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Molecular Packing Changes of Octadecylamine Monolayers on Mica Induced by Pressure and Humidity J. J. Benı´tez,† S. Kopta,| I. Dı´ez-Pe´rez,§ F. Sanz,§ D. F. Ogletree,| and M. Salmeron*,| Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla, Avda. Americo Vespuccio s/n, Isla de la Cartuja, Sevilla 41092, Spain, Laboratori d’Electroquı´mica i Materials, Universitat de Barcelona, Barcelona Spain, and Materials Science Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720 Received August 8, 2002. In Final Form: October 23, 2002 The molecular packing structure of octadecylamine molecules forming self-assembled monolayers on mica has been studied by AFM as a function of mechanical pressure and humidity. Upon increasing the load applied by the AFM tip, the thickness of the monolayer was observed to change discretely, which we explain as the result of stepwise tilting of the molecules. Thickness changes were also observed when the films were exposed to high humidity. In this case, we propose that the driving force for such a behavior is the relaxation of electrostatic repulsions between protonated amino groups.
1. Introduction In two recent papers we have shown that in air octadecylamine adsorbs on mica, either forming partial monolayers (islands) of nearly upright molecules when prepared from a relatively concentrated chloroform solution,1 or completely covering the surface with multilayers having alternating methyl and amino terminations when prepared from ethanol solutions.2 The partial coverage found with chloroform was shown to be the result of the displacement of the residual water film on mica by the hydrophobic chloroform solvent and octadecylamine adsorption on water patches. This problem disappears with the hydrophilic ethanol solvent, and full coverage is achieved. Micellar formation in the ethanol solvent was proposed as the mechanism leading to formation of stacks of layers with alternating end group termination. The packing is such that the molecules are in an alltrans configuration, tilted with respect to the direction normal to the surface and exposing the terminal -CH3 group in the monolayer islands prepared from chloroform solutions. This methyl-terminated film acts as a good lubricant as shown by the substantial lowering of the friction force relative to that of the uncovered mica substrate.3-5 Molecular packing can have a strong influence on the mechanical properties of the film6 and can also affect the resistance to chemical attack, for example, by controlling water penetration and the intercalation of foreign molecules. We have already shown how the packing of alkylthiol and alkylsilanes can vary under preparation conditions to yield flat-lying or upright molecules with †
Centro Mixto CSIC-Universidad de Sevilla. Universitat de Barcelona. | Lawrence Berkeley National Laboratory. §
(1) Benı´tez, J. J.; Kopta, S.; Ogletree, D. F.; Salmeron, M. Langmuir 2002, 18, 6096. (2) Benı´tez, J. J.; Ogletree, D. F.; Salmeron, M. Langmuir, in press. (3) Fujihira, M.; Morita, Y. J. Vac. Sci. Technol., B 1994, 12 (3), 1609. (4) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (5) Lio, A.; Charych, D. H.; Salmeron, M. J. Phys. Chem. B 1997, 101, 3800. (6) Salmeron, M. Tribol. Lett. 2001, 10, 69.
different tilt angles.7-9 The packing density was shown to change with applied load in these layers. Here we present new results showing the effects of various external parameters on the packing structure of alkylamine SAMs. In addition to load, we found that humidity also has a strong influence on the packing structure of the SAMs. 2. Experimental Section Octadecylamine (Fluka, >99%) was used as received and dissolved in chloroform (Aldrich, 99.8%) to obtain a 15 mM solution. Samples (3 × 1 cm) of muscovite mica (KAl2(Si3AlO10)(OH)2; Mica New York Corp.) were cleaved on both sides at ambient conditions (typically 20 °C and 40-50% RH) and quickly immersed in the solution for 30 s. The samples were subsequently removed and dried under a stream of dry nitrogen for several minutes. Samples were also allowed to ripen from a few hours up to a few days by storing them in a test tube under ambient conditions. This allowed for slow diffusion processes to take place and resulted in monolayer islands with a diameter ranging from 0.1 up to 0.7 µm depending on the ripening time and covering roughly half the surface of the mica substrate. For the measurements presented in section 3.3 a higher coverage was needed. We found that this can be achieved by adding a small amount of ethanol to the chloroform solution. As described in a previous paper,2 the ethanol solvent always led to much larger amounts of deposited material than could be achieved with the chloroform solvent alone. Contact images were obtained using either a Topometrix Explorer (TMX 2000) microscope working in air or a home-built instrument controlled by an RHK electronic unit (Troy, MI). The latter is enclosed in a chamber providing sound isolation and humidity control. Humidity was controlled by bubbling N2 through deionized water or by purging with dry nitrogen. Si3N4 cantilevers from NanoProbe (Digital Instruments, Santa Barbara, CA) with a nominal force constant of 0.12 N/m were employed in both cases. The z-distance scale (normal to the surface) was calibrated using a test silicon grating (TGZ01, Silicon-MDT, Moscow) with nominal steps of 25.5 nm and also in situ by (7) Barrena, E.; Kopta, S.; Ogletree, D. F.; Charych, D. H.; Salmeron, M. Phys. Rev. Lett. 1999, 82, 2880. (8) Barrena, E.; Ocal, C.; Salmeron, M. J. Chem. Phys. 2000, 113, 2413 . (9) Barrena, E.; Ocal, C.; Salmeron, M. J. Chem. Phys. 2001, 114, 4210 .
10.1021/la020705+ CCC: $25.00 © 2003 American Chemical Society Published on Web 12/31/2002
Molecular Packing Changes of Octadecylamine on Mica
Figure 1. Sequence of images (A-D) acquired at a load of -2 nN (the pull-off force is Fpull-off ≈ 20 nN), showing the production of damage in freshly prepared (4 h ripening at room conditions) islands of octadecylamine on mica. Damage is in the form of holes, which increase in size by aggregation of smaller holes and evolve until they reach the island edge. The gray scale corresponds to a 2.8 nm height range. generating 1 nm deep holes in the mica surface through wear after scanning with a sufficiently large load.10
3. Results and Discussion 3.1. Molecular Packing under Applied Pressure. The packing of molecules determines the height of the octadecylamine islands. Measuring changes in island height therefore provides a method to study packing changes as a function of pressure exerted by the tip. Freshly prepared islands (aged less than 12 h after preparation) have typical diameters of 0.3-0.5 µm and heights of 1.6 nm when imaged under very low load (near the pull-off point at approximately -20 nN). The islands are fragile, and even these low load conditions result in damage after successive scans, as shown in Figure 1. Damage is manifested in the formation of holes that become observable when their diameters are above a few nanometers, the limit of resolution in contact mode imaging. The holes aggregate as a result of scanning to form larger ones that extend toward the edge of the island, where they open up. Subsequent larger images show material accumulation at the edges of the scanned area. This eroding behavior is accelerated at high humidity, indicating that water plays an important role in the diffusion dynamics of the molecules. Films kept for a few days under ambient conditions are much more stable toward mechanical damage. Processes involving molecular diffusion and possible equilibration with water vapor (humidity) take place during this ambient ripening. One remarkable effect of ripening is the decrease in the height of the islands by 0.35 nm from the initial height of 1.6 nm. The height of these ripened films was measured as a function of applied load, and the results are shown in Figure 2. The various points for each load value correspond to measurements performed on the different islands shown in the image at the top. As can be seen the island height remains constant until a sharp decrease of about 0.25 nm is observed around -10 nN. (10) Kopta, S.; Salmeron, M. J. Chem. Phys. 2000, 13 (18), 8249 .
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Figure 2. Variation of the octadecylamine island height (h) as a function of applied load. Each point at a given load corresponds to one of the islands in the image shown in the inset at the top right. The height of freshly prepared islands (before ripening by several hours of exposure to ambient conditions) is shown as the first point (at -20 nN). Ripening results in a spontaneous height decrease of 0.35 nm. The value of h calculated for various tilt states (n) is indicated by the horizontal lines.
The height is then maintained until approximately 20 nN, above which a continuous decrease is observed. If the pressure is released and the islands are imaged at low load, the original height is fully recovered with no observable damage. These results show that ripened islands are more compact and resistant to damage than freshly prepared ones. This behavior can be explained with the same model used by Barrena et al. to account for the similar stepwise decrease of height vs load observed in monolayer islands of alkylthiols on gold and alkylsilanes on mica.6-9 In this model optimal packing of the methylene groups in the alkyl chains of adjacent molecules (assumed in an alltrans configuration) is maintained as the molecules tilt under the applied load. This imposes discrete values for the tilt angles dictated purely by the geometry of the molecules. In its simplest form the model predicts that the height (h) of a molecule can only adopt values of h ) L[1 + (na/d)2]-1/2, where L is the total length of the alkyl molecule, a the distance projected along the molecule axis between alternating carbon atoms (0.25 nm), d the separation between molecules in the direction perpendicular to their chain axis (0.47 nm), and n an integer starting from zero. The height values predicted by this simple model are in good agreement with the experimental ones, as indicated by the horizontal lines in Figure 2. No steps beyond n ) 4 have been observed although the island height continues to decrease under applied load. Clearly the pressure applied by the tip is enough to cause transitions between tilted states when it reaches critical values. The continuous height decrease observed above n ) 4 may indicate that other relaxation mechanisms in addition to rigid chain tilts might be operating at higher loads. In the load range investigated here, the process is reversible and the original situation is recovered as soon as pressure is released. 3.2. Molecular Packing vs Humidity. The first point in Figure 2 shows that a change of molecular packing is induced spontaneously by ripening in air, which is manifested by a decrease in height, i.e., by molecular tilting from the structure corresponding to n ) 2 to that corresponding to n ) 3, Figure 2. The process is induced by the humidity in the ambient air and can be extended by a more severe interaction with water. We thus exposed the sample to a high-humidity atmosphere (approximately
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Figure 3. Topography and friction images of an octadecylamine layer after exposure to high humidity (approximately 90% RH) followed by drying to 8% RH. The high-humidity exposure caused the aggregation of the initially small islands into large ones. Friction is high (bright contrast) over the uncovered mica. The regions of the island closer to the bare mica are higher that those farther inside. As shown in the line profile the height (h) decreases by discrete amounts, each corresponding to a specific value of the tilt angle determined by the tilt state (n).
90% RH) for several minutes. To avoid the increased damage caused by scanning under very high humidity, imaging was performed after drying back to 8% RH. The effects of such high-humidity exposure were several. The first and most obvious is the formation of large islands, many micrometers in size, due to aggregation of the original ones. This is the result of the higher mobility of molecules induced by water. The new islands are still one molecule thick but consist of regions with different heights, each height corresponding to one different tilt state n. Their distribution is such that the higher elevation states lie closer to the island boundaries, followed by more tilted states and finally the most tilted one in the center. This is shown in the image of Figure 3, where the regions with different heights appear with different shades of gray, and more quantitatively in the line profile drawn below. The friction image on the right-hand side shows the higher friction on mica (brightest area) and the uniform low friction on the island. The uniformity of the friction force indicates that the energy dissipation mechanisms are independent of tilt angle. This is because the load used for imaging is not sufficient to cause any additional tilting. Tilting results in a reduction in van der Waals cohesive energy because of the decrease in island height and of the length of the molecule in contact with its neighbors and is manifested in friction increases.6-9,11 Octadecylamine molecules are also expected to protonate to some extent due to their interaction with water.1 (11) Salmeron, M.; Kopta, S.; Barrena, E.; Ocal, C. Fundamentals of Tribology and Bridging the Gap between the Macro- and Micro/ Nanoscales; Kluwer Academic Publishers: Norwell, MA, 2001; pp 4152.
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Figure 4. Topographic images of an octadecylamine film on mica forming a nearly complete monolayer obtained using a 4% volume mixture of ethanol in the chloroform solvent. Dark patches correspond to residual uncovered mica. The brightest patches, shown by arrows in the left image (corresponding to the first scan), correspond to islands of a second layer made of completely vertical molecules (2.4 nm height). These secondlayer islands are amino terminated, while the first layer exposes the methyl groups. An entire bilayer is easily removed in a subsequent scan (right-hand-side image, scan 2). The first layer, covering most of the substrate, contains regions with two different heights, 2.42 and 1.66 nm, shown by the two gray levels. Arrows in the right diagram point to three of the 2.42 nm high regions. The cursor profile in the area marked by a circle illustrates the various heights before and after the second scan.
Protonation decreases cohesive energy due to electrostatic repulsion between charged headgroups. Since molecular tilting implies an increased distance between the protonated amino groups, it can compensate for some of this loss. Since the degree of protonation increases with the amount of water available, the expected system response is a corresponding increase in tilt angle as the humidity increases. These considerations can explain the observed spatial distribution of tilt states near the border of the islands, as shown in Figure 3. The regions nearest to the border are expected to have lost more water during drying and thus be less tilted. 3.3. Molecular Packing and Surface Coverage. In the previous sections we have shown how octadecylamine islands decreased in height (i.e., tilted) in response to perturbations such as humidity and applied load. These changes were always in the direction of an increased tilt toward the surface. An interesting question is whether it is possible to change the tilt angle in the opposite direction, away from the surface. One expects that this could be achieved by increasing the coverage. Lateral pressure should then force the molecules to cover a smaller fraction of the surface and thus adopt a more vertical position.
Molecular Packing Changes of Octadecylamine on Mica
Unfortunately, using chloroform solutions the alkylamine coverage was independent of experimental conditions such as immersion time or concentration in the solution, since it was limited by the amount of water present on the mica surface. A way around this difficulty is to add small amounts of ethanol to the chloroform solution. As we have shown in our previous paper,2 the deposition of octadecylamine from hydrophilic ethanol solutions results in multilayers. The result of the new preparation method using a mixed solvent is shown in Figure 4. As can be seen only small areas of mica remain uncovered (black spots in the images), while the alkylamine molecules cover most of the surface. In addition, small patches of second-layer islands are also formed, as shown by the brighter patches in the left image of the figure (pointed by arrows). These second-layer islands have a height of 2.4 nm, corresponding to completely vertical molecules (n ) 0). The first layer, covering most of the surface, is composed of regions with two different heights (regions pointed by arrows in the right-hand-side image). The highest one (light gray in the figure) also has a height of 2.4 nm, again indicating fully vertical molecules, i.e., in the n ) 0 tilt state. These n ) 0 regions are surrounded by the majority of molecules (the next gray level) with a height of 1.6 nm, the same found in the monolayer islands prepared from pure chloroform, and correspond to the n ) 2 tilt state. A cursor profile in the circled region of the two images, containing the various levels, is also shown in Figure 4. As we have shown in our previous paper,2 the molecules in the second layer are amine group terminated, which produces a higher friction and adhesion than the methyl termination. In that paper we also described the particular stability of bilayers made from molecules with back-toback methyl groups. In line with that result we found that the second-layer islands can be easily removed by the tip. This is shown in the image shown on the righthand side, corresponding to the next scan performed. The reason for the low stability of the bilayers is the higher chemical activity of the amino-terminated surface, which enhances the interaction with the tip. The removal of bilayers in one single sweep exposing the mica substrate is remarkable. It indicates that the interfacial energy between methyl-terminated films is stronger than that between the amine group and the mica surface. Meanwhile there is no damage to the methyl-terminated regions, which are chemically much more inert. 4. Conclusions We have shown that octadecylamine molecules form flat two-dimensional islands on mica made of mostly
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upright molecules. Several packing structures exist that are characterized by different discrete tilt angles. These angles are consistent with a simple geometrical model that maintains the densest packing geometry of the alkyl chains. The packing is most easily manifested in the height of the islands. The tilt state formed depends on the preparation conditions, in particular on the water content of the sample. Water also enhances the mobility of the molecules and gives rise to the formation of large islands. We propose a model where water increases the degree of protonation of the amino headgroups, thereby increasing electrostatic repulsion. This repulsion favors the formation of more tilted states, where the amino groups are farther apart. We have also shown that domains with different tilt states can coexist within an island, in particular after a drying process that appears to remove water in larger amounts from the edge regions of previously hydrated islands. As in the case of alkylthiols on gold and alkylsilanes on mica, tilting of the molecules can also be induced mechanically by the application of load with the AFM tip, an effect that has important implications on the tribological properties of the film. Lateral compression of the layer, achieved through high coverage, induces the formation of domains of molecules with smaller tilt states, including the completely vertical one with n ) 0. Finally we have shown the striking stability of bilayer islands formed by the back-to-back contact of molecules at the methyl ends. These bilayers can be mechanically removed by rupture of the mica-amine end group bonds, indicating that bonding at the aminowater-mica end is unexpectedly weaker than that between molecules through their methyl ends. This observation might be significant in understanding the stability of cell membranes, where the alkyl ends of the phospholipid molecules are bound together in the center. Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Material Science Division, of the U.S. Department of Energy under Contract No. DE-AC03-76F00098. J.J.B. gratefully acknowledges financial support from the Spanish Ministerio de Educacio´n, Cultura y Deporte under the “Movilidad del Profesorado Universitario e Investigadores” program. F.S. and I.D.-P. acknowledge financial support from the Gaspar de Portola` program (No. AGP 10) of the Generalitat de Catalunya. LA020705+
