Effect of Load on Structural and Frictional Properties of Alkanethiol

Feb 15, 2012 - Effect of Load on Structural and Frictional Properties of Alkanethiol Self-Assembled Monolayers on Gold: Some Odd–Even Effects. Leyla...
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Effect of Load on Structural and Frictional Properties of Alkanethiol Self-Assembled Monolayers on Gold: Some Odd−Even Effects Leyla Ramin and Ahmad Jabbarzadeh* School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW 2006, Australia ABSTRACT: We have conducted molecular dynamics simulations to study the frictional properties of alkanethiols CH3(CH2)n−1SH (Cn, 12 ≤ n ≤ 15) self-assembled monolayers (SAMs) on Au(111) surfaces, under various loading and shearing conditions. For the examined alkanethiols, we found some evidence of the friction coefficient being dependent on the number of carbon atoms in the molecule being odd or even. Alkanethiols with n = odd show consistently higher friction coefficients than those with n = even. Such odd−even effect seems to be independent of the sliding velocity. However, the effect is significant only at lower loads ( 2000 MPa the tilt angle remains almost constant. Using flat surfaces, earlier MD simulations of C16 by Tupper et al.7 have reported only tilting as a result of compression. Although the pressure could not be estimated in that work because of the lack of

Figure 4. Time-averaged tilt, θ, and tilt orientation, φ, angles plotted versus normal pressure. The results are shown for dodecanethiol SAM−Au contacts. The red dashed line is a guide to the eye showing three regimes of tilt angle variation.

information, we speculate this is at the lower pressure range. In experimental works, using AFM, STM, and sum-frequency generation, Salmeron48 has reported that at pressures higher than 100 MPa the collective tilt starts to increase. From the results in Figure 4, we can see that increasing the normal pressure changes not only the tilt angle but also the collective tilt direction φ. For 10 < PN < 500 MPa the orientation angle (φ) remains almost unchanged; this corresponds to the regime I. For 500 < PN < 2000 MPa, however, there is a shift from −70° to 120°. A drop in the tilt angle marks the onset of this structural change. Such changes in the tilt direction perhaps indicate solid−solid transitions in the packing structure of the monolayer. At P = 2000 MPa there is a sharp change in φ. In regime III (PN > 2000 MPa), φ remains stable at ∼−53°. We will 4105

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distribution shows a narrow distribution with a single peak. This indicates that dodecanethiol chains at PN < 500 MPa are tilted mostly in the same direction. At higher normal pressures, PN ≥ 500 MPa, the distribution becomes broader with two distinct peaks, corresponding to domains A and B shown in the snapshot. In group A, the orientation angle is distinctly different from that of group B. Furthermore, in group B chains are twisted and bended. This is a sign of a configurational transition due to the applied normal pressure. With increasing the pressure, the distribution becomes broader reflecting a more disordered system. 3.1.2. Film Thickness, Radius of Gyration, and Ellipsoid Aspect Ratio. Other structural and physical properties, including the film thickness, radius of gyration, and ellipsoid aspect ratio of the monolayer, were plotted against the normal pressure and are shown in Figure 6a,b. Here we can see an increasing trend in the energy as a function of applied normal pressure. This is due to the increase in potential energy because of the deformation of molecules through bond rotation and bending modes. In Figure 6a, the square radius of gyration (Rg2) against the normal pressure is plotted. Rg2 is indicative of the molecular shape and distribution of mass around its center. The results here show Rg2 remains almost constant up to a normal pressure of ∼500 MPa (regime I); however, at higher normal pressures, it drops from 0.27 to 0.25 nm2 and remains almost constant up to 2000 MPa (regime II). The aspect ratio of the ellipsoid semiaxes (a/b) follows a very similar trend to that of radius of gyration. These results suggest, as schematically shown in Figure 6c, with increasing the normal pressure molecules become more oblate. The results for ellipsoid axes ratio and Rg2 confirm that for PN < 500 MPa the chains are only slightly deformed under the load. However, as PN increases, a sharp decrease in a/b and Rg2 marks the onset of internal structural deformation. The results for the film thickness (wall to wall distance) in Figure 6b show a continuous decrease with increasing the normal pressure. This continuous drop in film thickness with the applied load is somehow in contrast to the results reported in AFM experiments by Salmeron.48 In that work when the normal pressure was increased, the film thickness changed at incremental stages, depending on the normal load. This, however, was suggested to be due to penetration of the sharp tip of AFM into the monolayer. In our work, however, the surfaces are flat, and this explains the difference in the dynamics of film thickness variation under pressure. We register a 30% decrease in the film thickness over the range of the normal pressures we examined. From the detailed structural information we obtained, we can conclude that at lower pressures, the decrease in the film thickness is accommodated by an increase in the tilt angle. This corresponds to regime I. For regime II, 500 < PN < 2000 MPa, the reduction in the film thickness is the result of the combined effects of the increased tilt and deformation of individual molecules in the monolayer and perhaps solid−solid structural transitions. This is reflected by the first reductions in Rg2 and a/b. In our simulation, a critical film thickness of 1.4 nm, corresponding to the normal pressure of 500 MPa can be seen, where the SAM structure starts to show structural deformation. As the normal pressure increases to 500 MPa, we observed a transition in the structure of the chains in terms of orientation and tilt angle, and a sharp transition in energy, radius of gyration, and aspect ratio (a/b) of the inertially equivalent ellipsoid. This is an indication of the P = 500 MPa marks the onset of internal structural deformation of individual molecules in the monolayer. However, more dramatic structural deformations take place at much higher pressures of PN > 2000 MPa.

see in the next section despite this relative stability in tilt angle and orientations, in regime III, other structural changes continue to occur in individual molecules. Houston and Kim28 used IFM method for characterization of hexadecanethiol (C16) systems on Au(111). Their findings suggest that the friction coefficient considerably increases when the normal pressure reaches ∼3.7 GPa and higher. They proposed that at this normal pressure chains suddenly undergo deformation as the tip is laterally displaced along the surface. The significant deformations seen for C12 at PN > 2 GPa is consistent with this analogy, and the lower threshold seen perhaps is due to the shorter length of molecule. Experimental studies have shown differences in packing ordering of alkanethiol SAMs under compression due to the internal structural changes. To explore such structural changes, a snapshot of dodecanethiol under normal pressure of 500 MPa is shown in Figure 5a. Clearly, there is evidence of two domains

Figure 5. (a) Snapshot of dodecanethiol under normal pressure of 500 MPa; (b) probability distribution of tilt orientation angle for different normal pressures. Domains A and B refer to the two groups of chains with same orientation angle. These are represented by two peaks in the probability distribution for P = 500 MPa.

where in each domain collective tilt direction is the same. We have labeled these domains by (A) and (B). Probability distribution plots of tilt orientation angle, at various normal pressures, are shown in Figure 5b. As it can be seen here, for normal pressures lower than 500 MPa, the probability 4106

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4.1. Stick−Slip Behavior. From the data obtained in the sliding stage of our simulations, clear stick−slip behavior was observed for all self-assembled monolayers. This stick−slip is characterized by periodic oscillations seen in the shear stress, film thickness, and total energy. As the system slides, the stress increases during the stick stage. Then, energy stored in the bonds during the stick level is transferred to the rest of contacting bodies and dissipates during the slip level. The results for C14 at two shear rates of 108 and 109 s−1 at a normal pressure of 700 MPa are shown in Figure 7. The periodic fluctuations in the energy are the result of dynamic instability in molecular motion.6 During the stick stage, energy is slowly stored leading to a maximum level represented by a peak as shown in Figure 7. Meanwhile, the shear stress follows a

Figure 7. Shear stress (σxz), film thickness (h), and total energy (e) as a function of time for C14 on gold under shear rates of (a) 108 s−1 and (b) 109 s−1. All systems are under a normal pressure of 700 MPa.

Figure 6. Physical properties of dodecanethiol self-assembled monolayers on gold under different normal pressures in the range of 10−5000 MPa: (a) energy (kJ/mol) and square radius of gyration (nm)2; (b) film thickness (nm) and aspect ratio of ellipsoid axes (a/b); (c) schematic representation of the effect of normal pressure on aspect ratio of the inertially equivalent ellipsoid for a single chain.

similar trend and increases to a maximum level as the energy reaches a peak. This is followed by a sudden slip accompanied by release of energy and a decrease in the shear stress. Film thickness follows an opposite periodic trend, albeit with the same frequency. Here minima in the film thickness are in registry with peaks in energy and shear stress. We can see from Figure 7b that similar patterns with higher frequency appear for the same system under a higher shear rate of 109 s−1. There are

4. SHEAR SIMULATIONS The configuration obtained at the end of a loading simulation at a given normal pressure was used as the starting point for the shear simulations of compressed monolayers. We applied shear, at various rates in the range of 108−1010 s−1. 4107

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Three important features can be seen here for both alkanethiol systems. First, the onset of both stick and slip stages is in synch with periodic behavior seen for θ and φ. Second, the stick stage where the shear stress grows is associated with an increase in the tilt angle. Third, tilt orientation angle φ, which is measured with respect to the shear direction, shows a similar periodic behavior; however, its correlation with shear stress depends on the shearing direction. In each stick−slip cycle, the peak in the shear stress coincides with the peak in the tilt angle and the tilt orientation angle that is the closest to the shearing direction. The synch lines in Figure 8 show that for C14 the shear stress and tilt angle are in phase with tilt orientation angle, φ, and out of phase for C15. This is simply a consequence of a positive tilt orientation angle, φ, for C15 and negative angle for C14. In both cases the shear stress is peaked when the tilt orientation shifts (decrease for C15 and increase for C14) closest to the shearing direction. Our results show the structural process during the stick−slip process not only involve tilting−untilting of the chains by ∼1°−2° but also a rotational mechanism where the tilt direction varies by as much as ∼5°−10°. This rotational periodic relaxation mode depends on the initial tilt orientation with respect to the shearing direction. During the stick stage, as the tilt direction rotates toward the sliding direction the shear stress increases, and in the slip stage the tilt direction returns back to its original position, while the shear stress drops. Despite the similarities in the structural variation during the stick−slip cycle, a distinct difference emerges in the peak values of the shear stress. Here we can see that the even system (C14) show distinctly lower peak values for the shear stress in the order of ∼25 MPa, compared to the peak value of ∼70 MPa seen for the odd system (C15). We can also see that variations in tilt and orientation angles are noticeably smaller for C14, indicating a smoother sliding for the even system. However, we find this distinct difference appear only at lower end of the pressures we examined here. In the following sections we will examine these odd−even effects more closely and will study the structural origin of this distinct difference. 4.3. Effect of Loading and Shear Rate on Friction Coefficient. Understanding dependence of friction force on the applied normal pressure at nanoscale is fundamental for the design of miniaturized devices with optimal mechanical performance. It is known for sometime through AFM and STM experiments that for nanoscale contacts the friction coefficient may be dependent on the applied load.49 Nakano et al.50 used pin-on-plate, FFM, and X-ray photoelectron spectroscopy (XPS) to study the tribological properties of SAMs on Au(111). They found a direct correlation between the frictional behavior and sliding speeds of the pin-on-plate tribometer and FFM, where faster sliding motion resulted in higher friction coefficient. To study this, we have performed various simulations at different normal pressures. For each pressure, the SAM−Au systems were subjected to shear at various shear rates. We have the calculated friction coefficients (μ) at various applied loads and shear rates for C12, C13, C14, and C15 SAM−Au contacts. In Figure 9, we have plotted friction coefficient versus the applied normal pressure for C12 and C15 for pressures in the range of 0.1−1.1 GPa and for three shear rates. At a pressure of 300 MPa, we can see almost for all systems, the friction coefficient increases with the shear rate. However, this increase with the shear rate is not as pronounced for systems under normal pressure of 700 MPa and higher. This stronger dependence of the friction on the shear rate at lower pressures is also reported by Chandross et al.

attractive and repulsive forces due to the interactions between the gold and the SAMs atoms. The end group of each molecule (CH3 group) is directly affected by these interaction forces. As the SAM slides, the Au and CH3 atoms stick to each other due to van der Waals forces. As the sliding continues, the shear stress rises to a peak. Once shear stress grows large enough to overcome these attractive forces and physical contacts, the slip takes place, the mechanical energy is converted into the dissipative energy, and the molecular displacements continue until the shear stress relaxes. As the sliding velocity of the gold surface increases, the frequency of force variation increases leading to a higher frequency in this periodic pattern. Similar stick−slip behavior is reported in other simulations done by Zhang et al.12 During stick−slip observed in these studies, structural variations were also seen in the tilt angle. We will investigate this in the next section to closely relate the stick− slip mechanism to the collective structure of the monolayer. 4.2. Structural Origin of Stick−Slip and Odd−Even Effects. In Figure 8, we have also shown the tilt and tilt

Figure 8. Shear stress, tilt (θ), and tilt orientation (φ) angles as a function of time during sliding at a shear rate of 109 s−1 for alkanethiols SAM−Au contacts systems an odd (C15) and even (C14) under the normal pressure of 300 MPa. The vertical dashed line is the synch line used to show the correlation between the peak in shear stress and position of tilt and orientation angles.

orientation angles variation during sliding and stick−slip process for two alkanethiol systems at a shear rate of 109 s−1 and PN = 300 MPa. Here we observe similar periodic patterns in behavior of tilt and tilt orientation angles for all systems. 4108

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that the friction coefficient despite small differences is mostly independent of the applied load. For C12, the calculated value of μ, at the shear rate of 109 s−1, are 0.027, and 0.026 respectively for PN = 100 and 500 MPa. Our calculated friction coefficient for C12 is in good agreement with Chandross et al. simulation result of ∼0.03552 over a comparable range of normal pressures (200−600 MPa) and slightly higher shear rate (∼1.3 × 109 s−1), which explains slightly higher friction coefficient in that work. In comparison, simulations by Park et al.,53 for a shorter molecule of C9, have reported a friction coefficient of 0.03 ± 0.01 in the range of 200−800 MPa and at a shear rate of ∼2 × 109 s−1. Although the conditions in these works are not exactly the same as those used by us here, there is a general agreement in the orders of magnitude of our calculated friction coefficient for C12. For C14 from Figure 10, we can see for PN > 100 MPa the friction coefficient is mostly independent of the applied load. The effect of the chain length on the frictional properties of alkanethiols has been studied by several groups.54−56 These experiment suggest friction coefficient is independent of the chain length for n > 8−10. Our results qualitatively agree with this, only at higher loads. We do not have an estimate of the normal pressure in these experiments; however, we suspect these have been conducted at high pressure region (>1 GPa) where we also find length independence for friction coefficient (Figure 10). There are differences between our simulation and AFM studies, due to varying contact area and nonuniformity of the normal pressure in experiments compared to an infinite contact area and uniform pressure in the simulations. However, we believe the effect of the load should be considered carefully in design of experiments. In contrast, the C13 and C15 show a decreasing trend in friction coefficient with increasing the normal pressure up to P = 700 MPa. It seems P = 700 MPa also act as the critical pressure where three remarkable effects can be seen. First, the friction coefficient becomes independent of the length of the molecule for P ≥ 700 MPa. Second, at pressures lower than 700 MPa the friction coefficients for the odd alkanethiols are much higher than those of even ones. Third, for P < 700 MPa the odd alkanethiols show higher sensitivity to the applied load. The reduction of friction coefficient with increasing the pressure can be attributed to the adhesion forces. Recent large scale MD simulations49 of dry single asperity contacts have shown that for non adhesive contacts the friction force dependence on the load is linear. However, the presence of the adhesion due to van der Waals interactions makes this relationship sublinear. At lower pressures, the adhesion force dominates the friction force and the high friction coefficient is associated with the high adhesion. With increasing the pressure, the friction is dominated by the pressure due to elastic forces, and as a result the friction coefficient decreases. To obtain estimate of the adhesion forces for all alkanethiol systems, in Figure 11 we have plotted the normal stress as a function of distance for various SAMs as the upper Au surface approaches the monolayer during the loading stage at a normal pressure of 300 MPa. The maximum positive peak in the normal stress (σzz) corresponds to a tensile stress due to adhesion forces exerted from the upper bare Au surface, as it is about to touch the monolayer. This is similar to the “pull in” force experienced in the AFM experiments on the SAMs.57 The calculated adhesion tensile stress from these IFM experiments using OHterminated glass tip against alkanethiol-coated gold substrate is 295 ± 95 MPa.51,57 Our calculated adhesion stress respectively for C12, C13, C14, and C15 are 450, 337, 360, and 288 MPa.

Figure 9. Friction coefficient versus the applied normal pressure for three shear rates for C12 (open symbols and dashed lines) and C15 (filled symbols and solid lines). The arrow shows the critical pressure where the friction coefficient becomes almost independent of the shear rate for both alkanethiol systems. The inset shows the half-log plot for C15 over a wider range of normal pressures.

in simulations of alkylsilane SAM−SAM contacts on silicon dioxide surfaces.51 In that work the results were only compared for two normal pressures of 0.2 and 2 GPa. So the critical pressure of shear rate independence is not clear. As can be seen for both systems the friction coefficient is more dependent on the shear rate for P < 700 MPa. This pressure seems to be the critical pressure where the effect of sliding velocity becomes smaller. We emphasize that this effect does not vanish but becomes less important. A half-log plot of the friction coefficient versus normal pressure for C15 in the inset of Figure 9 shows that even at higher loads up to 4 GPa the friction coefficient still is weakly dependent on the shear rate. We have plotted the friction coefficient as a function of normal pressure, at a shear rate of 109 s−1 for C12, C13, C14, and C15 in Figure 10. Our results for C12 in Figure 10 show

Figure 10. Friction coefficient versus the applied normal pressure for four alkanethiol systems C12, C13, C14, and C15. The arrow shows the critical pressure where the friction coefficient becomes almost independent from the length of the molecules. 4109

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Figure 11. Normal stress and tilt angle as a function of distance between the two gold surfaces during the loading of various alkanethiol monolayers C12, C13, C14, and C15 at P = 300 MPa. The positive normal stresses are a measure of attractive force of the gold surface on the monolayer and indicative of the adhesion.

Figure 12. Friction coefficient as a function of number of carbon atoms, at a normal pressure of 300 MPa, shown for three shear rates of 108, 109, and 1010 s−1. The odd−even effect can be seen regardless of shear rate.

friction coefficients are obtained for the chains with odd number of carbons. Higher friction coefficient for C13 SAM compared to the C14 SAMs are also reported by Mikulski et al.18 using all-atom MD simulations of n-alkane chains attached on a diamond (111) substrate with the packing density of 21.9 Å2 per chain, which is close to the packing density of n-alkanethiols on gold (111). Our results in Figure 12 show that this odd−even effect is independent of the sliding velocity for the range of shear rates examined here. To examine the effect of the load on this odd−even phenomenon, in Figure 13 we have plotted the friction coefficient as a

These values are in the range reported experimentally. We observe a trend where even SAMs show higher adhesion forces. Figure 11 also shows the tilt angle variation during approach and loading. Before compression, the chains had a tilt angle of 26.35 ± 0.32°, 30.09 ± 0.24°, 26.98 ± 0.25°, and 30.06 ± 0.16° respectively for C12, C13, C14, and C15.17 As the upper gold substrate approaches the monolayer, due to attractive forces between the gold and monolayer, chains gradually untilt to stand almost upright. Such untilting process in envisaged by Burns et al.57 in their experimental work, and our simulations confirm this process. As the distance decreases, the stress becomes compressive and the monolayer tilts to values higher than those before loading, of 34.6°, 32.3°, 34.4°, and 32° respectively for C12 to C15 systems. Since the adhesive tensile stress was larger for lower tilts (even systems), this implies that the adhesive forces depend on the tilt angle and that they decrease with increasing the tilt angle. For the odd systems after the loading the tilt angles are lower, so as a result the adhesion forces under loading for the odd systems should be higher. Also, one may speculate that at higher loads the adhesive forces both comparatively and quantitatively decrease because of increased tilt angle. This may provide an explanation as to why the friction coefficient decreases by increasing the load. This could also explain why C12 and C14 show lower friction coefficients at the same normal pressure and shear rate compared to those for C13 and C15. The role of adhesion seems a plausible explanation in the odd−even effects we have observed here. This also fits with the structural analysis we have done in 4.4.2 and higher wettability reported for odd systems in experiments and simulations.19,58 4.4.1. Odd−Even Effect on Friction Coefficient. Influence of Load and Shear Rate. The calculated friction coefficients versus the number of carbon atoms, n, at PN = 300 MPa, have been plotted in Figure 12 for three shear rates of 108, 109, and 1010 s−1. The fluctuations in the friction coefficient values obviously demonstrate that there is an odd−even effect. IFM and AFM measurements have shown similar zigzag pattern, as a signature of odd−even effect, for the friction force as a function of number of carbon atoms,16,22 where higher

Figure 13. Friction coefficient as a function of number of carbon atoms, shown for four different loads, for alkanethiols SAM−Au contacts. The shear rate for all cases is 109 s−1. Clearly the odd−even effect vanishes at P ≥ 700 MPa.

function of the number of carbon atoms for applied loads in the range of 100−1100 MPa. Here we can clearly see that the effect persists for pressures of 100 and 300 MPa; however, it becomes 4110

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To gain a deeper insight into the structural properties of individual chains when they are subjected to shear and pressure, the fraction of gauche defects17 in the dihedral angles along the molecular backbone for all odd and even systems under 300 MPa have been plotted in Figure 14. It can be clearly seen that for all cases the gauche defects of the last dihedral angle (tip of the molecules) are considerably larger than other dihedrals angles along the molecular backbone. Furthermore, for odd systems, the gauche defects of the last dihedral are 4−5 times larger than those for the even ones. The snapshots presented in Figure 14 show segments of the monolayer and the gold surface for C15 (odd) and C12 (even) systems under shear. These provide further evidence that the odd molecules show bending and deformation at their terminal end where they make contact with the opposing bare Au surface. Here, for the odd system two carbon atoms are in contact with the opposing wall whereas for the even system only one carbon atom touches the opposing gold surface. The increased area of contact between carbon atoms and the contacting gold surface results in larger friction coefficient. This seems to stem from the particular structure of odd alkanethiols where the terminal bond is almost perpendicular to the opposing gold surface and consequently more prone to be deflected at much lower pressures compared to those for even system.

5. CONCLUSIONS Molecular dynamic simulations have been used to study the effect of normal pressure and shear rate on the structural and frictional characteristics of alkanethiols self-assembled monolayers. Initially, the effect of normal pressure on the structure was studied for dodecanethiol SHCH2 (CH2)10CH3 monolayer, in stationary conditions, and for various normal pressures in the range of 10 MPa to 4 GPa. We found three regimes of structural changes under the load, for C12. Regime I, at 10 < PN < 500 MPa, where the monolayers responded to compression mainly through increased tilt angle. In regime II, at 500 ≤ PN < 2000 MPa, the pressure was accommodated by a combination of deformation of individual molecules and further increase in tilting. At PN > 2000 MPa, the SAM monolayer underwent through a progressive deformation of individual molecules. We showed the stick−slip dynamic involved a combination of tilting−untilting and periodic rotation of collective tilt direction toward shearing direction. At lower pressures where adhesion forces were dominant, for all SAMs (C12 to C15) the friction coefficient increased with the sliding velocity. We found however at pressures P ≥ 700 MPa the friction coefficient was only weakly dependent on the sliding velocity. We observed interesting odd−even effects for C12−C15 SAM−-Au alkanethiol contacts. These can be summarized as follows: (1) The odd alkanethiols show higher friction coefficient. (2) The odd−even effect observed is independent of the sliding velocities. (3) The odd−even effect (higher friction coefficient) becomes weaker as the applied load is increased, and the friction coefficient decreases with the applied load for the odd alkanethiols up to a normal pressure of ∼700 MPa. We believe at pressures below 700 MPa the forces of adhesion are dominant, and as a result a larger friction coefficient is observed for all system. The correlation we have seen between the adhesion and the tilt angle implies the odd alkanethiols under pressure have larger adhesion forces compared to even ones. Odd systems also deform much easier through increased gauche defects at their tips. Combination of

Figure 14. (a) Percentage of gauche defects in conformation of torsional angles along the molecule. The last carbon atom involved in each dihedral angle is represented by Nbond, the position of the carbon atoms along the backbone. So the first dihedral angle involves the sulfur group at one end and the third carbon group at the other end. The results are shown for four alkanethiols C12, C13, C14, and C15. All systems are under a normal pressure of PN = 300 MPa. Odd−even effects can be seen on the terminal bonds. (b) Snapshots of sections of odd (C12) and even (C15) monolayers clearly showing the gauche defects at the tip of odd molecules.

weaker with increasing the pressure. The effect almost vanishes at 700 MPa and higher pressures. 4.4.2. Structural Origins of Odd−Even Effect. Odd−even effects on surface wettability were also reported where higher wettability was observed for odd alkanethiols. According to the AFM studies, the structural differences between the odd and even chains mainly are due to the orientation of the terminal C−C bond in the molecule due to the steric constraints and different van der Waals interaction between the chains and AFM tip. This in turn is thought to affect the frictional and wetting properties.16 Larger exposure of methyl end group of chains with odd number of carbons results in larger van der Waals interactions between the monolayer and the AFM tip. This, in return, gives rise to a higher friction coefficient. 4111

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these factors results in larger friction coefficients for odd systems at lower pressures. At higher pressures differences arisen form structural factors become less important. Given the evidence presented in this work, we believe for alkanethiols, odd−even effects, length, and sliding velocity dependence of the friction coefficient are important at pressures below ∼700 MPa. Whether such odd−even effects persist for longer alkanethiols remains to be answered by further studies. We are investigating the effect for different contact types and will report our findings in future. Such conclusions may have implications on interpretation of the results from experiments.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of this study by an Australian Research Council Discovery Project grant and an Australian Postgraduate Award (APA) scholarship for the first author. We also thank the Australian Centre of Advanced Computing and Communications and also Australian National Computational Infrastructure Facility for the generous time allocated for computing.

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dx.doi.org/10.1021/la204701z | Langmuir 2012, 28, 4102−4112