M.M. thanks the Louisiana Board of Regents Louisiana Education Quality Support Fund for a fellowship. This paper was supported by the National Science Foundation (Grant CHE-0129807). LA035935I