Self-Assembled Monolayers of Cholesterol and Cholesteryl Esters on

Article pubs.acs.org/Langmuir

Self-Assembled Monolayers of Cholesterol and Cholesteryl Esters on Graphite Masahiro Hibino*,† and Hiroshi Tsuchiya‡,§ †

Department of Applied Science, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran 050-8585, Japan Department of Applied Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan

‡

ABSTRACT: The molecular arrangements of self-assembled monolayers (SAMs) of cholesterol, cholesteryl laurate, and cholesteryl stearate adsorbed on a graphite surface were studied using scanning tunneling microscopy (STM) at the liquid− solid interface. The STM images of the SAMs showed twodimensional periodic arrays of bright regions that corresponded to the sterol rings. However, individual sterol rings could not be observed in the bright regions in the STM images of the cholesterol monolayers. Nevertheless, by comparing the STM images and the crystallographic data, it is concluded that the cholesterol molecules are arranged in pairs oriented headto-head owing to the hydrogen bonds between the hydroxyl groups. These dimers, in turn, are oriented parallel to each other, owing to the interactions between the sterol rings. The STM images of cholesteryl ester monolayers had molecular resolution and showed pairs of cholesteryl ester molecules oriented in an antiparallel manner, with their fatty acid chains located in the central regions. Furthermore, the fatty acid chains of cholesteryl stearate were observed to be oriented in the (1120̅ ) zigzag direction of the graphite lattice, whereas those of cholesteryl laurate were oriented in the (101̅0) armchair direction. These observations reveal that the interactions between the fatty acid chains affect the structure of the SAMs. The molecular arrangements also depend on the lengths of the fatty acid chains of the cholesterol esters and hence on the interactions between the alkyl chains and the graphite surface. The self-assembly at the liquid−solid interface is therefore controlled by the interactions between sterol rings, between alkyl chains, and between alkyl chains and the substrate.

1. INTRODUCTION Cholesterol is an essential component of the cell membranes found in higher eukaryotes and accounts for as much as 25 wt % of the lipid fraction in the human erythrocyte membrane.1 The biologically important aggregates of cholesterol are known to be the low- and high-density lipoprotein cholesterols, which have been extensively studied.2 When the cholesterol level is abnormally high, single crystals tend to be deposited as cholesterol gallstones in bile. Previous studies of the crystal structures have revealed that cholesterol and its monohydrate crystallize in the rare triclinic space group P1, which has the lowest possible symmetry.3−5 Furthermore, the unit cell of each structure contains eight cholesterol molecules. Considering that these molecules are not related by crystallographic symmetry operations, it is important to understand the structural principles that govern their arrangements. Additionally, in fatty acid cholesteryl esters, which form ester bonds between the carboxylate groups of fatty acids and the hydroxyl groups of cholesterols, the factors that predominantly determine the crystal structures include changes in the relative importance of the cholesterol−cholesterol, cholesterol−alkyl chain, and alkyl chain−alkyl chain interactions. Two-dimensional (2D) crystals are excellent models for understanding the major factors that make one possible crystal structure preferred over other structures. In particular, the self© XXXX American Chemical Society

assembled monolayers (SAMs) adsorbed on the substrates at the liquid−solid interface are formed by combinations of the molecule−molecule and substrate−molecule interactions. Scanning tunneling microscopy (STM) has been used to characterize the structures of SAMs on substrates such as graphite.6−22 Although the structures of the physisorbed SAMs are affected by the substrate−molecule interactions at the liquid−solid interface, this model system is still considered to be dominantly affected by the inherent complexities of three-dimensional (3D) crystallization.23−26 In the present study, we used STM to investigate the molecular arrangements of SAMs of cholesterol, cholesteryl laurate, and cholesteryl stearate adsorbed on graphite at the liquid−solid interface. The STM images of the SAMs showed 2D arrays of bright regions corresponding to the sterol rings with a well-defined 2D lattice. On the basis of a comparison of these results with X-ray diffraction analyses of the 3D crystal structures, we report the effects of the steroid rings, fatty acid chains, and graphite surfaces on the 2D monolayers. Thus, this work contributes to a better understanding of the mechanisms of the self-assembly process. Received: March 11, 2014 Revised: May 12, 2014

A

dx.doi.org/10.1021/la500944t | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

2. EXPERIMENTAL METHOD The cholesterol (C27H46O) and phenyloctane used for this study were sourced from Sigma-Aldrich. Co., LLC (Milwaukee, WI). The cholesteryl laurate (C39H68O2) and cholesteryl stearate (C45H80O2) were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). All the chemicals were used without further purification. The solutions of the cholesterol and cholesteryl esters were prepared by dissolving them in phenyloctane using near-saturation concentrations. The monolayers were prepared by applying the solution to the basal plane of freshly cleaved, highly oriented pyrolytic graphite (HOPG) (ZYB grade, Advanced Ceramics Co., Cleveland, OH). Mechanically sharpened Pt−Ir (80/20) wires were used as STM tips. To image a monolayer, the STM tip was immersed in the solution and the liquid− solid interface was scanned using a Nanoscope II microscope (Digital Instruments, Santa Barbara, CA) equipped with a 14.6 μm × 14.6 μm scan head. The STM images were obtained in the constant-current mode under ambient conditions. The bias voltage was set with respect to the tip. The typical operating conditions were as follows: 0.27−0.80 nA current, 1.3−1.8 V bias voltage, and 19.5 Hz scan rate. Different tips and samples were used to ensure that the images were reproducible and free from artifacts. The dimensions of the images were measured using the STM images of graphite obtained in situ at a low bias voltage. All the procedures were executed at room temperature (24 ± 3 °C). Each of the STM images presented in this paper was flattened by second-order least-squares fitting of the selected segment and subsequent subtraction of the flattened segment from the scan line using the Nanoscope III software. This was done to remove the vertical offsets between each scan line and their tilt and bow. The unit cell parameters were determined by examining 30 images. The operating conditions (tunneling current Iset and sample bias voltage Vbias) are given in the captions of the respective figures.

3. RESULTS AND DISCUSSION 3.1. STM Images of Cholesterol. Figure 1 shows typical STM images of the SAMs formed by phenyloctane solutions containing cholesterol on the graphite surface. A 2D array of bright regions is clearly visible. The monolayer arrangements exhibited the same 6-fold symmetry as the substrate surface. Figure 1b is a high-magnification image of part of the region shown in Figure 1a. The 2D array of the bright elliptical regions reveals a well-defined 2D lattice with a unit cell defined by two vectors, namely, a1 = 2.44 ± 0.08 nm and a2 = 3.26 ± 0.15 nm, inclined at an angle φ = 55 ± 4° (Table 1). The area of the unit cell is 6.5 ± 0.4 nm2, which indicates that the unit cell is larger than a cholesterol molecule. The crystallographic data for the anhydrous cholesterol show that eight cholesterol molecules were packed into a triclinic unit cell with a = 1.4172 nm, b = 3.4209 nm, and c = 1.0481 nm and corresponding angles α = 94.64°, β = 90.67°, and γ = 96.32°.3 Based on the crystallographic data, an individual cholesterol molecule was estimated to be ∼1.7 nm long, ∼0.7 nm wide, and ∼0.5 nm thick. The product of the length and width of the cholesterol molecules varied between 1.0 and 1.2 nm2.3−5 The unit cell in Figure 1b was therefore considered to contain six molecules, which implies an area per molecule of 1.1 nm2. These values suggest that the cholesterol molecules were oriented with their length and width axes parallel to the surfaces of the graphite. The interpretational model in Figure 2 shows the packing of the cholesterol molecules in the monolayer. The cholesterol molecules are identified by skeletal formulas that were placed without optimization of the molecular structures using simulations. A unit cell in Figure 2 contains six cholesterol molecules. The flat-lying cholesterol molecules are arranged in pairs oriented head-to-head owing to the hydrogen bonds

Figure 1. STM images of a cholesterol monolayer adsorbed on HOPG. (a) Large-scale STM image. The image area is 75 nm × 75 nm, and the imaging conditions were Iset = 0.80 nA and Vbias = −1.0 V. (b) High-resolution STM image of part of image (a). The white rhombus identifies a unit cell occupied by six molecules. The image area is 15 nm × 15 nm, and the imaging conditions were Iset = 0.80 nA and Vbias = −1.3 V.

Table 1. Unit Cell Dimensions of Observed Cholesterol and Cholesteryl Ester Monolayers on HOPG substance

a1 (nm)

a2 (nm)

φ (deg)

cholesterol cholesteryl laurate cholesteryl stearate

2.44 ± 0.08 1.77 ± 0.07 2.58 ± 0.11

3.26 ± 0.15 2.35 ± 0.14 1.91 ± 0.07

55 ± 4 69 ± 4 78 ± 4

between the hydroxyl groups.3−5 The three dimers are also oriented in parallel because of the interactions between the sterol rings. It should be noted that the interactions between the hydrocarbon chains affect the arrangement of the cholesterol molecules on the HOPG. Thus, in the model, all the molecules are oriented head-to-head and tail-to-tail. Figure 3 shows the molecular model overlaid on the observed STM image of the monolayer. The bright elliptical regions and their neighboring dark regions in the STM images correspond to the six sterol rings and the hydroxyl groups, respectively. However, the accuracy of this arrangement of the six cholesterol molecules in the unit cell is not verified because individual B

dx.doi.org/10.1021/la500944t | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

number of molecules in a unit cell.9 Previous STM studies of cholesterol monolayers on an Au(111) surface have revealed that their unit cells also contains six molecules, but the molecular arrangement of the unit cell on Au is different from that on graphite.27 Although the difference between the arrangements on graphite and on Au is caused by the different interactions between the substrate and the molecules, all the molecules are considered to be oriented head-to-head and tailto-tail on both the substrate surfaces, since the interactions between the steroid rings, hydroxyl groups, and hydrocarbon chains are similar or the same. 3.2. STM Images of Cholesteryl Esters. Figure 4a shows a typical STM image of the cholesteryl laurate monolayer on HOPG. The bright regions are ordered periodically. Figure 4b is a high-magnification image of part of the region shown in Figure 4a. The bright regions in Figure 4a contain pairs of the asymmetrical bright regions in Figure 4b. Figure 4b contains roughly three structures: (1) bright regions corresponding to areas with higher local electrical conductance and/or

Figure 2. Schematic of the arrangement of a monolayer of cholesterol molecules adsorbed on HOPG. The cholesterol molecules are represented by the skeletal formulas. The molecules are arranged in pairs of black and red molecules oriented head-to-head. The rhombus in the model identifies a unit cell occupied by six molecules. The arrows in the right-bottom corner indicate the (112̅0) and (101̅0) directions of the graphite lattice.

Figure 3. STM image of a cholesterol monolayer (magnified view of Figure 1b) with an overlaid molecular model (from Figure 2). The image area is 9 nm × 9 nm.

sterol rings could not be observed by STM. The symmetry of the 2D array in the cholesterol monolayer requires six cholesterol molecules, according to the comparison between the STM images and the crystallographic data. Moreover, the large dark regions next to the bright elliptical regions in the STM images suggest that at least one of six molecules in the unit cell is tilted on the graphite surface. These observations cannot be satisfactorily explained by our model at the present stage. The interpretational model in Figure 2 is different from the previous model of cholesterol monolayers in terms of the

Figure 4. STM images of cholesteryl laurate monolayers adsorbed on HOPG. (a) Large-scale STM image with an area of 35 nm × 35 nm. (b) High-resolution STM image of part of image (a). The two white rectangles in (b) identify the approximate areas occupied by single molecules. The white rhombus in (b) identifies a unit cell occupied by two molecules. The image area is 15 nm × 15 nm. The imaging conditions for both (a) and (b) were Iset = 0.29 nA and Vbias = 1.7 V. C

dx.doi.org/10.1021/la500944t | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

Figure 5. Schematic of the arrangement of cholesteryl laurate monolayers adsorbed on HOPG. The molecules are represented by their skeletal formulas. The rhombus in the model identifies a unit cell occupied by two molecules. The arrows indicate the (101̅0) direction of the graphite lattice.

Figure 6a shows a typical STM image of the cholesteryl stearate monolayer on HOPG. The bright ellipsoidal regions are ordered periodically. Figure 6b is a high-magnification image of part of the region shown in Figure 6a. The white

topological effects in the vicinity of the sterol rings, (2) relatively dim thin bands corresponding to fatty acid chains extending linearly from the sterol rings, and (3) dark regions between the bright regions and the dim bands that lie between two molecules oriented in the same direction. Thus, the molecules appear as a combination of bright regions and thin bands in the STM images. Each white rectangle in Figure 4b approximately shows the area occupied by a single cholesteryl laurate molecule. The molecules in the two rectangles are arranged in an antiparallel orientation along the (101̅0) armchair direction of the graphite lattice and contain two antiparallel alkyl chains at their centers. The conformations of sterol rings and alkyl chains in both molecules are fully extended and seem to be straight. However, in this paper, the cholesteryl ester molecules in the model are represented by simple skeletal formulas, since the detailed molecular form, with or without a twist at the ester linkage, is not directly related to the number of molecules in the unit cell for this simple interpretational model. A unit cell of the cholesteryl laurate monolayer is shown in Figure 5. From the STM images, it is apparent that each unit cell contains two molecules and is defined by two vectors, namely, a1 = 1.77 ± 0.07 nm and a2 = 2.35 ± 0.14 nm, inclined at an angle φ = 69 ± 4° (Table 1). Figure 5 also shows that the black molecules are antiparallel to the red ones and that the unit cell contains a black and a red molecule. The interactions between the fatty acid chains of the black and red molecules in the unit cells affect the structure of the monolayer. However, the interactions in the monolayers in Figure 5 are different from those in the crystal structure of cholesteryl laurate.5,28,29 Based on crystallographic data, cholesteryl laurate is monoclinic and belongs to the space group P21, for which a = 1.2989 nm, b = 0.9008 nm, c = 3.2020 nm, and β = 91.36°. The monolayers projected along the b-axis contain two molecules, A and B, forming an asymmetric unit. Although all molecules have their long axes approximately parallel, the A molecules are oriented perpendicular to the B molecules. The A and B molecules make up a tetracyclic systems in which both the A and B molecules are arranged in antiparallel stacks. Moreover, the cholesteryl tails of the A molecules are close to the ends of the fatty acid chains of the B molecules. Therefore, the interactions between the alkyl chains have little effect on the structure of the monolayers in the crystal. In contrast, the interactions between the sterol rings and especially between the sterol rings and the alkyl chains affect the crystal structures. The difference between the interactions in the 2D monolayer on graphite and those in the 3D crystal structure indicates that molecular arrangements on graphite are affected by the graphite lattice.

Figure 6. STM images of cholesteryl stearate monolayers adsorbed on HOPG. (a) Large-scale STM image with an area of 35 nm × 35 nm. (b) High-resolution STM image of part of image (a). The white rectangle in (b) identifies the approximate area occupied by two molecules, while the white rhombus identifies a unit cell occupied by two molecules. The image area is 15 nm × 15 nm. The imaging conditions for both (a) and (b) were Iset = 0.32 nA and Vbias = 1.8 V. D

dx.doi.org/10.1021/la500944t | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

Figure 7. Schematic arrangements of cholesteryl stearate monolayers adsorbed on HOPG. The molecules are represented by their skeletal formulas. The rhombus in the model identifies a unit cell occupied by two molecules. The arrow indicates the (112̅0) direction of the graphite lattice.

stearate along the b-axis reveals that the molecules are arranged in 2D bilayer structures (see Figure 7). There is an almost regular packing of alkyl chains at the center of the 2D bilayer structures. The outer areas of the 2D bilayer structures consist of closely packed parallel sterol rings, and the projecting cholesteryl tails form the interfaces between the 2D bilayer structures. The 2D bilayer structures in the 3D crystals of cholesteryl stearate remain in 2D SAMs on HOPG, despite the dimension decrease from 3D to 2D. The investigation of 3D crystals and 2D SAMs on HOPG thus reveals that the interactions between the long alkyl chains of the cholesteryl stearate molecules significantly affect the molecular arrangements.

rhombus identifies a unit cell occupied by two molecules (see Figure 7). The bright ellipsoidal regions in Figure 6b correspond to pairs of asymmetrical sterol rings, and the relatively dim bands correspond to pairs of alkyl chains extending linearly from the sterol rings. The white rectangle in Figure 6b approximately identifies the area in the STM image occupied by two cholesteryl stearate molecules with opposite orientations. Two separate bright regions, a long dim band at the center of the rectangle, and two short dim bands on both sides of the rectangle correspond to two sterol rings, two fatty acid chains oriented in the (112̅0) zigzag direction of the graphite lattice, and two hydrocarbon chains at the tails of the cholesteryl esters, respectively. A unit cell of the cholesteryl stearate monolayer is shown in Figure 7. The STM images show that each unit cell contains two molecules and is defined by two vectors, namely, a1 = 2.58 ± 0.11 nm and a2 = 1.91 ± 0.07 nm, inclined at an angle φ = 78 ± 4° (Table 1). Figure 7 shows that the black molecules are antiparallel to the red ones and that a unit cell contains a pair of black and red molecules. Moreover, the molecules are arranged in 2D bilayer structures with all of the fatty acid chains in the central regions of the structures and all of the hydrocarbon chains of the cholesteryl tails at the interfaces between the structures. The interactions between the alkyl chains in the pairs of molecules affect the structure of the monolayer. The arrangement of the cholesteryl stearate monolayers is different from that of the cholesteryl laurate monolayers. Furthermore, the alkyl chains of the cholesteryl stearate are oriented in the (112̅0) zigzag direction of the graphite lattice, whereas those of the cholesteryl laurate are oriented in the (101̅0) armchair direction. Previous experimental6−22 and theoretical30,31 studies have revealed that the alkyl chains oriented in the zigzag direction of the graphite lattice have much lower formation energies than those oriented in the armchair direction. This is because the distance between a carbon atom and its secondneighbor carbon atom (second-neighbor carbon−carbon distance) in an alkyl chain with the zigzag conformation is close to the graphite lattice constant of 0.246 nm along the zigzag direction. The arrangement of the long alkyl chains of cholesteryl stearate in the zigzag direction is consistent with the findings of these previous reports. The self-assembly can therefore be controlled by the interaction between the alkyl chains and graphite surface. Based on crystallographic data, cholesteryl stearate has a monoclinic structure belonging to the space group P21, for which a = 1.020 nm, b = 0.755 nm, c = 5.750 nm, and β = 96°.5,32 A projection of the crystal structure of cholesteryl

4. CONCLUSIONS We presented STM images of SAMs of cholesterol, cholesteryl laurate, and cholesteryl stearate adsorbed on graphite at the liquid−solid interface. The comparison of the STM images and the crystallographic data indicates that the features of the 3D crystallization remain in 2D SAMs. A pair of cholesterol molecules in the SAMs or in the 3D crystals will orient itself head-to-head owing to hydrogen bonds between the hydroxyl groups. The dimers are oriented in parallel owing to the interactions between the sterol rings. However, a pair of cholesteryl ester molecules will orient antiparallel, while their fatty acid chains meet in the central regions in both the 2D and 3D structures. Moreover, the cholesteryl stearate molecules in the SAMs are arranged in bilayer structures that also appear in the plane projected along the b-axis of the 3D crystals. In addition, molecular arrangements of the SAMs on graphite are affected by the interactions between the substrate and the molecules. The long alkyl chains of the cholesteryl stearate were oriented in the zigzag direction of the graphite lattice, whereas those of the cholesteryl laurate were oriented in the armchair direction. These results show that the molecular arrangements of the cholesterol and cholesteryl esters at the liquid−graphite interface depend on three major interactions, namely, the interactions between sterol rings, between fatty acid chains, and between the long alkyl chains and the graphite surface. These findings will contribute to a better understanding of the mechanisms responsible for the self-assembly process and can be applied to the manipulation of interfacial self-assembly and the design of nanosensors. E

dx.doi.org/10.1021/la500944t | Langmuir XXXX, XXX, XXX−XXX

Langmuir

■

Article

(15) Giancarlo, L.; Cyr, D.; Muyskens, K.; Flynn, G. W. Scanning Tunneling Microscopy of Molecular Adsorbates at the Liquid−Solid Interface: Functional Group Variations in Image Contrast. Langmuir 1998, 14, 1465−1471. (16) Hibino, M.; Sumi, A.; Tsuchiya, H.; Hatta, I. Microscopic Origin of the Odd−Even Effect in Monolayer of Fatty Acids Formed on a Graphite Surface by Scanning Tunneling Microscopy. J. Phys. Chem. B 1998, 102, 4544−4547. (17) Giancarlo, L. C.; Flynn, G. W. Raising Flags: Applications of Chemical Marker Groups to Study Self-Assembly, Chirality, and Orientation of Interfacial Films by Scanning Tunneling Microscopy. Acc. Chem. Res. 2000, 33, 491−501. (18) De Feyter, S.; Gesquière, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Müllen, K. Scanning Tunneling Microscopy: A Unique Tool in the Study of Chirality, Dynamics, and Reactivity in Physisorbed Organic Monolayers. Acc. Chem. Res. 2000, 33, 520−531. (19) Florio, G. M.; Ilan, B.; Müller, T.; Baker, T. A.; Rothman, A.; Werblowsky, T. L.; Berne, B. J.; Flynn, G. W. Solvent Effects on the Self-Assembly of 1-Bromoeicosane on Graphite. Part I. Scanning Tunneling Microscopy. J. Phys. Chem. C 2009, 113, 3631−3640. (20) Thomas, L. K.; Kühnle, A.; Rode, S.; Beginn, U.; Reichling, M. Monolayer Structure of Arachidic Acid on Graphite. J. Phys. Chem. C 2010, 114, 18919−18924. (21) Hibino, M.; Tsuchiya, H. Cooperative Rotation of Alkylated Sulfide Molecules at the Liquid−Graphite Interface. J. Nanosci. Nanotechnol. 2014, 14, 2996−3000. (22) Hibino, M.; Tsuchiya, H. Coexistence of Alkylated Sulfide Molecules along Two Orthogonal Directions of Graphite Lattice. J. Phys. Chem. C 2014, 118, 1484−1491. (23) Stabel, A.; Heinz, R.; Rabe, J. P.; Wegner, G.; De Schryver, F. C.; Corens, D.; Dehaen, W.; Süling, C. STM Investigation of 2D Crystals of Anthrone Derivatives on Graphite: Analysis of Molecular Structure and Dynamics. J. Phys. Chem. 1995, 99, 8690−8697. (24) Kaneda, Y.; Stawasz, M. E.; Sampson, D. L.; Parkinson, B. A. STM Investigations of the Two-Dimensional Ordering of Perylenetetracarboxylic Acid N-Alkyl-diimides on HOPG and MoS2 Surfaces. Langmuir 2001, 17, 6185−6195. (25) Samorí, P.; Severin, N.; Simpson, C. D.; Müllen, K.; Rabe, J. P. Epitaxial Composite Layers of Electron Donors and Acceptors from Very Large Polycyclic Aromatic Hydrocarbons. J. Am. Chem. Soc. 2002, 124, 9454−9457. (26) Plass, K. E.; Kim, K.; Matzger, A. J. Two-Dimensional Crystallization: Self-Assembly, Pseudopolymorphism, and SymmetryIndependent Molecules. J. Am. Chem. Soc. 2004, 126, 9042−9053. (27) Sek, S.; Xu, S.; Chen, M.; Szymanski, G.; Lipkowski, J. STM Studies of Fusion of Cholesterol Suspensions and Mixed 1,2Dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/Cholesterol Vesicles onto a Au(111) Electrode Surface. J. Am. Chem. Soc. 2008, 130, 5736−5743. (28) Sawzik, P.; Craven, B. M. The Crystal Structure of Cholesteryl Laurate at 298 K. Acta Crystallogr. 1979, B35, 789−791. (29) Dahlen, B. The Crystal Structure of Cholesteryl Dodecanoate: Co-packing of Steroid Skeleta and Hydrocarbon Chains. Chem. Phys. Lipids 1979, 23, 179−188. (30) Yang, T.; Berber, S.; Liu, J.-F.; Miller, G. P.; Tománek, D. SelfAssembly of Long Chain Alkanes and Their Derivatives on Graphite. J. Chem. Phys. 2008, 128, 124709(8). (31) Prado, M. C.; Nascimento, R.; Moura, L. G.; Matos, M. J. S.; Mazzoni, M. S. C.; Cancado, L. G.; Chacham, H.; Neves, B. R. A. TwoDimensional Molecular Crystals of Phosphonic Acids on Graphene. ACS Nano 2011, 5, 394−398. (32) Barnard, J. A. W.; Lydona, J. E. A Crystallographic Examination of 14 Straight Chain Alkyl Esters of Cholesterol. Mol. Cryst. Liq. Cryst. 1974, 26, 285−296.

AUTHOR INFORMATION

Corresponding Author

*Ph +81-143-46-5771; Fax +81-143-46-5771; e-mail hibino@ mmm.muroran-it.ac.jp (M.H.). Present Address

§ Display Device Development Division, Sharp Corporation, 2613-1 Ichinomoto-cho, Tenri 632-8567, Japan.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors express their gratitude to Prof. I. Hatta for his encouragement and advice throughout this work. We also thank A. Sumi and J. Abe for their valuable cooperation on the experiments regarding the cholesterol and cholesteryl ester monolayers, respectively.

■

REFERENCES

(1) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; John Wiley & Sons: New York, 1973; Chapter 12, pp 94−111. (2) Pollard, H.; Scanu, A. M.; Taylor, E. W. On the Geometrical Arrangement of the Protein Subunits of Human Serum Low-Density Lipoprotein: Evidence for a Dodecahedral Model. Proc. Natl. Acad. Sci. U. S. A. 1969, 64, 304−310. (3) Shieh, H. S.; Hoard, L. G.; Nordman, C. E. Crystal Structure of Anhydrous Cholesterol. Nature 1977, 267, 287−289. (4) Craven, B. M. Crystal Structure of Cholesterol Monohydrate. Nature 1976, 260, 727−729. (5) Craven, B. M. Cholesterol Crystal Structures: Adducts and Esters. In Handbook of Lipid Research 4: The Physical Chemistry of Lipids; Small, D. M., Ed.; Plenum Press: New York, 1986; Chapter 6, pp 149− 182. (6) McGonigal, G. C.; Bernhardt, R. H.; Thomson, D. J. Imaging Alkane Layers at the Liquid/Graphite Interface with the Scanning Tunneling Microscope. Appl. Phys. Lett. 1990, 57, 28−30. (7) Rabe, J. P.; Buchholz, S. Commensurability and Mobility in TwoDimensional Molecular Patterns on Graphite. Science 1991, 253, 424− 427. (8) Hibino, M.; Sumi, A.; Hatta, I. Atomic Images of Saturated and Unsaturated Fatty Acids at Liquid/Graphite Interface and Difference of Tunneling Currents between Them Observed by Scanning Tunneling Microscopy. Jpn. J. Appl. Phys. 1995, 34, 610−614. (9) Hibino, M.; Sumi, A.; Hatta, I. Molecular Arrangements of Fatty Acids and Cholesterol at Liquid/Graphite Interface Observed by Scanning Tunneling Microscopy. Jpn. J. Appl. Phys. 1995, 34, 3354− 3359. (10) Venkataraman, B.; Breen, J. J.; Flynn, G. W. Scanning Tunneling Microscopy Studies of Solvent Effects on the Adsorption and Mobility of Triacontane/Triacontanol Molecules Adsorbed on Graphite. J. Phys. Chem. 1995, 99, 6608−6619. (11) Venkataraman, B.; Flynn, G. W.; Wilbur, J. L.; Folkers, J. P.; Whitesides, G. M. Differentiating Functional Groups with the Scanning Tunneling Microscope. J. Phys. Chem. 1995, 99, 8684−8689. (12) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. STM Investigations of Organic Molecules Physisorbed at the Liquid−Solid Interface. Chem. Mater. 1996, 8, 1600−1615. (13) Cyr, D. M.; Venkataraman, B.; Flynn, G. W.; Black, A.; Whitesides, G. M. Functional Group Identification in Scanning Tunneling Microscopy of Molecular Adsorbates. J. Phys. Chem. 1996, 100, 13747−13759. (14) Claypool, C. L.; Faglioni, F.; Goddard, W. A., III; Gray, H. B.; Lewis, N. S.; Marcus, R. A. Source of Image Contrast in STM Images of Functionalized Alkanes on Graphite: A Systematic Functional Group Approach. J. Phys. Chem. B 1997, 101, 5978−5995. F

dx.doi.org/10.1021/la500944t | Langmuir XXXX, XXX, XXX−XXX