Monolayer Study by VSFS - American Chemical Society

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Monolayer Study by VSFS: In Situ Response to Compression and Shear in a Contact Ahmed Ghalgaoui,⊥,† Ryosuke Shimizu,⊥,‡ Saman Hosseinpour,⊥ Rubén Á lvarez-Asencio,⊥ Clayton McKee,⊥,∥ C. Magnus Johnson,⊥ and Mark W. Rutland*,⊥,§ ⊥

KTH Royal Institute of Technology, Division of Surface and Corrosion Science, Drottning Kristinas Väg 51, SE-100 44 Stockholm, Sweden § SP Chemistry, Materials and Surfaces, SP Technical Research Institute of Sweden, Box 5607, SE-114 86 Stockholm, Sweden ABSTRACT: Self-assembled octadecyltrichlorosilane ((OTS), CH3(CH2)17SiCl3) layers on hydroxyl-terminated silicon oxide (SiO2) were prepared. The monolayers were characterized with atomic force microscopy (AFM) and contact angle measurements; their conformation was studied before, during, and after contact with a polymer (either PDMS or PTFE) surface using the vibrational sum frequency spectroscopy (VSFS) technique. During contact, the effect of pressure was studied for both polymer surfaces, but in the case of PTFE, the effect of shear rate on the contact was simultaneously studied. The VSFS response of the monolayers with pressure was almost entirely due to changes in the real area of contact with the polymer and therefore the Fresnel factors, whereas sliding caused disorder in the previously all-trans monolayer, as evidenced by a significant increase in the population of gauche defects.

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conformational change resulting in the molecules lying flat on the substrate at a pressure of 50 MPa. Bain14 also used VSFS to study a monolayer of zinc arachidate at the sapphire−silica interface and in contrast reported that the monolayer is resistant to pressure- and shear-induced conformational disorder. The authors claimed that there was intimate contact of the sapphire and silica over the entire contact region based on root-mean-square (rms) roughness measurements and contact mechanical theory. Both frequency shifts and changes in peak intensity ratios were observed and were ascribed to monolayer transfer between the surfaces. These observations are consistent with observations by Du et al.13 and Fraenkel et al.,15 who explained that the transfer of the monolayer between solid surfaces during contact could explain part of the observed drop in the VSFS signal intensity. The Dhinojwala16 group has studied the PDMS−PPMA interface for fresh and aged lenses. They observed an unusual increase in adhesion hysteresis and frictional forces for poly(dimethylsiloxane) (PDMS) lenses sliding on smooth, glassy surfaces after a period of aging. VSFS studies were carried out on the PDMS surfaces immediately after they slid out of contact. The increase in the VSFS signals from PDMS suggested significant ordering of the PDMS chains induced by sliding., The same group17 studied the interface of an oxidized (PDMSox) elastomer and a methyl-terminated selfassembled monolayer of OTS on sapphire substrates. A strong

INTRODUCTION Boundary lubricants form a thin barrier layer, typically a monolayer, preventing direct contact between two surfaces. Exactly how such a layer behaves under pressure and shear remains a topic of debate. A self-assembled silane layer1,2 provides a good model system for an in situ spectroscopic study. Octadecyltrichlorosilane (OTS) as a self-assembled monolayer (SAM) on silicon oxide has been widely studied by many groups in recent years.3−5 Such SAMs offer a facile means to alter and control the chemical nature of a surface. Some examples of current industrial uses of SAMs are lubricants, coupling agents, coatings, and templates.6 OTS is a particularly interesting type of SAM because it can bind to oxide surfaces and is thus of use in Si-based microelectromechanical systems, which have a native amorphous oxide layer,7 and where tribological and “stiction” issues are very important. The characteristics of these SAMs have been investigated extensively by various methods, such as FTIR, AFM, and ellipsometry.8,9 In this study, we are interested in the change in the molecular conformation of an OTS SAM in a tribological contact. In the literature, the molecular conformation of OTS SAM at the solid−solid interface as a function of pressure and sliding is limited to a theoretical study.10−12 However, Shen13 has experimentally studied the case of a monolayer of octadecyl alcohol confined between two equivalent solids (a quartz flat and a quartz lens). Vibrational sum frequency spectroscopy (VSFS) was employed, and the signal disappeared when the surfaces were placed in contact. This was interpreted as a © 2014 American Chemical Society

Received: November 13, 2013 Revised: February 13, 2014 Published: February 18, 2014 3075

dx.doi.org/10.1021/la4042474 | Langmuir 2014, 30, 3075−3085

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Figure 1. Schematic of the VHS/EHL system to permit in situ measurement of the molecular conformation of a SAM at a tribological contact. The experiment is performed in external reflection geometry. and the angular velocity of the rotating disc but was controlled from the instrument’s commercial software. A polymer lens (made in-house, see below) was attached to the load cell, and a purely sliding contact was achieved. VSFS Experiment. VSFS is a highly surface-sensitive technique that allows the identification of the surface molecules as well as a determination of their molecular conformation.20,21 Briefly, the VSFS technique relies on the spatial and temporal overlap of two laser pulses. One of these is tunable in the infrared, inducing coherence between vibrational levels, and the other one is in the visible, inducing Raman transitions of the excited vibrational level to a virtual excited level that then relaxes to the fundamental state, releasing a photon at the sum frequency (ωSFG = ωIR + ωVIS). It is the intensity of this sum frequency signal that is recorded and reveals the molecular information on the surface. The technique and our experimental setup are well described elsewhere.22 PDMS Preparation. Poly(dimethylsiloxane) (PDMS) lenses were prepared by using Sylgard 184 monomer supplied by Dow Corning (Midland). The recipe consisted of 1 part cross-linker to 10 parts monomer. The monomer and the cross-linker were mixed, and air bubbles were removed with a turbo vacuum pump. The lenses were cured in a vacuum oven for 4 h at 60 °C. The PDMS lenses were cleaned by rinsing in isopropanol and then ultrasonication in Milli-Q water for 15 min before each experiment. Liquid PDMS was obtained from Sigma-Aldrich (St. Louis, MO) with viscosity of 90−150 cSt. OTS Preparation. Octadecyltrichlorosilane (OTS) SAMs were prepared on silica discs (ISP Optics; diameter, 10 cm; thickness, 1.2 cm) and silica tablets (Thorlabs; diameter, ca. 1.27 cm; thickness, ca. 3.2 mm). These substrates were treated with piranha solution (H2SO4/H2O2 60:40) for 30 min at 90 °C to obtain reproducible OH termination. They were cleaned and sonicated several times in Milli-Q water and dried with nitrogen. To form a monolayer of octadecyltrichlorosilane (OTS) on silica, the substrates were immersed in a 2 mM solution of OTS in toluene overnight. This procedure was carried out in a water-vapor-free atmosphere. The silica disc was then rinsed several times with toluene and later rinsed with ethanol. The substrate was then placed in an oven at 120 °C for 1 h. AFM Measurement. The morphology of the silica disc surface before and after coating with OTS was studied using a Dimension 3100 atomic force microscope (Veeco). Images were collected in tapping mode using Ultrasharp Si tips (HQ35, MikroMasch, with nominal normal spring constants of 16 N/m). Before being imaged, the surfaces were cleaned by thoroughly rinsing with ethanol, followed by drying with N2 gas. The root-mean-square (rms) roughness was

methyl signal was observed in the case of the PDMSox/OTS interface, in contrast to the case of the PDMS/OTS interface. This was considered to be due to the generation of short PDMS chains that are ordered in contact with the methyl groups of the OTS layer. In this article, we focus on an OTS SAM attached to a silica disc. We compare in situ VSF spectra of the monolayer before contact, during contact, and after contact as a function of the normal load force and sliding speed. Two different polymers, PDMS and PTFE, are used as counter surfaces. Contact angle and AFM measurements are combined with VSFS results to allow the changes in the monolayer upon compression and sliding to be monitored. The effect of the change in Fresnel factors is explicitly considered, as is the effective contact area.

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EXPERIMENTAL SECTION

We modified a commercial elastohydrodynamic device (PCS Instruments) to allow a VSFS instrument to probe the contact between a silica disc and a polymer lens in situ (Figure 1). In this manner, the load applied to the contact could be both changed and measured, and the sliding speed could also be controlled (Figure 1). The equipment is designed to allow the simultaneous study of film thickness, contact size, and friction as a function of sliding velocity and applied load in liquids under both sliding and rolling conditions. The fact that the silica disc is transparent allows the optical interrogation of the contact position. For our purposes, the important features were the ability to control load and speed. The demands of the VSFS optical system precluded the use of the interferometric optical system for determining film thickness. The load force was varied from 0 to 50 N using the commercial instrument software and was calibrated according to the manufacturer’s instructions using an added weight method. This force determines the radius of the apparent contact, which was always kept larger than the beam size produced by the VSFS laser system. The relationship among the radius of the contact, the pressure, and the applied force is given by contact mechanical theory, for example, Hertz theory18 or JKR theory.19 Examples of such equations are shown in the supporting information. To allow the effect of shear on the contact to be studied, the silica disc was rotated continuously in a single direction so that the upper surface (with bound OTS SAM) was continuously refreshed. The speed was calculated from the radial position of the contact position 3076


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calculated by averaging 10 values from 1 μm2 areas on the AFM images using the AFM software (Nanoscope 6.13R1). Contact Angle Measurement. Contact angles of water droplets on the silica-disc surfaces before and after OTS coating were measured with a pocket goniometer (model PGX, FIBRO System AB). Measurements were performed at 10 different positions every time that the surface was modified. The rms average was calculated by averaging the values from three positions on the disc. The contact angle of a sessile water drop on an alkylsilane SAM or on a silica surface allows us to determine the quality of the coating and the substrate. A larger angle indicates a more hydrophobic and hence a more closely packed film. The contact angle of the “clean” silica was 0° after treatment with piranha solution, whereas for OTS the average contact angle was 110 ± 1.9°. The contact angle of the OTS SAM on a silicon wafer obtained is similar to those of OTS SAMs reported in the literature.2,5,23−26 An effective surface coverage can be estimated by the following equation assuming that the largest reported contact angle corresponds to full coverage.5,27

fOTS =

1 − cos θ 1 − cos θmax

monolayer, VSFS measurements were performed on the monolayer-coated disc at three different positions. Three different polarization combinations were employed at each position (PPP, SSP, and SPS). The letters refer to the polarization of the sum frequency, visible, and infrared beams, respectively. When these three polarization combinations are used, it is possible to determine the orientation of different functional groups.29 It is clear from the figures that there is no variation of the spectra with position. The spectra in Figure 3 show the presence of peaks at 2877, 2940, and 2962 cm−1, which are assigned to the symmetric methyl stretch, the Fermi resonance between the symmetric methyl stretch and the methyl bending overtone, and the antisymmetric methyl (CH3) stretch, respectively. These results are similar to that obtained by Guyot-Sionnest et al.30 The absence of any signal from the methylene (symmetric vibration at 2850 cm−1 and antisymmetric vibration at 2920 cm−117,31) groups is a clear indication of the absence of gauche defects and thus that a highly ordered monolayer is present on the surfaces.31 All three characterization techniquescontact angle, AFM, and VSFSare consistent with a homogeneous monolayer over the entire surface of the disc in an all-trans conformation. Such a characterization was performed at the beginning of each experiment. Any observed change in the OTS structure during the contact experiment could then be related to the effect of the load and/or sliding. Monolayers at the Solid−Solid Interface. OTS Monolayer at the Silica−PDMS Interface. Figure 4a−c shows VSF spectra of the OTS SAM at the PDMS−silica interface under PPP, SPS, and SSP polarization combinations, respectively. “Before contact” and “After contact” indicate measurements where there was no contact with the PDMS surface (i.e., corresponding to the air−silica interface). The OTS SAM on the silica surface is probed at the same position through the entire loading process (before, during, and after contact). In all of the polarization combinations, it was observed that the VSF signals decreased continuously as the conformable PDMS surface was brought into contact and the normal load force was increased. The signal levels then gradually returned to their initial levels when the applied force was decreased. The same spectral features as in Figure 3 are observed; once again the spectra at different loads have been offset for clarity. The apparent contact area at zero applied load was larger than the laser spot size (due to the adhesive interaction), and the visually observed contact area was observed to increase with the load. The systematic, reversible reduction of the signal intensity with load is the only clear observation to be made and, as will be discussed below, is of comparable magnitude in all polarization combinations. The methylene peaks (CH2) are once again absent in all of the spectra irrespective of the polarization combination during the pressure measurement, indicating that there are no gauche defects induced by the loading and thus that there is no change in the order of the monolayer. Monolayer at the PTFE−Silica Interface as a Function of the Normal Load Force. The PTFE lens is a much harder surface than PDMS. Increasing the normal load force induces a change in the maximum pressure but does not cause a discernible change in the contact area, which is nonetheless larger than the laser spot size. The effect of loading the contact on the OTS monolayer spectra is shown in Figure 5. The spectral features are the same as in Figure 4. Very few changes in the spectra can be observed. Some small blue shifts (1−4 cm−1) of the symmetric and antisymmetric methyl stretch can

(1)

where θ and θmax are the measured contact angle of the sample and the maximum contact angle measured for an OTS SAM, respectively. Taking θ = 110° and assuming θmax= 114°,2,5,26,28 the coverage is on the order of 99%.

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RESULTS Monolayer at the Silica−Air Interface. To study a monolayer of OTS at a silica−PDMS or silica−PTFE interface as a function of the load force and the speed of the silica disc, the monolayer must be homogeneous over the entire surface. A combined study was carried by AFM, VSFS, and contact angle measurement to assess the quality of the monolayer. We have previously shown how important it is to characterize monolayers with AFM before performing VSFS measurements.1 Figure 2a is a topographic image of the silica-disc surface before OTS coating, which shows a smooth surface with a

Figure 2. AFM images of the silica-disc surface (a) before and (b, c) after being coated with OTS.

measured rms surface roughness of 0.57 nm. The marks due to the surface polishing can be easily recognized in the image. Figure 2b shows the considerable surface changes after OTS coating. The polishing marks disappeared, and the measured surface roughness was reduced by almost 50% from 0.57 to 0.32 nm. This homogenizing effect was observed over the whole silica-disc surface, where an even larger topographic image, such as the one presented in Figure 2c, also displays a low surface roughness of 0.28 nm. Although some topography variation is visible (characteristic of the substrate), the local smoothness over submicrometer distances indicates a homogeneous monolayer. VSFS Results: Monolayer at the Solid−Air Interface. To establish the homogeneity and intrinsic ordering of the 3077


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Figure 3. Sum frequency spectra of octadecyltrichlorosilane (OTS) at the silica−air interface on three different positions of the sample recorded using (a) PPP, (b) SSP, and (c) SPS polarization combinations.

the maximum load, the signal had decreased about 4-fold. Unlike the previous cases, however, the monolayer did not completely recover on reducing the load and terminating the contact. A similar reduction in intensity with only partial signal recovery after separation was also observed for SPS and SSP polarizations. Under SSP conditions, it can also be observed that the relative intensity between symmetric and antisymmetric CH3 changed. At the maximum load, other bands appear at 2850 and 2920 cm−1, which are respectively characteristic of the symmetric and antisymmetric CH2 stretches. The presence of these peaks is indicative of gauche defects and thus of reduced order in the monolayer.31 Note that there is no possibility of a CH2 signal arising from the PTFE because it is entirely fluorinated. After contact, the two peaks assigned to the symmetric and antisymmetric CH2 were still present, the VSF intensity did not recover to the initial levels, and the relative intensity between CH3 bands was different from that of the spectra before contact. This indicates that irreversible changes to the monolayer occur as a result of sliding. After 3 days, the spectra are essentially indistinguishable from those taken immediately after separation (Figure 6d), indicating that there is no gradual healing and that the changes are essentially irreversible. Postcontact Characterization. The contact and sliding of the silica−PTFE surfaces had a marked effect on the surface properties. The average contact angle decreased from 110.0 ±

be observed in PPP and SSP at the high load force in Figure 5. These frequency shifts are ascribed to the change in the local environment of the methyl group when the contacting medium changes from air to PTFE. Such shifts have been seen in the VSF spectroscopy of self-assembly monolayers on gold when they are immersed in liquids,31 for a zinc arachidate monolayer at the sapphire silica interface,14 and in the IR spectra of a monolayer in contact with liquids.32 In each of the three polarization combinations, VSF spectra (Figure 5a−c) show a very small decrease in the intensity of the methyl group at 45 N, which is in stark contrast to the case of PDMS where the signal was highly load-dependent. As for the case of PDMS, however, when the load and contact were removed the spectra returned to their original precontact state, indicating that no irreversible change in the monolayer had occurred. Monolayer at PTFE−Silica: Effects of Both Loading and Sliding. In this part, we try to add the effects of both rotation and force on the monolayer structure. Figure 6a−c shows VSF spectra of the OTS SAM at the PTFE−silica interface before, during, and after contact for three different polarization combinations. In this case, the silica disc was rotated at constant speed at each load, leading to sliding in the contact. All spectra are recorded for a constant speed of v = 26 mm/s. The sliding leads to measurably different behavior. Under PPP polarization, a significant decrease in the intensity of the symmetric and antisymmetric CH3 stretches was observed; at 3078


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Figure 4. Sum frequency spectra of octadecyltrichlorosilane (OTS) at the silica−PDMS interface using (a) PPP, (b) SPS, and (c) SSP polarization combinations. (d) AFM image of the PDMS lens surface before contact with silica.

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1.9 to 97.6 ± 2.3°, which according to eq 1 would imply that the OTS coverage could be estimated to be 80%. Note that the use of eq 1, which is at best a guide given the results of Figure 7 below, implies that there is no disordering of the remaining monolayer. In reality and as indicated by the presence of the CH2 signal in the VSFS data, the disorder of the monolayer would also reduce the observed contact angle. The VSFS results show that the postcontact signal does not recover to that of the initial state. This observation could be explained either by OTS transfer from silica to PTFE or by a disordering of the monolayer or both, and indeed disorder presupposes that some material is lost. Given the magnitude of the monolayer reduction indicated by the contact angle data, it would appear that material transfer is the primary cause. Figure 7a is a topographic image of the silica-disc surface after OTS coating with a smooth surface of 0.29 ± 0.05 nm. In contrast, Figure 7b,c shows considerable surface changes after silica−PTFE contact and sliding, with roughness values of 3.5 ± 0.8 nm and 3.3 ± 1.0 nm, respectively. They also show that the silica surface is no longer manifest as a homogeneous monolayer but rather displays aggregates and fringes in the sliding direction that correspond to the transfer of PTFE to the monolayer surface. This is inferred from the fact that the sliding lines are in relief, indicating that it is not damage to the monolayer but rather a deposition of material from the counter surface.

DISCUSSION When the monolayer was placed between the PDMS lens and silica substrate, the spectra display only an intensity loss as the load (and thus pressure) is increased. The reduction in the VSF signal for PDMS was also greater than that for PTFE. Such an intensity change has been observed before.13,14,17 Pressure measurements were conducted for different interfaces (silica lens−sapphire,14 IR quartz lens−substrate,13 and PDMS− sapphire17) by using VSF spectroscopy. In particular, Shen13 concluded that the reduction in the VSF signal intensity (in their case, complete reduction) was due to molecular structural changes involving monolayer rearrangement such that the methyl groups were perfectly aligned with the surface and thus not visible by VSFS (i.e., a significant change in the monolayer tilt angle). (Note that only one polarization combination was used, so no other orientation information could be gathered.) Similar arguments could be applied to explain our results. However, there is a far more plausible explanation that can be quantitatively proven. This argument involves taking into account the reduced reflectance from the interface associated with the change in refractive index. This effect should be observed even if there is no change in the monolayer tilt angle. In Figure 8, the theoretical33 value of the SF intensity is plotted as a function of the methyl tilt angle for each of the polarization combinations and interfaces used (assuming intimate contact). For the calculation, the CH3 hyperpolarizability ratio R = βaac/ 3079


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Figure 5. Sum frequency spectra of octadecyltrichlorosilane (OTS) at the silica−PTFE interface using (a) PPP, (b) SPS, and (c) SSP polarization combinations. (d) AFM image of the PTFE lens surface before contact with silica.

βccc = βbbc/βccc is chosen to be 2.37 and βaac/βaca is chosen to be −1.05.17,34 The refractive index of PDMS is of course larger than that of air, so when the PDMS lens is in contact with the OTS layer, the SF intensity decreases, even without invoking the deformation of the OTS. Thus, the difference in the refractive indices between air and PDMS could explain the reduction in the VSF signal intensity. In fact, the observed reduction in intensity at the highest pressures in Figure 4 is in quantitative agreement with the a priori predictions of Figure 8 for reasonable tilt angles on the order of ∼35° (obtained from comparing the polarization intensities of the air−monolayer interface), which renders alternative arguments rather untenable. One simple further step is required to achieve a satisfactory explanation of both the results in Figure 4 and the previous observations in the literature. It involves taking into account the fact that local surface roughness causes the real contact area of the interface to be much lower than that estimated from contact mechanical theories or observed visually. Dhinojwala34 has previously shown that the true area of contact of PDMS, one of the most conformable materials available, against a flat surface is much lower than predicted. The contact zone contained welldefined regions without intimate contact. Thus, the VSF signal reflected from such a heterogeneous interface in our work

should contain signals from both the PDMS−silica and the air− silica interfaces. As the load increases, the local pressure must cause the PDMS to deform and increasingly conform to the silica, thus increasing the fraction of signal generated by the silica PDMS interface and reducing the overall signal level. At high load, the intensity should be asymptotic to that predicted by a pure PDMS−silica contact, but if no other factors are involved (i.e., no load-induced rearrangement), then there should also be no change in peak positions and no relative change in the magnitude of the different peaks observed. This is expected and appears to be what is observed in Figure 4. To confirm this hypothesis, we performed a control experiment. The PDMS lens was replaced by liquid PDMS under the assumption that intimate contact is achieved for a liquid. Assuming that the Fresnel factors for the OTS-PDMS interface are the same for solid and liquid PDMS, then it is reasonable to expect that the spectra obtained against the liquid interface should correspond to those obtained at high load against solid PDMS if the above arguments hold. These results are shown in Figure 9a−f, where a−c correspond to solid PDMS and panels d−f correspond to liquid PDMS. In each of the three polarization combinations, the spectrum in air is compared to that against PDMS. In both cases, the spectral intensity decreases irrespective of the polarization combination 3080


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Figure 6. VSF spectra of OTS at the silica−PTFE interface for a fixed sliding speed (26 mm/s) and as a function of increasing load for (a) PPP, (b) SPS, and (c) SSP polarization combinations. (d) SSP data from part c, together with data taken 3 days later, indicating that the induced change is irreversible.

lens at maximum pressure. (Note that to allow comparison the peak height of the CH3 stretch in SSP in air has been normalized to 1 in each case, and the peak intensities against PDMS have been scaled by the same factor.) In contrast, for PPP, the antisymmetric peak at ∼2962 cm−1 in contact with liquid PDMS is lower than that for solid PDMS. It may be unwise to place too much interpretation on this observation, given that the local environments of a methyl group vibrating against solid PDMS and liquid PDMS are expected to be slightly different and that small differences are always observed in different experiments using different monolayers.35 It is nonetheless interesting to consider whether these small changes could in fact reflect a difference in the monolayer due to a pressure-induced change in the solid PDMS case. The tilt angle of the methyl group of OTS on a silica surface (air−silica interface) has been determined at ∼40−50°.12 Figure 8 shows the theoretical variation of the VSF intensity as a function of the tilt angle of the terminal CH3 group. According to these calculations, it is predicted that the SSP (symmetric methyl stretch) and SPS (antisymmetric methyl stretch) intensities decrease whereas the PPP (antisymmetric methyl stretch) intensity increases when the tilt angle of the molecules is increased (e.g., by pressure). Thus, the slight discrepancies between the left (a−c) and right (d−f) panels of Figure 9 are in

Figure 7. AFM images of the OTS-coated silica disc against a PTFE surface (a) before and (b, c) after sliding.

as predicted by the theoretical calculations of Figure 8. Indeed, the magnitude of the reduction is quantitatively consistent with the ratios of intensities in Figure 8 over the expected range of monolayer tilts (∼35°). This observation strongly supports the contention that the signal reduction with pressure is due to changes in the contact area and that the overall signal reduction is satisfactorily explained by the Fresnel factor argument and not by molecular rearrangements. A careful examination of the spectra in Figure 9 reveals that the extent of signal reduction varies slightly between the different polarization combinations. In SSP and SPS, the peaks measured while in contact with the liquid PDMS are slightly higher than those measured while in contact with the PDMS 3081


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Figure 8. Theoretical estimation of SF intensity as a function of molecular tilt angle in (a) PPP antisymmetric, (b) SPS antisymmetric, and (c) SSP symmetric modes at air−silica and PDMS−silica interfaces. The theory for obtaining the curves is explained elsewhere.33

When the monolayer was placed at the PTFE−silica interface, a much smaller change in the intensity of the methyl signal was observed on loading (compared to that for PDMS). This small change can be explained by two factors. First, the “air gaps” alluded to in the previous part of the (PDMS) discussion almost certainly exist at the PTFE−silica interface. The surface roughness of the PTFE lenses was about 17 nm (the AFM image is shown in Figure 5d), which means tha the PTFE surface was rougher than the silica and it also has a higher elastic modulus than did PDMS. Therefore, the signals obtained in the measurements are expected to consist of contributions from both the air−silica interface and the PTFE− silica interface. Given the very small reduction in the SF spectra, the air−silica interface probably dominates in the contact area. Because the PTFE lens is harder than the PDMS lens, the air gaps are less likely to be removed by increased load, even though higher loads were used than for PDMS. (Note that the observed signal reduction for PDMS is larger than for PTFE despite the fact that the PDMS is more deformable and thus reflects lower pressures. Had the signal reduction been solely due to the tilting of the monolayer, then the opposite trend should have been observed, which further strengthens the arguments above.) Second, the different refractive indices of PDMS and PTFE result in different Fresnel factors and hence a dissimilar reduction in intensity. However, because the refractive indices are not known for all three beams for PTFE, a quantitative comparison between PDMS and PTFE is not possible. Thus,

fact consistent with a pressure-induced change in tilt angle, but the relative magnitudes indicate that such an orientational change is small. (In such case the change also occurs without any corresponding increase in disorder since there is no evidence for methylene signal.) Because some different kinds of CH3 stretching modes overlap and produce one peak, which makes it difficult to distinguish one stretching mode from another, it would probably be necessary to deuterate some of the CH3 groups if the spectrum were to be fully deconvoluted. According to the results of AFM measurement, the roughness of the PDMS lens surface (Figure 4d) was 9.48 nm and the roughness of the silica surface with OTS was about 0.30 nm on average. Hence, this fact is consistent with our hypothesis. We note, though, that solely quoting the rms roughness (which is typically what is done14) is not sufficient to indicate that a contact is intimate. It is the peak-to-valley roughness combined with the elastic moduli of the materials that should be considered. Thus, in the case of a silica−sapphire interface even very low roughness could preclude intimate contact as a result of the unconformable nature of the materials. Thus, the first three conclusions of this work are that one cannot assume intimate molecular contact over the entire apparent contact zone. One must take the change in Fresnel factors (refractive index) into account and must consider that there is no appreciable monolayer disruption over the pressures applied here. 3082


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Figure 9. Sum frequency spectra of octadecyltrichlorosilane (OTS) at the PDMS (lens)−silica interface in (a) SSP, (b) PPP, and (c) SPS and at the PDMS (liquid)−silica interface in (d) SSP, (e) PPP, and (f) SPS.

PDMS, increased disorder, or molecular transfer. The action of the disc sliding on the PTFE and the associated friction caused the PTFE surface to become smoother, which may have led to an increased true area of contact. In this case, the contribution from the PTFE−silica interface increased and the total VSF intensity decreased. If the molecules become highly disordered, then the signal must also decrease. However, the partial removal of the SAM could also explain the reduction in the VSF intensity. In fact, these two effects cannot be distinguished from one another, and an increase in disorder must be associated with a reduction in density by simple volume

the VSF intensities of OTS at the PTFE−silica interface do not show a very strong pressure-dependent reduction. When both the load normal force and sliding were simultaneously applied to the sample, significant changes occurred. (1) a decrease in the total VSF intensity, (2) the appearance of a CH2 symmetric mode (2850 cm−1) and a CH2 antisymmetric mode (2920 cm−1), indicating gauche defects and monolayer disorder, and (3) the phenomenon that the VSF intensities did not recover to their initial level. The decrease in the total intensity could be mainly related to the difference in refractive indices between air and PTFE, as now explained for 3083


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arguments. Consequently, the OTS molecules were partially removed from the surface, and the number of effective molecules that generated the VSF signals decreased. This is why the VSF intensity did not return to the initial level. The appearance of methylene stretch modes corresponding to the disordering of the molecules was probably derived from the molecules that were not removed but were deformed by the sliding. It is also impossible to determine whether the dislodged molecules remain associated with the surface in a disordered manner or are transferred to the PTFE surface because it is impossible to perform VSFS on the PTFE surface directly as a result of scattering and absorption effects. The observed signal is thus probably a combination of signals from regions of both undisturbed and disordered SAMs. It is also worth noting that the signal generated in situ during sliding arises from an interface that is constantly being refreshed because the SAM is attached to the rotating disc. The fact that the dynamic measurements essentially show spectra identical to those obtained from static measurements testifies to the reliability of this new technique. Further support for the partial removal of the SAM is provided by the contact angle reduction. The decrease in OTS coverage could be related to the monolayer transfer to the PTFE lens and the disordering of the monolayer. The increase in the CH2 intensity indicating the presence of gauche defects during and after sliding was due to the disordering of the monolayer, which is in agreement with AFM and contact angle measurements. These observations are consistent with conclusions from the Monte Carlo simulation of the self-assembly of a thiol monolayer,36 which shows that the molecular deformation is in the form of gauche defects in addition to an increase in the molecular tilt. A significant increase in the number of molecules with such an end-gauche defect with increasing pressure was observed. This molecular distortion has been shown to be reversible in several studies (i.e., recovery upon release of pressure; for example, that of Chen37) and AFM studies using a tip pressure up to 1 GPa,38−41 which is larger than the calculated maximum pressure used here. In our study of PTFE−silica, the molecules do not recover upon release of pressure, despite the lower pressures, and this cannot be linked to a larger relaxation time because 3 days later the molecular distortion was still present and the intensity was not reversible. Thus, the changes are due solely to the effects of shear in this case.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses †

Fritz Haber Institut der Max-Planck Gesellschaft, Department of Chemical Physics, Faradayweg 4-6, 14195 Berlin, Germany. ‡ Institute for Multidisciplinary Research of Advanced Materials, Tohoku University, Katahira 2-1-1, Aobaku, Sendai 980-8577, Japan. ∥ Bio-Rad Laboratories, Hercules, California 94547, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Johan Andersson at SP Chemistry Materials and Surfaces for assistance with the PDMS surfaces and the EHL. We also thank Connor Myant for sharing drawings of the lower surface mount for the EHL and invaluable advice. Eric Tyrode and Jonathan Liljeblad are gratefully acknowledged for stimulating discussions. VR and SSF are also acknowledged for early partial funding.

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REFERENCES

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CONCLUSIONS

The new experimental approach for the in situ measurement of the molecular conformational response to the confinement load and shear is presented and shown to be both feasible and powerful. The issue of in situ VSFS signal reduction for molecules confined between two surfaces has been resolved by considering for the first time both the true area of contact and the reduced reflectance associated with the refractive index of the second contacting surface. Consequently, we have been able to demonstrate that the effect of sliding in the system studied here in dry contact is to cause significant, measurable, and irreversible changes in both the molecular density and order of a confined, chemically bound monolayer. 3084


Langmuir

Article

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Monolayer Study by VSFS - American Chemical Society

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