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Influence of Order within Nonpolar Monolayers on Hydrophobic Interactions Hongseung Yeon,† Chenxuan Wang,†,‡ Reid C. Van Lehn,† and Nicholas L. Abbott*,† †

Department of Chemical and Biological Engineering and ‡Department of Chemistry, University of WisconsinMadison, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: We report an experimental investigation of the influence of molecular-level order (crystallinity) within nonpolar monolayers on hydrophobic interactions. The measurements were performed using gold film-supported monolayers formed from alkanethiols (CH3(CH2)nSH, with n ranging from 3 to 17), which we confirmed by using polarization−modulation infrared reflection−adsorption spectroscopy to exhibit chainlength-dependent order (methylene peak moves from 2926 to 2919 cm−1, corresponding to a transition from liquid- to quasi-crystalline-like order) in the absence of substantial changes in chain density (constant methyl peak intensity). By using monolayer-covered surfaces immersed in either aqueous triethanolamine (TEA, 10 mM, pH 7.0) or pure methanol, we quantified hydrophobic and van der Waals contributions to adhesive interactions between identical pairs of surfaces (measured using an atomic force microscope) as a function of the length and order of the aliphatic chains within the monolayers. In particular, we measured pull-off forces arising from hydrophobic adhesion to increase in a nonlinear manner with chain length (abrupt increase between n = 5 and 6 from 2.1 ± 0.3 to 14.1 ± 0.7 nN) and to correlate closely with a transition from a liquid-like to crystalline-like monolayer phase. In contrast, adhesion in methanol increased gradually with chain length from 0.3 ± 0.1 to 3.2 ± 0.3 nN for n = 3 to 7 and then did not change further with an increase in chain length. These results lead to the hypothesis that order within nonpolar monolayers influences hydrophobic interactions. Additional support for this hypothesis was obtained from measurements reported in this paper using long-chain alkanethiols (ordered) and alkenethiols (disordered). The results are placed into the context of recent spectroscopic studies of hydrogen bonding of water at nonpolar monolayers. Overall, our study provides new insight into factors that influence hydrophobic interactions at nonpolar monolayers.

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INTRODUCTION Hydrophobic interactions are water-mediated forces that arise between nonpolar molecules or surfaces. These interactions underlie phenomena as diverse as the separation of oil and water or the folding of proteins into functional complexes.1,2 Hydrophobic interactions arise from the dynamic structuring of water near nonpolar interfaces.3 For small nonpolar solutes (1 nm and macroscopic interfaces), the low curvature of the interface is hypothesized to inhibit formation of hydrogen-bonded networks, leading to an increase in entropy (and decrease in enthalpy) of the interfacial water.6,7 Overall, both simulations and experiments reveal that the thermodynamic signatures of hydrophobic interactions are sizedependent due to the subtle influence of interfacial curvature on the structuring of water. Hydrophobic interactions have also been found to be strongly modulated by chemical heterogeneity, reflecting the influence of charged and polar functional groups on the interfacial structuring of water.8−15 Molecular dynamic © XXXX American Chemical Society

simulations, for example, have been used to predict the influence of polar residues in proteins, or hydroxyl groups within nonpolar surfaces, on the local dynamic structure of water.8−13 In addition, recent experiments have unmasked the influence of proximal interfacial charges on hydrophobic interactions involving monolayers on surfaces and sequencespecific oligopeptides.14 Whereas the studies mentioned above address the influence of chemical heterogeneity of interfaces on hydrophobic interactions, here we move to consider the influence of molecular order (i.e., organization of aliphatic chains) within nonpolar monolayers on hydrophobic interactions. This focus is motivated by the observation that past studies of hydrophobic interactions at surfaces decorated with nonpolar monolayers have used a range of experimental systems, including monolayers formed from alkanethiols, octadecyltrichlorosilane (OTS), cetyltrimethylammonium bromide (CTAB), dioctadecyldimethylammonium bromide (DODAB), or surfaces of polydimethylsiloxane (PDMS).14,16−18 While Received: January 21, 2017 Revised: April 14, 2017 Published: April 18, 2017 A

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Langmuir these interfaces all mediate hydrophobic interactions, the organization of the nonpolar constituents (e.g., aliphatic chains) within these experimental systems differs in ways that potentially influence interfacial hydration and thus the strength of adhesive interactions in water (Table S1). For example, selfassembled monolayers formed from long-chain alkanethiols on gold films assume quasi-crystalline structures, whereas the organization of monolayers formed from OTS on metal oxides typically comprises both ordered and disordered regions (the organization of which is strongly dependent on the details of the procedures used to prepare the monolayers).19−21 At least one past study has speculated that these differences in organization may impact the strength of hydrophobic interactions.22 Herein, we report experiments that were designed to test the hypothesis that the degree of ordering within nonpolar monolayers influences the strength of hydrophobic interactions. This hypothesis is based on the prediction that the ordering of aliphatic chains within a nonpolar monolayer will impact the dynamic structuring of water near that interface (see below for additional discussion).23 We used self-assembled monolayers (SAMs) formed from alkanethiols on gold films in our experiments because the ordering of aliphatic chains within these monolayers can be manipulated systematically by changing the length of the alkanethiols (and, as shown below, also through changing the degree of saturation of the chains).19,24 These types of monolayers have been widely studied by scattering, scanning probe, and spectroscopic methods.19,24−27 Of particular relevance to our measurements, infrared (IR) spectroscopy provides easily measured signatures of changes in the ordering of the aliphatic chains, from liquid-like organization for short chains (asymmetric methylene stretch >2924 cm−1) to quasicrystalline-like organization (asymmetric methylene stretch 5), however, the pull-off forces increased rapidly to 14.1 ± 0.7 nN (n = 6) and larger values. Correlation between Order within Alkanethiol Monolayers and Hydrophobic Adhesion. As described in the Introduction, this study was motivated by the hypothesis that molecular order within nonpolar monolayers influences the strength of hydrophobic interactions. To characterize the degree of order within the monolayers used in our study, we used infrared spectroscopy (PM-IRRAS). Past studies have revealed that four C−H stretching modes are strongly correlated with the structure of SAMs formed from alkanethiols on gold, as illustrated in Figure 4a and summarized in Table 1.19,24 In brief, we analyzed the sharpness and position of the peak assigned to the asymmetric methylene stretch to characterize the degree of ordering of the aliphatic chains within the monolayers (Figure 4b). As shown in Figure 5a, we found the peak sharpness to increase with chain length, particularly between n = 5 and n = 13. These results are consistent with an increase in order of the aliphatic chains with chain length.19,24,44 In addition, as shown in Figure 5b, the peak positions were measured to shift from 2924 to 2919 cm−1, corresponding to a change from liquid-like to crystalline-like ordering with increasing chain length.19 Figures 5c and 5d correlate the order within the monolayers (as measured by using sharpness and the position of the asymmetric methylene peak in the IR spectrum) with the magnitude of the hydrophobic adhesive forces. Significantly, for both measures of order, the hydrophobic adhesion is found to be strongly correlated with the order within the SAMs (Figure 5c,d). Hydrophobic Interactions and Monolayers of Alkenethiols. To provide an additional test of the hypothesis that molecular order within nonpolar monolayers impacts hydrophobic adhesion, we formed monolayers from the alkenethiol (Z)-9-octadecen-1-thiol (unsaturated C18). This molecule contains a single double bond in the middle of the chain (Figure 1b). Because of the cis conformation of the double bond, we predicted that the fluidity of a SAM formed from the alkenethiol would be enhanced relative to SAMs formed from saturated chains with the same chain length. To confirm the formation of a SAM from unsaturated C18 prior to performing force measurements, we measured the ellipsometric thickness as shown in Figure 6a. The optical thickness of the unsaturated C18 SAM (25 ± 0.7 Å) was measured to be similar but slightly lower than that of the saturated C18 SAM (octadecanethiol, 27 ± 0.4 Å). We interpret the slightly lower ellipsometric thickness of the unsaturated C18 SAM to potentially reflect the influence of the double bond on the optical properties of the monolayers or the packing density of chains on the surface. Figure 6b compares the adhesive forces that arise from hydrophobic interactions using unsaturated C18 SAMs relative to saturated SAMs as a function of position of the asymmetric methylene peaks in the IR spectrum. To enable this comparison, we normalized all forces by the pull-off force of a saturated C16 SAM. Relative to the adhesive forces that arise from hydrophobic interactions generated by a C16 SAM (24.4 ± 0.8 nN), the adhesive forces generated by unsaturated C18 and saturated C18 SAMs were 67 ± 8% and 97 ± 5%, respectively.

excluded from our analysis according to the above criteria. No samples measured in methanol were excluded.) Second, we note that the force histograms are broad and that the widths of the histograms were measured to scale with the magnitudes of the mean forces. The width of the histograms, when normalized by the mean force, however, were found to be similar for all chain lengths used in our experiments (Figure S5). While we did not attempt to determine the factors that influence the widths of the force histograms, we note that the gold surfaces used in our experiments are polycrystalline, and thus the contact area between the AFM tip and surface will vary with the local geometry of the contact. Past studies have used the Johnson−Kendall−Roberts (JKR) theory to estimate the contact area between gold-coated AFM tips and gold films, and the contact radius is typically found to be substantially smaller than the apparent geometrical radius imaged by a scanning electron microscope (SEM).14 This difference has been attributed to the roughness of the gold films. Figure 3b shows mean adhesive forces measured between alkanethiol monolayers in aqueous TEA and methanol. Inspection of Figure 3b reveals two important points. First, we measured the adhesive interaction between the monolayers in the aqueous TEA to increase with chain length of the alkanethiols. In particular, we observed the increase to be particularly abrupt between n = 5 and n = 6 (where n indicates the number of methylene groups of the alkanethiol), although the adhesive interaction continued to increase with chain length above n = 6. Second, we measured the adhesive interaction in methanol to be much smaller than aqueous TEA and to change little in magnitude for values of n > 7. Below and in the Supporting Information (see Figure S8 and associated text), we provide additional analysis of these measurements. Past studies have revealed that adhesive interactions between nonpolar surfaces in methanol arise largely from by van der Waals forces.38−41 Accordingly, we interpret the adhesion measured in methanol (Figure 3b) to arise, at least in part, from van der Waals interactions between the alkanethiol monolayers (we return to this point in the discussion below).40,41 In contrast, adhesive interactions measured in aqueous TEA can potentially include contributions from van der Waals forces, charge interactions (electrical double layers), and structural (hydrophobic) forces. In our past studies, we measured adhesive forces between alkanethiol SAMs in water to be independent of pH.14 This result suggests that charge interactions do not significantly contribute to the adhesive forces in our system, although pH-dependent electrical doublelayer interactions were evident in approach curves.14 Accordingly, we interpret adhesion forces measured in aqueous 10 mM TEA solution to arise largely from van der Waals and hydrophobic interactions. The van der Waals forces (dispersion forces) are influenced by the refractive index (n) of a solvent. Water and methanol have very similar refractive indices (nH2O = 1.332542 and nCH3OH = 1.3314,43 both at 25 °C). By using a refractometer, we confirmed the refractive index of pure water, 10 mM TEA aqueous solution, and pure methanol at room temperature to be 1.3330, 1.3334, and 1.3278, respectively. Accordingly, we subtract the mean adhesion forces measured in pure methanol from those measured in 10 mM TEA aqueous solution to estimate the contribution of hydrophobic interactions to the adhesion measurements in water (Figure 3c). Because van der Waals forces make only a small contribution to the overall E

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varied).16,17,22,44−46 For instance, Noy et al. and Warszyński et al. separately reported water-mediated adhesive forces between SAMs formed from dodecanethiol and hexadecanethiol to be 12.5 ± 4.4 nN45 and 28.4 ± 9.4 nN,46 respectively. However, it is unlikely that the tip geometry was the same in these two studies. The same issue prevents us from making quantitative comparisons between our own results and these past studies. While we anticipate that the geometries of the AFM tips used in our study also vary across experiments, we emphasize that we performed multiple measurements for each chain length and used multiple tips to establish robust statistics that enable comparison of adhesion forces across different chain lengths (see Table S2). We note that the error bars shown in Figure 3b reflect, in part, uncertainty that arises from the variation in the AFM tip geometry. Additionally, we note that there is a shoulder in our adhesion data for n = 14, as shown in Figure 3b. We do not observe any IR spectroscopic features (Figure 5a,b) to correlate with the presence of the shoulder. Additional studies are needed to understand its origin. In addition to reporting adhesive forces in aqueous systems, we also describe measurements in methanol.44,47 In this context, we note that Nakagawa et al. measured adhesive forces between alkyltrichlorosilane-modified surfaces in ethanol (the AFM tip was chemically modified with OTS).47 Similar to our measurements, they observed adhesive forces to increase in magnitude with chain length, although the uncertainty of their measurements prevents us from determining if they observed the adhesive forces to exhibit a plateau in magnitude for longchains (n > 8), as seen in our measurements. According to Lifshitz theory48 (see Figure S9), the adhesive forces between SAMs (on gold) in the presence of methanol are predicted to decrease with increase in chain length. Specifically, the work of adhesion between two SAMs in methanol can be written as Wad = 2γAu − SAM − MeOH − γAu − SAM − SAM − Au

Our calculations using Lifshitz theory (see Supporting Information) reveal that γAu‑SAM‑SAM‑Au increases with increase in length of the alkanethiols (due to weaker Au−Au interactions across the SAM), whereas γAu‑SAM‑MeOH decreases with increase in length of the alkanethiols (due to a decrease in the strength of repulsive interactions between methanol and Au across the SAM). Importantly, although van der Waals interactions likely contribute to the adhesive forces in our measurements, these theoretical predictions of the effects of van der Waals forces cannot account for our observation of an increase in adhesion force with increase in chain length for n < 8 (Figure 3b) in methanol. Nakagawa et al. suggested that interdigitation of chains within their silane-based monolayers led to their observation of the chain-length-dependent increase in adhesion in ethanol.47 Monolayers formed from alkanethiols, however, are densely packed and unlikely to interdigitate unless self-assembled on surfaces with high curvature.49 An alternative possible explanation of our results shown in Figure 3b is that the adhesion measured in methanol also reflects, at least in part, chain-length-dependent order within the alkanethiol monolayer. For example, if liquid-like, short-chain SAMs lose conformational degrees of freedom upon adhesive contact, the decrease in entropy of the chains might weaken adhesion. We emphasize, however, that the magnitude of the chainlength-dependent change in adhesion measured in methanol is small compared to that measured in water in our measurements (see Figure 3b). Specifically, uncertainty regarding the contributions to the forces measured in methanol do not,

Figure 4. (a) Representative PM-IRRAS spectra of the C−H stretching modes of SAMs formed from octadecanethiol on gold. Overlapping peaks, νa(CH2) and νs(CH3, FR), were deconvolved by fitting to Gaussian curves. (b) Peak sharpness, defined as the absolute height from the baseline divided by the full width at half-maximum (FWHM) of the Gaussian curve.

We also note that the position of the asymmetric methylene stretch of the unsaturated C18 (2921.9 ± 0.5 cm−1) was shifted toward a liquid-like state as compared to saturated C18 (2919.3 ± 0.1 cm−1). Overall, these results support our hypothesis that the ordering of chains within nonpolar monolayers can modulate the strength of hydrophobic adhesion.

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DISCUSSION A key result reported in this paper is that adhesive forces measured in aqueous solution between nonpolar SAMs increase in strength with length of the alkanethiols used to form the SAMs. Here we note that a number of past studies have reported on adhesive forces between monolayers formed from various alkanethiols.16,17,22,44−46 It is, however, difficult to make definitive conclusions from comparisons between these past studies because the geometry of the AFM tips vary from one study to the next (the geometry of the tip influences the contact area and thus the magnitude of the adhesive forces F

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Figure 5. Peak sharpness (a) and position (b) for the νa(−CH2−, asymmetric) band as a function of the number of methylene groups in the alkanethiol used to form the SAMs. Adhesive force arising from hydrophobic interaction (hydrophobic force) plotted against the peak sharpness (c) and the peak positions (d) for νa (−CH2−, asymmetric). Red lines (b, d) indicate the peak positions for liquid-like and crystalline-like states as described in Table 1.

solvent-induced reorganization of SAMs formed from alkanethiols is observed was found to be dependent on the properties of the solvent. Specifically, for nonpolar solvents such as benzene and hexanes, evidence of “disordering” of the SAMs has been reported, as indicated by the appearance of methylene peaks in SF spectra.51,52 In contrast, for methanol, solvent-induced reorganization has not been reported.56 For example, Stole et al. reported that the organization of noctadecanethiol SAMs formed on gold films did not change upon contact with methanol (CD3OD) by using in situ infrared external reflection spectroscopy (no observable changes in C− H stretching of methylene chain).56 In summary, these results lead us to conclude that neither aqueous solutions (10 mM aqueous TEA and water at pH 7.0) nor methanol used in this study are likely to change significantly the ordering of the aliphatic chains within the SAMs relative to that measured in the presence of air. We wish to note that hydrophobic interactions are influenced by solvent-accessible surface area. If defects introduced to

therefore, impact our conclusions regarding forces measured in water (chain-length dependence of the hydrophobic adhesion). As shown in Figure 6b, we report that the order of the aliphatic chains within SAMs, as indicated by IR vibrations of the methylene groups, correlates closely with the magnitude of adhesive interactions measured in aqueous TEA, thus providing support for the hypothesis that hydrophobic interactions are influenced by the order within nonpolar monolayers. We caution that our measurements using PM-IRRAS were performed in air whereas the adhesion measurements were performed in aqueous TEA solution and methanol. Several past studies have investigated solvent-induced reordering of SAMs formed from alkanethiols on gold.50−56 In particular, when using aqueous solvents, a consensus conclusion of a wide range of measurements (using PM-IRRAS50 as well as infrared-visible sum frequency (SF) spectroscopy51−54) is that SAMs formed from alkanethiols (independent of chain length) do not significantly reorganize upon transfer from air into aqueous environments. For nonaqueous solvents, whether or not G

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single-component SAMs involve minimal changes in solventaccessible surface areas. A key conclusion of the study reported in this paper is that the ordering of aliphatic chains within nonpolar SAMs impacts hydrophobic adhesion. We interpret our results to suggest that the changes in the ordering of aliphatic chains within nonpolar monolayers influences the hydrogen bond network at the interface, thus leading to modulation of the strength of hydrophobic adhesion. In this context, we note that Tyrode et al. investigated the interfacial structure of water in contact with nonpolar SAMs (alkylsilanes formed on glass) by using SF spectroscopy.23 Although the interpretation of their measurements is complicated by the influence of the underlying glass substrate on the organization of water (likely, in part, due to the charge of the glass), a spectral feature corresponding to dangling or non-hydrogen-bonded OH groups was observed with ordered but not disordered SAMs. This observation hints at a possible mechanism by which the degree of ordering within SAMs formed from alkanethiols on gold impact hydrophobic adhesion: ordered SAMs cause water to adopt an organization that increases the number of non-hydrogen-bonded OH groups of water at the interface. We comment also, however, that dynamic properties of the monolayers may be important, as past simulation study has suggested that dynamic fluctuations in the local density of water play a key role in mediating hydrophobic interactions.58 Additional simulations are needed to test these hypotheses and elucidate detailed molecular-level mechanisms by which the order within a nonpolar monolayer impacts hydrophobic adhesion. The structure−property relationships reported in this paper provide a clear benchmark for future simulations.

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SUMMARY AND CONCLUSIONS We have measured hydrophobic interactions between pairs of gold surfaces modified with nonpolar SAMs of n-alkanethiols that differ in their chain length (from butanethiol to octadecanethiol). By combining use of AFM and PM-IRRAS, we observed the strength of the hydrophobic adhesion to correlate closely with a chain-length-dependent transition from a liquid-like to quasi-crystalline-like structural phase of the monolayers, leading to the hypothesis that the strength of hydrophobic interactions is strongly dependent on the degree of order within monolayers. This hypothesis is supported by additional force measurements conducted using a SAM formed from an alkenethiol, where an increase in fluidity within the SAM is correlated with a decrease in magnitude of hydrophobic adhesion. The measurements reported in this paper also generate a range of unanswered questions, including how static versus dynamic properties of SAMs influence hydrophobic adhesion at interfaces. For example, with increasing chain length of the alkanethiols, we anticipate that both the average number of gauche defects in the SAMs will decrease along with dynamic fluctuations of the conformations of the aliphatic chains.59,60 In future studies, it will be interesting to explore differences between disordered surfaces that differ in their dynamics, such as nonpolar liquid and glassy interfaces. It is likely also that insights into these unresolved issues can be obtained from molecular simulations in which the dynamics of interfacial molecular fluctuations can be manipulated independently of the time-average order of the assemblies. Overall, our results suggest that the ordering of nonpolar molecules at interfaces strongly modulates the strength of hydrophobic interactions mediated by those interfaces, thus providing new

Figure 6. (a) Ellipsometric thicknesses of monolayers formed from alkanethiols and octadecen-1-thiol. Blue and red represent the thicknesses for saturated and unsaturated C18 species, respectively. (b) Relative magnitude of forces arising from hydrophobic interactions plotted as a function of the peak positions of the νa (−CH2−, asymmetric) bands, for monolayers formed from alkanethiols and an alkenethiol (unsaturated C18).

change the ordering of a monolayer also result in substantial changes in solvent-accessible surface area, the change in solvent accessible area will also impact the hydrophobic interaction. This point confounds interpretation of experiments that change both the ordering within a monolayer and the solventaccessible surface area. For example, to explore this point, we prepared mixed SAMs formed from long and short alkanethiols (n = 9 and 15, 7:3 mixture; see Supporting Information). Past studies have reported that long alkanethiols in such mixed SAMs form an outer, disordered region.57 Although we measured the adhesion forces between the mixed SAMs (18.5 ± 0.5 nN) to be lower than those of the single-component SAMs (22.4 ± 2.6 nN for n = 9 and 25.4 ± 2.5 nN for n = 15) in aqueous solution, which is consistent with our hypothesis, we caution that the adhesion on the mixed SAM is likely influenced by a solvent-accessible area that is substantially greater than the single-component SAMs. In contrast, our measurements with H

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(8) Cheng, Y. K.; Rossky, P. J. Surface topography dependence of biomolecular hydrophobic hydration. Nature 1998, 392 (6677), 696− 699. (9) Factorovich, M. H.; Molinero, V.; Scherlis, D. A. Hydrogen-bond heterogeneity boosts hydrophobicity of solid interfaces. J. Am. Chem. Soc. 2015, 137 (33), 10618−0623. (10) Giovambattista, N.; Lopez, C. F.; Rossky, P. J.; Debenedetti, P. G. Hydrophobicity of protein surfaces: Separating geometry from chemistry. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 2274−2279. (11) Patel, A. J.; Varilly, P.; Jamadagni, S. N.; Hagan, M. F.; Chandler, D.; Garde, S. Sitting at the edge: How biomolecules use hydrophobicity to tune their interactiosn and function. J. Phys. Chem. B 2012, 116, 2498−2503. (12) Berne, B. J.; Weeks, J. D.; Zhou, R. Dewetting and hydrophobic interactions in physical and biological systems. Annu. Rev. Phys. Chem. 2009, 60, 85−103. (13) Giovambattista, N.; Debenedetti, P. G.; Rossky, P. J. Hydration behavior under confinement by nanoscale surfaces with patterned hydrophobicity and hydrophilicity. J. Phys. Chem. C 2007, 111, 1323− 1332. (14) Ma, C.; Wang, C.; Acevedo-Velez, C.; Gellman, S. H.; Abbott, N. L. Modulation of hydrophobic interactions by proximally immobilized ions. Nature 2015, 517 (7534), 347−350. (15) Li, I. T. S.; Walker, G. C. Signature of hydrophobic hydration in a single polymer. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 16527− 16532. (16) Israelachvili, J.; Pashley, R. The hydrophobic interaction is long range, decarying exponentially with distance. Nature 1982, 300, 341− 342. (17) Donaldson, S. H., Jr.; Røyne, A.; Kristiansen, K.; Rapp, M. V.; Das, S.; Gebbie, M. A.; Lee, D. W.; Stock, P.; Valtiner, M.; Israelachvili, J. Developing a general interaction potential for hydrophobic and hydrophilic interactions. Langmuir 2015, 31 (7), 2051−2064. (18) Shi, C.; Chan, D. Y. C.; Liu, Q.; Zeng, H. Probing the hydrophobic interaction between air bubbles and partially hydrophobic surfaces using atomic force microscopy. J. Phys. Chem. C 2014, 118 (43), 25000−25008. (19) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. Spontaneously organized molecular assemblies. 4. Structural characterization of n-alkyl thiol monolayers on gold by optical ellipsometry, infrared spectroscopy, and electrochemistry. J. Am. Chem. Soc. 1987, 109, 3559−3568. (20) Booth, B. D.; Vilt, S. G.; McCabe, C.; Jennings, G. K. Tribology of monolayer films: Comparison between n-alkanethiols on gold and n-alkyl trichlorosilanes on silicon. Langmuir 2009, 25 (17), 9995− 10001. (21) Barriga, J.; Coto, B.; Fernandez, B. Molecular dynamics study of optimal packing structure of OTS self-assembled monolayers on SiO2 surfaces. Tribol. Int. 2007, 40, 960−966. (22) Duwez, A.-S; Jonas, U.; Klein, H. Influence of molecular arrangement in self-assembled monolayers on adhesion forces measured by chemical force microscopy. ChemPhysChem 2003, 4, 1107−1111. (23) Tyrode, E.; Liljeblad, J. F. D. Water structure next to ordered and disordered hydrophobic silane monolayers: A vibrational sum frequency spectroscopy study. J. Phys. Chem. C 2013, 117, 1780−1790. (24) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. Spontaeously organized molecular assemblies. 3. Preparation and properties of solution adsorbed monolayers of organic disulfides on gold surfaces. J. Am. Chem. Soc. 1987, 109, 2358−2368. (25) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.; Parikh, A. N.; Nuzzo, R. G. Comparison of the structures and wetting properties of self-assembled monolayers of n-alkanethiols on the coinage metal surfaces, Cu, Ag, Au. J. Am. Chem. Soc. 1991, 113, 7152−7167. (26) Himmelhaus, M.; Eisert, F.; Buck, M.; Grunze, M. Self-assembly of n-alkanethiol monolayers. A study by IR-visible sum frequency spectroscopy (SFG). J. Phys. Chem. B 2000, 104, 576−584.

insight into the influence of nanoscale heterogeneity of nonpolar surfaces on hydrophobic interactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00226. Figure S1: comparison of adhesion forces in aqueous TEA solution and pure water; Figure S2: comparison of adhesion forces as a function of the contact time and the triggering threshold; Figure S3: representative example of a non-Gaussian force histogram; Figure S4: representative force histograms before and after thermal treatment; Figure S5: comparison of normalized standard deviation of force histograms; Figure S6: representative PM-IRRAS spectra of the SAMs formed from different chain length of alkanethiols; Figure S7: comparison of adhesion forces reported in a past study; Figure S8: connection between adhesion forces and contact angles of water; Figure S9: theoretical predictions of adhesion forces in methanol; Figure S10: AFM imaging of gold surfaces; Figure S11: calibration of AFM tip spring constants; Description of adhesion forces between mixed SAMs; Table S1: summary of adhesive forces and energies reported in past studies; Table S2: statistical data; Table S3: dielectric response constants (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (N.L.A.). ORCID

Nicholas L. Abbott: 0000-0002-9653-0326 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by the Army Research Office (W911NF-14-1-0140, W911NF-16-1-0154, and W911NF-15-1-0568) and the National Science Foundation (CBET-1263970 and the Wisconsin Materials Research Science and Engineering Center, DMR-1121288).

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REFERENCES

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DOI: 10.1021/acs.langmuir.7b00226 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.7b00226 Langmuir XXXX, XXX, XXX−XXX