Influence of Specific Intermolecular Interactions on ... - ACS Publications

A shortening of the alkyl chain to 11 methylenes has no effect on the amide-EG6 ...... stretching modes at 20 °C, before the heating cycle. The spect...
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J. Phys. Chem. B 2001, 105, 5459-5469

5459

Influence of Specific Intermolecular Interactions on the Self-Assembly and Phase Behavior of Oligo(Ethylene Glycol)-Terminated Alkanethiolates on Gold Ramu j nas Valiokas, Sofia Svedhem,† Mattias O 2 stblom, Stefan C. T. Svensson,† and Bo Liedberg* DiVisions of Applied Physics and Chemistry, Department of Physics and Measurement Technology, Linko¨ pings UniVersitet, S-581 83 Linko¨ ping, Sweden ReceiVed: December 7, 2000; In Final Form: March 19, 2001

A comparative study of the self-assembly and phase behavior of seven different oligo(ethylene glycol) (OEG)terminated alkanethiols on polycrystalline gold surfaces is presented. The general structure of the compounds is HS(CH2)m-X-EGn, where m ) 11, 15; n ) 2, 4, 6, and the linkages X are amide (-CONH-), ester (-COO-), or ether (-O-) groups. The amide and ester groups give rise to the intermolecular hydrogen bonding and dipole-dipole interactions, respectively, whereas the ether lacks specific interactions. The results from contact angle goniometry, null ellipsometry, and infrared reflection-absorption spectroscopy (IRAS) indicate that the intermolecular interactions can be partly used to control the conformation and order of the OEG portion of the self-assembled monolayers (SAMs). It is shown that the lateral hydrogen bonding stabilizes the all-trans conformation of the EG4 tails in the SAMs. Further on, the mechanism behind the thermal phase behavior of the OEG SAMs is investigated using temperature-programmed IRAS in ultrahigh vacuum. In the present study we show that the earlier reported helix-to-all-trans conformational transition at 60 °C in the ¨ stblom, M.; Svedhem, S.; Svensson, S. C. T.; Liedberg, B. SAM of HS(CH2)15CONH-EG6 (Valiokas, R.; O J. Phys. Chem. 2000, 104, 7565-7569.) is a result of the particular molecular design of the SAMs through the specifically built-in lateral hydrogen bonds. A shortening of the alkyl chain to 11 methylenes has no effect on the amide-EG6 phase behavior. Contrary, the ester- and ether- containing SAMs undergo a melting type of transitions at 52 and 68 °C, respectively, similar to that observed for poly(ethylene glycol).

Introduction This study focuses on lateral intermolecular interactions, specifically introduced into SAMs1,2 of oligomer-based compounds, and on their use to control the conformation and phase behavior of the SAMs. Oligo(ethylene glycol) derivatives, chemisorbed onto solid surfaces, is a good example of the oligomer-based SAMs. During the past decade,3 they have been used as model surfaces in studies of protein adsorption phenomena,4,5 thus mimicking properties of poly(ethylene glycol),6 a well-known protein- and cell-inert material.7,8 Different types of OEG-terminated SAMs on gold, silver3,9 and silicon10 have been investigated, including those assembled from the synthesized compounds and those prepared by reactions in situ on surfaces.11 Further on, soluble and flexible OEG chains in SAMs have been employed as a spacer for tethering of various biospecific groups in studies of molecular recognition events on surfaces.12-14 Similarly, OEG spacers are useful in the formation of the supported lipid membranes.15-18 In this case, the OEG SAMs serve as a hydrogellike matrix, separating the lipid phase from the solid support and thereby maintaining a liquid-crystalline structure of the biomimetic membrane. Therefore, the OEG SAMs also can be utilized in the design of interfacial layers for biosensors, DNA chips, and other bioanalytical devises.19-21 Although the OEG SAMs is an attractive approach in the discussed applications and could be of potential technological * Corresponding author: tel., +46 13 281877; fax, +46 13 137568; e-mail, [email protected]. † Division of Chemistry.

importance, obviously not enough has been done in order to understand their self-assembly mechanisms, structural properties, and stability. Harder et al.9 investigated structure of EG6 and EG3-containing SAMs, and demonstrated a way of controlling the conformation of metoxy tri(ethylene glycol)-terminated SAMs by varying the substrate lattice parameters. On the basis of the structural and protein adsorption studies, Grunze and coworkers proposed a mechanism, explaining how the resistance to protein binding depends on the conformation of the OEG.9,22 Further on, theoretical investigations of the conformational properties of the OEG SAMs have been reported.23,24 Recently, the effect of solvents on the conformation of the OEG SAMs has been studied.25,26 In addition, the self-assembly of a series of thiaoligo(ethylene glycol)-containing compounds has been investigated, showing another approach of SAM formation which takes an advantage of organized OEGs with well-defined conformations as a support for a monolayer of ordered aliphatic chains.27,28 A new class of OEG-terminated and amide group-containing alkanethiols was introduced by our group.29 The highly ordered SAMs, formed by these compounds, showed an OEG chainlength dependence of the oligomer conformation. The key feature of this type of OEG SAMs is the lateral hydrogen bonding between the amide groups, and we have suggested that the specifically introduced intermolecular interactions can affect the molecular conformation of the oligomer portion in the SAMs.29 Moreover, we have found an unusual temperaturedependent OEG phase behavior in the SAMs: the helical EG6 could be reversibly switched to the all-trans conformation at

10.1021/jp004441g CCC: $20.00 © 2001 American Chemical Society Published on Web 05/19/2001

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Valiokas et al. linkage) are employed to verify the effect of the alkyl spacer length vs the effect of the specific lateral interactions on the phase behavior of the OEG SAMs. We present here the results of contact angle goniometry, ellipsometry, IRAS, and temperature-programmed (TP) IRAS, discussing the differences in the OEG phases of the different compounds. We believe that this study will contribute to the understanding of the self-assembly of OEG and other oligomer-based monolayers, and to the development of SAMs for studies of protein/cell adsorption on interfaces, supported lipid membranes, and novel molecular architectures in general. Experimental Section

temperatures above 60 °C.30 Thus, our system relates the design of oriented oligomer (in this case OEG)- based interfaces to the fundamental problem regarding the role of the specific intermolecular interactions in the self-assembly. The influence of lateral hydrogen bonding on properties of SAMs has been already in focus of several studies. For example, the presence of terminal hydrogen-bonded groups was shown to increase SAM stability.31 In other studies, various amide containing- alkanethiolates and fluorinated alkanethiolates on gold were investigated.32-38 It was found that the lateral hydrogen bonds between the amide groups contribute to the stability of the SAMs against electrochemical39and thermal desorption33,37 and against the exchange from the solution,37 as compared to SAMs interacting exclusively through van der Waals interactions. On the other hand, it was suggested, that the lateral hydrogen bonds could affect the template and packing properties of the thiolates on the Au(111) lattice.32,34,35,37 Also, contradicting results from electrochemical studies have been reported concerning the influence of the amide groups in the SAMs on the electron transport across the interface.40,41 The present study is a continuation of our previous work,29,30 shading light on the role of lateral hydrogen bonding on the self-assembly of the OEG SAMs. We investigate in detail if the specific intermolecular interactions can determine the phase behavior of this system and if they can be used to fine-tune the conformation of the OEG SAMs. Our aim is also to reveal the mechanism behind the reported reversible helix-to-all-trans conformational transition in the EG6 SAM. To answer these questions, we have investigated OEG-terminated SAMs, prepared from a series of compounds with a varying length of the alkyl and oligomer parts, and three different linking groups between them, Scheme 1. As analogues to the previously studied hydrogen-bonded SAMs of amide- containing 1, 2, and 4,29 nonhydrogen-bonded SAMs are formed from compounds 3 and 5. A weaker dipole-dipole interaction, arising from the ester linkage, is expected in these SAMs. Also, 6 (hydrogen bonded through an amide) and 7 (no specific interaction from an ether

Compounds and Sample Preparation. Procedures for the synthesis of 1, 2, and 4 were schematically presented in a previous work.29 A detailed description for the synthesis of the investigated compounds 1-6 will be published separately.42 Compound 7 was synthesized following earlier published methods.3 For adsorption of SAMs, fresh ethanolic 20µM solutions of the compounds were prepared in plastic beakers from 1 mM stock solutions stored in glass vials at room conditions. Compound 5 displayed some degree of decomposition after several weeks storage of its ethanolic solutions, therefore only fresh solutions were used for the formation of this SAM. As a substrate for the SAMs, 2000 Å thick gold films were electron beam deposited via a 25 Å titanium adhesion layer on standard (100)-silicon wafers. The electron beam evaporation of the metals was done in a Balzers UMS 500 P system operating at a base pressure on the low 10-9 mbar scale and at an evaporation pressure of about 10-7 mbar. A constant evaporation rate of 10 Å/s was used for gold. The resulting polycrystalline gold film is heavily dominated by (111) texture.43 Prior to SAM adsorption, the gold surfaces were cleaned in a 5:1:1 mixture of deionized (Milli-Q) water, 25% hydrogen peroxide, and 30% ammonia for 5 min at 80 °C, followed by rinsing in deionized water. The efficiency of cleaning was tested before the adsorption of the SAMs. For this purpose, the optical characteristics of a gold check sample, taken from the cleaned batch, were measured using an automatic Rudolph Research AutoEL ellipsometer with a He-Ne laser light source of λ ) 632.8 nm, at an angle of incidence of 70°. The rest of the cleaned samples were soaked in ethanol and then transferred into the incubation beakers. After at least 48 h of adsorption, the samples were rinsed in ethanol, ultrasonicated for 3 min, and rinsed again. Finally, they were blown dry in nitrogen gas and immediately analyzed. Ellipsometry. For single-wavelength ellipsometry, average values of the refractive index of the clean gold sample, analyzed prior to the incubation, were used. The refractive index of the substrate and the results of the ellipsometric measurements on the SAMs were taken into a model “ambient/organic film/gold”, assuming an isotropic, transparent organic layer44 with the refractive index of n ) 1.5.45-47 The film thickness was calculated as an average of measurements at three different spots on at least 4 samples for each compound. Contact Angle Goniometry. Contact angles were measured with a Rame´-Hart NRL 100 goniometer, without control on the humidity in the ambient, using freshly deionized water from a Milli-Q unit. Taking into account the high surface energy of the hydrophilic surfaces, only one measurement of the advancing and receding contact angle was taken per sample. Infrared Reflection-Absorption Spectroscopy. The roomtemperature reflection-absorption (RA) spectra were recorded on a Bruker IFS 66 system, equipped with a grazing angle (85°)

Oligo(Ethylene Glycol)-Terminated Alkanethiolates

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TABLE 1: Ellipsometric Thickness d, Advancing (θa), and Receding (θr) Contact Angles of Water of the OEG-Terminated Alkanethiolates on Gold cHSC

15CONH-EG2 (1) HSC15CONH-EG4 (2) HSC15COO-EG4 (3) HSC15CONH-EG6 (4) HSC15COO-EG6 (5) HSC11CONH-EG6 (6) HSC11O-EG6 (7)

d [Å]a

θa [deg] (2b

θr [deg] (2b

28.4 ( 0.3 33.3 ( 1.1 30.9 ( 0.5 39.8 ( 1.7 38.6 ( 0.6 34.9 ( 1.1 33.0 ( 1.4

27 30 33 28 29 28 28

25 28 30 25 24 25 25

a Errors at 95% confidence level. b Maximum errors. c Results are taken from ref 29.

infrared reflection accessory and a liquid-nitrogen-cooled MCT detector. The measurement chamber was continuously purged with nitrogen gas during the measurements. The acquisition time was around 10 min at 2 cm-1 resolution and a three-term Blackmann-Harris apodization function was applied to the interferograms before Fourier transformation. A spectrum of a deuterated hexadecanethiolate (HS(CD2)15CD3) on gold was used as reference. Temperature-Programmed Infrared Reflection-Absorption Spectroscopy. The temperature-programmed spectroscopic measurements were performed in an ultrahigh vacuum (UHV) system which has been described in detail elsewhere.48 Briefly, a modified Bruker IFS PID 22 spectrometer aligned at a grazing angle of 82° was equipped with f/16 transfer optics and a liquidnitrogen-cooled MCT detector. The spectral resolution was 2 cm-1 and 500 scans were collected. The same apodization function was applied to the interferograms as in room-temperature IRAS. All spectroscopic data was further analyzed using Bruker OPUS software. The temperature of the sample was changed by resistive heating or liquid nitrogen-cooling of a solid copper sample holder in the UHV measurement chamber, at a base pressure on the scale of 10-10 mbar. The temperature was measured by a Pt100 element in the sample holder and controlled by an Eurotherm controller. After setting the temperature to the desired value, the system was allowed to stabilize for 5 min, then the sample spectrum was recorded. The temperature range was chosen as in our previous study,30 and the infrared RA spectra were taken every 5 °C. When the spectroscopic characterization of the sample at the different temperatures was completed, it was set back to room temperature to check the reversibility. Finally, the sample was transferred into a preparation chamber for neon ion sputtering and the background spectrum was recorded. Results Contact Angle and Ellipsometric Measurements. The contact angle goniometric and ellipsometric results are shown in Table 1. Almost all investigated SAMs display similar wettability, with advancing and receding contact angles around 28° and 25°, respectively. However, the highest contact angles are observed for 3, and also 2 has somewhat higher values than the rest of the SAMs. Generally, the studied OEG SAMs are less hydrophilic than the ordered long chain ω-hydroxyalkanethiolates on gold,49 indicating certain differences in the orientation and exposure of the terminal CH2-CH2-OH groups. The measured ellipsometric thicknesses correspond well to the expected molecular sizes of the investigated compounds. Nevertheless, it is interesting to compare in more detail the thicknesses of the SAMs with the same alkyl and OEG parts but with different linking groups. In the HS(CH2)15-X-EG4

Figure 1. Infrared reflection-absorption spectra of SAMs formed by compounds 1-7, taken at room temperature: the CH stretching region.

series, the SAMs of analogue 3 are thinner by 2.4 Å than those of 2. As discussed above, SAM 3 also has the highest contact angles, which correlate with the ellipsometric measurements, suggesting a conformational difference between these two SAMs. Further on, in the HS(CH2)15-X-EG6 series the difference between the average thicknesses of SAMs 5 and 4 is smaller, whereas in the HS(CH2)11-X-EG6 series, SAMs of 7 are thinner by ∼2 Å than SAMs of 6. Indeed, compound 6 is longer than compound 7. However, the ellipsometric thickness difference may equally well be due to variations in packing and order of the SAMs, or in their optical properties. Earlier, Harder et al.9 estimated the thickness of 7 on gold using X-ray photoelectron spectroscopy, by measuring the attenuation of substrate Au4f photoelectrons. They found that the measured thickness 25 Å was significantly lower than the estimated thickness 30.8 Å for 7 in helical conformation, and suggested that a significant degree of disorder existed in SAMs of 7 on Au. In contrast to their work, the measured ellipsometric thickness of SAM 7, 33.0 Å, is in reasonable agreement with the theoretical thickness. Moreover, the correlation between the ellipsometric thickness and the corresponding conformational properties of the SAMs is supported by the spectroscopic data. IRAS at Room Temperature. The room-temperature spectra of the investigated SAMs are presented in Figures 1 and 2. The spectra of SAMs 1, 2, and 4 have been reported previously.29 However, in this study are all reported spectroscopic results recorded on SAMs prepared by adsorption from micromolar solutions. Therefore, certain differences are seen in the spectral intensities and frequencies as compared to the SAMs prepared from millimolar solutions. Infrared RA spectra of SAMs prepared from micromolar solutions generally display sharper and stronger peaks consistent with an improved molecular packing and organization. CH Stretch Region. This region, shown in Figure 1, contains the asymmetric (νa) and symmetric (νs) alkyl CH stretching modes, which provide information about the molecular packing and the crystallinity of the alkyl part of the SAMs. For the entire C15 series, the νa peaks at 2918 cm-1 and νs at 2851 cm-1 suggest that the SAMs adopt an excellent all-trans crystalline

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TABLE 2: Assignments of the Main Peaks in the Infrared RA Spectra of the OEG-Terminated Alkanethiolates on Gold [cm-1] compound

alkyl CH, νa/νs

OEG CH, νsa)

HSC15CONH-EG2 (1) HSC15CONH-EG4 (2) HSC15COO-EG4 (3) HSC15CONH-EG6 (4) HSC15COO-EG6 (5) HSC11CONH-EG6 (6) HSC11O-EG6 (7)

2918/2850 2918/2851 2918/2851 2918/2851 2918/2851 2917/2851 2919/2851

broadc,i broadc,i broadd.i 2894b 2893b 2893b 2892b

linking group

CH2 scissora

1563e, 1253f 1560e, 1253f 1741g, 1205h, 1187 h 1553e, 1252f 1741g, 1204h, 1186h 1552e, 1252f

1467-1457c 1466-1459c 1466-1456d 1465b 1464b 1465b 1464b

CH2 waga,b CH2 twista,b

1349 1348 1349 1347

1243 overlap 1243 1243

C-O, C-C stretcha,b) CH2 rock,twista,b 1143c 1148c 1137d 1114 1116 1114 1115

964 964 964 964

a OEG peak assignments according to refs 6, 9, 23, and 52. b Helical EG conformation with the transition dipole moment parallel to the surface 6 normal. c All-trans conformation of EG4. d Amorphous conformation of EG4. e Amide II mode. f Amide III mode. g Ester mode ν(CdO). h Ester mode ν(C-O). i “Broad”: a broad band seen between the alkyl νa and νs peaks.

Figure 2. Infrared reflection-absorption spectra of SAMs formed by compounds 1-7, taken at room temperature: the fingerprint region.

structure in the alkyl underlayer.46,50 In the C11 series, the intensity of the alkyl peaks is lower, as expected, because of the shorter alkyl chains. Interestingly, the νa peak at 2917 cm-1 reveals the highest degree of crystallinity of the polymethylene chains in 6, whereas for the analogous compound 7 this peak appears at 2919 cm-1, indicating a slightly higher gauche content.46 Thus, the amide group, placed between the OEG and alkyl parts of 6, appears to improve the crystalliny of the alkyl portion in the SAM.51 Furthermore, all EG6 compounds have a very strong CH stretching peak around 2893 cm-1, originating from the νs mode of helical OEG.6,9 The transition dipole moment of this mode is parallel to the helical axis of the molecule.52 The 2893 cm-1 is strong in all EG6 spectra 4-7, suggesting that the helices are oriented preferentially along the surface normal. The absence of the peaks in this region for 1-3 reveals a different conformation of the shorter OEGs. Next, a broad shoulder around 2950 cm-1, seen for all SAMs, is expected to arise from the CH νa mode of OEG. Polarized IR spectra of crystalline PEG samples have shown that the transition dipole moment of the νa mode is oriented perpendicularly to the molecular axis.52 The low

intensity of the νa mode in all spectra is consistent with a parallel orientation of its dipole moment with respect to the surface. Peaks from the Linking Groups. The amide moiety, -CONH-, is expected to give rise to several amide peaks in the fingerprint region. We have previously discussed KBr and RA spectra of SAMs formed by 1, 2, and 4, as well as by the related compound HS(CH2)15CONH-EG1.29 The amide I mode, which is a CdO stretch, could be seen around 1640 cm-1 in the KBr spectra for this class of compounds, but not in their RA spectra. Instead, the RA spectra displayed only a strong amide II band around 1555 cm-1, and the same peak can be also seen here for the compound 6, Figure 2. The amide II mode has its main contribution from C-N-H in-plane bending and C-N stretching, and the transition dipole moment is oriented along the C-N axis.53 The presence of a strong amide II peak and the absence of the amide I in the RA spectra indicates an excellent alignment of the amide groups, where the C-N bond is parallel to the surface normal, and the CdO moiety has an orientation parallel to the surface plane in the SAM. Also, a weaker peak around 1252 cm-1 is found in the low-frequency region, and, in analogy with the study by Clegg et al.,35 this peak is assigned to the amide III mode, also a combination of C-N stretching and C-N-H in-plane bending. The alignment of the transition dipole moments from the amide modes and the observed frequencies of the modes suggest that lateral hydrogen bonding exists in the SAMs.32,37 Compounds 3 and 5 have an ester (-COO-) as a linking group between the alkyl and OEG parts. This group gives rise to a CdO stretch peak at 1742 cm-1, as well as to C-O stretch peaks at around 1205 and 1185 cm-1. Also, a series of CH2 progression bands appear between 1340 and 1220 cm-1, as has been observed in analogous methylester-containing alkanethiols on gold.54 Contrary to the amide-containing compounds 1, 2, 4, and 6, the presence of the ester carbonyl stretch peak reveals that the CdO group is not aligned parallel to the surface plane in SAMs of 3 and 5. Finally, the ether linkage between the EG6 and the alkyl parts of compound 7 has no specific peaks in the fingerprint region. The modes from the -O- group overlap with those arising from the polyether chain of EG6. Fingerprint Peaks of the Short OEG SAMs. The spectra of the amide-containing, hydrogen-bonded SAMs of 1 and 2 have very strong COC stretch peaks at 1143 and 1148 cm-1, respectively. The peaks differ from those arising from the helical and amorphous phases of OEG, and have been assigned29 to an all-trans conformation of the OEG portion.9,23 Recently, Vanderah et al.27 reported spectra of SAMs on gold formed by HS(EG4)(CH2)9CH3, and they also concluded that EG4 adopted the all-trans conformation. However, they observed the tetra(ethylene glycol) COC peak at 1143 cm-1. We have also found

Oligo(Ethylene Glycol)-Terminated Alkanethiolates from our studies that the COC peak appears at somewhat lower frequencies when a higher concentration or shorter adsorption times are used during the SAM formation,29 and the same effect is obtained by raising the temperature, as evidenced by TPIRAS.30 We therefore propose that the shift of the skeletal COC mode of EG4 toward the lower frequencies is associated with the increase in disorder (increasing amount of gauche conformers). Interestingly, SAM 3 has the COC peak at much lower frequency, 1137 cm-1, and its intensity is significantly decreased as compared to SAM 2. The low COC stretch frequency together with the higher contact angle values and the smaller ellipsometric thickness suggest that the EG4 part of 3 adopt an amorphous-like conformation. Fingerprint Peaks of the EG6 SAMs. As can be seen in Figure 2, the fingerprint region of all EG6-terminated SAMs 4-7 contains features typical for a helical EG6 phase: the peaks are assigned to CH2 scissoring 1464, wagging 1349, twisting 1243, and rocking 964 cm-1 modes, respectively, as well as to the strong skeletal COC stretching mode around 1114 cm-1.6,9,28,30 The transition dipole moments of these modes are all oriented along the helical axis of EG6,9,23,52 and the appearance of the corresponding peaks in the infrared RA spectra confirm a preferential alignment of the helical axis along the surface normal. A more detailed comparison reveals that there is no detectable differences between the main helical peaks in the spectra of the SAMs 4 and 6, neither in terms of intensity, nor in the spectral position. Thus, it can be concluded that cutting away the four methylene units from the alkyl chain does not influence the self-assembly of the EG6 moiety. However, the replacement of the hydrogen-bonded amide group in 4 by the ester linkage in 5 causes certain changes. Namely, the helical peaks at 1349, 1116, and 964 cm-1 are reduced in intensity, correlating with the reduced intensity of the helical CH νs peak at 2893 cm-1. In addition, the relative intensity of the shoulder around 1128 cm-1 on the main skeletal COC peak is much stronger for 5. The strongest helical peaks in the fingerprint and CH stretch regions are observed for SAM 7, suggesting that it forms a highly crystalline and/or preferentially ordered assembly with the helices in an almost perfect parallel orientation with respect to the surface normal. Temperature-Programmed IRAS. Short OEG SAMs. Figure 3 shows the evolution of the CH stretch and fingerprint peaks with temperature for the shorter SAMs 1-3, which do not form the helical OEG phase at room temperature. The results for 2 are taken from our previous study.30 For all three SAMs, only marginal changes are seen on going from 20 °C to 75 °C; therefore, the spectra recorded between are omitted for simplicity. The alkyl νa and νs (Figure 3a) decrease in intensity, most probably due to the changes in tilt/rotation angles of the alkyl chains.50,55 The shift seen in the CH νa mode from 2918 to 2919 cm-1, suggests a partial disordering of the alkyls, corresponding to the presence of approximately one gauche defect per alkyl chain during the temperature treatment. The fingerprint region displays no significant changes for the SAMs 1-3. A comparison of the peaks from the linking groups reveals that very weak red shifts of the amide II can be identified upon increasing temperature for 1 and 2 (they will be discussed separately below), whereas the peaks due to the ester linkage in 3 (data not shown) are not affected at all. For the OEG portion, only small changes can be observed from the skeletal COC modes, Figure 3b. The SAMs 1 and 2 with the all-trans phases of EG2 and EG4, behave similarly, undergoing small and fully reversible low-frequency shifts of the COC stretch from 1143 to 1140 cm-1 and from 1148 to 1143 cm-1, respectively.

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Figure 3. Temperature-induced changes in the RA spectra of SAMs 1-3 in ultrahigh vacuum: A, the CH stretching region, and B, the COC skeletal modes. The spectra at 20 °C are in thick lines, and those taken at 75 °C in thin lines.

This finding is consistent with a slight increase of disorder in the all-trans OEG chains. However, no changes at all are seen for the COC peak of SAM 3, it remains close to 1137 cm-1. This is in agreement with amorphous-like EG4 chains observed already at room temperature. EG6 SAMs. The spectra in Figures 4 and 5 are reported within the temperature range of 20-75 °C for SAMs 4-6. However, the helical phase of 7 disappears at higher temperatures, and a temperature range of 20-90 °C was chosen for the TP-IRAS study of this SAM. In the CH stretch region, Figure 4, the main spectral changes are found for the ether νs peak at 2893 cm-1 which disappears upon increasing the temperature to 75 °C. Also, a weak increase in the ether νa shoulder around 2950 cm-1 is seen. The alkyl chain peaks are observed with somewhat reduced intensities for SAMs 4 and 5, and the νa mode shifts toward 2919 cm-1. A similar, but a slightly weaker decrease of the alkyl νa peak is seen for 6. However, for the SAM of 7, the spectral changes from both the alkyl and EG6 parts display marked differences, as compared to the other EG6 SAMs. First, as mentioned above,

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Figure 4. Temperature-induced changes in the RA spectra of SAMs 4-7 in ultrahigh vacuum: the CH stretching region. The spectra of SAMs 4-6 are recorded from 20 to 75 °C, and those of 7 from 20 to 90 °C. The temperature ramp is in 5 °C steps. The spectra at the highest temperatures are in thick lines.

the overall temperature range for the existence of the helical conformers is extended up to 85-90 °C: the disappearance of the intensity around 2893 cm-1 occurs when approaching these temperatures. The maximum of the alkyl νa peak moves toward 2920 cm-1 at around 75 °C, which indicates a further increase in alkyl chain disorder. The shift is also associated with a broadening of the νa peak. A similar broadening of the CH stretch peaks is observed in the spectra of disordered alkylthiolate SAMs on gold.46,51 It is also possible that the overlapping CH peak intensities from the EG6 phase contribute to the broadening. The temperature-dependent evolution of all helical peaks in the fingerprint region was previously reported for compound 4.30 In the present study, we do not show the entire spectral

Figure 5. Temperature-induced changes in the RA spectra of the EG6 SAMs 4-7 in ultrahigh vacuum: the skeletal COC modes. The spectra of SAMs 4-6 are recorded from 20 to 75 °C, and those of 7 from 20 to 90 °C. The temperature ramp is in 5 °C steps. The peaks corresponding to the helical EG6 phase are around 1115 cm-1, and the spectra taken at the highest temperatures are in thick lines.

range of the SAMs 4-7. Instead we focus on the narrow range of the COC skeletal modes, Figure 5. The essential differences in the phase transitions are seen here for the different EG6terminated compounds. Figure 5 shows that for SAMs 4 and 6 the helical COC peak at 1114 cm-1 is replaced by strong peaks at 1144 and 1145 cm-1, respectively. We previously assigned this transformation of the COC peak to a conformational change of the EG6 portion from the helical to the all-trans state, because

Oligo(Ethylene Glycol)-Terminated Alkanethiolates

Figure 6. Temperature dependence of normalized integrated intensities of the EG6 TP-IRAS fingerprint peaks of SAMs 4-7 due to the helical (open symbols), all-trans (filled circle), and amorphous (squares) EG6 phases. The intensities were integrated as follows: wagging modes 1360-1338 cm-1(triangles), skeletal COC stretching modes 12001050 cm-1(circles and squares), and rocking modes 980-950 cm-1(diamonds).

of the similarity with the corresponding spectral signature of 2.30 However, the EG6 phases behave differently in SAMs 5 and 7, which are not laterally hydrogen bonded. The helical COC peak of 5 transforms into a broader and weaker peak with a maximum around 1138 cm-1. It can be compared to the similar peak at 1137 cm-1 from the spectra of EG4 in SAM 3. Likewise, the helical peak in the spectra of 7 undergoes transformation to a weaker and broader peak as for 5, but this happens at higher temperatures. The resulting peak has a maximum shifted to around 1133 cm-1, and it resembles the broad COC peak, observed for the disordered EG3 phases in SAMs on Au.9 On the basis of that, and on the analogy with the spectra of SAM 3, we conclude that EG6 in the SAMs 5 and 7 exist in an amorphous-like phase at the higher temperatures. The difference in the spectral position of their COC stretch peaks suggests that the EG6 portion probably retains a higher degree of order in 5, due to slightly stronger interactions between the longer C15 alkyl chains and/or ester groups, whereas 7 becomes completely disordered at the highest temperatures. Figure 6 shows an analysis of the EG6 phase behavior in the SAMs 4-7. Integrated normalized intensities of the peaks, arising from the appearance/disappearance of the corresponding conformers in the phases of EG6, are plotted against temperature. The intensities were obtained following the same procedure as described previously.30 Prior to integration in the skeletal modes region 1200-1050 cm-1, the spectral components are separated by taking the spectrum at the respective temperature and subtracting it by the spectrum at the highest temperature, to reveal the dynamics of the helical phase: Ihelix(T) ) I(T) IT)Tmax. Likewise, to reveal the dynamics of the resulting new phase at higher temperatures, the multicomponent peak at particular temperature is subtracted by the spectrum at room temperature: Inew phase(T) ) I(T) - IT)20°C. From the plotted diagrams, the phase transition temperature can be defined as a

J. Phys. Chem. B, Vol. 105, No. 23, 2001 5465 temperature at which the curves due to the two different phases intersect. It also can be defined as a temperature at which the intensity of the particular mode increases/decreases to 50% of its final/initial value. A comparison of the EG6 phase diagrams (Figure 6) confirms the similarity between SAMs 4 and 6, showing identical dynamics of the corresponding peaks. It can be seen from Figure 6 that the intensities of the peaks due to the helical modes at 1349, 964, and 1114 cm-1 display the same temperature dependence in both SAMs, with a fast drop in intensity above ∼50 °C (the helical peaks at around 1465 and 1243 cm-1 are not analyzed, because of the overlap with the modes from the alkyl chains and the amide groups, respectively). At the same time, the decreasing helical peaks are exchanged by the alltrans COC peak around 1144 cm-1, which increases rapidly above 50 °C, and finally replaces the helical peaks at 70-75 °C. Note that the attribution of the intensity Inew phase to the alltrans phase is not straightforward below 50 °C, since the integrated peaks contain components which could be associated with the appearance of an amorphous-like phase. Therefore, the initial steps of the evolution of the all-trans peak intensity are not exactly represented in the diagrams (Figure 6). Nevertheless, from these diagrams, the same phase transition temperature at 60 °C is found for the two EG6 SAMs, 4 and 6. The intensity analysis for SAM 5 reveals that the helical phase decreases less steeply within the entire range of temperatures. This phase is gradually replaced by an amorphous-like phase which manifests itself in the broad peak around 1138 cm-1. The phase transition temperature is found to be around 52 °C, remarkably lower than in the SAMs 4 and 6. Further on, for SAM 7, which also lacks the hydrogen bonds, all helical peaks gradually decrease in intensity, with a faster drop above 60 °C (the accessible intensity of the twisting mode at 1243 cm-1 was also analyzed, but it is not shown in Figure 6, as it follows exactly the behavior of the wagging mode at 1347 cm-1). However, for this SAM the helical phase is finally lost at 90 °C. The loss of the helical peaks is paralleled by the growth of the COC peak around 1133 cm-1, which is assigned to the amorphous state of EG6. From the diagram, the phase transition temperature is found to be 68 °C for SAM 7. It is also important to mention, that the phase transitions of the EG6 portions of the SAMs are associated with certain rearrangements of the linking groups. For example, the peaks due to the ester group in 5 are somewhat reduced in intensity upon increasing temperature. For the CdO stretch at 1741 cm-1 the integrated intensity drops by ∼8% in the range of 20-75 °C. Moreover, the amide II bands of 4 and 6 undergo weak but well-resolved blue shifts, which are discussed in detail in the Discussion part. Reversibility of the Phase Transitions in the EG6 SAMs. Since the helical EG6 phases in the four different SAMs 4-7 undergo dramatic conformational changes during the temperature cycle, it is interesting to compare their reversibility. This was done in a separate measurement series, by choosing a temperature range of 20-70 °C for 4-6, and 20-90 °C for 7. The criterion for the choice of the upper temperature limit was the loss of the helical phase at around 70 and 90 °C for the respective SAMs. After the heating ramp, the temperature was set back directly to 20 °C and stabilized for 5 min. Figure 7 illustrates the reversibility for the skeletal COC modes. Generally, the helical phase is recovered in all SAMs after the thermal cycle. The best reversibility, in terms of the restored COC helical modes, is observed for SAM 7. It is somewhat lower in 4 and 6, with a slight loss in the intensity of the 1114 cm-1 peak and

5466 J. Phys. Chem. B, Vol. 105, No. 23, 2001

Figure 7. Reversibility of the temperature-induced conformational changes of the EG6 SAMs 4-7. The thick lines show the COC stretching modes at 20 °C, before the heating cycle. The spectra in thin lines are taken at 20 °C, after heating in 5 °C steps up to 70 °C (SAMs 4-6) or to 90 °C (SAM 7).

an increase of the shoulder on the high-frequency side. The lowest reversibility of the spectral signature of is achieved for 5. Finally, it is worth to mention, that for all EG6-terminated SAMs 4-7, the CH stretch region indicates exactly the same structure of the supporting alkyl underlayer after the thermal treatment as before (not shown). Discussion The results from this study allow us to categorize the studied OEG SAMs into the following two main groups: the short nonhelical OEG SAMs, and the helical EG6 SAMs. In the first group, the EG2 and EG4 oligomers of the hydrogen bonded SAMs 1 and 2, respectively, form the all-trans phase, and they do not change conformation within the investigated range of the temperatures. Only a slight increase in the amount of gauche conformers along the OEG chains is observed at temperatures close to 75 °C. Compound 3 has a different linking group, as compared to 1 and 2, and it influences oligomer portion of the SAM to assemble in an amorphous-like conformation. A rough comparison of the strength of lateral interactions can be done in order to relate the differences in the OEG conformations to the energetic contributions from the linking groups in the SAMs 2 and 3. An oligo(ethylene glycol) chain, when it is at least three EG units long, is known to relax in a helical conformation.9 However, it has been estimated that the differences in conformational energies per EG unit between the helical, all-trans, and amorphous conformations are less than 2 kJ/mol.23 This energy should be compared with the energy of hydrogen bond formation between the amide groups, which can be as large as 20-25 kJ/mol.56,57 The ester group, on the other hand, has an effective dipole moment varying from 0.7 D in polymers to 1.7 D in low-molecular weight liquids.58 An estimation of the interaction between such dipoles in a vacuum, assuming that they are optimally (linearly) aligned at a separation of 4.6 Å, gives electrostatic energies less than 4 kJ/mol. Indeed, it has been found that the contribution of the ester group

Valiokas et al. to the cohesive energy of polymers is up to 4.5 times smaller than that of the amide group,58 i.e., the intermolecular dipoledipole interaction is significantly weaker than the hydrogen bond. Although the actual energetic contribution from the different linking groups depends on their alignment in the SAM, this simple way of reasoning clearly points out that lateral hydrogen bonding can play an important role for the stabilization and conformational behavior of the various OEG phases in SAMs. We believe that it is the lateral hydrogen bonding that favors the formation of the all trans instead of the expected helical phase in the EG4 part of SAM 2. However, neither of these phases is dominating in the SAM 3. While the overall contribution from the ester linkages appears to be too weak to drive the oligomers into the all-trans conformation, it is plausible that dipolar interactions, together with the van der Waals contribution from the long alkyl chains are sufficiently strong to destabilize the helical conformation. As a result, the EG4 portion of SAM 3 is trapped in an amorphous-like state. The infrared analysis of the EG6-terminated SAMs 4-7 at room temperature also reveals structural variations between the hydrogen bonded SAMs of 4 and 6 and their analogues 5 and 7. The SAM of compound 7, for example, displays the strongest and sharpest helical peaks in the RA spectra, Figures 1 and 2. Thus, the SAM that possesses the weakest lateral interactions in the underlying part (short alkyl chain and weakly interacting ether groups) appears, at the same time, to offer advantageous conditions for the EG6 portion to relax in a well-defined crystalline conformation. The slightly increased disorder (mobility) of the alkyl chains in SAM 7 as compared to 6 (same chain length) may in fact facilitate such a process. The amidecontaining compounds 4 and 6 also form well-ordered SAMs on gold. However, the helical peaks in the RA spectra of 4 and 6 are reduced as compared to those in 7. The lateral hydrogen bonding induced by the amide groups in 4 and 6 may very well bring certain constrains into the SAM so that a fraction of the EG6 oligomers is forced to adopt other conformations than the helical, e.g., the amorphous conformation. An increasing fraction of amorphous conformers in the EG6 part will in the RA spectrum show up as a growing shoulder on the high-frequency side (at ∼1130 cm-1) of the main COC peak. The intensities of the helical (axial) peaks are also reduced for 5, and the 1128 cm-1 shoulder becomes even more pronounced than for 4 and 6. As discussed before, the ester-containing SAM 3 has the most disordered EG4 tail. Thus, it is plausible that even in 5 the dipoles of the -COO- groups tend to align so that the helical conformation of EG6 is perturbed. However, it is also possible that the observed lower intensity of the helical (axial) peaks of 4, 5, and 6 is associated with rather subtle changes in the tilt angle of the helices, due to the introduction to different linking groups and/or alkyl chain lengths. For example, an increase in the alkyl chain length recently was found to increase the tilt angle of the perfluorocarbon chain helices in a series of semifluorinated alkylthiolates on gold.59 It should also be pointed out that the helical conformers of poly(ethylene glycol) have radial COC modes (transition dipole moments orthogonal to the helical axis) that are expected to appear near 1119 cm-1.52 Thus, a slight increase in the tilt angle of the helix will increase the intensity of the high frequency shoulder because of a more favorable alignment of the transition dipole moment of the radial modes parallel to the surface normal. Unfortunately, the present set of data does not allow us to conclusively exclude any of the two explanations. We merely conclude that the EG6 helices form a more defined crystalline or oriented (helical) structure in the SAM of 7, as compared to 4 and 6 and ultimately to 5.

Oligo(Ethylene Glycol)-Terminated Alkanethiolates

J. Phys. Chem. B, Vol. 105, No. 23, 2001 5467

Figure 8. Temperature-dependent changes of the amide II peaks in the spectra of SAMs 1, 2, 4, 6. The arrow indicates the direction of the thermal evolution of the peak. For 1 and 2, the spectra are shown from 20 to 70 °C with 10 °C intervals, and the spectra in thick lines are at 75 °C. For 4 and 6, the thick lines indicate the spectra at the start point (20 °C) and the end point (75 °C), the spectra between are shown in 5 °C steps.

The most obvious differences in the contributions from the linking groups are observed from the study of the temperaturedriven EG6 phase behavior of the SAMs 4-7. The SAMs 4 and 6 display an unique helical-to-all-trans phase transition at a phase transition temperature Tp ) 60 °C. The replacement of the amide group in 4 by an ester in 5 prevents the transition to the all-trans phase. Likewise, in the C11 series, the same effect is obtained upon replacing the amide 6 with an ether linkage 7. However, a shortening of the alkyl chain in 6 (C11), as compared to 4 (C15), does not influence the helix-to-all-trans conformational transition, confirming that the driving force comes from the specific interactions, as the structure of the alkyl underlayer of the SAMs remains intact during the temperature cycle. On the other hand, the highest EG6 phase transition temperature is found for 7, Tp ) 68 °C. This SAM undergoes a type of melting transition where the EG6 part adopts an amorphous-like conformation at temperature >Tp. The same type of phase behavior is observed for SAM 5, but at a significantly lower temperature, Tp ) 52 °C. The fact that the highest Tp is observed for 7 suggests that the shorter alkyl chain (C11) and/or the ether linkage provides the SAM with the most stable helical crystalline EG6 phase. Before going into the details of the mechanism behind the observed helix-to-all-trans phase transition in SAMs 4 and 6, one more important observation from the RA spectra of the amide-containing compounds has to be discussed in detail. Namely, small but distinctive oligomer chain-length-dependent variations of the amide II modes are seen for the amidecontaining compounds 1, 2, 4, and 6, Figure 8. The spectra of SAMs 1 and 2, terminated by EG2 and EG4, respectively, at room temperature (Figure 2) display the amide II peaks at 1563

and 1560 cm-1, respectively (Table 2). The exact position of the amide II mode is related to the strength of NH‚‚‚OdC hydrogen bonding. It has been observed that the amide II peak shifts from 1510 cm-1 for compounds in a non-hydrogen bonded state to 1550 cm-1 and higher frequencies in the spectra of the same compounds in hydrogen bonded SAMs.32,37 For example, in the spectra of amide group containing- and well ordered alkanethiolate SAMs on gold, the maxima of the amide II peaks appear at 1561-1563 cm-1,32-35 and they are identical to those found the RA spectra of SAM 1 and 2. However, for the EG6terminated SAMs 4 and 6, the amide II peaks appear at 1553 cm-1. Therefore, it can be concluded with confidence that in the SAMs of 4 and 6 the hydrogen bonding network is somewhat perturbed by the helical EG6 oligomers, most likely because of steric reasons, whereas the lateral interactions are maximized in SAMs 1 and 2. This interpretation is consistent with the geometrical considerations about the cross sectional area of the helical and all-trans conformers.9 Earlier IR studies of globular proteins also have established correlation between the red shift of the amide II peak and a lower degree of hydrogen bonding.60 When the temperature of the substrate increases, a different behavior of the amide II peak is observed for the EG6 and the shorter OEG SAMs. A detailed comparison of the amide II peaks from TP-IRAS spectra is shown in Figure 8. For SAMs 1 and 2, no significant changes in the amide II peaks are seen, except for gradual, weak red shifts from 1563 to 1560 cm-1 and from 1560 to 1558 cm-1, respectively. This loss in the strength of hydrogen bonding is paralleled with a small disordering of these SAMs, as shown above for the alkyl peaks in TP-IRAS, Figure 4. However, for the SAMs 4 and 6, the

5468 J. Phys. Chem. B, Vol. 105, No. 23, 2001 temperature-dependent spectral shifts of the amide II peaks are completely different. The shape and position of the peaks remain unchanged during the first few steps of the temperature cycle until approximately 60 °C, when a sudden jump toward higher frequencies occurs. The overall frequency shift from 1553 to 1559 cm-1 at 75 °C is observed in parallel with an increase in the relative intensity of the peaks by 15%. This observation suggests that a higher degree of lateral hydrogen bonding is achieved in the SAMs,60 because of a more favorable alignment of the NH‚‚‚OdC bond. Also, the full width at half-maximum of the peaks is reduced from 26 to 23 cm-1, indicating a lower dispersion of the hydrogen bonded states of the amide groups. Generally, the amide II peaks of the EG6-terminated SAMs 4 and 6 at 75 °C become identical to those of the EG4-terminated SAM 2. We would also like to point out that the onsets of the observed shifts of the amide II peaks in the RA spectra of 4 and 6, Figure 8, are identical for both SAMs, and occur at 60 °C, i.e., at the EG6 phase transition temperature of the SAMs. Thus, the shifts of the amide II peaks for SAMs 4 and 6 follow exactly the changes in the molecular conformation (helix to alltrans) of the EG6 part. The observed correlation between the behavior of the amide II and the EG6 peaks suggests the following mechanism of the helix-to-all-trans phase transition in hydrogen bonded SAMs 4 and 6. At room temperature, the EG6 portion relaxes predominantly into the expected helical conformation. The bulkiness of the EG6 helices and their mutual interactions in the formed crystalline phase could bring certain steric constrains to the SAMs so that the lateral hydrogen bonding network becomes perturbed. This is consistent with the red shifts of the amide II peaks in the TP-IRAS spectra of 4 and 6 as compared to 1 and 2, Figure 8. Bearing in mind the strong directionality and distance dependence of the hydrogen bond,61,62 a weaker interaction is therefore obtained between the amide groups than the estimated 20-25 kJ/mol. Further on, as the temperature increases, the concentration of gauche conformers is increasing in the EG6 chain, and the existing barrier for optimized lateral hydrogen bonding is gradually lowered. At around 60 °C the EG6 portion becomes amorphous-like and more flexible, enabling the amide groups to align so that a higher degree of hydrogen bonding is achieved, maximizing the lateral interaction. Responding to the stronger hydrogen bonding and the better alignment of the amide groups, the SAM has to undergo a certain rearrangement of the EG6 chains. Since the conformational energy difference between the different OEG conformers is less than 2 kJ/mol per EG unit,23 it is reasonable that the gain in the optimized hydrogen bonding network is strong enough to drive SAMs 4 and 6 into a metastable state with a better ordered EG6 phase. It turns out, that the hydrogen bonds snap the oligomers together, forcing them to adopt predominantly an all-trans conformation, as evidenced by the appearance of the distinctive COC stretch peaks at ∼1145 cm-1 in the infrared RA spectra at 75 °C. The suggested mechanism is based only on a rough estimation of the interplaying energetic contributions in the EG6-terminated, amide-containing SAMs. Nevertheless it is consistent with the main observations of this study. An exact account of the intermolecular interactions, arising from the linking groups, would require an extensive computational study. Also, it remains to be explained in detail in what type of two-dimensional structure the lateral hydrogen bonding network is organized in the OEG SAMs. Particularly, this is of importance for further applications of the OEG SAMs, as the properties of the hydrogen bond could influence a broad range of characteristics of the OEG

Valiokas et al. SAMs, including the crystalline lattice parameters, the tilt and rotation angles, packing density, island/domain formation, concentration of defects, and the overall stability. Conclusions This comparative study of the OEG-terminated alkanethiolate SAMs on gold shows, that the self-assembly and phase behavior of the oligomers in the SAMs can be partly fine-tuned by specific lateral intermolecular interactions. The formation of a hydrogen bonding network is an example of such an approach, which from a synthetic point of view is very convenient to realize. First, we conclude that the lateral hydrogen bonding facilitates the molecular organization in long linear molecule-based SAMs, as it is evidenced by formation of very ordered SAMs with the dominating all-trans phases of EG2 and EG4. Second, the oligomers of EG6 relax predominantly in helical conformation in SAMs 4-7 at room temperature, independently of the strength of the lateral interaction. However, the most pronounced spectral features, related to the crystalline helical phase, are seen for SAM 7 which is not hydrogen-bonded. The present study does not give the answer, whether the somewhat weaker spectral features for the SAMs 4-6 are mainly due to a different orientation of the EG6 helices with respect to the surface normal, or due to a higher degree of disorder of the oligomer portion of the SAM. Third, the lateral hydrogen bonding is responsible for the unique phase behavior of EG6 terminated and amide group containing SAMs: the reversible, temperature-driven switching between the helical and all-trans phases. We show that a fine balance between the intermolecular and intramolecular interactions can be achieved by an appropriate design of the compounds, and that it allows a construction of novel self-assembled architectures with specifically controlled oligomer phase behavior. Additional structural aspects of the laterally hydrogen bonded SAMs along with their applications as protein resistant interfaces and as templates for supported lipid membranes will be reported in subsequent papers. Acknowledgment. R.V. and S.S. were supported by the Graduate School Forum Scientum, founded by the Swedish Foundation for Strategic Research (SSF). The authors also thank the Swedish Research Council for Engineering Sciences (TFR). References References and Notes (1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (2) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (3) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12-20. (4) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164-1167. (5) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714-10721. (6) Bailey, F. E., Jr.; Koleske, J. V. Poly(ethylene oxide); Academic Press: New York, 1976. (7) Lee, J. H.; Kopecek, J.; Andrade, J. D. J. Biomed. Mater. Res. 1989, 23, 351-368. (8) Desai, N. P.; Hubbell, J. A. Biomaterials 1991, 12, 144-153. (9) Harder, P.; Grunze, M.; Dahint, R.; Whitesides, G. M.; Laibinis, P. E. J. Phys. Chem. 1998, 102, 426-436. (10) Lee, S. W.; Laibinis, P. E. Biomaterials 1998, 19, 1669-1675. (11) Chapman, R. G.; Ostuni, E.; Yan, L.; Whitesides, G. M. Langmuir 2000, 16, 6927-6936. (12) Mrksich, M.; Grunwell, J. R.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 12009-12010. (13) Lahiri, J.; Isaacs, L.; Grzybowski, B.; Carbeck, J. D.; Whitesides, G. M. Langmuir 1999, 15, 7186-7198. (14) Lahiri, J.; Isaacs, L.; Tien, J.; Whitesides, G. M. Anal. Chem. 1999, 71, 777-790.

Oligo(Ethylene Glycol)-Terminated Alkanethiolates (15) Lang, H.; Duschl, C.; Vogel, H. Langmuir 1994, 10, 197-210. (16) Raguse, B.; Braach-Maksvytis, V.; Cornell, B. A.; King, L. G.; Osman, P. D. J.; Pace, R. J.; Wieczorek, L. Langmuir 1998, 14, 648-659. (17) Williams, L. M.; Evans, S. D.; Flynn, T. M.; Marsh, A.; Knowles, P. F.; Bushby, R. J.; Boden, N. Langmuir 1997, 13, 751-757. (18) Burns, C. J.; Culshaw, P.; Field, L. D.; Islam, M.; Morgan, J.; Raguse, B.; Ridley, D. D.; Wieczorek, L.; Wilkinson, M.; Vignevich, V. Aust. J. Chem. 1999, 52, 1071-1075. (19) Cornell, B. A.; Braach-Maksvytis, V. L. B.; King, L. G.; Osman, P. D. J.; Raguse, B.; Wieczorek, L.; Pace, R. J. Nature 1997, 387, 580583. (20) Brockman, J. M.; Frutos, A. G.; Corn, R. M. J. Am. Chem. Soc. 1999, 121, 8044-8051. (21) Lahiri, J.; Kalal, P.; Frutos, A. G.; Jonas, S. J.; Schaeffler, R. Langmuir 2000, 16, 7805-7810. (22) Wang, R. L. C.; Kreuzer, H. J.; Grunze, M. J. Phys. Chem. 1997, 101, 9767-9773. (23) Pertsin, A. J.; Grunze, M.; Garbuzova, I. A. J. Phys. Chem. 1998, 102, 4918-4926. (24) Pertsin, A. J.; Grunze, M.; Kreuzer, H. J.; Wang, R. L. C. Phys. Chem. Chem. Phys. 2000, 2, 1729-1733. (25) Zolk, M.; Eisert, F.; Pipper, J.; Herrwerth, S.; Eck, W.; Buck, M.; Grunze, M. Langmuir 2000, 16, 5849-5852. (26) Pertsin, A. J.; Grunze, M. Langmuir 2000, 16, 8829-8841. (27) Vanderah, D. J.; Pham, C. P.; Springer, S. K.; Silin, V.; Meuse, C. W. Langmuir 2000, 16, 6527-6532. (28) Vanderah, D. J.; Meuse, C. W.; Silin, V.; Plant, A. L. Langmuir 1998, 14, 6916-6923. (29) Valiokas, R.; Svedhem, S.; Svensson, S. C. T.; Liedberg, B. Langmuir 1999, 15, 3390-3394. (30) Valiokas, R.; O ¨ stblom, M.; Svedhem, S.; Svensson, S. C. T.; Liedberg, B. J. Phys. Chem. 2000, 104, 7565-7569. (31) Cooper, E.; Leggett, G. J. Langmuir 1999, 15, 1024-1032. (32) Clegg, R. S.; Hutchison, J. E. Langmuir 1996, 12, 5239-5243. (33) Clegg, R. S.; Reed, S. M.; Hutchison, J. E. J. Am. Chem. Soc. 1998, 120, 2486-2487. (34) Clegg, R. S.; Reed, S. M.; Smith, R. K.; Barron, B. L.; Rear, J. A.; Hutchison, J. E. Langmuir 1999, 15, 8876-8883. (35) Clegg, R. S.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 53195327. (36) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610-4617. (37) Tam-Chang, S. W.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir 1995, 11, 4371-4382. (38) Zhang, J.; Zhang, H. L.; Chen, M.; Zhao, J.; Liu, Z. F.; Li, H. L. Phys. Chem. Chem. Phys. 1998, 102, 701-703.

J. Phys. Chem. B, Vol. 105, No. 23, 2001 5469 (39) Sabapathy, R. C.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E.; Hussey, C. L. Langmuir 1998, 14, 124-136. (40) Sek, S.; Misicka, A.; Bilewicz, R. J. Phys. Chem. 2000, 104, 53995402. (41) Sumner, J. J.; Weber, K. S.; Hockett, L. A.; Creager, S. E. J. Phys. Chem. 2000, 104, 7449-7454. (42) Svedhem, S.; Hollander, C.-Å.; Shi, J.; Konradsson, P.; Liedberg, B.; Svensson, S. C. T. J. Org. Chem. 2000. Accepted for publication. (43) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141-149. (44) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45-52. (45) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (46) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (47) Shi, J.; Hong, B.; Parikh, A. N.; Collins, R. W.; Allara, D. L. Chem. Phys. Lett. 1995, 246, 90-94. (48) Engquist, I.; Lundstro¨m, I.; Liedberg, B. J. Phys. Chem. 1995, 99, 12257-12267. (49) Bain, C. D.; Whitesides, G. M. Science 1988, 240, 62-63. (50) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (51) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767-773. (52) Miyazawa, T.; Fukushima, K.; Ideguchi, Y. J. Chem. Phys. 1962, 37, 2764-2776. (53) Bamford, C. H.; Elliott, A.; Hanby, W. E. Synthetic Polypeptides: Preparation, Structure, and Properties; Academic Press: New York, 1956. (54) Engquist, I.; Lestelius, M.; Liedberg, B. Langmuir 1997, 13, 40034012. (55) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-66. (56) Doig, A. J.; Williams, D. H. J. Am. Chem. Soc. 1992, 114, 338343. (57) Kang, Y. K. J. Phys. Chem. 2000, 104, 8321-8326. (58) Van Krevelen, D. W. Properties of Polymers; Elsevier: Amsterdam, 1997. (59) Fukushima, H.; Seki, S.; Nishikawa, T.; Takiguchi, H.; Tamada, K.; Abe, K.; Colorado, R.; Graupe, M.; Shmakova, O. E.; Lee, T. R. J. Phys. Chem. 2000, 104, 7417-7423. (60) Koenig, J. L.; Tabb, D. L. In Analytical Applications of FT-IR to Molecular and Biological Systems; Durig, J. R., Ed.; D. Reidel Publishing Company: Dordrecht, 1980; p 241-255. (61) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1992. (62) Grzybowski, B. A.; Ishchenko, A. V.; DeWitte, R. S.; Whitesides, G. M.; Shakhnovich, E. I. J. Phys. Chem. 2000, 104, 7293-7298.