Effect of Substrate Morphology on the Odd–Even Effect in

Article pubs.acs.org/Langmuir

Effect of Substrate Morphology on the Odd−Even Effect in Hydrophobicity of Self-Assembled Monolayers Zhengjia Wang,†,⊥ Jiahao Chen,†,‡,⊥ Symon M. Gathiaka,§ Stephanie Oyola-Reynoso,† and Martin Thuo*,†,‡,∥ †

Department of Materials Science and Engineering, Iowa State University, 2220 Hoover Hall, Ames, Iowa 50011 United States Micro-Electronic Research Center, Iowa State University, 133 Applied Sciences Complex I, 1925 Scholl Road, Ames, Iowa 50011 United States § Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California, San Diego, Pharmaceutical Sciences Building, 9500 Gilman Drive, La Jolla, California 92093, United States ∥ Biopolymer and Biocomposites Research Team, Center for Bioplastics and Biocomposites, Iowa State University, 1041 Food Sciences Building, Ames, Iowa 50011 United States ‡

S Supporting Information *

ABSTRACT: Surface roughness, often captured through root-mean-square roughness (Rrms), has been shown to impact the quality of self-assembled monolayers (SAMs) formed on coinage metals. Understanding the effect of roughness on hydrophobicity of SAMs, however, is complicated by the odd−even effecta zigzag oscillation in contact angles with changes in molecular length. We recently showed that for surfaces with Rrms > 1 nm, the odd−even effect in hydrophobicity cannot be empirically observed. In this report, we compare wetting properties of SAMs on Ag and Au surfaces of different morphologies across the Rrms ∼ 1 nm limit. We prepared surfaces with comparable properties (grain sizes and Rrms) and assessed the wetting properties of resultant SAMs. Substrates with Rrms either below or above the odd−even limit were investigated. With smoother surfaces (lower Rrms), an inverted asymmetric odd−even zigzag oscillation in static contact angles (θs) was observed with change from Au to Ag. Asymmetry in odd−even oscillation in Au was attributed to a larger change in θs from odd to even number of carbons in the n-alkanethiol and vice versa for Ag. For rougher surfaces, no odd−even effect was observed; however, a gradual increase in the static contact angle was observed. Increase in the average grain sizes (>3 times larger) on rough surfaces did not lead to significant difference in the wetting properties, suggesting that surface roughness significantly dominated the nature of the SAMs. We therefore infer that the predicted roughness-dependent limit to the observation of the odd−even effect in wetting properties of n-alkanethiols cannot be overcome by creating surfaces with large grain sizes for surfaces with Rrms > 1 nm. We also observed that the differences between Au and Ag surfaces are dominated by differences in the even-numbered SAMs, but this difference vanishes with shorter molecular chain length (≤C3).

I. INTRODUCTION Self-assembled monolayers (SAMs) are widely used to fabricate surfaces and interfaces with defined compositions, structures, and thicknesses.1−9 Due to their ability to control interfacial structure at molecular scales, SAMs have significant implications in fundamental and technological studies.1,10−18 Interestingly, properties of n-alkanethiolate SAMs depend on whether the molecule has odd or even number of methylene units (carbons in the n-alkanethiols).11,13,15,17,19−25 This odd−even effect manifests as zigzag alternations in chemical, physical, and interface properties in areas like electronic properties, friction behavior, hydrophobicity, and electrochemical properties.17,19,26−45 We,11,13,46 and others,20,28,31 are interested in understanding odd−even effect in wetting properties of n-alkanethiolate SAMs to further inform applications of these foundational platform/ technologies. This work is part of a series13,46−48 aimed at informing design and developments in SAM-based technologies like molecular tunneling junctions, field-effect transistors, © 2016 American Chemical Society

molecular diodes, bioanalytical devices, and surface coating, among many others.10,15,19,23,25,49−51 As previously discussed46 and for simplicity, brevity, and clarity, we utilize static contact anglesa representation of the equilibrium state between the surface and wetting liquid that avoids complexity that may arise from other measurements like advancing and receding angles (e.g., effect of disjoining pressure on a receding liquid film and the need for Derjaguin isotherm for comprehensive understanding of role of residual films in local wetting/dewetting). The odd−even effect in hydrophobicity is believed to be caused by the varying tilting angles of the terminal ethyl group (structural effect) and/or the effective surface-normal dipole (electronic effect) of the SAMs.38,39,52,53 Figure 1a schematically shows an n-alkanethiolate SAM on Au and Ag having odd Received: May 2, 2016 Revised: September 16, 2016 Published: September 19, 2016 10358

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well-known that longer linear alkanes (carbon number ≥19) tend to back-fold,17 in part due to energy minimization in the absence of external or next-neighbor(s) perturbations.60 Smooth surfaces, however, may promote formation of well-ordered SAMs from relatively long linear n-alkanethiols where intermolecular distance is dictated by its footprint on the substrate. On rough substrates, however, this maximized packing is not possible and molecules are likely to be further apart (due to curvature of surface asperities)and hence likely to back-fold and form poorly ordered SAMs (Figure 1c). Despite significant advances in SAMs, understanding the effect of surface roughness on basic interfacial properties of monolayers is ongoing. The use of root-mean-square roughness (Rrms), for example, as a measure of substrate roughness is common, often with the inference that surfaces with comparable Rrms should yield SAMs with comparable properties. We recently showed that for substrate Rrms > 1 nm the difference in static contact angles between SAMO and SAME (θsE − θsO) ≈ 0°, indicating that the odd−even effect in hydrophobicity cannot be observed (Figure 1d).11,46 On the other hand, we also demonstrated that the ultimate contribution of the odd−even effect to the hydrophobicity of n-alkanethiolate SAMs on Au is ∼3° (Figure 1d).11 Unlike in hydrophobicity (polar liquids), the odd−even effect in oleophilicity (nonpolar liquids) of n-alkanethiolate SAMs (wetting with hexadecane) seems to be independent of the surface roughness.53,61 This dichotomy that is dependent on the nature of the probe liquid, coupled with observation of the odd− even effect in other SAM properties across surfaces with different morphologies,17−19,22−24,62,63 calls for a need to further investigate the role of substrates in SAM properties. In this work we investigate the role of substrate identity and Rrms across the odd−even limit in understanding SAM properties, herein exemplified in wetting properties. By comparing the wetting properties across two substrates, SAMs with different tilt angles, we delineate the origin of the odd−even effect and the role of the substrate (SAM tilt angle) in the odd−even effect in SAM hydrophobicity. Unlike in previous studies, herein, the Au and Ag surfaces being compared have similar Rrms, allowing for reliable comparisons.

Figure 1. Summary of the structure and property of SAMs relevant to the wetting properties of SAMs. (a) Schematic illustration of molecular structure of n-alkanethiols on Au and Ag surfaces showing the molecular (dotted line) and the headgroup (solid line) dipole. (b) Comparison of changes in calculated molecular dipoles due to molecular orientation or bending. (c) Schematic illustration of SAM structure on the surfaces with small and large grains. (d) The linear fits of contact angles on SAMO and SAME showing a decaying odd−even effect that converge at Rrms = 1 nmthe roughness-dependent limit.

(SAMO) and even (SAME) number of carbon atoms, highlighting the structural differences across the substrates. The orientation, hence dipole moment, of the terminal CH2−CH3 group differs with odd−even variations in chain length leading to changes in surface energy. These observations are, however, predicated on assuming smooth substrates, onto which wellordered (idealized, defect-free) SAMs are formed. It is, however, well-known that properties of SAMs depend on the quality and properties of the substrates onto which they are fabricated, as well as the chain length of the molecules that build the SAMs.5,11,13,54−58 For short chain alkanethiols (C9), in the absence of steric encumbrance (that is, in a free unrestricted molecule), the thermodynamically favorable conformation is bent, which in a SAM leads to changes in the effective surface normal dipole.60 Figure 1b shows the energy minimized structure of n-alkanethiols in the all-trans-extended form and in the bentconformer analogous to that predicted by Goodman.60 We infer that these types of difference will dominate SAM properties when rough and smooth surfaces are used, with the rough surface likely leading to more bending and, hence, disordered SAMs. Increase in grain sizes, however, may lead to more ordered regions of the SAM,55 hence mitigate the detrimental effects of the rougher regions of the substrate as illustrated in Figure 1c. As depicted in the comparison in Figure 1c, large grains may lead to formation of local superflat areas while the average Rrms of the whole surface remains almost the same. The effect of grains size on SAMs structure and the odd−even effect in wetting on SAM modified substrates have, however, not been reported. It is

II. EXPERIMENTAL SECTION Materials. All alkanethiols reagents were purchased from SigmaAldrich except n-tridecyl mercaptan (C13) which was obtained from Pfaltz & Bauer. Ethanol (200 proof) was purchased from Decon Laboratories, Inc. All chemicals and reagents were used as received. Nitrogen and argon gas (industrial grade) were purchased from Airgas and used as supplied. Substrates and Template Stripping. All substrates were custom evaporated either in-house or by Substrata Thin Film Solutions Inc. and used as deposited or template stripped as previously reported.17 To template strip the metal surface, glass pieces and metal films were first cleaned with ethanol and dried with a stream of nitrogen gas. A piece of glass (∼1 cm × 1 cm) was then attached on top of the metal film using a ∼10 μL drop of optical adhesive (Norland Optical Adhesive 61). Possible bubbles were gently removed by tapping the glass support. After 12 h curing under long-wave length UV light, the supported film was stripped using a razor blade. Preparation of Monolayers. As deposited AuAD and templatestripped AuTS, AgAl‑TS, and AgFe‑TS were washed with ethanol and dried with a stream of nitrogen gas before forming SAMs. As previously reported,5 SAMs were prepared by immersing the metal substrates into a vial containing 3 mMol alkanethiol in 5 mL of ethanol for 3 h under inert atmosphere (Argon gas). The SAM was then rinsed with ethanol and dried with a stream of N2. 10359

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Figure 2. Surface analysis of AuTS and AgAl‑TS. Atomic force microscopy (AFM) images of (a) AuTS and (b) AgAl‑TS surfaces, from which the roughness root-mean-square (Rrms), average grain area, surface coverage, power spectral density (PSD), and bearing volume (BV) were estimated or calculated.

Figure 3. Wetting behavior of SAMs on AuTS and AgAl‑TS surface. (a, b) Hydrophobicity measured through contact angles formed between water and SAMs on AuTS (compared with reference data) and AgAl‑TS. (c, d) Oleophilicty measured through contact angles formed between hexadecane (HD) and SAMs on AuTS and AgAl‑TS. Atomic Force Microscopy (AFM) Characterization of Surface Parameters. A Bruker Multi-Mode AFM was used in tapping mode to characterize the surface features of all surfaces. All samples were scanned immediately after the lift-off. The Rrms, PSD, and BV of all surfaces were analyzed automatically using the NanoScope v1.5 software. Measuring the Contact Angle. Contact angles of deionized (DI) water, hexadecane (HD), and diethylene glycol (DEG) on fabricated SAMs were measured using a Rame-hart goniometer with tilting base. A droplet of 1.5 μL of liquid was dispensed onto the SAMs using an

automated syringe pump. Images of droplet on SAMs were analyzed with the accompanying DropImage software.

III. RESULTS AND DISCUSSION All metal surfaces were fabricated through e-beam evaporation of a 200 nm film onto an ultraflat Si(100) with its native oxide, and where needed, a 10 nm adlayer was sputtered onto the films.64 The films were then template-stripped, and surface roughness was characterized by atomic force microscopy (AFM). We 10360

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(AgAl‑TS), suggesting that the magnitude of the odd−even effect in oleophilicity on AuTS is significantly larger than on AgAl‑TS. This phenomenon (for both water and HD) is likely induced by differences in molecular orientation and packing density of the thiols across the two substrates. III.a.iii. Origin of the Substrate Discrepancy in Hydrophobicity. To better understand the hydrophobicity on SAMs that are fabricated on AgAl‑TS and AuTS surfaces, we compare the water contact angle across the two types of surfaces (Figure 4a).

observed that the surface morphologies of template-stripped Au (AuTS) and template-stripped Ag with an Al adlayer (AgAl‑TS) were comparable. Figures 2a and 2b show atomic force microscopy (AFM) images of template-stripping gold, AuTS (Rrms = 0.41 ± 0.04 nm) and the aluminum ad-layer reorganized silver surface, AgAl‑TS (Rrms = 0.43 ± 0.05 nm). The Rrms values are not statistically significant different (student t test, p > 0.1) allowing us to compare resulting SAM properties without accounting for differences in roughness. Through an image analysis process (see Supporting Information), we estimated the relative grain area, which is defined as a flat region within 1 nm of the Rrms midline on a surface, and then calculated the surface coverage of the grains. AuTS and AgAl‑TS surfaces have comparable average grain areas (AuTS = 0.05 ± 0.00; AgAl‑TS = 0.10 ± 0.01) and percentage surface coverage (AuTS = 98.35 ± 0.02; AgAl‑TS = 98.96 ± 0.01). Power spectral density (PSD) and bearing volume (BV) were used to further characterize the surface defects (Figure 2). The PSD (nm2) and the BV (μm2) for both surfaces are not statistically different (t test with p > 0.1). These data (grain areas, % surface coverage, Rrms, PSD, and BV) suggest that the surface morphologies of AuTS and AgAl‑TS are comparable. III.a. Wetting Properties of n-Alkanethiolate SAMs within the Odd−Even Effect Region (Rrms ∼ 0.4 nm). III.a.i. Hydrophobicity. Wetting properties of SAMs on these two substrates with comparable surface morphologies were investigated. Static contact angle, θs, formed by water on the SAMs showed an odd−even effect on both AuTS and AgAl‑TS substrates (Figure 3a and 3b). The water contact angle on SAM on AuTS agrees well with our previously reported data.11 When surface Rrms was ∼0.4 nm (inside the 1 nm odd−even limit11), the odd− even effect in hydrophobicity was observed. The zigzag oscillation was, however, inverted when the substrate changed from Au to Ag, as expected.5,13 We observe that the odd−even oscillation is asymmetric leading to an overall increase in θs with increase in the length of the molecule. The increase in θs is due to an asymmetry in the magnitude of the oscillation, with the , being not equal to the change change from odd to even, ΔθO→E s . For AuTS surfaces, ΔθE→O = 1.3° while from even to odd, ΔθE→O s s = 2.6°, hence an increase in θs by 1.3° for every 3 carbons ΔθO→E s − ΔθE→O | = 1.3°). For AgAl‑TS the overall (that is | ΔθO→E s s increase in θs was slightly lower than in AuTS since ΔθE→O = 1.2° s = 0.5 (hence | ΔθE→O − ΔθO→E | = 0.7°). For while ΔθO→E s s s as a, and ΔθO→E as b in AuTS, and brevity, we abbreviate ΔθE→O s s Al‑TS (Figure 3a and 3b). We observe that a ≈ a′ as a′ and b′ in Ag while b > b′ (2.6 vs 0.5, respectively) suggesting that the difference in wetting across the two surface is also asymmetric. III.a.ii. Oleophilicty. Besides hydrophobicity, we observed an odd−even effect in oleophilicity (Figure 3c and 3d) across the two surfaces. When hexadecane was used as a probe liquid, an odd−even oscillation on both substrates was observed with an inversion, analogous to one obtained with hydrophobicity, when substrate was changed from Au to Ag. The C13 thiol was an anomaly, and we suspect that either (i) there are minor impurities in the thiol that are not readily removed (the molecule was sourced from a different company than the other molecules and had organic impurities as supplied which were not removed by silica gel column chromatograph, see Supporting Information Figure S4) and they interfere with the SAM formation process, or (ii) in light of spectroscopic studies, C13 falls in a transition region between waxy phase and crystalline phase, leading to a mixed phase that is significantly different than other members of this homologous series.47 Disregarding C13, we obtained an average a = 5.1° and b = 5.8° (on AuTS), while a′ = 2.6° and b′ = 1.3°

Figure 4. Summary of the odd−even effect in the hydrophobicity of SAMs fabricated on AuTS and AgAl‑TS surfaces. (a) Summary of all the data including the reference data, (b) contact angles derived from SAMO on AuTS and AgAl‑TS surfaces, and (c) contact angle derived from SAME on AuTS and AgAl‑TS surfaces.

We observe that there is no significant difference between the hydrophobicity of SAMO across the two surfaces (Figure 4b), but there is a significant difference in SAME (Figure 4c). Unlike in our earlier report where we compared the relative degree of smoothness (Rrms = 0.4 vs 0.6 nm), in the current study not only are the Rrms similar, but the grain sizes and surface coverages are statistically indistinguishable. Since surface morphologies are comparable and the main differences in hydrophobicity are from 10361

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Figure 5. Characterization of Au and Ag surfaces with similar RRMS but different morphology (PSD). (a, b) AFM image of “as-deposited” Au surface (AuAD) and template-stripped AgFe‑TS surface. (c, d) i. The line profiles (along the dotted line) derived from AFM images, showing the topological side view of the surface morphology. ii. Hypothetical structures of SAMs formed on the surfaces along the dotted lines are shown below them to capture the expected differences on the quality of SAMs across these two surfaces. The asperity-dominated AuAD surface (higher PSD) has a potentially highly disordered monolayer while the AgFe‑TS has some ordered assemblies on the relatively flat grains areas.

Characterization. To investigate the effect of substrate morphology on SAMs wetting outside the odd−even limit, Au and Ag surfaces were fabricated with similar Rrms but different grain sizes and surface coverage. An “as-deposited” gold surface, AuAD, was fabricated as previously reported.11,13 Similar to AgAl‑TS, we managed to tune the roughness of the templatestripped silver surface by adding a thin (10 nm) Fe ad-layer on to a predeposited Ag film. The resultant silver surface, AgFe‑TS, undergoes a reorganization, leading to a rougher surface. Figures 5a and 5b show AFM images of AuAD and AgFe‑TS, with Rrms = 2.23 ± 0.15 and 2.13 ± 0.43 nm, respectively. Though the rms values roughnesses of the two surfaces are indistinguishable (student t test, p > 0.1), they have different morphologies (grain size and surface coverage). AgFe‑TS has larger average grain area (0.064 ± 0.03 μm2) compared to AuAD (0.019 ± 0.01 μm2). Due to the existence of large grains, the surface coverage (% of the surface occupied by the flat regions) of AgFe‑TS (57.63 ± 6.72%) is, therefore, much larger than that of AuAD (30.90 ± 3.03%). Power spectra density (PSD) and bearing volume (BV) were also measured for these surface to highlight defect, especially grain boundaries, on these surfaces (Figure 5a, b). Comparing the two surface AuAD (PSD = 5.28 ± 0.78 nm2 and BV of 0.045 ± 0.006 μm3) and AgFe‑TS (PSD of 2.94 ± 1.41 nm2 and a BV of 0.057 ± 0.011 μm3) shows that AuAD has a statistically significantly larger PSD than AgFe‑TS (p < 0.012), while the BV are comparable (p > 0.05). This may be translated to imply that differences in the two surfaces are dominated by asperity differences and not grain boundaries. Figures 5c and 5d show line profiles across the highlighted (dotted white line) to highlight differences in surface morphologies. We schematically illustrate how thiols can assemble across the highlighted regions

evens, with the odds lying in between, we observe that the difference increases with molecular length. Extrapolating the linear fits to θs data derived from SAME leads to convergence at ∼C3 (Figure 4c and Supporting Information Figure S2). We infer that the difference in wetting disappears for n-alkanethiols with 2 nm). Others have investigated wetting properties of SAMs on surfaces with Rrms comparable to ours.13,17,20,61,66 In our case, SAMs on AgFe‑TS and AuAD did not show odd−even effect in hydrophobicity. The values of water θs, however, show a gradual increase with molecular length. This phenomenon is important considering that conformation of n-alkanethiolate SAM

(Figure 5c.ii and Figure 5d.ii). In the asperity-dominated regions, no well-ordered SAMs would form. This inference is also supported by spectroscopic evidence communicated in an upcoming paper.47 Wetting properties of SAMs fabricated on AgFe‑TS and AuAD were investigated by measuring the contact angle on SAMs using water, HD, and diethylene glycol (DEG) as probe liquids (Figure 6a−f and Table 1). 10363

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intensity and peak-width of the CH3asym in sum-frequency generation analysis of SAMs on rough and smooth surfaces.47 The water contact angles on SAMs formed on AuAD and AgFe‑TS show a significant difference (Figure 7). Average contact

molecules primarily depends on the substrate type and roughness.5,7,14 We define the degree of the gradual increase in θs as Δθs = |θs|max − |θs|min. For AuAD, Δθs = 4.6° (108.5 ± 0.8° to 113.1 ± 0.4°) while for AgFe‑TS, Δθs = 5.1° (103.9 ± 0.8° to 109.0 ± 0.5°). The rate of change is higher for shorter thiols, but this increase seems to level off at C14 thiol for AuAD (C15 for AgFe‑TS), which could suggest a transition to a more crystalline SAM with an increase in molecular chain length as noted above. III.b.iii. Wetting with Less Polar Liquids. In addition to water (polar liquid), a nonpolar liquid (hexadecane, HD) and a less polar liquid (diethylene glycol, DEG) were used as probe liquid. Contact angle from HD shows an odd−even effect on both substrates while that of DEG does not, which is similar to our previous work.46 Due to the better contact (wetting) of HD molecules to the SAM, it has a higher sensitivity to subtle changes in the overall SAM structure (bulk and interface) and shows an odd−even effect irrespective of surface roughness.20 An analogous observation was recently reported using spectroscopy that supports our inference.47 As expected,11,13 there was a gradual increase in values of θs with increase in molecular length; for HD Δθs, AuAD = 3.9° and Δθs, AgFe‑TS = 4.8° while for DEG Δθs, AuAD = 4.7° and Δθs, AgFe‑TS = 3.7°. Similar to water, the values of Δθs for HD and DEG are not significantly different between AuAD and AgFe‑TS suggesting that this could be due to a molecule-dominated phenomenon as opposed to substrate. The adsorbed molecules, therefore, behave like “grease”67  disordered hydrocarbons on a surface as previously suggested.62 This result, therefore, suggests that solvent−molecule interactions are dominated by surface roughness in the absence of well-ordered SAMs, while Δθs is a molecule-dominated effect. III.c. Effect of Surface Morphology. To comprehend the effect of surface morphologies, we note that the total percentage of surface occupied by grains on AgFe‑TS (57.63 ± 6.72%) is almost twice that of AuAD (30.90 ± 3.03%). In terms of quality of the SAM, the large grains on AgFe‑TS compensate for the equally large grain boundaries (BV = 0.057 μm2 vs 0.045 μm2). In the asperity-dominated surfaces (AuAD, PSD = 5.28 ± 0.78 nm2 vs 2.94 ± 1.41 nm2), the effective contact area between a liquid and the surface is highly limited. Thus, for a high surface tension liquid (e.g., water), capillary effects and the cohesive forces term in the spreading parameter will dominate. Hexadecane is the only liquid that shows an odd−even effect, albeit characterized by very small differences in θs. Focusing on the wetting properties of HD between the two surfaces, as expected,13,22,23,25 the odd−even oscillation inverts when the substrate changes from Au to Ag (Figures 6c and 6d). The previously observed asymmetry in zigzag oscillation, being an interface-dominated phenomenon, would strongly manifest in well-ordered SAMs and would at least be poorly captured on roughness-dominated surfaces. For the AuAD surfaces, ΔθE→O s (a) = 1.0° while ΔθO→E (b) = 1.8° (that is |Δθs| = |ΔθE→O − s s ΔθO→E | = 0.8°). For AgFe‑TS surface, the differences in odd−even s oscillation were slightly larger than on AuAD with ΔθE→O (a′) = s (b′) = 1.8° (that is |Δθ | = 0.9°). The magnitude 2.7° and ΔθO→E s s of odd−even oscillation is much smaller than those on smooth AuTS and AgAl‑TS surfaces (Figure 2), where a = 5.1°, b = 5.8° on AuTS while a′ = 2.6°, b′ = 1.3° on AgAl‑TS. We, however, note that the Δθs on these surfaces are comparable, suggesting that the evolution in the nature of the surface adsorbed molecule is independent of the surface. This is in line with the inference that Δθs is associated with increase in molecular order at the interface. The inference drawn here is in line with observed changes in the

Figure 7. Effect of surface morphology on SAMs wetting. The water contact angle on AuAD minus the effect of packing density compared to that on AgFe‑TS.

angles derived from the AgFe‑TS surface are slightly lower than those derived from analogous SAMs on AuAD surfaces (average |θs‑Au − θs‑Ag| ≈ 4.2°). The difference in θs due to difference in packing density of n-alkanethiolate SAMs on gold and silver, estimated to be ∼1.6°, suggests that there is more than the effect of packing density in the difference in wetting across rough Ag and Au. We, therefore, infer that there is a significant effect (∼2.6°) due to differences in surface morphology between these two surfaces.

IV. CONCLUSION By comparing wetting properties between n-alkanethiolate SAMs on AuAD with AgFe‑TS (Rrms ∼ 2.0 nm) surfaces and AuTS with AgAl‑TS (Rrms ∼ 0.4 nm), we infer the following: a. Substrate surface morphology has significant effect on the wetting properties of SAMs. By deducting the effect of packing density from measurements on smooth Ag and Au surfaces, the effect of surface morphology on wetting properties of SAMs was estimated. For the rough substrates used in this study, the contribution of contact angle change due to surface morphology is approximately 2.6°, which confirms our observations. b. The odd−even effect, and associated asymmetry, either diminishes or is lessened by substrate roughness. No odd−even effect in hydrophobicity was observed for rougher surfaces, further indicating that there is a roughness-dependent limit, beyond which it is empirically not possible to observe the odd−even effect in hydrophobicity. c. When substrates have comparable surface properties, SAMs on Au show a larger odd−even oscillation than analogous series on Ag. On Ag and Au surfaces that have similar roughness and grain area, the magnitude of odd− even oscillation is significantly different. We infer this to be primarily due to differences in molecule packing density and SAM tilt angle. With increase in packing density, the static contact angle decreasesthat is, it tends toward polyethylene, while with lower packing density SAMs become more hydrophobic probably due to a slight change in surface topology. d. The difference in wetting between smooth Au and Ag surface disappears for n-alkanethiols with ≤3 carbons. On Ag and Au surfaces that have comparable 10364

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Langmuir properties, extrapolated linear fits to water θs derived from SAME converge right around C3, indicating that the difference in hydrophobicity between Au and Ag disappears for n-alkanethiols with ≤3 carbons. These lack of difference in hydrophobicity can be attributed to lack of order for shorter n-alkanthiolate SAMs, which perturbs the order of the terminal groups, and hence the lack of a well-defined interface. Alternatively, this could be due to orbital mixing at the metal−molecule interface, with concomitant inductive effect (large electronegativity differences) promoting adhesion of the polar liquid. Ratner and co-workers investigated this effect in silicon and suggested that there is significant extension of mixing of the molecule and metal orbitals at the point of attachment.68−70 e. The rms is limited in describing surface roughness and, hence, the correlation with surface properties. We observe that although the rougher surfaces had comparable Rrms, the surface morphologies were significantly different, hence the need for more descriptive parameters to capture differences in grain sizes, grain boundaries, and other surface asperities. A combination of Rrms, PSD, and BV was sufficient to capture differences between the two otherwise very different surfaces.

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(5) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103−1170. (6) Nijhuis, C. A.; ter Maat, J.; Bisri, S. Z.; Weusthof, M. H. H.; Salm, C.; Schmitz, J.; Ravoo, B. J.; Huskens, J.; Reinhoudt, D. N. Preparation of Metal-SAM-Dendrimer-SAM-Metal Junctions by Supramolecular Metal Transfer Printing. New J. Chem. 2008, 32, 652−661. (7) Nijhuis, C. A.; Reus, W. F.; Whitesides, G. M. Molecular Rectification in Metal-SAM-Metal Oxide-Metal Junctions. J. Am. Chem. Soc. 2009, 131, 17814−17827. (8) Siegel, A. C.; Tang, S. K. Y.; Nijhuis, C. A.; Hashimoto, M.; Phillips, S. T.; Dickey, M. D.; Whitesides, G. M. Cofabrication: A Strategy for Building Multicomponent Microsystems. Acc. Chem. Res. 2010, 43, 518−528. (9) Nijhuis, C. A.; Reus, W. F.; Siegel, A. C.; Whitesides, G. M. A Molecular Half-Wave Rectifier. J. Am. Chem. Soc. 2011, 133, 15397− 15411. (10) Akkerman, H. B.; Mannsfeld, S. C.; Kaushik, A. P.; Verploegen, E.; Burnier, L.; Zoombelt, A. P.; Saathoff, J. D.; Hong, S.; Atahan-Evrenk, S.; Liu, X.; Aspuru-Guzik, A.; Toney, M.; Clancy, P.; Bao, Z. Effects of Odd−Even Side Chain Length of Alkyl-Substituted Diphenylbithiophenes on First Monolayer Thin Film Packing Structure. J. Am. Chem. Soc. 2013, 135, 11006−11014. (11) Chen, J.; Wang, Z.; Oyola-Reynoso, S.; Gathiaka, S. M.; Thuo, M. Limits to the Effect of Substrate Roughness or Smoothness on the Odd−Even Effect in Wetting Properties of N-Alkanethiolate Monolayers. Langmuir 2015, 31, 7047−7054. (12) Glavan, A. C.; Martinez, R. V.; Subramaniam, A. B.; Yoon, H. J.; Nunes, R. M. D.; Lange, H.; Thuo, M. M.; Whitesides, G. M. Omniphobic ″Rf Paper″ Produced by Silanization of Paper with Fluoroalkyltrichlorosilanes. Adv. Funct. Mater. 2014, 24, 60−70. (13) Newcomb, L. B.; Tevis, I. D.; Atkinson, M. B. J.; Gathiaka, S. M.; Luna, R. E.; Thuo, M. Odd−Even Effect in the Hydrophobicity of NAlkanethiolate Self-Assembled Monolayers Depends upon the Roughness of the Substrate and the Orientation of the Terminal Moiety. Langmuir 2014, 30, 11985−11992. (14) Oyola-Reynoso, S.; Heim, A. P.; Halbertsma-Black, J.; Zhao, C.; Tevis, I. D.; Cinar, S.; Cademartiri, R.; Liu, X.; Bloch, J.-F.; Thuo, M. M. Draw Your Assay: Fabrication of Low-Cost Paper-Based Diagnostic and Multi-Well Test Zones by Drawing on a Paper. Talanta 2015, 144, 289− 293. (15) Stoliar, P.; Kshirsagar, R.; Massi, M.; Annibale, P.; Albonetti, C.; de Leeuw, D. M.; Biscarini, F. Charge Injection across Self-Assembly Monolayers in Organic Field-Effect Transistors: Odd−Even Effects. J. Am. Chem. Soc. 2007, 129, 6477−6484. (16) Thuo, M. M.; Martinez, R. V.; Lan, W.-J.; Liu, X.; Barber, J.; Atkinson, M. B. J.; Bandarage, D.; Bloch, J.-F.; Whitesides, G. M. Fabrication of Low-Cost Paper-Based Microfluidic Devices by Embossing or Cut-and-Stack Methods. Chem. Mater. 2014, 26, 4230− 4237. (17) Thuo, M. M.; Reus, W. F.; Nijhuis, C. A.; Barber, J. R.; Kim, C.; Schulz, M. D.; Whitesides, G. M. Odd−Even Effects in Charge Transport across Self-Assembled Monolayers. J. Am. Chem. Soc. 2011, 133, 2962−2975. (18) Thuo, M. M.; Reus, W. F.; Simeone, F. C.; Kim, C.; Schulz, M. D.; Yoon, H. J.; Whitesides, G. M. Replacing −CH2CH2− with −Conh− Does Not Significantly Change Rates of Charge Transport through Agts-SAM//Ga2O3/Egain Junctions. J. Am. Chem. Soc. 2012, 134, 10876−10884. (19) Baghbanzadeh, M.; Simeone, F. C.; Bowers, C. M.; Liao, K.-C.; Thuo, M.; Baghbanzadeh, M.; Miller, M. S.; Carmichael, T. B.; Whitesides, G. M. Odd−Even Effects in Charge Transport across NAlkanethiolate-Based SAMs. J. Am. Chem. Soc. 2014, 136, 16919− 16925. (20) Srivastava, P.; Chapman, W. G.; Laibinis, P. E. Odd−Even Variations in the Wettability of N-Alkanethiolate Monolayers on Gold by Water and Hexadecane: A Molecular Dynamics Simulation Study. Langmuir 2005, 21, 12171−12178.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01681. Figures, table, and additional details as described in the text (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1-515-294-8581. Author Contributions ⊥

These authors contribute equally to the manuscript.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Iowa State University through startup funds. M.T. acknowledges support through a “Black and Veatch building a world of difference” faculty fellowship. J.C. was supported in part by a Catron fellowship from the Catron solar energy center. S.O.-R. was supported by a GMAP fellowship. We thank Dr. Ian Tevis and Rafael Luna (Luna scientific storytelling) for helping with the manuscript.

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REFERENCES

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