Surface Energy Differences in Monolayers Prepared with the Isomers

Oct 21, 2003 - Wesley Sanders and Ricardo Vargas , Mark R. Anderson. Langmuir ... Alyza A. Azmi , Iraklii I. Ebralidze , Steven E. Dickson , J. Hugh H...
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Surface Energy Differences in Monolayers Prepared with the Isomers 3- and 4-(12-Mercaptododecyl)phenol Francisco Cavadas and Mark R. Anderson* Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0212 Received April 17, 2003. In Final Form: September 4, 2003 Monolayers of 3-(12-mercaptododecyl)phenol and 4-(12-mercaptododecyl)phenol are prepared to investigate the relationship between the interfacial molecular structure and the surface energy of modified interfaces. These modified interfaces are characterized by reflection absorption infrared spectroscopy, reductive desorption, and contact angle measurements. Contact angle measurements with water (γ ) 72.0 mN/m), glycerol (γ ) 63.2 mN/m), and ethylene glycol (γ ) 48.1 mN/m) show that monolayers of 4-(12mercaptododecyl)phenol have a higher surface energy (∼42 mN/m) than 3-(12-mercaptododecyl)phenol monolayers (∼31 mN/m). Furthermore, Fowkes analysis of the contact angle data show that only ∼8% of the surface energy of the 4-(12-mercaptododecyl)phenol monolayer arises from dispersive interactions, while ∼30% of the total surface energy of 3-(12-mercaptododecyl)phenol monolayers comes from dispersive interactions. These surface energy results along with the spectral data suggest that the hydroxyl group with monolayers of 4-(12-mercaptododecyl)phenol is oriented away from the substrate and into the adjacent phase more than is found with monolayers of 3-(12-mercaptododecyl)phenol.

Introduction Preparation of well-defined interfacial structure by molecular self-assembly has been an actively studied area for many years.1 The opportunity to generate well-defined structures at interfaces that can subsequently be used in fundamental studies (e.g., electron transfer)2-4 or in practical applications (e.g., adhesion promotion5 or corrosion protection)6-9 provides much of the interest in this field. For these applications, one is taking advantage of the fact that the molecular structure of the self-assembled monolayer is providing some desirable physical property at the interface. Presumably, one could control the interfacial structure and, consequently, properties by careful design of the molecules used to assemble the monolayer. Our interest is in understanding the role that small changes in the structure of the molecules used to assemble the monolayer has on the properties of the interface.10,11 The role of molecular structure on interfacial physical properties is illustrated by several previous studies. Wells et al. show that the chemical behavior of monolayers prepared with isomers of mercaptobenzoic acid is different dependent on the placement of the carboxylic acid group * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932-950. (2) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173-3181. (3) Miller, C.; Cuendet, P.; Gratzel, M. J. Phys. Chem. 1991, 95, 877886. (4) Miller, C.; Gratzel, M. J. Phys. Chem. 1991, 95, 5225-5233. (5) Glodde, M.; Hartwig, A.; Hennemann, O. D.; Stohrer, W. D. Int. J. Adhes. Adhes. 1998, 18, 359-364. (6) Yamamoto, Y.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1993, 140, 436-443. (7) Haneda, R.; Aramaki, K. J. Electrochem. Soc. 1998, 145, 18561861. (8) Aramaki, K. Corros. Sci. 1999, 41, 1715-1730. (9) Zamborini, F. P.; Crooks, R. M. Langmuir 1998, 14, 3279-3286. (10) Taylor, C. D.; Anderson, M. R. Langmuir 2002, 18, 120-126. (11) Anderson, M. R.; Baltzersen, R. J. Colloid Interface Sci. 2003, 263, 516-521.

on the ring.12 Here, the substitution pattern of the aromatic ring alters the lateral interactions within the monolayer as well as the ability of the acid group to interact with basic molecules in the adjacent phase. A more subtle influence of the interfacial structure on the physical properties is illustrated by the “even-odd” effect that a homologous series of n-alkyl chains have on the contact angle that a water droplet makes with the modified surface.13-16 These investigations all show that the change in the projection of the terminal methyl group with changing alkyl-chain length influences the measured contact angle. Lee et al. report that monolayers prepared from phenylterminated n-alkyl chains form densely packed, wellordered structures.17 In this interfacial structure, they found that the phenyl group organizes in a herringbone fashion at the interface. Interestingly, they also find a dependence of the contact angle that the monolayers make with methylene iodide and nitrobenzene on whether there is an even or an odd number of carbon atoms in the alkyl chain. Lee et al.’s result is contrasted with the apparent disorder that we find with monolayers of 12-phenoxydodecylmercaptan.10 This result suggests that the ether oxygen, present in the 12-phenoxy-dodecylmercaptan monolayer but absent in the 12-phenyl-dodecylmercaptan monolayer, disrupts that ability of the monolayer to assume a well-ordered, crystalline structure. To continue investigating how small molecular changes influence the structural and physical properties of monolayers, we prepare monolayers from the isomers 3- and 4-(12-mercaptododecyl)phenol and compare their interfacial structure to that of monolayers prepared with 12(12) Wells, M.; Dermody, D. L.; Yang, H. C.; Kim, T.; Crooks, R. M.; Ricco, A. J. Langmuir 1996, 12, 1989-1996. (13) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350-4358. (14) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (15) Wong, S.-S.; Takano, H.; Porter, M. D. Analytical Chemistry 1998, 70, 5209-5212. (16) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370-2378. (17) Lee, S.; Puck, A.; Graupe, M.; Colorado, R.; Shon, Y.-S.; Lee, T. R.; Perry, S. S. Langmuir 2001, 17, 7364-7370.

10.1021/la034666t CCC: $25.00 © 2003 American Chemical Society Published on Web 10/21/2003

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phenyl-dodecylmercaptan. Lee et al.17 show that monolayers of 12-phenyl-dodecylmercaptan form well-ordered interfacial structures, so differences in the structures of the hydroxyl-substituted monolayers are attributed to the influence that the hydroxyl group has on the ability of the molecules to organize within the monolayer. We also investigate how these interfacial structural changes influence the surface energy of the modified interface. Experimental Section 4-Bromoanisole, 3-bromoanisole, phenyllithium, 1,12-dibromododecane, thiourea, and BBr3 (Aldrich) were reagent grade and used without further purification. Methanol (HPLC grade) was obtained from VWR and used without further purification. Diethyl ether and methylene chloride (HPLC grade) were obtained from Aldrich, dried over activated alumina, and degassed prior to use. Chloroform was obtained from Aldrich and used without further purification. Anhydrous magnesium sulfate was obtained from VWR and used as received. 3-(12Mercaptododecyl)phenol, 4-(12-mercaptododecyl)phenol, and 12phenyl-dodecylmercaptan were synthesized from 3-bromoanisole, 4-bromoanisole, and phenyllithium, respectively. Details of the synthetic procedure and analytical data for these molecules are given in the Supporting Information. The 1 × 1 in. gold substrates were purchased from Evaporated Metal Films, Inc. (Ithaca, NY). The substrates consisted of 1000-Å vapor-deposited Au on top of 50-Å vapor-deposited Cr, used as an adherent, on glass substrates. Monolayers of 12-phenyldodecylmercaptan, 3-(12-mercaptododecyl)phenol, and 4-(12mercaptododecyl)phenol were prepared by spontaneous adsorption from chloroform solutions containing 1.0 × 10-3 M concentrations of the mercaptan. Immediately prior to adsorption, the gold substrates were cleaned in “piranha” solution (75% of concentrated H2SO4/25% of 30% H2O2) for approximately 1 min, rinsed with water, and dried in a stream of dry nitrogen. Caution must be exercised with piranha solutions because they are potentially explosive. Following usage, piranha solutions should be properly disposed. The clean substrates were kept in the adsorption solution for 24 h to prepare the monolayers. When removed from the adsorption solution, the modified gold surfaces were rinsed with ethanol followed by distilled water and then dried by blowing with a rapid stream of dry nitrogen. After preparation, the monolayers were characterized by reflectionabsorption infrared spectroscopy (RAIRS), contact angle goniometry, and electrochemical reductive desorption. Infrared spectroscopy was conducted with a Nicolet model 710 FTIR (Madison, WI) spectrometer using a liquid nitrogen cooled HgCdTe detector (MCT-A detector). Reflection spectra of the monolayers were collected at 4-cm-1 resolution and 512 interferometer scans using a Spectra Tech (Stamford, CT) FT80 Specular Reflectance attachment with incident radiation polarized perpendicular to the substrate plane with a BaF2 wire-grid polarizer (IGP228, Cambridge Physical Sciences). Vibrational band assignments used in this analysis were aided using normal mode calculations conducted with Titan Molecular modeling software (Wavefunction, Irvine, CA). Contact angle measurements were conducted using the sessile, static drop method with a Rame-Hart NRL contact angle goniometer. Drops of 5 µL of various probe liquids were used for these measurements, and the values reported are the average of a minimum of four drops per sample. Ethylene glycol and glycerol were analytical grade from Aldrich (Milwaukee, WI) and used as received. Deionized water was purified using a Nanopure II (Barnsted) water purification system. Thin-film thicknesses were measured using a Beaglehole picometer ellipsometer (Wellington, New Zealand). Beaglehole system software was used to estimate the thin-film dimensions from the experimental ∆ and Ψ values. Electrochemical measurements were made using a CH Instruments model 600A potentiostat (Austin, TX) and a drop electrochemical cell described previously.18-19 With this elec(18) Zhang, M.; Anderson, M. R. Langmuir 1994, 10, 2807-2813. (19) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-691.

Figure 1. RAIRS spectra of monolayers prepared with (A) 12-phenyl-dodecylmercaptan, (B) 3-(12-mercaptododecyl)phenol, and (C) 4-(12-mercaptododecyl)phenol. Table 1. Peak Position and Widths of the Asymmetric and Symmetric Methylene Vibrational Modes for the Phenyl-Terminated Monolayers

12-phenyl-dodecylmercaptan 3-(12-mercaptododecyl)phenol 4-(12-mercaptododecyl)phenol dodecanethiol

CH2,asy position

CH2,asy width

CH2,sym position

2918 2924 2923 2917

16 23 24 15

2850 2852 2852 2850

trochemical cell, the contact area that a drop of electrolyte solution makes with the modified substrate, for these measurements 0.65 cm2, defines the surface area of the working electrode. The surface roughness was found to be 1.15 ( 0.05 using Porter et al.’s method, and the surface coverages are reported accounting for this roughness.20 Solution resistance was compensated 90% using positive feedback IR compensation for all measurements. Potentials are measured relative to an aqueous Ag/AgCl reference electrode.

Results and Discussion RAIRS spectra for monolayers of 4-(12-mercaptododecyl)phenol, 3-(12-mercaptododecyl)phenol, and 12-phenyldodecylmercaptan are shown in Figure 1. The methylene modes for the 12-phenyl-dodecylmercaptan monolayers are found at 2918 and 2850 cm-1 (Table 1). These peak positions are consistent with values reported by Lee et al. and also with values reported for a well-ordered, nalkanethiol monolayer structure.17 There are two additional features in the C-H portion of the spectrum of 12-phenyl-dodecylmercaptan, found at 3068 and 3039 cm-1. These are assigned to aromatic C-H stretching modes. The appearance of these features in the spectrum is additional evidence of a well-ordered monolayer structure. Lee et al. also see these aromatic C-H features and report that monolayers of 12-phenyl-dodecylmercaptan are well-ordered and arrange with the phenyl group in a herringbone structure at the interface.17 Others also find that monolayers of benzenethiolate and benzyl mercaptan form a herringbone interfacial structure of the terminal phenyl ring.21-22 A herringbone interfacial structure for these monolayers is similar to a combination of the stable structures reported for benzene dimers, the edge-to-face (T-shaped) arrangement and the parallel-displaced struc(20) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (21) Jung, H. H.; Won, Y. D.; Shin, S.; Kim, K. Langmuir 1999, 15, 1147-1154. (22) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M. L.; Rubinstein, I. Langmuir 1993, 9, 2974-2981.

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Table 2. Reductive Desorption Peak Potentials and Calculated Surface Coverages for Monolayers of the Isomers of (12-mercaptododecyl)phenol and 12-phenyl-dodecylmercaptan peak potential coverage (V vs Ag-AgCl) (×1010 mol/cm2) 12-phenyl-dodecylmercaptan 4-(12-mercaptododecyl)phenol 3-(12-mercaptododecyl)phenol

-1.09 ((0.01) -1.05 ((0.01) -1.05 ((0.01)

4.6 ((0.2) 4.9 ((0.4) 5.0 ((0.4)

ture, and is representative of a well-ordered interfacial structure.23-24 In contrast, the asymmetric methylene and symmetric methylene stretching modes for 4-(12-mercaptododecyl)phenol and 3-(12-mercaptododecyl)phenol are found at a higher energy: 2926 and 2853 cm-1 for 4-(12-mercaptododecyl)phenol, and 2926 and 2852 cm-1 for 3-(12mercaptododecyl)phenol. The aromatic C-H stretching modes observed with the 12-phenyl-dodecylmercaptan monolayer are not found in the infrared spectra of the isomeric (12-mercaptododecyl)phenol monolayers. These observations suggest that the presence of the hydroxyl functional group disrupts the ability of these molecules to form well-ordered interfacial structures. Additional evidence for the relative molecular order of these monolayers is found by examining the width of the methylene modes (full width at half-maximum) of these monolayers. The width of the asymmetric CH2 mode for 12-phenyl-dodecylmercaptan is approximately 16 cm-1, while it is 23-24 cm-1 for the 4-(12-mercaptododecyl)phenol and 3-(12-mercaptododecyl)phenol monolayers. The narrow peak width for the 12-phenyl-dodecylmercaptan monolayer is consistent with values reported for monolayers of n-alkanethiols and is suggestive of better organization of this layer relative to that of the isomeric (12-mercaptododecyl)phenol monolayers.19 The ellipsometry measurements indicate that the monolayers have similar dimensions: 18.4 ( 1.5 Å for 4-(12-mercaptododecyl)phenol, 15.7 (1.5 Å for 3-(12mercaptododecyl)phenol, and 18.5 ( 1.4 Å for 12-phenyldodecylmercaptan. This result suggests that, although the isomeric (12-mercaptododecyl)phenol monolayers are structurally disordered, the molecules that comprise the monolayer on average are extending away from the substrate surface. The reductive desorption measurements conducted on each of these monolayers show that the interfacial structural differences are not due to significantly different coverages (Table 2). The calculated coverage of each monolayer, approximately 5.0 × 10-10 mol/cm2, is slightly below the saturation coverage measured for n-alkanethiols.25 This result is in contrast with the atomic force microscopy images of phenyl-terminated monolayers published by Lee et al., which show that the sulfur atoms for this monolayer have a 4.9-Å spacing at the interface, a value indistinguishable from the spacing found for monolayers prepared with n-alkanethiols.17 Similar coverages of the isomeric (12-mercaptododecyl)phenol monolayers suggest that the lower structural order, compared with that of monolayers of 12-phenyl-dodecylmercaptan, may be due to intermolecular interactions within the monolayer introduced by the hydroxyl group. The hydroxyl substitution on the terminal aromatic ring (23) Endo, T.; Iida, T.; Furuya, N.; Yamada, Y.; Ito, M. M. J. Chem. Software 1999, 5, 82-91. (24) Hobza, P.; Selzle, H. L.; Schlog, E. W. J. Phys. Chem. 1993, 97, 3937-3938. (25) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359.

introduces the potential for either steric or hydrogen bonding interactions affecting the overall organization of the monolayer. The lower order of the monolayers should be manifested by changes in the lateral interaction energy for the isomeric monolayers compared to that found for monolayers of 12-phenyl-dodecylmercaptan. Imabayashi et al. show that the desorption potential is characteristic of the stabilization energy of the monolayer.26 In their model, a more positive desorption potential is representative of less lateral stabilization within the monolayer. The desorption peak potential for 4-(12mercaptododecyl)phenol and 3-(12-mercaptododecyl)phenol are approximately 35 mV positive of the desorption potential of 12-phenyl-dodecylmercaptan. Using Imabayashi et al.’s method, monolayers prepared with 4-(12mercaptododecyl)phenol and 3-(12-mercaptododecyl)phenol are 6.7 kJ/mol less stable than the monolayer prepared with 12-phenyl-dodecylmercaptan. The diminished lateral stabilization of the isomeric monolayers of (12-mercaptododecyl)phenol suggest that the hydroxyl substitution introduces interactions that contribute to the lower order of these monolayers. The hydroxyl substitution introduces the possibility of either steric or hydrogen bonding interactions within the monolayer that may influence the ability of the molecules to order. Steric influences would prevent the molecules within the monolayer from efficient packing, preventing favorable van der Waals interactions among the methylene chains and aromatic interactions among the phenyl groups of these molecules. In either case, we would anticipate that the desorption potential would be positive of that observed for the well-ordered 12-phenyl-dodecylmercaptan monolayer. Lateral hydrogen bonding interactions among the molecules that comprise the monolayer would have a more complex influence on the lateral stabilization energy of the monolayer. The alkyl portion of the monolayer would likely have to reorganize to allow the molecules to hydrogen bond. Efficient van der Waals interactions among the alkyl chains would be lost, having the influence of lowering the lateral stabilization energy. Imabayashi et al. estimate that the reductive desorption potential shifts by 15 mV as a result of the van der Waals interactions from each methylene group.26 Hydrogen bonds, however, are comparatively strong interactions and should result in the desorption potential shifting to more negative values. We found previously that monolayers containing internal amide bonds capable of hydrogen bonding had reductive desorption potentials 120 mV more negative than the reductive desoprtion potential of 11-mercaptoundecanoic acid monolayers.11 The net influence of hydrogen bonding on the reductive desorption potential, therefore, depends on the extent of the loss in van der Waals stabilization relative to the additional monolayer stabilization due to the presence of the hydrogen bonding. We cannot predict, therefore, whether the desorption potential would shift to more positive or negative values for the 3- or 4-(12mercaptododecyl)phenol monolayers. On the basis of the behavior of the monolayers of 11-mercaptoundecanoic acid reacted with 1,4-phenylenediamine, however, we believe that hydrogen bonding interactions would dominate the lateral stabilization energy and shift the desorption potential to more negative values. Endo et al. calculate that the interaction energy of the T-shaped structure of a benzene dimer is 2.64 kcal/mol (11.05 kJ/mol).23 Likewise, NMR studies showed that the (26) Imabayashi, S.; Iida, M.; Hobara, D.; Feng, Z. Q.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1997, 428, 33-38.

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benzene-benzene interaction energy is approximately 2.0 kcal/mol (8.4 kJ/mol).27 The parallel-displaced arrangement of aromatic rings is reported to have a similar stabilization energy to that of the T-shaped arrangement.24 The decreased monolayer stabilization of the 3- and 4-(12mercaptododecyl)phenol monolayers (6.7 kJ/mol) measured by reductive desorption is on the order of these values, suggesting that disruption of the aromatic interaction energy may account for the structural differences between the 3- and 4-(12-mercaptododecyl)phenol monolayers and the 12-phenyl-dodecylmercaptan monolayer. Vibrational features found at lower energies with the isomeric (12-mercaptododecyl)phenol monolayers and with 12-phenyl-dodecylmercaptan monolayers are characteristic of aromatic ring modes. The three monolayers differ significantly from each other in this region of the spectrum (Figure 1). Lee et al. propose a structure for monolayers of 12phenyl-dodecylmercaptan monolayers in which the plane of the aromatic ring is perpendicular to the plane established by the aliphatic methylene chain.17 This conformation of the individual molecules combined with the (x3 × x3)R30° arrangement of the molecules on the Au(111) substrate is thought to lead to the energetically stable herringbone pattern of the terminal phenyl rings within the monolayer. With this monolayer structure, many of the aromatic C-C vibrations will either be weak or not be observed in the RAIRS spectrum because the transition dipole moment for many of these ring vibrations lie parallel to the substrate. Monolayers of 12-phenyldodecylmercaptan have only one moderate feature in the low energy region of the spectrum, found at 1497 cm-1. This feature is assigned to vibration 19a (using the Wilson numbering convention), an aromatic C-C ring stretching vibration.28,29 The transition dipole of vibration 19a is along the C1C4 axis of the phenyl ring. This direction is predominantly parallel to the substrate surface in the monolayer structure proposed by Lee et al. for 12-phenyl-dodecylmercaptan and would not be observed in the reflection spectrum of the monolayer.17 The small intensity of the feature at 1497 cm-1 suggests that the phenyl ring is slightly tilted out of the substrate plane. The transition dipole moment for vibration 8a is also along the C1-C4 axis and should be relatively weak in the 12-phenyl-dodecylmercaptan monolayer spectrum. In our RAIRS spectra, this vibration is not observed. There are two weak features in the low energy region for the reflection spectrum of 3-(12-mercaptododecyl)phenol at 1612 and 1457 cm-1. The feature at 1612 cm-1 is assigned to a symmetric C-C ring mode (8a) and that at 1457 cm-1 to another C-C ring mode.28-29 None of the spectral features for the 3-(12-mercaptododecyl)phenol monolayer are as intense as those observed with the 4-(12mercaptododecyl)phenol monolayer. In particular, the low intensity of the feature at 1612 cm-1 and the absence of a feature assignable to mode 19a suggest that the aromatic ring for this monolayer is largely in the plane parallel to the substrate, as is observed with monolayers of 4-(12mercaptododecyl)phenol. In contrast, monolayers of 4-(12-mercaptododecyl)phenol have a very intense spectral feature in the reflection (27) Slejko, F. L.; Drago, R. S.; Brown, D. G. J. Am. Chem. Soc. 1972, 94, 9210-9216. (28) Varsanyi, G. Vibrational Spectra of Benzene Derivatives; Academic Press: New York, 1969. (29) Pretsch, R.; Clerc, T.; Siebl, J.; Simon, W. Tables of Spectral Data for Structure Determination of Organic Compounds; SpringerVerlag: Berlin, 1989.

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spectrum, found at 1515 cm-1 (vibration 19a), and several features of moderate intensity at 1618 cm-1 (vibration 8a), 1597 cm-1, and 1266 cm-1.28,29 The feature at 1597 cm-1 is assigned to vibration 8b.28 The feature at 1266 cm-1 is assigned to a C-H bend. The transition dipole of the 1515 cm-1 vibration is along the C1-C4 axis of the aromatic ring. The intensities of vibrations 19a and 8a suggest that the phenyl ring C1-C4 axis is oriented at an angle intermediate between the surface normal and the plane of the substrate. The quantitative determination of the interfacial structure of the 3-(12-mercaptododecyl)phenol and 4-(12mercaptododecyl)phenol monolayers was not conducted because spectral features attributable to three orthogonal transition dipoles could not be identified in the RAIRS spectra.30-31 The similarity of the peak positions and integrated areas of the symmetric and asymmetric methylene modes, however, suggests that the methylene portion of these isomeric monolayers are comparably structured. The intensities of vibrations 19a and 8a in the reflection spectra allow some interpretation of the orientation of the phenyl ring. The strong intensity of the aromatic ring feature at 1515 cm-1 and the moderate intensity of the symmetric ring breathing mode at 1612 cm-1 for monolayers of 4-(12mercaptododecyl)phenol suggest that the C1-C4 axis of this monolayer lies at an angle intermediate between the surface normal and the substrate surface. In this structural model, the hydroxyl group of 4-(12-mercaptododecyl)phenol monolayers is exposed at the interface, with the degree that it is exposed dependent on the angle that the phenyl ring C1-C4 axis makes with the surface normal. In contrast, these vibrational modes are weak or are not observed in the reflection spectrum of 3-(12-mercaptododecyl)phenol. This spectral behavior is similar to that observed with monolayers of 12-phenyl-dodecylmercaptan, and suggests that the aromatic ring plane lies largely parallel to the plane of the substrate. These different orientations of the terminal aromatic ring orientation for the monolayers prepared with the isomeric 12-mercaptododecyl-phenol monolayers suggests that they should have measurably different surface energies. Lee et al., Tao et al., and Porter et al. all demonstrate that the contact angle that a water droplet makes with monolayers prepared with pendant homologous n-alkyl chains varies dependent on whether the alkyl chain has an even or odd number of carbon atoms.13-15,17 This “evenodd” effect on the interfacial contact angle is representative of the surface energy differences that are found because the terminal methyl group has a different orientation with respect to the surface normal depending on whether there is an even or an odd number of carbon atoms in the alkyl chain. We anticipate that a similar phenomenon will occur with the isomeric (12-mercaptododecyl)phenol monolayers because the different substitution appears to change the orientation of the phenyl portion and the hydroxyl group of the monolayer. The surface energy of a substrate may be quantitatively determined by measuring the contact angle that a series of probe liquids make with the substrate.32 Figure 2 shows a plot of the cosine of the contact angle versus the surface tension for a series of probe liquids (Table 3). Following the analysis of Zisman, estimates of the surface energy are obtained by extrapolating the trend line of these plots (30) Kwan, W. S. V.; Atanasoska, L.; Miller, L. L. Langmuir 1991, 7, 1419-1425. (31) Cammarata, V.; Atanasoska, L.; Miller, L. L.; Kolaskie, C. J.; Stallman, B. J. Langmuir 1992, 8, 876-886. (32) Zisman, W. A. Ind. Eng. Chem. 1963, 55, 18-38.

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Figure 2. Plot of cosine of the contact angles versus surface tension that water, glycerol, and ethylene glycol make with substrates modified with (A) 12-phenyl-dodecylmercaptan, (B) 3-(12-mercaptododecyl)phenol, and (C) 4-(12-mercaptododecyl)phenol. Table 3. Surface Tensions and Their Dispersive Components for the Three Probe Liquids Used in the Zisman and Fowkes Analysesa

water glycerol ethylene glycol a

γL (mN/m)

γdL (mN/m)

72.0 63.2 48.1

10.8 20.5 17.5

Figure 3. Fowkes analysis of the contact angle data to extract estimates of the dispersive contribution to the total surface energy. Plot of the cosine of the contact angles that probe liquids make versus xγdL/γtotal for substrates modified with (A) 12L phenyl-dodecylmercaptan, (B) 3-(12-mercaptododecyl)phenol, and (C) 4-(12-mercaptododecyl)phenol. The slopes of the trend lines (11.84 for 12-phenyl-dodecylmercaptan, 6.15 for 3-(12mercaptododecyl)phenol, and 3.56 for 4-(12-mercaptododecyl)phenol) are equal to 2xγsurface . Error bars for the contact angle d data are given on the plot.

Values taken from Panzer.33

to the point where the cosine of the contact angle is equal to 1.32 This point, called the critical surface tension, represents the surface tension at which a probe liquid would completely spread over the surface. For 4-(12mercaptododecyl)phenol, the critical surface tension is found to be approximately 42 ((8) mN/m, and for 3-(12mercaptododecyl)phenol, it is found to be 31 ((6) mN/m. The large errors in the critical surface tensions are due to the errors in the slope and the need to extrapolate the trend line. Nevertheless, the trend of the data shows that the 4-(12-mercaptododecyl)phenol monolayer has a higher surface energy than monolayers prepared from 3-(12mercaptododecyl)phenol. The lower surface energy for the 3-(12-mercaptododecyl)phenol monolayer is consistent with the hydroxyl group being less available for interaction with the adjacent phase and suggests that the monolayer orients the hydroxyl group into the monolayer structure. For comparison, Zisman analysis was also conducted with monolayers prepared from 12-phenyl-dodecylmercaptan (Figure 2). For this monolayer, the surface energy is expected to be lower than either of the isomeric (12mercaptododecyl)phenol monolayers because of the absence of the hydroxyl substitution on the terminal ring. The surface energy is found to be 26 ((5) mN/m, consistent with the trend established with the monolayers formed with the isomers of (12-mercaptododecyl)phenol. Fowkes demonstrated that the surface tension could be separated into contributions from polar and dispersive components.34 An estimation of the dispersive contribution to the total surface energy may be obtained by plotting probe /γtotal versus the cosine of the contact angle, where xγprobe d probe γd is the dispersive portion of the probe surface tension probe is the total surface tension of the probe and γtotal liquid.34-35 Fowkes shows that the slope of this plot is (33) Panzer, J. J. Colloid Interface Sci. 1973, 44, 142-161. (34) Fowkes, F. M. Ind. Eng. Chem. 1964, 56, 40-52. (35) Georges, E.; Georges, J.-M.; Hollinger, S. Langmuir 1997, 13, 3454-3463.

equal to 2xγsurface , where γsurface is the dispersive contrid d bution to the surface energy of the modified interface. Using Fowkes’ method, the contact angle data may be analyzed to extract an estimate of the dispersive component of the surface energy for these interfaces (Figure 3). From the slope of the trend line of these plots, the dispersive contribution to the surface energy for 4-(12mercaptododecyl)phenol is 3.2 ((0.7) mN/m and 10 ((3) mN/m for 3-(12-mercaptododecyl)phenol. The small dispersive component of the surface energy (only approximately 8% of the total surface energy) for the 4-(12mercaptododecyl)phenol is consistent with the model that the polar hydroxyl group is easily available at the interface to interact with the adjacent phase. In this orientation, the surface energy has a large contribution from the polar hydroxyl group. With the 3-(12-mercaptododecyl)phenol monolayer, the dispersive component of the total surface energy is nearly 30% of the total. This value is consistent with the model of the monolayer structure in which the hydroxyl group is oriented away from the surface normal, having less ability to interact with the adjacent phase. With the 3-(12-mercaptododecyl)phenol monolayer, the phenyl ring contributes significantly to the total surface energy. For comparison, the Fowkes analysis was conducted with the contact angle data obtained with the 12-phenyldodecylmercaptan monolayers (Figure 3). From these data, the dispersive contribution to the surface energy for monolayers of 12-phenyl-dodecylmercaptan is found to be 35 ((7) mN/m. This value is higher than the estimate of the total surface energy found for this monolayer by the Zisman analysis (although, within the error), suggesting that errors in the measurement (especially those associated with extrapolating the trend line) limit the confidence placed in the quantitative values obtained. Nevertheless, this result indicates that the surface energy of 12-phenyldodecylmercaptan monolayers is dominated by dispersive interactions, as expected. The result for the 12-phenyldodecylmercaptan surface energy is consistent with the interpretation that the aromatic ring is largely parallel

Surface Energy Differences

to the substrate plane and suggests that the large dispersive contribution to the surface energy for monolayers prepared with 3-(12-mercaptododecyl)phenol is due to exposure of the aromatic ring at the interface. From the infrared spectroscopy, reductive desorption, and contact angle measurements, the interfacial structures of monolayers prepared with 3-(12-mercaptododecyl)phenol and 4-(12-mercaptododecyl)phenol are proposed. The well-ordered structure of the 12-phenyldodecylmercaptan monolayer arranges the terminal phenyl group in a herringbone pattern with the phenyl ring plane close to parallel to the substrate.17 This arrangement provides favorable interactions among the phenyl rings that help to stabilize the well-ordered monolayer structure. This interfacial structure is characterized by a low surface energy dominated by dispersive interactions. Hydroxyl substitution on the aromatic ring apparently disrupts the structure of the monolayer. This is most evident with monolayers of 4-(12-mercaptododecyl)phenol, where strong vibrational features (not seen with monolayers of 12phenyl-dodecylmercaptan) attributable to C-C ring modes are observed. The similar coverage for the monolayers suggests that the disruption of the aromatic structure may be due to reorientation of the phenyl ring within the monolayer to accommodate either the steric bulk of the hydroxyl group or hydrogen bonding interactions between hydroxyl groups. The restructuring of the aromatic ring places the hydroxyl group in an orientation in which it can have significant interaction with the adjacent phase; consequently, this monolayer has higher surface energy than that found for the 3-(12-mercaptododecyl)phenol. Similar monolayer destabilization is measured for monolayers of 3-(12-mercaptododecyl)phenol; however, the infrared spectra for this monolayer does not have evidence for a dramatic change in the projection of the aromatic ring from that suggested for monolayers of 12-phenyldodecylmercaptan. Disruption of the monolayer order occurs with these monolayers, as suggested by the position of the methylene absorptions; however, the restructuring apparently maintains the plane of the aromatic ring largely parallel to the substrate surface. Contact angle analysis for monolayers prepared with 3-(12-mercaptododecyl)phenol suggest that the final structure assumed by the monolayer has a significant contribution from dispersive forces to the total measured surface energy. Summary Monolayers prepared with isomers of (12-mercaptododecyl)phenol are found to have different surface energies.

Langmuir, Vol. 19, No. 23, 2003 9729

We attribute these surface energy differences to the role that the hydroxyl group has in changing the orientation of the terminal phenyl ring within the monolayer. When in the para position, the hydroxyl group causes the ring to be oriented away from the plane of the substrate. This also has the effect of allowing the hydroxyl group more access to the adjacent phase. Hydroxyl substitution in the meta position apparently leaves the phenyl ring largely parallel to the substrate. In this orientation, the hydroxyl group is not as accessible to the adjacent phase, and the total surface energy of this modified interface is similar to that found for monolayers of 12-phenyl-dodecylmercaptan. The surface energy of monolayers of 4-(12mercaptododecyl)phenol is primarily due to polar interactions, supporting the interpretation that the hydroxyl group is available at the interface. For 3-(12-mercaptododecyl)phenol monolayers, a significant portion of the surface energy arises from dispersive interactions, consistent with the structure of the aromatic ring portion of the monolayer being similar to that of 12-phenyl-dodecylmercaptan. This result suggests that the hydroxyl group is not as exposed at the interface as is found with the 4-(12-mercaptododecyl)phenol isomer. These measurements are consistent with the interpretation that interfacial structural differences can lead to measurable differences in surface energy. In this research, the presence of the hydroxyl group and its position on the terminal ring influence the measured surface energy. Both the projection of the hydroxyl group and the orientation of the phenyl ring in the isomeric monolayers contribute to the measured surface energy differences. This result is consistent with the conclusions of others that the molecular structural details present at monolayer modified interfaces influences the physical and chemical behavior of the interface.15-17 We are currently extending our research in this area by using these monolayers to determine if these structural differences impact other macroscopic interfacial phenomena and by investigating how other changes in molecular structure influence the macroscopic properties of modified interfaces. Acknowledgment. We thank Professor Paul Deck for help with synthetic methods. Supporting Information Available: Details of the synthetic procedure and analytical data. This material is available free of charge via the Internet at http://pubs.acs.org. LA034666T