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Empirical Evidence for Roughness-Dependent Limit in Observation of Odd–Even Effect in Wetting Properties of Polar Liquids on n-Alkanethiolate Self-Assembled Monolayers Zhengjia Wang, Jiahao Chen, Stephanie Oyola-Reynoso, and Martin M. Thuo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02159 • Publication Date (Web): 22 Jul 2016 Downloaded from http://pubs.acs.org on July 23, 2016

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Empirical Evidence for Roughness-Dependent Limit in Observation of Odd–Even Effect in Wetting Properties of Polar Liquids on n-Alkanethiolate SelfAssembled Monolayers Zhengjia Wang1, Jiahao Chen1,2, Stephanie Oyola-Reynoso 1, Martin Thuo1,2,3* 1

Department of Materials Science and Engineering, Iowa State University, 2220 Hoover Hall,

Ames, IA 50011 USA 2

Micro-electronic research center, Iowa State University, 133 Applied Sciences Complex I, 1925

Scholl Road, Ames, IA 50011 USA 3

Biopolymer and Biocomposites Research Team, Center for Bioplastics and Biocomposites,

Iowa State University, 1041 Food Sciences Building, Ames, IA 50011 USA Abstract Substrate roughness influences the wetting properties of self-assembled monolayers (SAMs), but details on this dependency at the sub-nanometer level are still lacking. This study investigates the effect of surface roughness on interfacial properties of an n-alkanethiolate SAMs, specifically wetting, and confirms the predicted limit to the observation of the odd–even effect in hydrophobicity. This article studies static contact angles of polar and non-polar probe liquids on a series of n-alkanethiolate SAMs on surfaces with tunable roughness. We prepared Ag surfaces with root-mean-square roughness (Rrms) of ∼0.6–2.2 nm and compared the wetting properties of n-alkanethiolate SAMs fabricated on these surfaces. We measured the static contact angles, θs,

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formed between SAM and probe liquids [water, glycerol, and hexadecane]. Hexadecane showed an odd–even effect on all surfaces irrespective of the degree of roughness. Polar liquids (water and glycerol), however, showed a dependency on the roughness of the substrate with an odd– even effect observable only on smooth, but not rougher (Rrms ≥ 1.15 nm), surfaces. These results confirm that the previously predicted limit to observation of the odd–even effect in hydrophobicity (here extended to polar liquids) is real. From the results with glycerol, we infer that this limit is not limited just to hydrophobicity but may extend to other polar liquids. Results from hexadecane, however, suggest that this limit may not be a universal property of the SAM.

1. Introduction The ability and nature of a liquid to establish contact with a surface depends on the resultant interfacial surface energies, which, in turn, depend heavily on the nature of surface asperities.1 In self-assembled monolayers (SAMs) on smooth surfaces, surface asperities—often expressed in terms of the root-mean-square roughness (Rrms) are way below the diameter or the capillary length of the wetting liquid.2-8 In this regard, the contact angle—an equilibrium state independent of the shape, volume, and external field—emerges as a simple yet powerful tool to delineate surface property evolution with subtle changes in SAM surface morphology. This equilibrium represents a balance between the droplet capillary energy and the change in substrate energy upon placement of the droplet and can be captured in terms of Gibbs free energy as an integral over the substrate area (Equation 1). This expression of the change in free energy, though exhaustive in its capture of interfacial phenomena, is challenging to realize experimentally, illustrating the complexity in delineating the nature of the probe liquid–substrate interface. [, ] = ∬[ + ∆ ] = ∬1 + ∇ℎ + !" − !$ %& %'(.…………(1)


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where G is the Gibbs free energy, h(xy) is the height of the droplet at a point x,y on the substrate, E is energy, and γ is the surface tension. A simpler approach to understanding wetting on textured surfaces is to assume that such an equilibrium, expressed in Gibbs free energy (above), can be captured by simplified descriptors like Young’s equation (Equation 2), the spreading parameter (Equation 3), and the Cassie-Baxter equation, among others, through the evolution of the static contact angle with changes in surface roughness. In the case of SAM on metal surfaces, the metal would dominate the surface roughness, although the so-called odd–even effect captures interfacial variations in molecular orientation: ) = + ) *+,-….………… (2) where θ is the contact angle and ) , , and ) are surface tension between three phases (solid, vapor and liquid). In a more simplified version, the equilibrium between a liquid droplet and a surface is, by nature, a balance between cohesive and adhesive forces. The spreading parameter, S, is a balance between work of adhesion (Wa) and work of cohesion (Wc) (Equation 3).9 For a non-wetting liquid (S < 0), the contact angle (θ) is thus dictated by ) and S (Equation 4)9: . = /0 − /* = ) − + ) ….………… (3) . = ) *+,- − 1 ……………………………….(4). The relations above illustrate that for non-wetting surfaces, the static contact angle is highly informative on the nature of the interfaces generated when a liquid contacts a surface and thus can be used to effectively study wetting. Although these parameters capture the global evolution in free energy at the interface, the degree of order in an n-alkanethiolate SAM imparts differences in wetting properties in part due to the so-called odd–even effect (Figure 1). The


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odd–even effect, an oscillation in the values of static contact angles related to the total number of carbon atoms in the n-alkanethiol, was recently predicted to be observable up to a certain level of substrate roughness.7 To advance our understanding of wetting, and hence expand the applicability of the above parameters, it is necessary to empirically validate this limit. This paper provides the first experimental data in support of this limitation.

Figure 1. The contact angle of water on n-alkanethiolate SAMs fabricated on ultra-flat silver surfaces, showing an odd–even oscillation. The tilting angle of the terminal group, relative to the surface normal, of alkanethiols on Ag varies with the number of carbons in the molecule chain being odd or even, which is approximately 10° and 20°, respectively.10 Background: SAMs are widely studied for tuning surface structure and properties at the molecular scales and hence have major fundamental and technological implications.11-19 A small change in molecular structures can result in variations in physical properties of materials, especially in alkanethiolate SAMs where properties can depend on the number of -CH2- units: for example, in the odd–even effect, which manifests in the chemical, physical, surface, and, interfacial properties of both bulk and nanomaterials. Affected properties include chemical reactivity, electronic property, friction behavior, wetting, and electrochemical properties.14-15, 18,


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We,10, 23-26 and others16-17, 22, 27-30 have empirically and theoretically observed the odd–even

effect in SAMs. It is generally agreed that the odd–even effect in SAMs is caused by varying tilting angles of terminal groups (CH3 in n-alkanethiols), hence its surface dipole in well-ordered SAMs.25 This terminal moiety orientation and overall structure of SAMs, however, significantly depends on the quality of the monolayers, which in turn depends on the roughness of the substrate and metal-head group (S for thiols) interface. We have recently shown that when the substrate Rrms was larger than 2 nm, we did not observe the odd–even effect in hydrophobicity (a zigzag oscillation in values of static water contact angle, θs).25 With a smoother substrate (Rrms ≤ 0.6 nm), however, where a more uniform and less defective SAM was expected to form, we did observe an odd–even effect.23 Using a variety of ultra-flat surfaces (Rrms = 0.2–0.4 nm), and extrapolating over long range, we predicted a limit to the observation of the odd–even effect at substrate Rrms ≈1 nm.23 From the same study, we predicted the maximum observable difference in static contact angles between odd- and even-numbered n-alkanethiols SAMs and water on an atomically flat substrate was 3°.23 Because the odd–even effect in SAMs wetting could be due to a combination of interfacial effect from structure of the SAMs and surface normal dipole effect,20-21 it is important to expand the library of probe liquids such that the polar and dispersive components in surface tension can be decoupled. Contributions of dispersive components (dominated by molecular vibrations at the interface, hence weak secondary interactions) can be deduced from non-polar liquids (e.g., hexadecane), while polar liquids (water and glycerol in this work) can inform the role of the polar component in the wetting properties. Combining this understanding with the ability to fabricate template-stripped substrates with tunable surface roughness and morphologies, we extend our work to rougher surfaces to i) establish whether the


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predicted limit to the observation of the odd–even effect is real, and ii) extend our studies beyond hydrophobicity to further inform the nature of the generated interfaces. 2. Methods and Experiments Materials: Alkanethiol reagents (decanethiol-to-hexadecanethiol, C10-to-C16) were purchased from Sigma-Aldrich, except for n-Tridecyl mercaptan (C13), which was purchased from Pfaltz & Bauer. 200-proof ethanol was purchased from Decon Laboratories, Inc. All chemicals and reagents were used as received. Nitrogen and argon gases were purchased from Airgas and used as supplied. Ag surface preparation: All metal films were custom-prepared by Substrata Thin Film Solutions Inc. and used as received. A 200nm Ag film was first e-beam evaporated onto a 4-inch silicon wafer, followed by a sputtering of 10nm of a different metal adlayer(either iron (Fe), zinc (Zn), copper (Cu) and titanium (Ti)). The metal films were then template-stripped to reveal a fresh Ag surface, as previously reported.23-25 Atomic force microscopy (AFM) characterization: We used a Bruker Innova AFM in tapping mode to characterize the surface features of template-stripped Ag surfaces. Images obtained ranged from 3 to 5 μm in length and width at the rate of 1 Hz. We measured all samples immediately after template-stripping. We used the data analysis software Gwyddion to process and automatically analyze the images and obtain the roughness. Preparation of monolayers:

Freshly template-stripped AgTS or AgM-TS were washed with

ethanol and dried with a stream of nitrogen gas before forming SAMs. As previously reported,2325

SAMs were prepared by placing the template-stripped metal substrate into a vial containing

3mMol alkanethiol in 5ml 200 proof ethanol. The surface and thiol solution were incubated for at least 3 hours under inert atmosphere (Argon gas). The SAM was then rinsed with copious ethanol and dried with nitrogen gas.


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Measuring the contact angle: Static contact angles formed between the SAMs and probe liquids (deionized water, hexadecane [HD], and glycerol) were measured using a Goniometer (RaméHart 100-00) with a tilting base. A droplet of probe liquid (1.5µL) was dispensed onto the SAMs through an integrated syringe pump. Images of the droplets on SAMs were generated and analyzed with a DropImage® software. A minimum of 8 measurements were made for each monolayer to obtain the average values of θs. 3. Results and discussion To study the dependence of SAM wetting on substrate roughness, we first fabricated Ag surfaces with various morphology and roughness through surface reorganization. To induce this surface reorganization, we designed and applied an interfacial stress on the Ag film by sputter coating a thin ad-layer (10nm) on the film before template-stripping. Surface roughness of the templatestripped silver substrates (with metal adlayer, AgM-TS, and without adlayer, AgTS) were characterized by atomic force microscopy (AFM). Figure 2 shows the AFM images and their corresponding 3D views of the prepared surfaces. AgTS had comparatively larger grains and was smoother than other AgM-TS surfaces. The 3D topography indicated that the grains lie deeper in the substrate than the asperities. We observe that AgTS has lowest Rrms (0.63 ± 0.08 nm), while AgZn-TS, AgTi-TS, AgCu-TS and AgFe-TS surfaces have average Rrms of 1.15 ± 0.17 nm, 1.53 ± 0.13 nm, 1.89 ± 0.18 nm, and 2.18 ± 0.41 nm, respectively (Figure 2). The wetting properties of medium chain length n-alkanethiolate SAMs (C10-C16) formed on AgTS and AgM-TS were investigated by measuring the static contact angle, θs, formed with different probe liquids. Three probe liquids (water, glycerol, and HD) were used for investigating the wetting properties of the formed SAMs. All contact angle data are summarized in supporting information (Figure S1 and Table S1-S3).


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TS

Figure 2. Characterization of surface roughness analysis of Ag and Ag including both 2D topology images and the tilted 3D-view images.


M-TS

surfaces by AFM,

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General Trends: As previously demonstrated, we observed that the values of θs gradually increased as molecular length increased, irrespective of presence or absence of the odd–even effect. The odd–even effect, a zigzag oscillation in the values of θs, was observed between all probe liquids and SAMs on the smooth substrate (Rrms = 0.6 nm). Interestingly, HD showed the odd–even effect across all substrates, but the polar liquids failed to show similar results with surfaces with RRMS > 1 nm. Table 1 summarizes observed odd–even effect across different probe liquids and substrates. From these data, we can therefore infer the following: i) With smooth AgTS surface (Rrms ~ 0.63±0.08 nm), all probe liquids give an odd–even oscillation in θs on medium chain-length SAMs (C10-C16). ii) We observed no odd–even effect on AgM-TS (Rrms ~ 1.15–2.18 nm) with the polar liquids (water and glycerol). iii) With non-polar liquid (HD), all surfaces showed an odd–even effect in θs. As Laibinis et al. argued,30 a closer proximity of HD molecules to the SAM leads to greater sensitivity to the terminal methyl group orientations and hence smaller roughness dependence. iv) Despite the slight discrepancy in the absolute values of θs from one substrate to the other, we observed a general increase in the values of θs with increasing molecular length of nalkanethiol forming the SAM (Figure S1). The general trend in hydrophobicity seemed to plateau for longer n-alkanethiolate (> C13) SAMs on the smooth AgTS but not on the other substrates. As we previously reported,23, 25 this is likely because the SAMs became more crystalline, forming a more rigid and uniform interface, resulting in no significant change in the θs (Table S1-3 and Figure S1).


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Table 1. Summary of wetting properties of n-alkanethiolate SAMs with polar and non-polar liquids. Surfaces with different degrees of roughness (Rrms) were used to prepare SAMs of different lengths with the odd-even effect in hexadecane (HD, non-polar) wetting being observed on all surfaces. Polar liquids (Water and glycerol) only showed the odd-even effect on smooth surfaces. The respective surface tension (γP = polar, γd=dispersive) components for each liquid are given for clarity. Detailed summaries of contact angles are given in the supporting information.

Odd–Even Effect of Wetting Substrates 2

Ag

2

HD

= 22 mN/m = 51 mN/m

= 24 mN/m = 41 mN/m

= 28 mN/m = 0 mN/m

Zn-TS

Rrms ~1.15

Ag

Ti-TS

Rrms ~1.53

Ag

Glycerol

2

TS

Rrms ~0.63

Ag

Water

Cu-TS

Rrms ~1.89

Ag

Fe-TS

Rrms ~2.18

Effect of substrate roughness on the odd–even effect: We previously predicted an Rrms ≈ 1nm limit in the observation of the odd–even effect in SAM hydrophobicity.23 The results in Table 1 confirmed that for the water contact angle, we could observe the odd–even effect on substrates with Rrms=0.63±0.08 nm (AgTS), but not on those with Rrms≥1.15±0.17 nm (AgZn-TS). This finding indicates that the experimental limit in odd–even effect lies between substrate Rrms 0.63– 1.15nm, close to the predicted limit of 1nm. Besides water, we observed a similar trend with glycerol but not HD, indicating that this limit exists for polar liquids but not non-polar liquids.


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Based on spectroscopic studies (Sum-frequency generation),31 we observed that there are two main changes occurring in the nature of the SAM with changing molecular length viz; i) The SAM becomes more rigid hence an increase in the vibration intensity of the terminal moiety ii) the local environment of the terminal group (and hence that of the methylene spacers) changes with increase in molecular length. We believe that hexadecane being non-polar (γP=0), hence wets the SAM, will be sensitive to the later while non-wetting (polar) liquids will not be. Effect of n-Alkanethiol Chain Length on the odd–even effect: We have previously shown that the odd–even oscillation in θs is asymmetrical because of changes in the nature of a well-ordered SAM with increasing molecular length.23,

25-26

To further understand the effect of substrate

roughness on the odd–even effect, we introduced two parameters, a and b, the change of contact angle with change in overall length by one CH2 unit. We define a as the average absolute value of the change in contact angle from even- to odd-numbered SAMs (0 =

7→9 | ∑: ;

, where

∆-?→@ = -ABC − -A with n being an even number and N the total number of ∆-?→@ considered), while b is the analogous change from odds to evens (D =

9→7 | ∑: ;

, where ∆-@→? = -ABC −

-A with n being an odd number and N the total number of ∆-@→? considered). Figure 3 shows the values of a and b parameters with respect to substrate roughness (see Supporting Information Table S4). From Figure 3a, it is obvious that a values decreased by ~1° with increase in surface roughness but asymptotes with Rrms > 1nm. Similarly, Figure 3b indicates an analogous behavior for the value of b for water and glycerol but not for HD (no significant differences). We observe that the values of a are higher than those of b, which we attribute to the differences in the nature of the SAM with increasing molecular length.7 We observe that the difference is larger on the smoother surfaces, where we also observe the odd–even effect, than on the rougher surfaces.


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Figure 3. Understanding variations in asymmetry in the odd-even effect. The a and b values, derived from contact angle of three liquids, are compared with regard to substrate roughness. a) Correlation between a and the substrate roughness. b) Correlation between b and the substrate roughness. Dispersive vs. Polar Components of Surface Tension: The spreading of a liquid on a surface depends on the dispersion and polar components of surface tension (see Equation 3). To further delineate the role of these two components in SAM wetting, we quantify a liquid property by calculating the ratio of and 2 , viz, / 2 . For example, the surface tension of water at 293K is 72.8 mNm-1 with a dispersion contribution ~ 22 mNm-1 and a polar contribution ~ 51 mNm-1, hence a ratio of 2.3.32 All dispersion force and polar force for water, glycerol, and HD are listed in Table 1,33-34 giving / 2 = 2.3, 1.7, and 0 for water, glycerol, and HD, respectively. Figure 4 compares the a and b parameters and the overall change in contact angle, ∆θs (= θs max - θs min), with the change in the / 2 for the three liquids. We observe that a, b, and ∆θs decreased with increasing / 2 analogous to the relations observed when the a and b parameters were compared to roughness (Figure 4a). For comparison purposes, we also made a similar plot for AuTS based on literature data (Figure 4b). We observed a more pronounced decrease with an increase in / 2 for AuTS. We attribute this pronounced relationship to the larger odd–even


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oscillation in the contact angle in AuTS SAMs, which also captures the difference in tilt angles for molecules on these two surfaces and hence the orientation in the terminal CH3 group. Because the dependency of a and b on dispersive and polar components is linear (Figure 4), we can infer that the large drop in roughness dependency—and the eventual asymptote (Figure 3)— are due to the effect of surface roughness on the contact angle and hence confirm a transition in the wetting behavior at/or around Rrms ≈ 1 nm. For non-wetting liquids, this transition leads to the loss of an ability to observe the odd–even effect for medium sized SAMs. We also observed a slight but insignificant inflection on the wetting HD, but we express caution in over interpreting this inflection as it is within margins of error; we further observed a reliable linear fit for the HD data in Figures 3.

Figure 4. Comparing the a and b parameters and ∆θs of water contact angles for AgTS and AuTS. a) The correlation between the three parameters with probe liquid properties ( / 2 ) on AgTS. b) The correlation between the three parameters with probe liquid properties ( / 2 ) on AuTS (data derived from literature). Effect of Grain sizes and surface morphology: To delineate the role of the substrate surface morphology on wetting properties of SAMs, we compared the grain size of all substrate surfaces. ImageJ was used to analyze and estimate the average grain area, as shown in Figures 5a and 5b, where the black regions were considered flat grain area (details are available in Supporting


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Information). Among all substrates, AgTS has the largest grain area, indicating fewer defects (primarily grain boundaries) on the surface. In comparison, AgTi-TS with smaller grain and hence more grain boundaries has wetting properties comparable to AgZn-TS and AgCu-TS. AgZn-TS, whose Rrms is close to the predicted 1 nm limit in hydrophobicity (Rrms~1.15nm), has considerably larger average grain size (Figure 5a) but still does not show the odd–even effect in hydrophobicity. In agreement with our previous report, the large grains on the surface do not significantly enhance the overall SAMs structure to allow for observation of the odd–even effect. This finding implies that the overall surface roughness, and not necessarily the surface morphologies, plays a greater role in observation of the odd–even effect. As in Figure 5c, the overall change in contact angle for all three liquids depends on the surface roughness. However, we exercise caution in overgeneralizing the effect of large grains, as a more robust systematic study is needed to make this general statement.

Figure 5. Surface texture of substrate and wetting properties of SAMs on the substrates. a) Images of grains on AgTS and AgM-TS surfaces. b) Estimated average grain area of AgTS and AgM-


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TS

surfaces. c) Comparing the substrate roughness with the overall contact angle changes of water, glycerol, and HD. Dashed lines are fitted lines showing the effect of surface roughness on different contact liquids. Conclusion This article advances our understanding of the odd–even effect in wetting properties of SAMs formed on silver surfaces (0.6 nm < Rrms < 2.5 nm) using various probe liquids (both wetting and non-wetting). The results bringing to the forefront an understanding of some fundamental properties of n-alkanethiolate SAMs that can be extended beyond wetting. From this study, we reached infer that: i) Static contact angle is an information-rich measurement for understanding subtle changes at liquid-substrate interfaces: We have used static contact angles to demonstrate that subtle changes in both the substrate and the SAM can lead to significant consequences in the wetting properties and hence capture changes at the liquid-SAM interface. Deployed in a comparative physical-organic study, in which case the study is self-referencing, we observed general trends in the wetting properties resulting from changes in surface roughness or the properties of probe liquid. Variations in the absolute values of the contact angles with changes in molecule size allowed us to capture changes in the order of the SAM, which has been reported to increase with molecular length increase.14, 19, 22 ii) The odd–even effect in hydrophobicity cannot be observed on Ag surface with Rrms ≥ 1.15 ± 0.17 nm: From experimental results, the odd–even effect in hydrophobicity is observed only when Rrms ≤ 0.63 ± 0.08 nm, but not for Rrms ≥ 1.15 ± 0.17 nm (range 1.0–1.3 nm). This empirical evidence supports the idea that there is a roughness beyond which the odd–even effect in hydrophobicity, and by extension in wetting for polar liquids, cannot be observed. From the current study, this transition point is at least around or below Rrms = 1 nm.


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iii) Hexadecane shows no roughness dependency in the odd-even effect: unlike with polar liquids, HD shows an odd-even effect across all interfaces and also wets all the surfaces. We believe that this is due to the changing local environment of whatever moiety (potentially CH2 for disordered, rough surface SAMs) is exposed to the interface. This inference is partially informed by spectroscopic data (submitted manuscript) that indicate that the structure of the SAM evolves with increase in molecular length and manifests differently when SAM is on smooth vs rough surfaces. Hexadecane is sensitive to changes in local environments of the surface exposed moiety, in part due to establishment of a better contact (wetting) and its non-polar nature. Polar liquids, however, are predominantly influenced by the orientation of the terminal moiety and structure of the SAM. iv) There is no significant dependency of ∆θs of non-wetting liquid on the surface roughness of the substrates but a heavy dependency of overall ∆θs of wetting liquid on the roughness of the surface. By comparing data from surfaces with different probe liquids, we observed that the non-wetting property depends more on interface chemistry while the wetting property depends more on the SAM structure (induced by the substrate structure). As a result, on surfaces of various roughness, ∆θs for non-wetting liquids are comparable because of the long-chain molecules while ∆θs for non-wetting liquids change significantly because SAM structure varies with substrate surface roughness. v) Wetting and non-wetting probe liquids show dissimilar behaviors in the transition of the odd–even effect: We observed a transition in the ability to observe the odd–even effect for probe liquids with significant contribution of a polar component in their surface tension. This indicates that although the odd–even effect can be observed in both wetting and nonwetting liquids, the driving forces are different, and thus the two similar phenomena should


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be differentiated. From the definition of the spreading parameter, we believe that the odd– even effect in non-wetting surfaces demonstrates domination by cohesive forces resulting from the large interface surface tension mismatch. In wetting liquids, however, the spreading parameter is dominated by adhesive forces because a more favorable interface has been created.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Materials Science and Engineering, Iowa State University, 2220 Hoover Hall, Ames, IA 50011 USA. Email: [email protected]. Phone: +1-515-294-8581.

Acknowledgments This work was supported by Iowa State University through startup funds. MT acknowledges support through a Black and Veatch building a world of difference faculty fellowship, and MT and JC were supported in part by a Catron fellowship from Catron Solar Energy Center. SOR was partially supported by a GMAP fellowship from Iowa State University. References 1. Bormashenko, E. Y. Wetting on real surfaces. De Gruyter: Berlin, 2013. 2. Yuan, L.; Thompson, D.; Cao, L.; Nerngchangnong, N.; Nijhuis, C. A. One Carbon Matters: The Origin and Reversal of Odd-Even Effects in Molecular Diodes with SelfAssembled Monolayers of Ferrocenyl-Alkanethiolates. J. Phys. Chem. C 2015, 119 (31), 1791017919. 3. Nurbawono, A.; Liu, S.; Nijhuis, C. A.; Zhang, C. Odd-Even Effects in Charge Transport through Self-Assembled Monolayer of Alkanethiolates. J. Phys.Chem. C 2015, 119 (10), 56575662.


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4. Jiang, L.; Wang, T.; Nijhuis, C. A. Fabrication of ultra-flat silver surfaces with submicro-meter scale grains. Thin Solid Films 2015, 593, 26-39. 5. Jiang, L.; Sangeeth, C. S.; Nijhuis, C. A. The Origin of the Odd-Even Effect in the Tunneling Rates across EGaIn Junctions with Self-Assembled Monolayers (SAMs) of nAlkanethiolates. J. Am. Chem. Soc. 2015, 137 (33), 10659-67. 6. Yuan, L.; Jiang, L.; Thompson, D.; Nijhuis, C. A. On the Remarkable Role of Surface Topography of the Bottom Electrodes in Blocking Leakage Currents in Molecular Diodes. J. Am. Chem. Soc. 2014, 136 (18), 6554-6557. 7. 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 nAlkanethiolate Monolayers. Langmuir 2015, 31 (25), 7047-7054. 8. 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 n-Alkanethiolate Self-Assembled Monolayers Depends upon the Roughness of the Substrate and the Orientation of the Terminal Moiety. Langmuir 2014, 30 (40), 11985-11992. 9. Kalin, M.; Polajnar, M. The correlation between the surface energy, the contact angle and the spreading parameter, and their relevance for the wetting behaviour of DLC with lubricating oils. Tribol. Int. 2013, 66, 225-233. 10. Wang, Z.; Chen, J.; Oyola-Reynoso, S.; Thuo, M. The Porter-Whitesides Discrepancy: Revisiting Odd-Even Effects in Wetting Properties of n-Alkanethiolate SAMs. Coatings 2015, 5 (4), 1034-1055. 11. Dubi, Y. Transport through Self-Assembled Monolayer Molecular Junctions: Role of InPlane Dephasing. J. Phys. Chem. C 2014, 118 (36), 21119-21127. 12. Flynn, N. T.; Tran, T. N. T.; Cima, M. J.; Langer, R. Long-term stability of selfassembled monolayers in biological media. Langmuir 2003, 19 (26), 10909-10915. 13. Kudelski, A. Structures of monolayers formed from different HS—(CH2) 2—X thiols on gold, silver and copper: comparitive studies by surface‐enhanced Raman scattering. J. Raman Spectrosc. 2003, 34 (11), 853-862. 14. Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Selfassembled monolayers of thiolates on metals as a form of nanotechnology. Chem. Rev. 2005, 105 (4), 1103-1170. 15. Love, J. C.; Wolfe, D. B.; Haasch, R.; Chabinyc, M. L.; Paul, K. E.; Whitesides, G. M.; Nuzzo, R. G. Formation and structure of self-assembled monolayers of alkanethiolates on palladium. J. Am. Chem. Soc.2003, 125 (9), 2597-2609. 16. Ramin, L.; Jabbarzadeh, A. Effect of load on structural and frictional properties of alkanethiol self-assembled monolayers on gold: Some odd–even effects. Langmuir 2012, 28 (9), 4102-4112. 17. Ramin, L.; Jabbarzadeh, A. Effect of compression on self-assembled monolayers: a molecular dynamics study. Model. Simul. Mater. Sci. Eng. 2012, 20 (8), 085010. 18. Whitesides, G. M.; Grzybowski, B. Self-assembly at all scales. Science 2002, 295 (5564), 2418-2421. 19. Ulman, A. Formation and structure of self-assembled monolayers. Chem. Rev. 1996, 96 (4), 1533-1554. 20. Colorado, R.; Graupe, M.; Takenaga, M.; Koini, T.; Lee, T. R. In Surface dipoles influence the wettability of terminally fluorinated organic films, MRS Proceedings, Cambridge Univ Press: 1998; p 237.


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21. Graupe, M.; Takenaga, M.; Koini, T.; Colorado, R.; Lee, T. R. Oriented surface dipoles strongly influence interfacial wettabilities. J. Am. Chem. Soc. 1999, 121 (13), 3222-3223. 22. Tao, F.; Bernasek, S. L. Understanding odd-even effects in organic self-assembled monolayers. Chem. Rev. 2007, 107 (5), 1408-1453. 23. 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 nalkanethiolate monolayers. Langmuir 2015, 31 (25), 7047-7054. 24. 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 (9), 2962-2975. 25. Newcomb, L. B.; Tevis, I. D.; Atkinson, M. B.; Gathiaka, S. M.; Luna, R. E.; Thuo, M. Odd–even effect in the hydrophobicity of n-alkanethiolate self-assembled monolayers depends upon the roughness of the substrate and the orientation of the terminal moiety. Langmuir 2014, 30 (40), 11985-11992. 26. Chen, J.; Tevis, I.; Gathiaka, S.; Thuo, M. Stereo-Electronic Effects in Tunneling Junctions: Revisiting the Platform. Procedia Eng. 2016, 141, 138-143. 27. Nurbawono, A.; Liu, S.; Nijhuis, C. A.; Zhang, C. Odd–Even Effects in Charge Transport through Self-Assembled Monolayer of Alkanethiolates. J. Phys. Chem. C 2015, 119 (10), 56575662. 28. Jiang, L.; Sangeeth, C. S.; Nijhuis, C. A. The Origin of the Odd–Even Effect in the Tunneling Rates across EGaIn Junctions with Self-Assembled Monolayers (SAMs) of nAlkanethiolates. J. Am. Chem. Soc. 2015, 137 (33), 10659-10667. 29. Ramin, L.; Jabbarzadeh, A. Effect of water on structural and frictional properties of self assembled monolayers. Langmuir 2013, 29 (44), 13367-13378. 30. 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 (26), 12171-12178. 31. Jiahao Chen, J. L., Ian D. Tevis, Richard S. Andino, Christina M. Miller, Lawrence D. Ziegler, Xin Chen, Martin Thuo. Spectroscopic Evidence for the Origin of Odd-Even Effects in Self-Assembled Monolayers and Effect of Substrate Roughness. Submitted to Langmuir 2016. 32. Jańczuk, B.; Białopiotrowicz, T.; Wójcik, W. The surface tension components of aqueous alcohol solutions. Colloids Surf. 1989, 36 (3), 391-403. 33. Hołysz, L.; Chibowski, E. Surface tension components of some organic liquids, tenside. Surf. Deter 1988, 25, 337-339. 34. Jańczuk, B.; Białopiotrowicz, T.; Wójcik, W. The components of surface tension of liquids and their usefulness in determinations of surface free energy of solids. J. Colloid Interface Sci. 1989, 127 (1), 59-66.


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TOC Graphic


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The contact angle of water on n-alkanethiolate SAMs fabricated on ultra-flat silver surfaces, showing an odd–even oscillation. The tilting angle of the terminal group, relative to the surface normal, of alkanethiols on Ag varies with the number of carbons in the molecule chain being odd or even, which is approximately 10° and 20°, respectively.10 Figure 1 185x145mm (120 x 120 DPI)


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Characterization of surface roughness analysis of AgTS and AgM-TS surfaces by AFM, including both 2D topology images and the tilted 3D-view images. Figure 2 159x286mm (120 x 120 DPI)


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Understanding variations in asymmetry in the odd-even effect.The a and b values, derived from contact angle of three liquids, are compared with regard to substrate roughness. a) Correlation between a and the substrate roughness. b) Correlation between b and the substrate roughness. Figure 3 391x158mm (120 x 120 DPI)


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Comparing the a and b parameters and ∆θs of water contact angles for AgTS and AuTS. a) The correlation between the three parameters with probe liquid properties (γ^p/γ^d) on AgTS. b) The correlation between the three parameters with probe liquid properties (γ^p/γ^d) on AuTS (data derived from literature). Figure 4 470x190mm (120 x 120 DPI)


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Surface texture of substrate and wetting properties of SAMs on the substrates. a) Images of grains on AgTS and AgM-TS surfaces. b) Estimated average grain area of AgTS and AgM-TS surfaces. c) Comparing the substrate roughness with the overall contact angle changes of water, glycerol, and HD. Dashed lines are fitted lines showing the effect of surface roughness on different contact liquids. Figure 5 612x347mm (120 x 120 DPI)


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Table of content graphic TOC 255x170mm (96 x 96 DPI)


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Empirical Evidence for Roughness-Dependent ... - ACS Publications

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