Self-Assembled Monolayers of Positional Isomers of - American

C. Douglas Taylor and Mark R. Anderson*. Department of Chemistry, Virginia Polytechnic Institute and State University,. Blacksburg, Virginia 24061-021...
0 downloads 0 Views 82KB Size
Download PDF
120

Langmuir 2002, 18, 120-126

Self-Assembled Monolayers of Positional Isomers of (12-Mercaptododecyloxy)phenol: The Role of Molecular Structure on Interfacial Properties C. Douglas Taylor and Mark R. Anderson* Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0212 Received May 18, 2001. In Final Form: October 25, 2001 Self-assembled monolayers of 3-(12-mercaptododecyloxy)phenol and 4-(12-mercaptododecyloxy)phenol are prepared in an effort to probe the role of the orientation of the terminal hydroxyl group on interfacial properties. Differences in the interfacial properties of these isomeric monolayers are characterized by reflection-absorption infrared spectroscopy, reductive desorption, and contact angle measurements. The coverage of 4-(12-mercaptododecyloxy)phenol is 40%-60% lower than that of 3-(12-mercaptododecyloxy)phenol, suggesting that structural differences between these monolayers contribute to the monolayer organization. This is supported by differences in the infrared spectra which show that the monolayers have different organization at the interface. Contact angle measurements using solutions buffered at pH 7 show that monolayers of 4-(12-mercaptododecyloxy)phenol have a lower surface energy than monolayers of 3-(12-mercaptododecyloxy)phenol. Contact angle titrations of these monolayer-modified surfaces also have distinctly different behavior. The titration curve of 3-(12-mercaptododecyloxy)phenol has a clear transition from low pH to high pH with an inflection near pH 9, while 4-(12-mercaptododecyloxy)phenol does not reach a limiting value at high pH. These results are interpreted in terms of interfacial structural differences between the isomeric monolayers.

Introduction Molecular modification of interfaces has been an active area of research for many years.1,2 Using the LangmuirBlodgett method, silane chemistry, or the spontaneous adsorption of mercaptans to modify substrates, many research groups demonstrate that the presence of chemical overlayers can dramatically alter the interfacial properties of a substrate.1-8 Structural investigations show that small changes of the interfacial molecular structure may result in relatively large changes in how the substrate interacts with the adjacent media.9-11 Consequently, much effort has gone toward understanding how molecular modification of interfaces can be used in adhesion, lubrication, and corrosion protection applications. The ability to alter the properties of the interface makes surface modification particularly attractive for use in adhesion promotion.1,6-8 In this application, the adhesion promoter is intended to alter the surface energy of the * Corresponding author. E-mail: [email protected]. Phone: (540)231-3869. (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) Watkins, B. F.; Behling, J. R.; Kariv, E.; Miller, L. L. J. Am. Chem. Soc. 1975, 97, 3549-3550. (3) Glodde, M.; Hartwig, A.; Hennemann, O. D.; Stohrer, W. D. Int. J. Adhes. 1998, 18, 359-364. (4) Tesoro, G.; Wu, Y. J. Adhes. Sci. Technol. 1991, 5, 771-784. (5) Tsuji, N.; Nozawa, K.; Aramaki, K. Corros. Sci. 2000, 42, 15231538. (6) Kim, H.; Jang, J. Polymer 2000, 41, 6553-6561. (7) Inagaki, N.; Tasaka, S.; Onodera, A. J. Appl. Polym. Sci. 1999, 73, 1645-1654. (8) Gonon, L.; Chabert, B.; Bernard, A.; Van Hoyweghen, D.; Ge´rard, J. F. J. Adhes. 1997, 61, 271-292. (9) Roush, J. A.; Thacker, D. L.; Anderson, M. R. Langmuir 1994, 10, 1642-1646. (10) Sun, L.; Kepley, L. J.; Crooks, R. M. Langmuir 1992, 8, 21012103. (11) Sun, L.; Crooks, R. M.; Ricco, A. J. Langmuir 1993, 9, 17751780.

substrate to promote favorable interactions between the substrate and the adhesive. Young and Dupre´ gave the relationship between the surface energy and adhesion strength in the 1800s as the work-of-adhesion:12

wAB ) γB(1 + cos θ)

(1)

where wAB is the thermodynamic work-of-adhesion between a substrate (A) and an adhesive (B), γB is the surface energy of the adhesive, and θ is the contact angle between a liquid and the substrate. Large values of the work-ofadhesion are characteristic of conditions needed to form a strong adhesive bond. Qualitatively, this relationship comes from recognizing the importance of the intermolecular forces that exist between the substrate and the adhesive. Determination of changes in the surface energy brought about by chemical modification of the interface provides a measure of how modifications of the interfacial structure may influence the thermodynamic work-ofadhesion. The presence of different functionality at the chain terminus of the monolayer is perhaps the most straightforward way of altering the chemical properties of the interface.9,13-15 This effect is demonstrated experimentally by the contact angle that water droplets have with differently functionalized monolayers.13,16 The experimental contact angle is representative of the macroscopic influence that modification has on the interfacial properties.13,17-21 More intriguing than the influence of different functionality on surface energy, however, is the observation that the contact angle of water droplets on some self-assembled monolayers changes depending on whether the monolayer is formed from an alkanethiol with an even or an odd number of carbon atoms in the alkyl chain.18,21-25 Tao et al.,21 Porter et al.,18 and Lee et al.23 (12) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces; John Wiley and Sons: New York, 1997; pp 452-459.

10.1021/la0107392 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/02/2002

SAMs of Isomers of (12-Mercaptododecyloxy)phenol

demonstrate this “even-odd effect” dependence of the surface energy using sessile drop contact angle measurements for methyl-terminated adsorbates. This result is representative of the subtle influence that changing interfacial structure has on the surface energy of modified substrates.21 More recently, Porter illustrated the “even-odd effect” of adsorbed alkanethiols on changing the surface energy using frictional force measurements of gold substrates modified by contact printing regions of even-chain-length alkanethiols separated from regions of odd-chain-length alkanethiols.25 Here, the frictional force image, characteristic of interfacial regions with different surface energies, replicates the image stamp used to prepare the modified interface. These results with different monolayers suggest that surface energy, and consequently the thermodynamic work-of-adhesion, is related to the identity and projection of the terminal functionality at the interface. These results also suggest that one may be able to relate interfacial molecular features (e.g. structure) to macroscopic behavior. The even-odd effect has been demonstrated for interfaces modified with monolayers that have pendant alkyl chains.21,22,25 Presumably, the effect of terminal group orientation on surface energy would be even larger if the terminal functionality were more polar than a methyl group. For ω-functionalized alkanethiols, however, the terminal functionality often introduces other intermolecular interactions that may disrupt the overall organization of the self-assembled monolayer.13 This disruption of the structural organization of the monolayer may prevent the systematic variation of the polar terminal functional group projection at the interface based on the even-odd effect. Studying the relationship between surface energy and functional group orientation, therefore, requires systems that do not rely on the even-odd effect of homologous alkane chains for changing the projection of the functional group. Wells et al. show that the positional isomers of mercaptobenzoic acid can be used to systematically vary the projection of the carboxylic acid functionality at the interface.26 In their research, monolayers of o-, m-, and p-mercaptobenzoic acid were prepared and their interaction with vapor phase n-decylamine was studied. They found that the ability of the monolayer to interact with the adjacent vapor phase amine was a strong function of the orientation of the carboxylic acid group at the interface. They also found that the ability of the carboxylic acid

Langmuir, Vol. 18, No. 1, 2002 121

Figure 1. Structure of 3-(12-mercaptododecyloxy)phenol and 4-(12-mercaptododecyloxy)phenol molecules used to prepare the isomeric monolayers.

group to laterally hydrogen bond was strongly influenced by the projection of the carboxylic acid group, with the meta and ortho isomers having spectral evidence of greater extents of lateral hydrogen bonding than the para isomer. The absence of strong lateral hydrogen bonding within the monolayer prepared with p-mercaptobenzoic acid was also reported by Creager and can be attributed to steric interactions preventing the carboxylic acid groups from being close enough to form hydrogen bonds.27 Although they did not report contact angles, the research by Wells et al. suggests that a monolayer with a pendant phenyl group may provide an avenue for changing the orientation of functionality at the interface and for altering the surface energy of a modified interface.26 In our research, we are interested in preparing molecules for use as adhesion promoters and understanding their interfacial behavior. Here we prepare isomeric monolayers based on differing substitution patterns around a terminal phenyl ring, using the molecules 3-(12mercaptododecyloxy)phenol and 4-(12-mercaptododecyloxy)phenol (Figure 1) to modify interfaces. The presence of a long alkyl chain spacer between the gold surface and the phenyl group is intended to insulate the phenyl ring electronically from the metallic substrate and to contribute to the structural organization of the molecules at the interface. By using these positional isomers of (12mercaptododecyloxy)phenol, we investigate whether the projection of the terminal functionality can be systematically altered using isomeric monolayers of this type. We also investigate how the interfacial properties of the modified substrate change on the basis of the substitution pattern of the terminal phenyl ring. The ability to systematically alter the physical properties of the interface by changing the substitution on the terminal phenyl ring may provide an alternate mechanism for studying how the projection of structural features of a monolayer impacts the macroscopic behavior of the interface. Experimental Section

(13) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-691. (14) Miller, C.; Cuendet, P.; Gratzel, M. J. Phys. Chem. 1991, 95, 877-886. (15) Zhang, M.; Anderson, M. R. Langmuir 1994, 10, 2807-2813. (16) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365-385. (17) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (18) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370-2378. (19) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1990, 112, 570-579. (20) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990-1995. (21) Tao, Y.-T. J. Am. Chem. Soc. 1993, 115, 4350-4358. (22) 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. (23) Colorado, R.; Villazana, R. J.; Lee, T. R. Langmuir 1998, 14, 6337-6340. (24) Green, J. D.; McDermitt, M. T.; Porter, M. D. J. Phys. Chem. 1995, 99, 10960-10965. (25) Wong, S.-S.; Takano, H.; Porter, M. D. Anal. Chem. 1998, 70, 5209-5212. (26) Wells, M.; Dermody, D. L.; Yang, H. C.; Kim, T.; Crooks, R. M.; Ricco, A. J. Langmuir 1996, 12, 1989-1996.

Hydroquinone, resorcinol, thioacetic acid, and 1,12-dibromododecane (Aldrich) were reagent grade and used without further purification. Ethanol (200 proof) was obtained from Midwest Grain products and stored over molecular sieves. Potassium hydroxide and sodium sulfate were obtained from Mallinckrodt and used as received. Hexane, ethyl acetate, diethyl ether, CHCl3, and CCl4 were obtained from VWR and used as received. 3- and 4-(12-Mercaptododecyloxy)phenol were synthesized from resorcinol or hydroquinone, respectively, and 1,12-dibromododecane. Details of the synthetic procedure are given in the Supporting Information. Monolayers of these isomers were prepared by standard self-assembly procedures from chloroform solutions.28 Gold substrates (1 in. × 1 in.) 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. Monolayer preparation was conducted by spontaneous adsorption from chloroform solutions containing between 2.0 × 10-5 and 1.0 × 10-3 M concentrations (27) Creager, S. E.; Steiger, C. M. Langmuir 1995, 11, 1852-1854. (28) Anderson, M. R.; Gatin, M. Langmuir 1994, 10, 1638-1641.

122

Langmuir, Vol. 18, No. 1, 2002

of the mercaptan. Immediately prior to adsorption, the gold substrates were cleaned in “piranha” solution (75% concentrated H2SO4s25% 30% H2O2) for approximately 1 min, rinsed with water, and dried in a stream of dry nitrogen. Caution must be exercised with piranha solutions, as they are potentially explosive. Following usage, piranha solutions should be properly disposed of. The clean substrates were kept in the adsorption solution for 24 h to prepare the monolayer. 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 reflection-absorption infrared spectroscopy, contact angle goniometry, ellipsometry, and reductive desorption measurements. Infrared spectroscopy was conducted with a Nicolet model 710 FTIR (Madison, WI) spectrometer using a liquid nitrogen cooled HgCdTe detector. Reflection spectra of the monolayers were collected at 2 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 ZnSe 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). Sessile drop contact angles were measured using the quartz crystal microbalance method described by Ward et al.29 AT cut quartz crystal oscillators were obtained from International Crystal Manufacturing (Oklahoma City, OK). The quartz crystals were resonant at a fundamental frequency of 5 MHz and had 0.25 cm diameter vapor-deposited Au electrodes. Each electrode was coated with a monolayer of the compound of interest using self-assembly procedures previously described. A 10 µL syringe was obtained from Hamiliton and was fitted with a Chaney adapter to reproducibly deliver a constant volume of liquid to the center of the quartz crystal oscillator. Deionized water from a Barnstead Nanopure II water purification system with a measured resistivity of 17.3 MΩ‚cm was used to prepare solutions for the contact angle measurements. Buffer solutions used in the contact angle titrations were prepared using phosphoric acid, NaH2PO4, Na2HPO4, or Na3PO4. The pH of the individual buffer solutions was adjusted to the desired value with 0.1 M NaOH. Sodium chloride was added to the buffer solutions to maintain constant ionic strength (0.5 M). The chemicals were reagent grade and used as received. During the contact angle titrations, the probe liquid pH was randomly varied. After each measurement, the substrate was rinsed with a pH 7 buffer and then dried by blowing with dry nitrogen. If the substrate frequency did not return to approximately the same initial value ((2 Hz) following the rinse, then the substrate was replaced with a freshly prepared sample for subsequent measurements. The quartz crystal microbalance contact angle measurement was calibrated by comparison to optical goniometry using Ward’s method.29 The output of the crystal oscillator circuit (locally constructed, based on Buttry’s design30) was connected to a Hewlett-Packard (Palo Alto, CA) 3932B frequency counter, and frequency measurements were made with a gate time of 1 s and a resolution of 0.01 Hz. The frequency counter was interfaced to a computer via a GPIB-488 (Measurement Computing Corp., Middleboro, MA), and the data were collected using a locally written program. Ellipsometry measurements were conducted with a Gaertner ellipsometer (Chicago, IL). Thickness values are the average of a minimum of five measurements using monolayers prepared by adsorption for 24 h from 1.0 × 10-3 M solutions of the mercaptan. A value of 1.45 was used as the real part of the refractive index in the thickness calculations. Reductive desorption experiments were conducted using an EG&G Princeton Applied Research model 273 galvanostat/ potentiostat controlled by EG&G Princeton Applied Research (Princeton, NJ) model 270 software. A drop electrochemical cell similar to that described by Chidsey was used for these measurements.13 The inside diameter of the O-ring holding the (29) Lin, Z.; Ward, M. D. Anal. Chem. 1996, 68, 1285-1291. (30) Melroy, O.; Kanazawa, K.; Gordon, J. G.; Buttry, D. Langmuir 1986, 2, 697-700.

Taylor and Anderson

Figure 2. Reflection-absorption infrared spectra of monolayers prepared from (A) (12-mercaptododecyloxy)benzene, (B) 3-(12-mercaptododecyloxy)phenol, and (C) 4-(12-mercaptododecyloxy)phenol. 0.5 M KOH electrolytic solution in place defined the geometric area, 0.646 cm2, of the working electrode. The gold substrates were found to have a surface roughness of 1.2 using the method of Porter et al.17 In the reductive desorption experiments, the potential was swept between -0.200 and -1.2 V versus AgAgCl at a rate of 100 mV/s. All solutions were deaerated with nitrogen for 30 min prior to measurement.

Results and Discussion Reflection infrared spectra of monolayers of 3-(12mercaptododecyloxy)phenol, 4-(12-mercaptododecyloxy)phenol, and (12-mercaptododecyloxy)benzene prepared by self-assembly on gold substrates are shown in Figure 2. The monolayers have spectral differences that cannot be attributed solely to the different aromatic substitution patterns of these molecules. For example, the methylene modes for the three monolayers differ from each other, decreasing in absorbance intensity for (12-mercaptododecyloxy)benzene, 3-(12-mercaptododecyloxy)phenol, and 4-(12-mercaptododecyloxy)phenol, respectively. This result is most likely due to steric interactions introduced by the terminal phenyl group that prevent efficient organization of the alkyl chains for these monolayers. Although each of these monolayers has steric interactions due to the terminal phenyl ring, the position of the hydroxyl on the phenyl ring apparently changes the lateral interactions among these monolayers, influencing the overall organizational structure of the alkyl portion of the monolayer. The methylene modes are stronger for the (12-mercaptododecyloxy)benzene monolayer than for the monolayers of the phenol isomers. This result suggests that the hydroxyl group has a significant role in establishing the intermolecular interactions that contribute to the monolayer organization. This was surprising, as we anticipated

SAMs of Isomers of (12-Mercaptododecyloxy)phenol

Langmuir, Vol. 18, No. 1, 2002 123

Table 1. Surface Coverage, as Determined by Reductive Desorption Measurements, of 3-(12-Mercaptododecyloxy)phenol and 4-(12-Mercaptododecyloxy)phenol Adsorbed from Different Concentration Solutions solution conc (mol/L) 3.4 ×

10-4

1.7 × 10-4 6.8 × 10-5 1.7 × 10-5

monolayer

Γ ((0.005 × 10-10 mol/cm2)

peak position (V vs Ag/AgCl)

3-(12-mercaptododecyloxy)phenol 4-(12-mercaptododecyloxy)phenol 3-(12-mercaptododecyloxy)phenol 4-(12-mercaptododecyloxy)phenol 3-(12-mercaptododecyloxy)phenol 4-(12-mercaptododecyloxy)phenol 3-(12-mercaptododecyloxy)phenol 4-(12-mercaptododecyloxy)phenol

5.197 3.701 4.030 2.105 2.777 1.091 1.064 0.753

-1.149 -1.013 -1.103 -0.993 -1.051 -0.970 -0.994 -0.960

that the steric interactions of the phenyl ring alone would dominate the lateral interactions present at the interface and possibly isolate the hydroxyl from interacting with neighboring molecules. The methylene peak positions (2854 and 2924 cm-1) for each of the monolayers indicate that the alkyl portion of each monolayer is in a more disordered environment than is found for straight chain alkanethiol monolayers.17 This result suggests that the lateral interactions from the terminal phenol dominate the organization of the interface. The different relative intensities of the methylene modes may be characteristic of a different net orientation of the alkyl portion of the monolayer for these molecules. The low intensity of these modes in the 4-(12-mercaptododecyloxy)phenol monolayer indicates that its alkyl chain may, on average, be oriented more normal to the substrate than those of 3-(12-mercaptododecyloxy)phenol and (12-mercaptododecyloxy)benzene monolayers. Qualitatively, these spectral results suggest that the presence and position of the hydroxyl group influence the overall monolayer structure. Vibrational features at lower energy are assigned to aromatic ring modes and C-O modes associated with the ether or the phenol. Differences between the monolayer spectra of the isomers may be due to both the orientation of the molecules at the interface and the substitution pattern of the hydroxyl group on the ring. The monolayer prepared with 4-(12-mercaptododecyloxy)phenol has two dominant features in the reflection spectrum: one at 1501 cm-1 and the other at 1245 cm-1. The feature at 1501 cm-1 is tentatively assigned to a ring C-C stretching mode (vibration 19b by the Wilson numbering convention).31,32 The feature at 1245 cm-1 is assigned to an asymmetric φ-O-C vibration of the aromatic ether.33 A less intense absorption is found at 1187 cm-1 that is assigned to the aromatic C-O vibration of the phenol.31,34 The reflection spectrum of monolayers of 3-(12-mercaptododecyloxy)phenol has more features than observed with monolayers of the para isomer, not all of which have been assigned. Absorptions at 1611 and 1502 cm-1 are assigned to C-C ring stretching modes (vibrations 8a and 19b, respectively, by the Wilson numbering convention).31,32 The asymmetric aromatic φ-O-C of the aromatic ether is assigned to a feature at 1190 cm-1, and the phenol C-O is assigned to a feature at 1157 cm-1.31,33,34 When the 3-(12-mercaptododecyloxy)phenol and 4-(12mercaptododecyloxy)phenol isomers are adsorbed from chloroform solutions of the same concentration (ranging from 2.0 × 10-5 to 5.0 × 10-4 M), the coverage of the para isomer is consistently 40-60% less than the coverage of (31) Pretsch, E.; Clerc, T.; Siebl, J.; Simon, W. Tables of Spectral Data for Structure Determination of Organic Compounds; SpringerVerlag: Berlin, 1989. (32) Varsanyi, G. Vibrational Spectra of Benzene Derivatives; Academic Press: New York, 1969. (33) Nyquist, R. A. Appl. Spectrosc. 1991, 45, 1649-1651. (34) Keresztury, G.; Billes, F.; Kubinyi, M.; Sundius, T. J. Phys. Chem. A 1998, 102, 1371-1380.

the meta isomer (Table 1). The saturation coverage of both isomers (5.2 × 10-10 mol/cm2 for 3-(12-mercaptododecyloxy)phenol and 3.7 × 10-10 mol/cm2 for 4-(12-mercaptododecyloxy)phenol, respectively) is also significantly less than that of straight chain alkanethiols (literature values average approximately 9 × 10-10 mol/cm2 for dodecanethiol35,36). The experimentally determined coverage for 3-(12-mercaptododecyloxy)phenol and 4-(12-mercaptododecyloxy)phenol is consistent with the coverage of other alkanethiols that are terminated with a bulky, aromatic functionality.36 These results imply that the terminal phenyl ring and hydroxyl group introduce steric interactions that prevent efficient surface coverage by the (12-mercaptododecyloxy)phenol isomers. The interpretation that the bulky terminal phenol group disrupts the organization of the underlying alkyl chains is consistent with the position of the methylene modes (2854 and 2924 cm-1) in the reflection infrared spectra, and with the interpretation that the position of the hydroxyl group alters the extent of lateral interactions that exist within the monolayers. The thicknesses of the 3-(12-mercaptododecyloxy)phenol and 4-(12-mercaptododecyloxy)phenol monolayers, as determined by ellipsometry, are 22 ( 2 and 18 ( 2 Å, respectively. These values are consistent with the monolayer thicknesses reported for other alkanethiols.17 Had the monolayers assumed a well-packed, dense structure, the measured thicknesses of the isomeric monolayers should be approximately the same. Because the monolayers do not assume a well-ordered structure, the smaller value for the 4-(12-mercaptododecyloxy)phenol monolayer thickness may be characteristic of a less densely packed monolayer than found with the 3-(12-mercaptododecyloxy)phenol, consistent with the coverage trends found by the reductive desorption measurements.17 Comparing the reductive desorption behavior of 3-(12mercaptododecyloxy)phenol and 4-(12-mercaptododecyloxy)phenol when both have approximately the same surface coverage (e.g. approximately 1.0 × 10-10 mol/cm2, Table 1) reveals that the reductive desorption peak of the 4-isomer is at a potential 24 mV positive of the reductive desorption peak of the 3-isomer (Figure 3). Reductive desorption at the more positive potential is characteristic of the monolayer having less lateral stabilization. Under these experimental conditions (0.5 M KOH), it is anticipated that the hydroxyl groups are ionized; therefore, the more positive reductive desorption peak potential of 4-(12-mercaptododecyloxy)phenol is taken as being representative of more destabilization of the monolayer due to electrostatic interactions than is found for monolayers prepared with 3-(12-mercaptododecyloxy)phenol.35,37 Using Imabayashi’s model, a 24 mV difference in the reductive desorption peaks represents a 2.3 kJ/mol (35) Imabayashi, S.; Iida, M.; Hobara, D.; Feng, Z. Q.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1997, 428, 33-38. (36) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C. K.; Porter, M. D. Langmuir 1991, 7, 2687-2693.

124

Langmuir, Vol. 18, No. 1, 2002

Taylor and Anderson

Figure 4. Sessile drop contact angle measurements as a function of pH for monolayers of (12-mercaptododecyloxy)benzene. Data points are the average of three measurements at each pH acquired using several substrates.

Figure 3. Reductive desorption cyclic voltammograms for monolayers of (A) 4-(12-mercaptododecyloxy)phenol and (B) 3-(12-mercaptododecyloxy)phenol. Measurements were conducted in 0.50 M aqueous KOH using a 100 mV/s scan rate. Table 2. Experimentally Determined Contact Angles Measured by Sessile Drop Optical Goniometry monolayer

contact angle

(12-mercaptododecyloxy)benzene 4-(12-mercaptododecyloxy)phenol 3-(12-mercaptododecyloxy)phenol 4-thiophenol

80 ( 4 72 ( 2 57 ( 3 57 ( 2

difference in the monolayer stabilization energy between the isomers of (12-mercaptododecyloxy) phenol.35 These apparent interfacial structural differences suggested by the infrared spectra and the reductive desorption measurements are also qualitatively manifested in the contact angle that each of these monolayers makes with water (Table 2). Despite the presence of the terminal hydroxyl group of 4-(12-mercaptododecyloxy)phenol, the contact angle of this monolayer (72°) is comparable to that of an analogous monolayer that lacks the hydroxyl group, (12-mercaptododecyloxy)benzene (80°). These results should be contrasted with the contact angle of 3-(12mercaptododecyloxy)phenol (57°), which is approximately the same as that measured with a monolayer prepared with 4-hydroxythiophenol (57°). Differences in the contact angles along with the infrared spectra of monolayers prepared from the isomers of (12-mercaptododecyloxy)phenol are suggestive of different interfacial structures of these isomers, perhaps related to the projection of the hydroxyl group at the interface. These interfacial differences between the isomeric monolayers are also manifested in contact angle titrations for these modified interfaces. As expected, the contact angle of (12-mercaptododecyloxy)benzene monolayers does not significantly change with changing pH of the aqueous sessile drop during the titration (Figure 4). This behavior is consistent with that of alkanethiol monolayers, and is the expected result, as this monolayer does not have an (37) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359.

ionizable group. With monolayers prepared from 3-(12mercaptododecyloxy)phenol and 4-(12-mercaptododecyloxy)phenol, however, there is an ionizable phenol and the contact angle is expected to change with the pH of the aqueous sessile drop. The pKa values for the isomers are estimated to be ≈9.6 for 3-(12-mercaptododecyloxy)phenol and ≈10.2 for 4-(12-mercaptododecyloxy)phenol, by analogy to literature pKa values for m- and p-alkoxyphenols, respectively.38,39 The titration curve for 3-(12-mercaptododecyloxy)phenol has a clear transition in the measured contact angle as the pH of the sessile drop is varied from acidic to basic values (Figure 5). If the pH where the measured contact angle is halfway between the low pH and high pH limiting values is taken as being representative of the pK1/2 of the monolayer (e.g. is the point at which 50% of the confined molecules have been deprotonated40,41), then the experimental pK1/2 of the 3-(12-mercaptododecyloxy)phenol monolayer is approximately 9. Contact angle titrations for 4-(12-mercaptododecyloxy)phenol were considerably different than those for the meta isomer. No clear transition in the measured contact angle of 4-(12-mercaptododecyloxy)phenol was observed in the titration (Figure 5). At pH values greater than 10, however, there is a general trend toward lower contact angle. This behavior is consistent with contact angle titrations of ω-mercaptoalkanoic acids where a slow trend to lower contact angle for sessile drops with higher pH was also observed.40,41 For monolayers prepared with ω-mercaptoalkanoic acids, the absence of a clearly defined transition in the contact angle titration curve has been attributed to strong lateral interactions between the terminal groups of the monolayer stabilizing the terminal acid functionality.40,41 The estimated pKa of the 4-(12-mercaptododecyloxy)phenol is higher than that of the meta isomer (10.2 vs 9.6); however, a pKa of 10.2 is well within the pH range investigated, and a pK1/2 transition in the titration curve should be observed if its interfacial behavior is like that of 3-(12-mercaptododecyloxy)phenol. The similarity of the experimental contact angle titration behavior for 4-(12(38) Epstein, J.; Plapinger, R. E.; Michel, H. O.; Cable, J. R.; Stephani, R. A.; Hester, R. J.; Billington, C.; List, G. R. J. Am. Chem. Soc. 1964, 86, 3075-3084. (39) Wehry, E. L.; Rogers, L. B. J. Am. Chem. Soc. 1966, 88, 351354. (40) Creager, S. E.; Clarke, J. Langmuir 1994, 10, 3675-3683. (41) Lee, T. R.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741-749.

SAMs of Isomers of (12-Mercaptododecyloxy)phenol

Figure 5. Sessile drop contact angle titrations of (A) monolayers of 3-(12-mercaptododecyloxy)phenol and (B) monolayers of 4-(12-mercaptododecyloxy)phenol. Data points are the average of five measurements at each pH acquired using several substrates.

Figure 6. Sessile drop contact angle titration of monolayers of 4-hydroxythiophenol. Data points are the average of five measurements at each pH acquired using several substrates.

mercaptododecyloxy)phenol with that of ω-mercaptoalkanoic acid monolayers suggests that this isomer has stronger lateral interactions at the interface than the meta isomer that prevent efficient titration of the acidic proton. The contact angle titration of 3-(12-mercaptododecyloxy)phenol and 4-(12-mercaptododecyloxy)phenol should be contrasted with that of a monolayer of 4-hydroxythiophenol (Figure 6). As with the 3-(12-mercaptododecyloxy)phenol, monolayers prepared with 4-hydroxythiophenol have a clear transition as the pH of the sessile drop is altered from acidic to basic values. The estimated pK1/2 of 4-hydroxythiophenol from these contact angle

Langmuir, Vol. 18, No. 1, 2002 125

titrations (≈10) is consistent with values found in the literature for phenol. Wells et al. as well as Creager showed that the carboxylic acid groups in a monolayer prepared with 4-mercaptobenzoic acid are isolated from each other and do not have extensive lateral hydrogen bonding interactions with neighboring carboxylic acid functionalities.26,27 By analogy to monolayers of 4-mercaptobenzoic acids, the phenolic OH of 4-hydroxythiophenol monolayers may also be isolated from, and not have strong lateral interactions with, neighboring hydroxyls. The similarity between the contact angle behavior of 4-hydroxythiophenol and 3-(12-mercaptododecyloxy)phenol suggests that the phenolic OH functionality in each of these monolayers experiences a similar environment. The absence of a transition in the titration of 4-(12-mercaptododecyloxy)phenol as well as the monolayer’s apparent lower surface energy suggests that the phenolic OH for this monolayer experiences a different environment at the interface than the OH of both 4-hydroxythiophenol and 3-(12-mercaptododecyloxy)phenol. From the infrared spectroscopy, the reductive desorption results, ellipsometry, and the contact angle data found for the isomers of (12-mercaptododecyloxy)phenol, we propose a qualitative model of the structure of these monolayers. In the model for the 3-(12-mercaptododecyloxy)phenol monolayer, the phenolic hydroxyl is oriented nearly normal to the interface. In this orientation, the hydroxyl groups are likely to be isolated from each other, because of steric interactions with neighboring phenyl rings, and to have fewer interactions with the bulk of the monolayer; therefore, ionization will have less of a destabilizing effect on this monolayer. This is consistent with Wells’ results for 4-mercaptobenzoic acid monolayers where the carboxylic acid group is more easily ionized than the other isomeric monolayers.26 Observation of a clear transition during the contact angle titration, and the correspondence between the estimated pKa values for the bulk 3-(12-mercaptododecyloxy)phenol and the experimentally determined pK1/2 from the contact angle titration for the monolayer are consistent with the hydroxyl being available for interaction with the adjacent phase and not being stabilized by lateral interactions within the monolayer. A normal orientation of the hydroxyl for the 3-(12-mercaptododecyloxy)phenol monolayer is consistent with the contact angle data and also places the hydroxyl in an orientation similar to that expected for the hydroxyl of 4-hydroxythiophenol (e.g. normal to the substrate), a monolayer whose contact angle behavior the 3-(12-mercaptododecyloxy)phenol closely approximates. The model structure of the 4-(12-mercaptododecyloxy)phenol monolayer orients the phenolic hydroxyl away from the surface normal. In this conformation, the hydroxyl may extend along or into the monolayer interface, exposing more of the aromatic C-H and C-C at the interface than found with the model 3-(12-mercaptododecyloxy)phenol monolayer structure. This schematic model is consistent with the lower surface energy measured with 4-(12mercaptododecyloxy)phenol. In this orientation, the hydroxyl will participate in more lateral interactions among the molecules that comprise the monolayer than would be expected with the 3-(12-mercaptododecyloxy)phenol. If the hydroxyl in the 4-(12-mercaptododecyloxy)phenol monolayer extends into the monolayer, as proposed, a destabilizing effect may be expected if the layer becomes ionized, consistent with reductive desorption results. This interfacial structure may also account for the lower surface coverage found for 4-(12-mercaptododecyloxy)phenol due to steric interactions attributable to the hydroxyl group

126

Langmuir, Vol. 18, No. 1, 2002

that are not present with the 3-(12-mercaptododecyloxy)phenol monolayer. Summary For monolayers prepared with the isomers of (12mercaptododecyloxy)phenol, there are clear differences in the interfacial behavior that we attribute to the structural differences of the monolayers. The 3-isomer has a higher surface energy that we attribute to the hydroxyl being oriented more normal to the substrate. We propose that the hydroxyl on the 4-isomer is projected away from the surface normal and may lie along the monolayer interface. In this orientation, more of the aromatic C-H or C-C is exposed at the interface, leading to a lower surface energy than is found for the 3-isomer. The monolayers also have different surface coverages. This result is also attributed to the different orientations of the terminal hydroxyl group altering the amount of lateral interactions that exists among the molecules that comprise the monolayer. In the case of the 4-isomer, the projection of the hydroxyl group along the monolayer surface possibly introduces additional steric interactions that prevent efficient packing of the molecules on the surface. As a consequence, the surface coverage of the 4-isomer is typically 40-60% less that is found with the 3-isomer.

Taylor and Anderson

The monolayers prepared from the positional isomers of (12-mercaptododecyloxy)phenol have different interfacial properties, as shown by the contact angle measurements. Infrared spectra of the monolayers suggest that the interfacial structures are different, and we attribute the observed differences in the contact angle behavior to these structural differences. It is not clear from our data, however, if the interfacial differences are due to the substitution of the terminal aromatic ring or to the different abilities of these layers to assemble at the interface. The positions of the methylene modes and the different intensities of these modes in the reflection infrared spectra of the monolayers suggest that the apparent structural differences are due to the latter. Acknowledgment. This research was partially funded in the form of a fellowship to C.D.T. by the Adhesion and Sealant Council Education Foundation and the Center for Adhesive and Sealant Science. Supporting Information Available: Text describing the syntheses of 3- and 4-(12-mercaptododecyloxy)phenol and transmission infrared spectra of these two compounds. This material is available free of charge via the Internet at http://pubs.acs.org. LA0107392