Article pubs.acs.org/Langmuir
Competitive Adsorption on Graphite Investigated Using FrequencyModulation Atomic Force Microscopy: Interfacial Liquid Structure Controlled by the Competition of Adsorbed Species Takumi Hiasa* and Hiroshi Onishi* Department of Chemistry, Graduate School of Science, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan S Supporting Information *
ABSTRACT: The competitive adsorption of long-chain (C18 and C24) carboxylic acids versus n-decanol on graphite was investigated using frequency-modulation atomic force microscopy. A long-range-ordered monolayer of the solute (stearic acid or lignoceric acid) developed in saturated decanol solution, whereas an ordered decanol monolayer was deposited from dilute solutions. The piconewton-order tip−surface force was observed in solutions as a function of the vertical and lateral coordinates, together with the topography of the monolayers. The tip−surface force was periodically modulated, which was interpreted with a solution structure commensurate with the ordered assembly of adsorbed monolayers. These results show that the solution structure at the interface was controlled by the competitively adsorbed species and thus was sensitive to the composition of the bulk solution.
1. INTRODUCTION The adsorption of long-chain hydrocarbons and their derivatives onto graphite has been investigated as a simple adsorption from liquid. A molecular monolayer of these compounds is easily deposited from solution.1 When the liquid consists of a solute in a solvent, the two compounds are competitively adsorbed on the surface. The assembly of adsorbed solute can be modified by the solvent as summarized in recent papers.2,3 It is thought that the attractive interaction strength of each adsorbate to the substrate controls the fractional coverage of adsorbates. The attractive or repulsive interaction of one adsorbate to the other adsorbate determines the adsorbate assembly, such as domain separation. The liquid that is in contact with the adsorbates may play an additional role. It is thus necessary to determine the structure of liquids at interfaces under competitive adsorption. Here, we studied n-decanol solutions of stearic acid (C17H35COOH) or lignoceric acid (C23H47COOH). Longchain carboxylic acids are known to form ordered monolayers on graphite.3−10 Pure aliphatic alcohols, however, produce adsorbed monolayers with lamellae on this substrate.11−13 It is expected that the solute and solvent are competitively adsorbed. The structure of solutions in real space and the topography of adsorbed monolayers were observed using advanced frequency-modulation atomic force microscopy (FMAFM). Recent technical developments in FM-AFM14 have enabled piconewton-order force sensitivity in liquid environments. The cross-sectional15,16 and volume17 distributions of the tip−surface force have been successfully probed and related to the structure of the interfacial liquids at a number of liquid− solid interfaces, including pure decanol on graphite.13 © 2013 American Chemical Society
2. EXPERIMENT 2.1. Imaging Solutions. Saturated solutions of stearic acid (Nacalai Tesque, >99%) and lignoceric acid (Tokyo Chemical Industry, >96%) were prepared with n-decanol (Wako, >97%) as received. The solute was added to a known quantity of solvent, heated until dissolved, and then cooled at 20 °C. When precipitates in the cooled solution were identified by sight, solvent was added to the solution. The cycle was repeated until the precipitates disappeared. The saturated concentration thereby obtained was 200 mM with stearic acid and 9 mM with lignoceric acid. The saturated solutions were diluted 20-fold to give 10 mM stearic acid and 0.5 mM lignoceric acid. 2.2. Microscope. A commercial microscope (Shimadzu, SPM9600) was modified with a low-noise optical deflection sensor. Bandpass filters were inserted in the feedback loop for cantilever oscillation amplitude regulation to stabilize resonant oscillation in the viscous liquids. A highly oriented pyrolytic graphite wafer (NT-MDT, ZYA Quality, 10 × 10 × 1.2 mm3) was fixed on an open cell, cleaved using scotch tape, and covered with a droplet of the saturated or dilute solution. Silicon cantilevers backside coated with aluminum (Nanosensors, NCH-R) were resonantly oscillated in the solutions at frequencies of 130−150 kHz with a quality factor of around 5. The absolute deflection of the cantilever was calibrated using the theoretical amplitude of thermally induced Brownian motion. Imaging scans were conducted at room temperature (20−25 °C).
3. RESULTS AND DISCUSSION 3.1. Topography in Saturated Solutions. The topography was traced with the regulation of the tip−surface distance to keep the shift of the resonant oscillation frequency Received: February 14, 2013 Revised: April 14, 2013 Published: April 15, 2013 5801
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Figure 2. Lignoceric acid monolayer on graphite deposited from the saturated decanol solution. Large-area and narrow-area topographies are shown in a and b. Cantilever oscillation amplitude: 0.1 nm. Δf set points: (a) +600 and (b) +700 Hz.
mismatch of the lignoceric acid structure with the substrate lattice. The mismatch is negligible with one ethylene unit but significant when accumulated along long chains.18 The probability of finding a domain boundary in the adsorbate monolayer is accordingly enhanced. This is what was observed in Figure 2a. The molar ratio of the solute over the solvent was quite small, even in the saturated solutions. Nevertheless, a monolayer of solute was exclusively present on the surface. This is driven by the larger adsorption energy of the C18 and C24 compounds compared to that of the C10 solvent. The heat of adsorption of C32H66, 120 kJ/mol, is almost twice of that of the 64 kJ/mol of C7H16.11 3.2. Topography in Dilute Solutions. Decanol solvent was exclusively adsorbed in the dilute solutions, in contrast to solute adsorption in the saturated solutions. Figure 3a,b present typical topography obtained in the dilute solutions of stearic acid and lignoceric acid, respectively. The two topographic images were composed of parallel stripes with one bright line in each stripe. The width of the stripes was 2.5 nm regardless of the solutes, although they were obtained in the presence of the different solutes. The identical topographies show that the decanol solvent, instead of the carboxylic acid solute, was adsorbed on the substrate. Although carboxylic acid molecules adsorbed with the molecular axis orthogonal to the lamellar axis, decanol formed a herringbone structure (Figure 3c). In the solutions examined here, the decanol monolayers frequently showed a topography of parallel stripes with one bright line in each stripe, as shown in Figure 3. A less frequently observed topography, which was found in repeated scans, is presented in the Supporting Information. In the minor
Figure 1. Stearic acid monolayer on graphite deposited from the saturated decanol solution. The large-area and narrow-area topographies are shown in a and b. Cantilever oscillation amplitude: 0.1 nm. Δf set point: +700 Hz. The assembled structure of stearic acid proposed in ref 7 is illustrated in c.
(Δf) constant. Figure 1a presents large-area topography observed in the saturated solution of stearic acid. The graphite surface was covered with stripe features. The width of the stripes was 2.3−2.4 nm, consistent with the length of all-trans stearic acid. One ethylene (−CH2−CH2−) unit in alkyl chains occupies a length of 0.25 nm when adsorbed on graphite, according to an early STM study.7 Narrow-area topography is shown in Figure 1b. Individual molecules were arrayed with their molecular axis perpendicular to the stripe axis. The observed stripe structure is consistent with previous STM studies4,5,8 in which a stearic acid monolayer was deposited on graphite from solutions with nonpolar solvents. The assembled structure suggested in the previous study7 is illustrated in Figure 1c. The bright edges of the stripes may be assigned to dimerized COOH groups or the methyl end of the molecule. Adsorbed lignoceric acid presented similar stripes in the saturated solution. The distance between adjacent stripes was 4.1−4.3 nm, as seen in the large-area topography of Figure 2a. Lignoceric acid contains three more ethylene units than stearic acid. The stripe width of all-trans lignoceric acid is expected to be 3.1−3.2 nm on the basis of the suggested formula.6 The larger than anticipated distance suggests some gap between adjacent stripes. The gap, if any, should result from the 5802
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lignoceric acid, as shown in Figure 2. These results suggest that the concentration relative to saturation is important in determining the composition of the adsorbate. The absolute concentration does not matter. This is reasonable because the free-energy gain induced by adsorption onto graphite should have a positive relationship with the free-energy gain by precipitation. The exclusive adsorption of one compound over the other was observed in the current study. A similar phenomenon is known in a three-component solution.19 When graphitized carbon was exposed to decanol and docosane (C20H42) dissolved in n-heptane, a decanol monolayer was deposited with a decanol/docosane molar ratio greater than 10. The docosane monolayer was developed with a ratio of less than 10. Note here that the selectivity also depends on the adsorption ability of solute molecules. The molecular layer of a carboxylic acid derivative, 15-hydoxypentadecanoic acid (HOC15H30COOH), was not developed even from the saturated decanol solution.20 This may be because the additional OH group and/or the short alkyl chains of 15hydoxypentadecanoic acid reduced the adsorption ability compared to that of stearic acid and lignoceric acid. 3.3. Solution Structure at Interfaces. The cross-sectional distribution of the tip−surface force was observed on planes perpendicular to the axis of the topographic stripes created by the organic adsorbates. The oscillating cantilever was vertically scanned from the bulk solution to the surface, and Δf was recorded as a function of the vertical coordinate. When the vertical scans at different lateral positions were repeated, a slice of the Δf distribution was constructed. The slice represents the cross-sectional distribution of the tip−surface force. The repulsive tip−surface force induces a positive shift, though the absolute shift is not proportional to the force strength. A quantitative relationship between Δf and the tip−surface force is given by Sader and Jarvis.21 3.3.1. Dilute Solutions. Figure 4a,b presents Δf distribution slices observed in the dilute solutions. They were made on planes perpendicular to the axis of the topographic stripes of the monolayer. When the tip made near contact with the adsorbed monolayer, the two solid objects repelled each other. The brightest thin region present at the bottom of each slice is ascribed to the repulsive force in the near-contact regime. The envelope of the brightest region hence represents the topography of the monolayer. The envelope was periodically corrugated along the lateral coordinate as expected. The peakto-peak distance of the lateral corrugation was 1.2−1.3 nm, as shown in Figure 4a,b. This length corresponds to the half width of the stripe observed in the topography of Figure 3a,b. The stripe was interpreted with the herringbone structure of adsorbed decanol, as illustrated in Figure 3c. The half-width corrugation, which appeared in Figure 4, is ascribed to each of the decanol columns paired in the herringbone structure. Dark and bright layers appeared alternately from the bottom to the top in the solution over the adsorbed monolayer. The oscillating brightness along the vertical coordinate indicates the oscillating strength of the force, which has been interpreted as an oscillating liquid density.22,23 A quantitative relationship between the tip−surface force and liquid density has not yet been established, however. The vertical spacing from one dark (or bright) layer to the next dark (or bright) layer was 0.6 nm. This length is identical to the layer spacing observed in ndodecane liquid or n-hexadecane liquid on a CH3-terminated thiolate monolayer,24 suggesting that the aliphatic solvent and
Figure 3. Decanol monolayer on graphite deposited from the dilute solutions containing (a) 10 mM stearic acid and (b) 0.5 mM lignoceric acid. Cantilever oscillation amplitude: 0.1 nm. Δf set point: (a) +1000 and (b) +800 Hz. The assembled structure of decanol is illustrated in c according to ref 25.
topography, both sides of the stripes are terminated with bright lines. In our previous study of pure decanol,13 parallel stripes terminated by two bright lines appeared on graphite as the major appearance of FM-AFM topography. However, stripes with only one bright line, similar to those shown in Figure 3, made only a minor appearance. Both these topographies, stripes with one bright line inside or stripes terminated by two bright lines, were observed in the dilute solutions and in pure decanol. This supports our assignment of the adsorbate shown in Figure 3 to the decanol monolayer. The inverted frequency of appearance, from major in the solutions to minor in pure decanol, can be ascribed to tip changes. A detailed description of the minor appearance is in the Supporting Information. Stearic acid was reported to be adsorbed on graphite from a phenyloctane solution with a concentration of 1.7 mM.8 The same compound was not adsorbed from the 10 mM decanol solution. These results do not conflict. Phenyloctane is not adsorbed on graphite in pure phenyloctane liquid, whereas decanol is adsorbed to form a monolayer. Stearic acid in phenyloctane does not compete with the solvent in adsorption. A decanol monolayer was formed in the presence of stearic acid at a concentration of 10 mM, as shown in Figure 3a. However, decanol was not adsorbed in the presence of 9 mM 5803
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or lignoceric acid. The absence of solute contributions is reasonable because of the low molar ratio of the solutes. 3.3.2. Saturated Solutions. Δf slices were then observed in saturated solutions. Figure 4c,d shows the results. The brightest regions at the bottom represents the topography of the adsorbed stearic acid and lignoceric acid monolayers. The brightest regions were periodically corrugated with repetition lengths consistent with the topographic corrugations of the corresponding monolayers recognized in Figures 1 and 2. Dark and bright layers appeared over the monolayers with a layer spacing of 0.6 nm. Layers in the solution oscillated laterally with different repetition lengths, 2.5 nm in the stearic acid solution and 4.3 nm in the lignoceric acid solution, as evidenced by the Δf−distance curves obtained along the first dark layer. These repetition lengths are nearly identical to the stripe widths, 2.3− 2.4 nm with stearic acid and 4.1−4.3 nm with lignoceric acid. These results clearly show that the solution structure at the interface was controlled by the solute concentration via the competition of adsorbed species. Hydrogen bonding and van der Waals interactions are probably responsible for projecting the structure of the adsorbed species into the solution, as suggested in our study of the decanol−graphite interface.13 When liquid decanol is on top of the adsorbed solute, the OH groups of the former can form a hydrogen bond with the COOH group of the latter. The van der Waals interaction of the stacked alkyl chains provides an additional energy gain. By packing the aliphatic molecules with their molecular axes parallel to one another, the excluded volume per molecule is minimized. The other molecules possess the freedom to occupy the released volume, and the number of ways that they can be distributed in the available volume increases. An entropic gain of free energy is hence additionally expected. A similar entropic gain has been recognized in the statistical mechanics of liquids, an essential role of which was recently proposed in protein folding.26 It is difficult at this stage to estimate the composition of the structured solutions. A natural assumption is that the solution at the interface contains the solute and solvent as in the bulk solution. The decanol solvent should have been structured as seen in Figure 4c,d because the molar fraction of the solutes was small, even in the saturated solutions. However, the concentration of the solutes may be enhanced in the interfacial region of the solution because the local number density of the solutes is high in the adsorbed solute monolayer on the substrate. When this is the case, a finite contribution by the solute can be expected in structuring the solution at the interface.
Figure 4. Cross-sectional Δf distributions observed in (a) 10 mM stearic acid solution, (b) 0.5 mM lignoceric acid solution, (c) saturated stearic acid solution, and (d) saturated lignoceric acid solution. Large (small) positive Δf is shown as bright (dark) stripes. The tip was scanned in a cross-sectional plane perpendicular to the axis of the topographic stripes on the surface. The lateral Δf−distance curves obtained along the first dark layer are shown below each distribution. The vertical coordinate for which the lateral curve was made is marked by an arrow. Cantilever oscillation amplitude: 0.1 nm.
solute lie with their molecular axes parallel to the graphite surface. The brightness of each layer oscillated along the lateral coordinate. To determine the repetition length of the lateral oscillation, a Δf−distance curve was obtained along the first dark layer and is shown below each slice. The vertical coordinate on which the lateral curve was made is marked by the arrow on the left side of the slice. The Δf−distance curves of the two dilute solutions exhibited a common repetition length of 1.2−1.3 nm, which corresponds to the half-width of the topographic stripe in the monolayer. Early STM studies10,11,25 showed that aliphatic alcohol molecules are paired via hydrogen bonding and that the paired molecules create columns that are epitaxial to the graphite lattice, as illustrated in Figure 3c. Because the solution was structured on the paired-decanol columns, it is natural that the solution density was periodically modulated with the half column width. Liquid decanol preferred to be on top of an adsorbed decanol rather than in interstitial positions. When liquid decanol is present on top of physisorbed decanol, the OH groups of the two molecules can form a hydrogen bond. An additional energy gain by the van der Waals interaction is expected from the stacked alkyl chains. The Δf features described in the preceding paragraphs were identical to those observed in pure decanol on graphite.13 There was no sign of a contribution by the solute, stearic acid,
4. CONCLUSIONS The adsorption of stearic acid or lignoceric acid dissolved in decanol was studied on graphite to show the deposition of long-range-ordered monolayers of solute from saturated solutions. However, monolayers composed exclusively of the decanol solvent were found for the dilute solutions. The density distribution of the solutions was oscillated periodically with a repetition length of 0.6 nm along the vertical distance from the solid surface, showing the parallel orientation of the aliphatic solvent and solute molecules. The first-layer density of the solutions was further modulated along the lateral coordinate by the adsorbate−solution interaction. The repetition length of the lateral structuring was sensitive to the composition of the adsorbed monolayer. It was thus demonstrated using FM-AFM that the solution structure was controlled at the interface by the 5804
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solute concentration via the competitive adsorption of materials from the solution. The two limiting regimes of competitive adsorption were examined in the dilute and saturated solutions. Exploring an intermediate regime, where the two competing types of monolayer are thermodynamically nearly equally stable, is reserved as an interesting issue for future studies.
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ASSOCIATED CONTENT
* Supporting Information S
The minor topographic appearance of the decanol monolayer. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (T.H.);
[email protected] (H.O.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The microscope used in this study was developed by the Advanced Measurement and Analysis Project of the Japan Science Technology Agency in collaboration with Kenjiro Kimura, Masahiro Ohta, Kazuyuki Watanabe, Ryohei Kokawa, Noriaki Oyabu, Kei Kobayashi, and Hirofumi Yamada. This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (477) “Molecular Science for Supra Functional Systems.” T.H. was supported by a Japan Society for the Promotion of Science fellowship.
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REFERENCES
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