Scanning Tunneling Microscopy Studies of Solvent Effects on

May 12, 1994 - of Triacontane/Triacontanol Molecules Adsorbed on Graphite ... that the triacontane and triacontanol molecules phase separate on the su...
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J. Phys. Chem. 1995, 99, 6608-6619

6608

Scanning Tunneling Microscopy Studies of Solvent Effects on the Adsorption and Mobility of TriacontanelTriacontanol Molecules Adsorbed on Graphite Bhawani Venkataraman, John J. Breen,' and George W. Flynn* Department of Chemistry and Columbia Radiation Laboratory, Columbia University, New York, New York 10027 Received: May 12, 1994; In Final Form: February 14, 1995@

Solutions of triacontane/triacontanol in different solvents have been investigated to determine the importance of the long chain hydrocarbon functional group and the solvent in mediating the probability of sticking to a graphite surface. The scanning tunneling microscopy images obtained on graphite for these mixtures suggest that the triacontane and triacontanol molecules phase separate on the surface into "pure" alcohol and alkane regions with a preferential deposition of alcohol molecules over alkane molecules. The nature of the solvent has a dramatic effect on the mobility and sticking probability of the molecules on the surface.

Introduction Scanning tunneling microscopy (STM) is a powerful tool for obtaining information on the packing order of molecular adsorbates on a surface. Data obtained from STM images can help to provide an understanding of the relative importance of molecule-molecule and molecule-substrate interactions as well as the types of forces responsible for a particular packing order on the surface. The molecular details of processes like epitaxial growth of thin films, chromatography, lubrication, and microelectronics fabrication, each of which involves interactions between a molecule and a surface, as well as between molecules on a surface, can be investigated in this way. The physisorption of long chain hydrocarbons on graphite surfaces has been studied as a model system for elucidating the forces that govem ad~orption.'-~Thermodynamic measurements indicate that these molecules have a high affinity for graphite and that the heat of adsorption increases with chain length.3 Experiments of this kind also suggest that the molecules are adsorbed with their long axis parallel to the graphite surface. These results have been confirmed in studies of STM images of hydrocarbons on graphite, which reveal that the molecules lie flat on the graphite surface and exhibit a high degree of STM experiments performed with n-alkanes, alcohols, and carboxylic acids indicate that the short and long range packing order depends on the functional group at the end of the hydrocarbon chair^.^-^ However, these images also suggest that alcohols, alkanes, or carboxylic acids cannot be distinguished by looking at an individual molecule since all appear to have the same contrast pattern along their length independent of functional group. Experiments measuring voltage dependent STM contrast ratios along an alcohol molecule indicate that it is possible to differentiate between the hydroxyl end and the hydrocarbon end of the molecule, suggesting that this may be a way of distinguishing between functional groups in a mole~ule.~~'~ The experiments presented here are aimed at furthering our understanding of the relative importance of moIecule-molecule and molecule- substrate interactions in adsorption processes. In particular, by the study of mixtures of alcohols and alkanes,

* To whom all correspondence

should be addressed. + Permanent address: Deoartment of Chemistrv. Indiana UniversitvPurdue University at Indianapolis, 402 N Blackfdrd Street, Indianapiis, IN 46202-3274. Abstract published in Advance ACS Abstracts, April 1, 1995. @

0022-365419512099-6608$09.00/0

information on the relative importance of different functional groups in determining the sticking probability on a graphite surface can be obtained. In the case of alcohols the influence of hydrogen bonding between the alcohol molecules, and its role in physisorption on graphite, can be investigated. The compounds used in this study were triacontanol (C30H610H) and triacontane (C30H62). Although the difference between the two molecules (the terminal OH or CH3 group) is a small part of the overall structure, the presence of the OH group can have a significant effect on the adsorption characteristicsthrough the influence of hydrogen bonding and polarity. Indeed earlier studies of these systems, which were largely sensitive to the adsorption energy, found little difference between alcohols and alkanes with more than 18 Nevertheless, significant variations in the geometric ordering of alcohol and alkane molecules at the graphite-liquid interface are readily observed in STM experiment^.^-^ By the study of a mixture of the two compounds, the effect of the OH group on the adsorption process can be investigated. For example, is the adsorption dominated by the long hydrocarbon chain or the presence of the OWCH3 functional end group? Since the STh4 images of alcohols and alkanes indicate that they have different packing orders on a graphite surface, this difference can be used to distinguish between the two molecules in a mixture allowing a number of issues affecting adsorption to be studied. The present STM experiments were performed in solution, and hence, there is a competition between the molecules depositing on the surface and remaining in solution. This phenomenon has been studied using fast imaging techniques which demonstrate the reorganization of domains on the The experiments presented here demonstrate that the solvent also plays a role in determining which component of a mixture will preferentially deposit on a graphite surface suggesting selective deposition may be possible with appropriate choice of solvent. These experiments also indicate that the nature of the solvent affects the amount of time the molecule stays on the surface before going back into solution.

Experimental Section Experiments were performed with a Digital Instruments Nanoscope III STM under ambient conditions. Triacontane and triacontanol were purchased from Aldrich and used without further purification. For single-component solutions, concentrations of approximately 1 mglmL were prepared in phenyloctane 0 1995 American Chemical Society

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Figure 1. (a) STM image of triacontanol in phenyloctane on graphite (top). The angle between the molecular axis and the troughs between two rows is 60”. Tunneling conditions were -1400 mV (sample negative) and 80 pA in the constant current mode. (b) STM image of triacontanol in phenyloctane on graphite showing the long range packing order (bottom). Note the change in direction of a row by -120’. Tunneling conditions were -1400 mV (sample negative) and 80 pA in the constant current mode.

(Aldrich). For two-component alcohollalkane solutions, concentrations of either 1:l or l:lO, with the alcohol at lower concentration, were prepared. A drop of the solution was deposited on a piece of freshly cleaved highly ordered pyrolytic graphite (HOPG), purchased from Advanced Ceramics Corporation. Images were obtained by immersing a 0.01 in. diameter Pt/Rh (87/13) tip, snipped with wire cutters, into solution. Typical tunneling conditions used were 1.2- 1.5 V (sample negative) and 60-120 pA with the STM operating in the constant current mode. Some images were also obtained in the constant height mode at higher tunneling currents around 150 to 200 PA. The scan rates varied depending on the size of the area being imaged. Images were obtained with different tips and samples to check for reproducibility and to ensure that the images were free from artifacts caused by the tip or sample.

Results Single Component Solutions. The STM images of the triacontanol and triacontane solutions indicate a high degree of order with the molecules arranged in rows and the molecular axis parallel to the graphite surface. The alcohol and alkane molecules cannot be differentiatedin an obvious way by looking at the image of an individual molecule, but the aggregates of the two species display different packing orders on the surface.

Figure l a shows a high resolution STM image of the triacontanol molecules. Each bright band in the image corresponds to an individual alcohol molecule. The angle between the molecular axis and the troughs between two rows is about 60”. This angle of 60” is also observed in the three-dimensional crystal structure of the alcohol. Additional structural detail is observed along the length of the molecule with a periodicity of about 0.25 nm and is attributed to the presence of the methylene groups which form the hydrocarbon backbone. Images of larger areas of the alcohol molecules show long range packing order (Figure lb). The alcohol rows form “zigzag” patterns on the graphite surface with a row changing its direction by 120°, typically every few nanometers. These “zigzag” patterns are not stable with time and tend to change directions. The “zigzag” patterns appear to be randomly positioned on the surface, possibly driven by entropy maximization in the system. These observations agree with published STM results for alcohols adsorbed on Figure 2a shows an STM image of the triacontane molecules. The alkanes differ from the alcohols in that the angle between the molecular axis and the trough between two rows is 90°, and instead of forming “zigzag” patterns on the graphite, they form straight rows (Figure 2b). These straight rows of alkanes extend over areas of at least 300 nm x 300 nm, which were

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Figure 2. (a) STM image of triacontane in phenyloctane/phenylpentanolon graphite (top). The angle between the molecular axis and the troughs between two rows is 90". Tunneling conditions were -1300 mV (sample negative) and 166 pA in the constant height mode. (b) STM image of triacontane in phenyloctane/phenylpentanol on graphite showing the long range packing order (bottom). The alkanes form straight rows over areas of at least 300 x 300 nm2. Tunneling conditions were -1235 mV (sample negative) and 166 pA in the constant height mode.

the largest areas that could be scanned while still resolving the rows. The above results are also in agreement with previously published STM results of alkanes on graphite s ~ r f a c e s ? ~As ~*~ with the images of the alcohols, structural detail is visible along the length of each alkane molecule with the same periodicity of 0.25 nm. On a few occasions, for both alcohol and alkane solutions, the STM images revealed areas with domains where molecules in adjacent domains were oriented differently. The domain boundaries were always at 60" or 120" with respect to each other which reflects the symmetry of the graphite substrate. The interfaces between two domains were hard to image since molecules along the interface were the most mobile, presumably due to weaker intermolecular interactions between molecules in adjacent domains. On observing an area of the surface which was covered with more than one domain, we find that one of the domains would grow at the expense of the others and in time would cover regions which were at least on the order of 300 nm x 300 nm (which were the largest areas scanned). This suggests that having all molecules in the same orientation (Le., one domain) with respect to each other is preferred since molecules can then optimize lateral interactions between adjacent molecules. As can be seen in Figure 1, the troughs between two alcohol rows were always well-defined, the distance between two rows

being defined by optimizing hydrogen bonding interactions between adjacent OH groups. Images where alcohol molecules traversed across a trough were not observed. Unlike the alcohols, troughs between two rows of alkanes (Figure 2) appear less well-defined with some degree of interdigitating. However, as with the alcohols, images in which alkane molecules traversed troughs were not observed once again emphasizing that lateral interactions between molecules in a row are very important in stabilizing the monolayers on the surface. That is, the molecules minimize their free energy by being parallel to each other over their entire length. Two-Component Solutions. 1:I Mixtures. The images of the 1:1 mixtures of triacontanolhriacontanesuggest that the two species from separate regions on the graphite surface. Most of -' the time regions of either alcohols or alkanes are imaged. These regions are at least 300 x 300 nm2 in area. The different regions are identified by their characteristic long and short range packing arrangements as described above. Even though the solutions contained an equal number of alcohol and alkane molecules, the alcohol regions were imaged greater than 70% of the time. On a few occasions, while imaging alkane regions, the image blurred and, after a few minutes, the images cleared to reveal alcohol molecules, suggesting that the alcohols had displaced the alkanes on the graphite surface. Since the alcohols seemed to cover a larger area of the surface, the interfaces between

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Figure 3. (a) STM image of a 1:l triacontanehiacontanol mixture in phenyloctane showing the alcohol-alkane interface (top). The alcohol region is in the top left comer, as identified by the “zigzag” pattern, and the alkane region is in the lower right comer as identified by the straight rows. Tunneling conditions were -1200 mV (sample negative) and 60 pA in the constant current mode. (b) STM image of a 1:lO triacontanoy triacontane mixture in phenyloctane (bottom). Note the presence of multiple domains (both alcohol-alcohol and alcohol-alkane domains) which are absent in the 1:1 mixtures of Figure 3a. Tunneling conditions were - 1400 mV (sample negative) and 60 pA in the constant current mode.

alcohol and alkane regions were hard to find. However, on a few occasions the interface was imaged. Figure 3a shows the interface between an alcohol and an alkane region, where the alcohol region is identified at the top left by its characteristic “zigzag” pattern and the alkane region to the lower right is identified by its straight rows. Each region was also imaged at molecular resolution (smaller scan sizes) to determine the orientation of the molecules relative to the troughs in order to further ascertain whether the regions were alcohols or alkanes. To determine the molecular orientation of the alcohols and alkanes along the interface, the scan size was reduced and the tip translated over the interface region. However, attempts to image the interface at higher resolution always resulted in a loss of resolution. When the system stabilized and the resolution was restored to normal, the images in what was formerly the interface region appeared to be exclusively those of alcohols. Along the alcohol-alkane interface, the alcohol rows were oriented at 60” or 120” relative to the alkane rows. In general mixtures were harder to image than singlecomponent solutions and produced noisier STM topographs. This is most likely due to an increase in mobility of the solute molecules in the mixtures, which have a tendency to form either pure alcohol or alkane regions on the surface. In addition, the

alcohol molecules preferentially displace the alkane molecules off the surface, which also contributes to the increased mobility of the solute molecules. For the STM to image these molecules they must reside on the surface while the tip scans over them. Molecular motion will result in poorer, noisier images. Hence, for the mixtures, the increased mobility of the solute molecules is most likely responsible for the poorer resolution compared to that of the single-component solution. 1:10 Mixtures. In order to increase the surface coverage of the alkanes on the graphite surface, the alcohol concentration in solution was lowered by a factor of 10 relative to the alkane concentration. As with the 1:l solutions, “pure” regions of alcohols or alkanes were imaged. The alkane regions were imaged more often relative to the alcohols; however, alcohol regions were observed greater than 9% of the time. On a few occasions, areas with domains on the order of 30 x 30 nm2 were observed (Figure 3b). These relatively small domains were not observed in the 1 :1 alcohoValkane mixtures. Each domain was comprised of the same type of molecules, Le., either alcohol or alkane molecules, as distinguished by the different packing order. Along the interface of two domains, rows in one domain were oriented at 60” or 120” with respect to the rows in the neighboring domain.

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Figure 4. Orientation of the alcohol molecules with respect to each other and the graphite substrate. Note how the translation of an alcohol molecule results in the formation of "zigzag" patterns while maintaining the network of hydrogen bonds and the lattice match with the graphite substrate (see text). The angle at which the rows change direction is about 120" (as denoted by the solid lines in the figure). The dotted lines connecting the hydrogens and oxygens of neighboring molecules indicate hydrogen bonds between these molecules.

Discussion Single Component Solutions. The high packing order exhibited by these molecules on the graphite surface can be attributed to a combination of molecule-molecule and moleculesubstrate interactions. Volumetric experiments, which measure the surface excess mass, and heat of adsorption measurements for these long chain alcohol and alkane molecules on graphite indicate that they have a high affinity for the graphite surface.'-3 This affinity has been attributed to the almost perfect match of the distance between alternate methylene groups along the hydrocarbon chain (0.251 nm) and the holes in the hexagonal lattice of graphite (0.246 nm).3 The STM images show that the molecules lie parallel to each other suggesting that the lateral interactions between the molecules are also an important factor in determining the stability of the system. As can be seen in the STM images of the alcohol and alkane molecules, the short and long range packing order depend strongly on the functional group at the end of these hydrocarbon molecules. X-ray data for the crystal structure of alcohols indicate that the 60" angle between the molecular axis of the molecules and the troughs between two rows is a consequence of the hydrogen bonding between neighboring alcohol molecules.'' This offset of 60" between two molecules in a row will reduce the van der Waals interactions between adjacent

partners since the molecules do not overlap each other over their entire length. However, the stabilizationgained from hydrogen bonding must exceed this loss allowing the 60" offset structure to be a lower energy configuration than that having complete overlap along the entire length of the molecule. In addition for the alcohols on a graphite surface, if the methylene groups that form the hydrocarbon backbone lie in the hollows of the graphite lattice, then the distance between the oxygen atoms in the OH groups of two alcohol molecules in adjacent rows allows the formation of strong hydrogen bonds.' This results in a network of hydrogen bonds in the troughs, as shown in Figure 4. This model was proposed to explain the adsorption of alcohols on graphite and the fact that alcohols with chain lengths less than 18 carbons have a higher affhity for graphite compared to an alkane of the same chain length.' The alkane molecules, on the other hand, lie at 90" with respect to the troughs and hence overlap neighboring molecules over their entire length. This results in optimum van der Waals interactions between molecules. For the alkanes an offset by 60" would reduce these interactions with nothing to be gained energetically since they cannot hydrogen bond to each other. In order for the alkane molecules to form "zigzag" patterns, as observed with the alcohols, the molecules must lie at 60", with respect to the troughs, at the point where the rows change

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Figure 5. Alkane molecules on graphite indicating the orientation of molecules with respect to each other and the graphite surface. For the alkane rows to form “zigzag” patterns, the alkane molecules will have to lie at 60” with respect to the troughs at the point where the row changes direction, instead of the observed 90” (see text).

direction (see Figure 5). However, as mentioned above, this would result in an unstable configuration, thereby precluding the formation of “zigzag” rows. Figure 4 shows a portion of the hydrocarbon chain, in lattice match with the underlying graphite substrate, and the OH groups at the end of the chain. If the top five molecules are considered to be a part of two rows running in the direction determined by a line joining the oxygen atoms, then there are two equivalent positions for the next alcohol molecule, one being the position that would continue the row and the other being a position obtained by translating the molecule to the position shown in the lower portion of Figure 4. Due to the symmetry of the system, the two positions should be energetically almost equivalent. The consequence of a translation like this is that the row changes orientation by about 120” and hence results in the formation of the “zigzag” patterns. In spite of this “zigzag” formation, the lattice match between the alcohol hydrocarbon chain and the graphite substrate as well as the hydrogen bonding network is maintained. Rabe and co-workers have suggested that these “zigzag” patterns are formed because the symmetry of the alcohol molecules lying on the graphite surface is the same as that of the graphite itself‘ which would also agree with the above explanation. In addition, as is shown below, for the network of hydrogen bonds to be maintained as the rows change direction,the alcohol molecules must rotate by 180”about their

molecular axes at the point at which the row direction changes (see Figure 4). Figure 4 was constructed under the following constraints. (1) The molecules in two adjacent rows are interleaved; i.e., the “heads” of two molecules in adjacent rows do not line up. This feature is evident in the high-resolution STM images of the alcohols (Figure 1). (2) The hydrogen bonded 0-He 0angle was kept as close to 180” as possible since that results in the formation of strong hydrogen bonds. (3) The molecules lie flat on the graphite with all 30 carbons in contact with the surface. To maintain optimum hydrogen bonding interactions between the molecules and satisfy the above constraints, the molecules are required to rotate by 180’ about their molecular axis right at the point where the row changes its direction as shown in Figure 4. Also, the row shifts by one lattice position (of the graphite substrate) with respect to the preceding row. As a consequence of this, the angle between the molecular axis of a molecule right at the turning point and the trough of the preceding rows is now about 70” instead of 60”. This is borne out in the STM images where this angle is measured to be between 65” and 70”. From Figure 4 the rows change orientation by about 125 to 130”, which is larger than the 120” measured from the STM images. The experimentally measured 120” angle is actually obtained by drawing a line along the troughs as the rows change orientation. However, on drawing

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;

,

. i -

Figure 6. (a) STM image of the triacontanol molecules on graphite in phenyloctane where the presence of a longer chain impurity (C40) is observed in the lower right of the image (15 nm on the abscissa and 7 ,nm on the ordinate) (top). Note that the packing order of the molecules is still maintained even around these longer chains. Tunneling conditions were -1400 mV (sample negative) and 80 pA in the constant current mode. (b) STM image of the long chain impurity (see part a) in the triacontanol solution showing that the gaps (lower left) that are formed by mixing short and long chain molecules are occupied by (C30) alcohol molecules (bottom). Tunneling conditions were - 1400 mV (sample negative) and 80 pA in the constant current mode.

a line in the STM images through the “heads” of the molecules, the angle obtained when the rows change orientation is between 125 and 130°, which agrees with that obtained from Figure 4. This model also suggests that there may be two types of hydrogen bonds formed between the OH groups, one a stronger shorter bond and the other a longer weaker bond. This has been observed for methanol adsorbed on graphite where the X-ray data suggest two types of hydrogen bonds.12 We have confirmed that the “zigzag” pattern exhibited by the alcohols is not due to defect sites on the graphite by changing the tunneling parameters and imaging the graphite substrate in the regions where rows change orientation. These images show that the graphite surface in these regions is free from defects. Also, the “zigzag” patterns are observed to change with time when scanning the same region repeatedly, suggesting that these patterns are not formed over defect sites. Since the experiments described here are all performed in solution, there is, of course, a competition for a molecule to either stay on the surface or go into solution. This dynamic process results in changes of the row orientation as the molecules are readsorbed from solution. Figure 6a shows a 30 nm x 30 nm region of alcohol molecules where the long range, “zigzag” row packing order can be seen clearly. At the lower right hand portion of the figure

(coordinates 15 nm on the abscissa and 7 nm on the ordinate), there appear to be longer molecules mixed in with the C30 alcohol chains. The observation of longer chain impurities in a sample of shorter hydrocarbon chains has been previously reported in the literature.’ If this is the correct interpretation of these images, the length of these longer molecules would suggest that they are approximately a C40 species which is probably an impurity in our C30 sample (96%). Note that the system still maintains a high degree of order, with the longer chains lining up together but packing in beside the shorter chains. In fact the gaps formed by a mixture of long and short chains are nicely filled by alcohol molecules (see Figure 6b), once again indicating the importance of lateral interactions between molecules in determining the stability of the system. The images of the alkanes and alcohols show a series of “bumps” along the molecular axes with a periodicity of about 0.25 nm. In Figures l a and 2a the number of bumps along the molecular axes equals approximately half the number of carbon atoms in the hydrocarbon chain. This structural detail has also been reported in earlier STM results on these molecule^^^^^^ The work of Thomson et al. suggests that the bumps correspond to those graphite lattice sites that are strongly perturbed by the presence of the adsorbed molecule^.^ If this interpretation is

STM Studies of Adsorption and Mobility correct, the STM images are not those of the molecules themselves but reflect the underlying substrate electronic structure perturbed by the presence of the adsorbate molecules. Suggestions have been made that the hydrocarbon chains are oriented with the plane containing all the carbon atoms parallel to the graphite surface4 and perpendicular to the graphite ~ u r f a c e . ~More , ~ recent work indicates that the plane of the carbon atoms can flip back and forth between these two orientations, and that the bumps visible in the STM images may correspond to the positions of the hydrogen atoms protruding out from the hydrocarbon molecule^.'^ The STM images in this work reveal a double row of bumps for every molecule (unlike the single row of bumps visible in Figures l a and 2a).I3 We have also obtained images of alcohols and alkanes that reveal a double row of bumps for every molecule, where one row appears to have a higher tunneling current than the adjacent row.I4 Measurements of the distances and angles between these bumps indicate that they correspond either to the positions of the hydrogen atoms protruding out from the molecules or to those graphite lattice sites perturbed by the presence of the adsorbed molecules. In addition, our results suggest yet another orientation for these molecules on the graphite surface, namely that the plane of the carbon atoms can lie at an angle of about 32” with respect to the surface. The scans shown in Figures l a and 2a simply lack sufficient resolution to image all the “bumps” along the carbon backbone; therefore, the number of bumps equals approximately half the number of carbon atoms in the molecules. Two-Component Solutions. The results obtained with the alcohohlkane mixtures indicate that the two components separate out to form “pure” alcohol and alkane regions. There do not seem to be strong attractive or repulsive interactions between the alcohol and alkane molecules since both the short and long range packing order of each species are maintained in the mixture. As mentioned above, molecular packing details along the interface were hard to obtain and attempts to image the interface at high resolution resulted in loss of resolution suggesting that the interface was unstable. It is possible that this instability is due to the asymmetry along the interface. A molecule that lies along the interface does not have molecules lying parallel to it on both sides thereby obtaining only half the stability of typical molecules which have lateral interactions on two sides. Tip-surface interactions in these unstable regions probably further disturbed the system resulting in a loss of resolution. Measurements of the surface excess mass of hydrocarbons on graphite indicate that for chain lengths less than 18 carbons an alcohol molecule has a higher surface excess mass than an alkane of the same chain length.’ The additional stabilization for the alcohols arises from the hydrogen bonding between the OH groups of neighboring molecules. However, these measurements also suggest that as the chain length increases, the contribution to the stability of the system from the match between the methylene groups on the hydrocarbon chain and the underlying graphite surface lattice should dominate over the contribution from hydrogen bonding. For chain lengths greater than 18 the surface excess mass of the alcohol was found to be the same as that of an alkane molecule of the same chain length. For chain lengths of 30 carbons, the STM experiments described here indicate a predominance of the alcohol molecules on the graphite surface over the alkane molecules. For the 1:1 solutions the alcohols were observed greater than 70% of the time and greater than 9% of the time for the 1:lO solutions. These results suggest that the alcohols have a higher affinity for the graphite surface than do the alkane molecules. This observation would seem to contradict the volumetric measure-

J. Phys. Chem., Vol. 99, No. 17, 1995 6615 ments described above. However, there is an important difference between the volumetric experiments and the experiments described here. The volumetric experiments were performed with neat samples while the STM images shown here have been obtained in solution. The preferential adsorption of the alcohols observed here may be due to the presence of the solvent. Since the STM experiments are performed in solution, there is an equilibrum between the molecules staying on the surface and those going back into solution. For adsorption of a solute A from a solution in solvent X the adsorption process can be expressed asi5

+ X(so1vent in solution, N,) t A(so1ute in solution, NA)+ X(adsorbed solvent, I?,) (1)

A(adsorbed solute, I?,)

where f l is the mole fraction of species i on the surface and Ni is the mole fraction of species i in solution. In this model a solute molecule can only be adsorbed on the surface if it displaces a solvent molecule off the surface. The equilibrium constant, K, for this process can be expressed as

where ai is the activity of species i in solution. The fractional coverage of A(&) on the surface can then be expressed as

6 .=

Kaja, - ba, -1 Kai/ax 1 ba,

+

+

(3)

where b = Hax. Equation 3 is reminiscent of the Langmuir model for the adsorption of a gaseous molecule onto a solid surface. For n noninteracting solutes competing for sites on the surface, eq 3 can be generalized to the following form

biai

e, =

(4)

1

+ cb,aj j= I

where bi = KJaX and Ki is the equilibrium constant for the ith component (solute) in solution undergoing the process in eq 1. In the STM experiments described here, the solution contains an equimolar mixture of alcohol molecules and alkane molecules. Since the images indicate that the alcohol molecules and alkane molecules form separate regions on the surface, we can assume that there is no interaction between an alkane and alcohol molecule, which is an essential requirement for the validity of eq 4. Therefore, the ratio of the surface coverage of the alcohol molecules to that of the alkane molecules can be expressed immediately (using eq 4)as

where the subscript “1” refers to the alcohol and “2” to the alkane. From eq 3,

For the 1:l alcohoYalkane mixtures, the STM images indicate that the ratio of the area of the alcohol region on the surface to that of the alkane region is 7/3. This implies that the ratio of O1/& is also 7/3. Since the solution contains an equimolar

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mixture of the alcohols and the alkanes and assuming that the activities, ai, can be replaced by their respective concentrations (dilute solution limit),

(7) Also,

K = exp(-AGlRn

(8)

where AG is the change in free energy for the process in eq 1. Therefore.

-K_, - exp(-(AG, K2

7 - AGJIRn = 3

(9)

Hence, A(AG) = AGl

- AG2 = 0.5 kcal/mol

At room temperature then, the difference in the free energy for the alcohol molecules undergoing process 1 to that for the alkane molecules undergoing process 1 is only 0.5 kcal/mol. The above analysis shows that a shift in the energetics of the adsorption process by only 0.5 kcallmol can result in a preferential deposition of the alcohol molecules over the alkane molecules by a factor of 2. This small 0.5 kcaYmol shift in the free energy could be due either to a difference between the heats of adsorption of the alcohol and alkane molecules on graphite or to a difference in the interaction energies between the alcohol or alkane molecules and the solvent molecules. This assumes that the change in entropy of the alcohol molecules undergoing the process as expressed in eq 1 is the same as that for the alkane molecules undergoing this process. The heat of adsorption for dotriacontane (C32H66) on graphite has been measured to be 15-30 kcallmol depending on the solvent from which the alkane molecules are ab~orbed.~ The experiments measuring the surface excess mass suggest that for chain lengths of 30 the heats of adsorption of the triacontanol and triacontane molecules on graphite should be the same.’ The STM experiments indicate that a difference of only 0.5 kcall mol between the heats of adsorption of the alcohol molecules on graphite and the alkane molecules would explain the preferential deposition of the alcohol molecules on the surface (assuming no entropy effects). This is certainly within the experimental error for the measured heats of adsorption for these molecules on graphite. The presence of the solvent could also shift the energetics of the adsorption process. At low concentrations, and in nonpolar solvents, alcohol molecules remain unassociated or associate to form small clusters of about two to four molecule^.^^-'^ This can be observed in the IR spectra of alcohols in nonpolar solvents where the presence of a “free” O-H stretch can be observed.I6-ls In the experiments described above, the triacontanol and triacontane solution concentrations were approximately 0.002 M. At such concentrations the triacontanol molecules are most likely unassociated in solution. This was confirmed by recording the IR spectra of the alcohoValkane solution in phenyloctane. The strongest peak in the O-H stretching region was found to be that of the unassociated or “free” O-H stretch. The unassociated state of the alcohol molecules is an energetically unfavorable configurationsince these species prefer being hydrogen-bonded to each other. The nonpolar alkane molecules, on the other hand, can interact more favorably with the nonpolar solvent and hence are expected to have a lower

energy in solution compared with that of the alcohol molecules. The difference of 0.5 kcaYmol between AG for the alcohol molecules undergoing the process in eq 1 to that of AG for the alkane molecules also undergoing this process could simply be due to the higher solvation energy of the alcohol molecules. Therefore, the driving force behind the preferential deposition of the alcohol molecules onto the surface may be due to unfavorable interactions between alcohol and solvent molecules. Phase Separation of Alcohols and Alkanes on the Surface. These results suggest the interesting possibility of using the solvent to control which component of a mixture will preferentially come out of solution and deposit onto the surface. As demonstrated here, in favorable cases very small shifts in the interaction energies between solute and solvent molecules are required to achieve preferential deposition. In the case of the alcohoYalkane mixtures, the alcohol molecules remain unassociated in solution and have an unfavorable interaction with the nonpolar solvent molecules due to the presence of the polar OH group; the alkane molecules on the other hand interact more favorably with the solvent molecules. If the alcohol molecules are unassociated in solution, then the phase separation into “pure” alcohol and alkane regions on the graphite surface must occur due to the influence of the surface. When an alcohol molecule comes out of solution onto the surface, it must find another alcohol molecule with which to form a hydrogen bond. If it happens to adsorb onto the surface in an alkane rich region, it will go back into solution, and this process will repeat itself until it has found an alcohol region. For the 1:l alcohoValkane mixtures the surface is predominantly covered with alcohols, and hence alcohoYalcoho1 association on the surface is easy. However, for the 1:lO alcohoYalkane mixtures, where the alcohol is at a lower concentration, this is not the case. Since there are more alkanes on the surface, it takes longer for an alcohol molecule to find an alcohol rich region and also longer to displace the alkanes off the surface. In these 1:lO mixtures the presence of the alkanes on the surface interferes with the formation of large domain “pure” alcohol on the surface. Hence, the formation of smaller domains is probably an intermediate step toward complete phase separation into alcohol and alkane regions on the surface. Effect of Solvent on the Adsorption Process. In order to determine whether the preferential deposition of alcohols over alkanes on the graphite surface can be affected by the nature of the solvent, STM experiments were performed using a 1:l solvent mixture of phenylpentanol and phenyloctane. Phenylpentanol was chosen as the second solvent component since the solute triacontanol molecules can form hydrogen bonds with this solvent. At the low tunneling current set points needed to image adsorbed molecules, no images were observed of any solute molecules in phenylpentanol alone. To overcome this difficulty, solutions of the solute molecules in a 1:l solvent mixture of phenylpentanol and phenyloctane were used. Typical tunneling currents that were used to image the solute molecules in phenyloctane had to be increased in order to engage the STM tip and obtain images in the solutions with the mixed solvents. In order to obtain images of the solute molecules in the mixed solvent solutions tunneling currents between 150 and 170 pA were used in the constant current mode and between 150 and 200 pA in the constant height mode. A possible explanation for the need of an increased tunneling current while imaging solutions of the mixed solvents may be that the higher polarity of phenylpentanol allows for a higher concentration of dissolved ionic impurities in solution. If the solution contains ionic impurities, application of a bias between the tip and the sample will result in an ionic current flow between the two. If the

S T M Studies.of Adsorption and Mobility

J. Phys. Chem., Vol. 99,No. 17, 1995 6617

Figure 7. STM image of 1:l triacontane/triacontanol mixture in a solvent of I: 1 phenylpentanol/phenyloctane. Note the broken rows and the lack of order compared with the images in phenyloctane solutions (e.g., Figure 6a). The high intensity areas are probably regions where the molecules are moving in or out of solution resulting in low resolution. Tunneling conditions were -1247 mV (sample negative) and 171 pA in the constant height mode.

magnitude of this ionic current exceeds that of the set point tunneling current, then it will not be possible to obtain an STM image. While measurements of the conductivity of the mixed solvents were not performed, the above speculation is an appealing explanation for the higher tunneling currents used to image the solute molecules in the mixed solvents. This would also explain why solutions with only phenylpentanol as the solvent could not be used for these experiments. STM images were difficult to obtain for solutions of 1:l tiacontane/triacontanol in 1:1 phenylpentanol/phenyloctane (Figure 7) since the molecules did not seem to form ordered structures on the graphite surface. Even when the rows of molecules could be resolved, they often disappeared after one scan. This was observed even in the constant height mode where it is possible to scan faster compared to the constant current mode (typically around 4 times faster). The lack of good images with the mixed solvents could be due to a loss in system order or due to the movement of alcohol and alkane molecules in and out of solution at a rate faster than the scan rate of the STM. To shed further light on this problem, experiments were performed with solutions of the triacontane in the mixed solvents and the triacontanol in the mixed solvents. Images of triacontane in 1:1 phenylpentanoVphenyloctane indicate that the alkanes do form ordered rows as in pure phenyloctane. However, on monitoring an area of alkanes, the orientation of the alkane rows appeared to change with time (Figure 8). This reordering of the rows was observed only on initial engagement of the STM tip and more often with the triacontane in the mixed solvents than the tricontane in phenyloctane. In the case of the triacontane in the mixed solvents, the solutions are more viscous compared with pure phenyloctane due to the higher viscosity of phenylpentanol. The increased viscosity of the solutions could slow the movement of molecules in and out of solution, which is required for the alkane rows to orient themselves along a given direction. This explanation is consistent with the observation that the initial reordering of the alkane rows was observed less frequently with solutions of triacontane in 1:4 phenylpentanoVphenyloctane, compared with solutions in 1:1 phenylpentanol/phenyloctane. In addition, the alkanes are readily imaged (without blurring) in the mixed solvent experiments as would be expected if they spend a long time on the surface (see Figure 2). Another possible explanation for the apparent slower movement of alkanes in the mixed solvents in that the phenylpentanol is

adsorbed onto the surface, and the energy required for the triacontane molecules to displace this solvent species off the surface is higher than the energy required to displace phenyloctane from the surface. However, STM experiments performed with solutions of just 1:1 phenylpentanoVphenyloctanedeposited on the graphite resulted in no discernible adsorbate surface features. This suggests either that the solvent molecules do not strongly interact with the surface or that (at room temperature) the molecules are not stationary on the surface long enough for the STM to image them. Behavior of Triacontanol in PhenylpentanoVPhenyloctane Solutions. The STM images (in both the constant current and constant height modes) of the triacontanol in phenylpentanoV phenyloctane showed far more disorder and mobility than triacontane in the mixed solvents and triacontanol in phenyloctane. The alcohol molecules form broken rows in the mixed solvents, and the orientation of these rows changes from one scan to the next. Blurred regions of high intensity are observed which are probably regions of high mobility that would result in lower resolution during imaging. To ascertain that these observations were not due to tip artifacts, tips were first checked by imaging the alcohol in phenyloctane. Only tips that resulted in images with molecular resolution for the alcohol in phenyloctane solutions were used. Samples showing blurred images also showed resolved alcohol molecules where there is some order suggesting that the tips were capable of producing images having molecular resolution. The blurred images are also not due to changes in the tip as it is scanned over an area, since these images revealed resolved molecules in different regions of the scanned area as the images changed from one scan to the next. Images of this type were obtained with different tips and different samples. The loss in resolution in the images of alcohol molecules in the mixed solvents is probably due to a change in mobility of the alcohol molecules in these solvents compared with the phenyloctane solutions. The alcohol molecules in the mixed solvents appear to spend less time on the graphite surface resulting in poor STM resolution. Model for the Difference in Behavior of Triacontanol in Phenyloctane and the Mixed Solvents. The change in the mobility of the alcohol molecules in going from a nonpolar solvent where the interaction between the solute and solvent molecules is unfavorable to a polar solvent where the interaction between solute and solvent molecules is more favorable can be understood by examining the kinetics of the process described

6618 J. Phys. Chem., Vol. 99, No. 17, 1995

Venkataraman et al.

Figure 8. STM image of triacontane in 1: 1 phenylpentanoVphenyloctane showing the change in orientation with time of the alkane rows. Part b (bottom) was taken about 15 min after part a (top). Note how the orientation of rows in part b has changed compared with that in part a. Tunneling conditions were -1372 mV (sample negative) and 166 pA in the constant current mode.

f-r

in eq 1. Fluctuations about the equilibrium position of this reaction can be expressed as

9 = -[(Ai

+ X,) + A,~ +]X i

k

$

dt where x corresponds to the fluctuation of the concentration of solute on the surface from its equilibrium value, K is the equilibrium constant, and kf the rate constant for the forward reaction. The subscript “0” corresponds to the equilibrium concentration of each species and the superscript “s” the concentration of a species adsorbed on the surface. The rate constant for the forward reaction can then be expressed as

where A is a pre-exponential factor, Eat, the activation energy for the process in eq 1, R the gas constant, and T the temperature. A detailed understanding of the kinetic behavior of the alcohol molecules in the two solvents (phenyloctane and the mixed solvent) requires a knowledge of the barrier height (if any) to adsorption. Given that this information is unknown at present, the simplest picture of the energetics for physisorption of a molecule on a surface would be to assume that there is no barrier for a molecule in solution to be adsorbed onto a surface. This

I

moleculein solution

lAH

molecule on

Figure 9. Schematic showing the energetics for physisorption of a molecule from solution onto a surface. In this model it is assumed that there is no barrier to physisorption and that a solute molecule is energetically more stable on the surface than in solution. Since there is no barrier to physisorption, the activation energy is equivalent to the change in enthalpy (AH) for the process.

is schematically shown in Figure 9 where we assume that the molecules in solution have a higher energy than those on the surface. This is a reasonable assumption since the graphite surface is completely covered with molecules suggesting that the energy of the molecules is lower on the surface than in solution. Since it is assumed there is no barrier to the process

J. Phys. Chem., Vol. 99, No. 17, 1995 6619

STM Studies of Adsorption and Mobility in reaction 1, the activation energy can be equated to the change in enthalpy (AH) for this process. In order for the rate at which a molecule goes in and out of solution to change, the activation energy, or in this simple model case AH, must change. For the alcohol molecules in the two different solutions to have different rates of desorption, the solvents must be shifting the energetics of the process (Le., changing AH) since the solute is the same in both solutions. The energetics of the reaction can be changed if the energy of interaction between an alcohol molecule and a solvent molecule is different in the two solutions, or if the heat of adsorption of an alcohol molecule in phenyloctane or in the mixed solvents is different. The interaction energy between an alcohol molecule and a solvent molecule must be different for the two solutions. As mentioned above, the alcohol molecules in phenyloctane are unassociated and therefore energetically in an unfavorable configuration. However, in the mixed solvents, where one of the species is an alcohol, the solute molecule can hydrogen bond to the solvent phenylpentanol resulting in a larger heat of solvation. Therefore, the energy of an alcohol molecule in the mixed solvent is expected to be lower than that of an alcohol molecule in phenyloctane. In a similar manner the adsorption energy for the solute on the surface can be affected by the solvent. The most probable effect is a destabilization of the solute-surface interaction by phenylpentanol compared to phenyloctane. Both of these effects would result in a lower AH (see Figure 9) for an alcohol molecule in the mixed solvents and therefore a lower activation energy for the forward reaction in eq 1. Consequently, the time spent on the surface will be shorter for an alcohol molecule in the mixed solvent compared to those in phenyloctane solution. This would explain why the images of the alcohol molecules in the mixed solvents are blurred. If the rate at which the surface adsorbed alcohol molecules go back into solution is comparable to or greater than the rate at which the STM tip scans the area being imaged, the resulting images will have poor resolution. These results once again illustrate the dramatic effect the solvent can have on the energetics of adsorption for a solute molecule onto a surface. It must be pointed out that the model used above, where there is no barrier to adsorption of a solute molecule from solution, is quite simple. In reality energy is required to displace solvent molecules from the surface before the solute molecule can be adsorbed as well as for the solute molecule to overcome the interaction energy between it and the solvent in solution before it can be adsorbed onto the surface. As a result, the process of physisorption may exhibit at least a small barrier, and such a barrier could well be sensitive to the nature of the solvent and to the strength of solvent-solute interactions. Nevertheless, the simple model used here contains the essential elements needed to understand why alcohol molecules adsorbed from phenylpentanollphenyloctane solutions spend less time on the surface than alcohol molecules in a phenyloctane solution. A more detailed model will require a knowledge of the height of the barrier to physisorption from solution for both systems, the interaction energy between the solute and solvent molecules, and the heat of adsorption of the solute molecules from both solutions onto the graphite surface.

Conclusions STM experiments have been performed which demonstrate that different components of a mixture adsorbed on a surface can be identified by the differences in long and short range packing order of each component on the surface. STM images of equimolar triacontanehiacontanol mixtures in phenyloctane

indicate “pure” alcohol or alkane regions form readily on the graphite surface, with the alcohols preferentially (2: 1) depositing on the surface. A shift in the free energy of adsorption of only 0.5 kcdmol for an alcohol molecule compared to that of an alkane molecule can explain the preferential deposition of the alcohol molecules on the graphite surface. The poor solvent interaction between a polar alcohol solute molecule and a nonpolar phenyloctane solvent molecule is the most reasonable explanation for the alcohol molecule being less stable in solution than a nonpolar alkane molecule. These results illustrate the possibility of controlling which component of a mixture will preferentially deposit on a surface by changing the solventsolute interactions. The effect of the nature of the solvent on these adsorption processes was further investigated using a mixture of phenylpentanollphenyloctane as the solvent. The solute molecules exhibit very different behavior in mixed solvent compared to that in pure phenyloctane. Solutions of the alkane in the mixed solvents appear to form ordered rows on the surface, whereas the alcohols appear to be less ordered with “broken” rows. The loss of order for the alcohol molecules on the surface has been attributed to an increase in their mobility in the mixed solvent, presumably due to a change in solvent-solute interactions. The alcohol molecules interact more favorably with the polar phenylpentanol solvent molecules than with the phenyloctane solvent molecules. This results in a lowering of the energy of an alcohol molecule in the mixed solvent compared with an alcohol molecule in phenylpentanol.

Acknowledgment. The authors gratefully acknowledge discussions with Professors Bruce Beme and Brian Bent. This work is supported by The Joint Services Electronics Program (US.Army, Navy, and Air Force DAAL03-91-C-0016), the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the National Institute of Health (1 R03 RR06987-01A1). Equipment support was provided by the National Science Foundation (CHE-91-18782). References and Notes (1) Findenegg, G. H. J . Chem. Soc., Faraday Trans. 1973, 69, 1069. (2) Findenegg, G. H. J . Chem. Soc., Faraday Trans. 1972, 68, 1799. (3) Groszek, A. J. Proc. R . Soc. London 1970, A314, 473. (4) McGonigal, G. C.; Bemhardt, R. H.; Thomson, D. J. Appl. Phys. Lett. 1990, 57, 28. (5) McGonigal, G. C.; Bemhardt, R. H.; Yeo, Y. H.; Thomson, D. J. 3. Vac. Sei. Technol. 1991, B9, 1107. (6) Yeo, Y. H.; McGonigal, G. C.; Thomson, D. J. Langmuir 1993, 9, 649. (7) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (8) Buchholz, S . ; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1992, 31, 189. (9) Yeo, Y. H.; Yachoboski, K.; McGonigal, G. C.; Thomson, D. J. J . Vac. Sei. Technol. 1992, AIO, 600. (10) Yachoboski, K.; Yeo, Y. H.; McGonigal, G. C.; Thomson, D. J. Ultramicroscopy 1992, 42-44, 963. (11) Abrahamsson, S.; Larson, G.; Von Sydow, E. Acta Crystallogr. 1960, 13, 770. (12) Morishige, K.; Kawamura, K.; Kose, A. J . Chem. Phys. 1990, 93, 5267. (13) Liang, W.; Whangbo, M.; Wawkuschewski, A,; Cantow, H. Adv. Mater. 1993, 5, 817. (14) Venkataraman, B.; Flynn, G. W. Submitted for publication. (15) Adamson, A. Physical Chemistry of Surfaces, 4th ed.; John Wiley and Sons: New York, 1982; pp 369-373. (16) Van Ness, H. C.; Van Winkle, J.; Richtol, H. H.; Hollinger, H. B. 3.Phys. Chem. 1967, 71, 1483. (17) Fletcher, A. N.; Heller, C. A. J . Phys. Chem. 1967, 71, 3742. (18) Fletcher, A. N. J . Phys. Chem. 1969, 73, 2217. (19) Tucker, E. E.; Christian, S. D. J . Phys. Chem. 1977, 81, 1295.

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