Effect of Water on Structural and Frictional Properties of Self

Article pubs.acs.org/Langmuir

Effect of Water on Structural and Frictional Properties of Self Assembled Monolayers Leyla Ramin and Ahmad Jabbarzadeh* School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia

ABSTRACT: Self-assembled monolayers (SAMs) of n-alkanethiols [(CH3(CH2)n−1, n = 14, 15] on Au(111) in the presence of water have been simulated by molecular dynamics simulation. The behavior and effects of compression on structural characteristics and water penetration into monolayers under different ranges of normal pressures have been investigated. Frictional properties of hydrated SAM systems under various sliding velocities, and loading conditions are examined to explore correlation between the amount of water penetration and friction. Simulations for one odd and one even SAM (C14 and C15) systems have revealed interesting odd−even effects in water penetration and frictional properties. We have also compared the frictional and structural properties of hydrated systems to that of dry SAM−Au (one surface of gold is covered by SAM) and SAM−SAM (both gold substrates are covered by SAM) contacts. The results reveal that the even hydrated SAM (C14) shows lower friction coefficient compared with the odd hydrated SAM (C15). We found the presence of water reduces the friction only at lower pressures; and at higher pressures, dry SAM−Au contacts offer lower friction. It was interesting to see that the lubricity effect of water was much stronger for the odd system and persisted to slightly higher pressures (300 MPa for the even SAM and 700 MPa for the odd SAM). At higher pressures, for both odd and even systems, the presence of water increased the friction. We also found that at low sliding velocities and higher pressures apparent water viscosity was enhanced by up to 3 orders of magnitude, indicating possible solidification.

1. INTRODUCTION In the past two decades, self-assembled monolayers (SAMs)1−8 and their interfacial properties with metals have been very active research topics. These films form covalent bonds with a variety of metal substrates such as Au, Ag, Cu, Pt, Pd, Hg, and Si. Gold has often been used as a substrate due to its strong interaction energy with sulfur, inertness, and ease of patterning.7 Some of the SAM potential applications are in lubrication,9−14 surface engineering, adhesion modification, micro/nanofabrication, thin-film nonlinear optics, corrosion prevention, and microlithography. With the emergence of applications and devices relying on nanotechnology, lubricant films with monomolecular layer thickness are desired. Miniaturization in the form of nano/microelectromechanical (NEMS/MEMS) devices requires working with extremely small parts whose adhesion/mass ratio is significantly larger than those in macroscale applications. Among the various types of SAM coatings, alkanethiols [CH3(CH2)n−1SH], are the simplest thiol-based SAMs and are good candidates for MEMS lubrication because they can form chemical bonds and highly ordered monolayers on different © 2013 American Chemical Society

types of surfaces. Since MEMS/NEMS may be operated in different environmental conditions, it is important to investigate the effects of humidity on friction of SAM systems.15 Hydrated SAM systems have been studied in the past by experimental and simulation methods. Using Monte Carlo simulation, Henda and co-workers,16 studied the effect of structural modification of oligo(ethylene glycol) (OEG)terminated alkanethiols SAMs on water penetration. Their results show structural dependency of these types of SAMs and resistance against water penetration. Examination of the water structure through the number of hydrogen bonds per water molecule and orientational order parameters show that the water structure becomes bulklike by about 5 Å above the SAM surface. Using molecular dynamics (MD), Lane et al.17 observed the orientation of water molecules and film thickness of hydrated carboxyl-terminated alkanethiol SAM−SAM on gold change Received: August 30, 2013 Revised: September 30, 2013 Published: October 1, 2013 13367

dx.doi.org/10.1021/la403321a | Langmuir 2013, 29, 13367−13378

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Figure 1. (a) Snapshot of the model Au(111) substrate, (b) different groups of a pentadecanethiol (C15) chain, (c) hexagonal arrangement of sulfur atoms on the Au surface, which produces packing density of 21.6 Å2/chain, (d) model of water molecule, and (e) a hydrated SAM−Au contact setup for a C15 monolayer attached to a gold substrate.

measured friction corresponds to the increase in hydrogen bonding and suggested that the cooperative effect of hydrogen bonding by water molecules between the two S(CH2)15COOH SAMs is responsible for the high viscosity calculated from the experimental data. They have however reported no diffusion or viscosity data. Lorenz and co-workers26,27 have simulated water confined between two hydrophilic alkylsilane SAMs on silica and have calculated the friction coefficient and water viscosity,26 showing although the water viscosity was enhanced the structure remained bulk-like. This paper aims to gain insight into the behavior of hydrated SAM−Au contacts under various loading and shearing conditions. There is also major interest in the studies of water on hydrophobic surfaces and its structure formed on these surfaces. There are a number of important questions concerning SAM−water interfaces. For instance, how does the presence of water in SAM−Au systems affect the structural deformation under different pressures? What is the difference between the frictional properties of dry SAM−Au and SAM−SAM contacts with hydrated SAM−Au system? How these properties can be affected by the SAM being odd or even numbered alkanethiol? Since the original work of Hautman and Klein,28 molecular simulation methods have been widely used as an alternative tool to study SAMs at the molecular level. In this paper, we present our comprehensive molecular dynamics simulation results on frictional, structural, and compression properties of alkanethiols on Au(111), where hydrated C14 and C15 alkanethiols have been simulated to obtain insight on the effect of hydration and chain length (odd−even effect)8 on water penetration and friction. We will compare frictional properties of hydrated and dry SAMs at various shear rates and pressures. Here we will show the frictional properties of hydrated SAMs depend not only on the applied pressure and shear but also on the water penetration and chain being odd or even.

under compression and that affects the diffusion characteristics. Using IR spectroscopy Marsalek et al.18 have suggested that chemical structure of interface interacting with water molecules is less important than the physical presence of the interface. Although some limited works have been done on hydrated thiol based SAM systems, understanding the interaction of SAMs with water in nanoscale in a sliding contact has not been adequately addressed. This area of research is of considerable interest both fundamentally and technologically. It is known from the results obtained by different studies that there is a correlation between water presence and frictional properties of SAMs. Liu et al.19 found a significant increase in the friction force of SAMs on silicon oxide as the relative humidity increased from 10 to 50%. Khatri and Biswas20 suggested that, at this range of humidity, friction increases as the defects build up in the monolayer. In another study, however, Xiao and Qian21 reported that in hydrated silane coated systems there is no correlation between the friction and adhesion when number of water molecules is low (RH < 70%). Fujihira et al.22 also have suggested that adhesive and frictional forces of carboxylic acid SAMs are independent of relative humidity. Binggeli and Mate23 have suggested that the friction of the silane-coated interfaces decreases only when humidity reaches high values (>70%). Atomic force microscopy (AFM) experiments by Qian et al.15 indicate that friction coefficient decreases at higher humidity for N-octadecyltrimethoxysilane (OTE, CH3(CH2)17Si(OCH33) on SiO2(OTE/SiO2), and n-alkanethiols on Au(111). They have attributed this to the lubricating action of a condensed thin layer of water on the hydrophobic SAM surface. STM experiments by Li et al.24 have shown clear effect of hydration on friction coefficient of mixed SAMs of dodecanethiol and 11-mercapto-1undecanol on Au(111). However, this was only observed when the humidity was low (RH < 50%). Using Monte Carlo simulations, Major et al.25 reported that the increase in the 13368


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We used the TIP3P model40,41 and SHAKE algorithm for simulation of water molecules. This model is employed in MD simulations of water, and has been shown to provide reasonable description of water,41 including reproducing of first hydration shell and the energetic of liquid water.40 It has also been used the in presence of SAMs that is reported by Lorentz et al.42 Since we have made some comparison to that work, we have also adopted this model in our work. The TIP3P water model as implemented in CHARMM40 specifies a three-site rigid water molecule with charges and Lennard−Jones parameters assigned to each of the three atoms. Ewald method was used to compute the long-range Coulomb interactions.43 The parameters used for TIP3P with a longrange Ewald are adopted from Price and Brooks44 and are listed in Table 1.

2. MODEL AND METHODOLOGY 2.1. Modeling the Hydrated Alkanethiols and Gold Surfaces. The hydrated systems in this work are simulated by LAMMPS (Largescale Atomic/Molecular Massively Parallel Simulator).29 Here the alkanethiol molecules are simulated by a united atom model in which groups of CH2, CH3 and S are treated as single interaction sites. Intramolecular architecture including bond stretching, angle bending and torsional potentials are included in the model. The parameters for the intramolecular and intermolecular potentials, involving the alkyl part of the molecule, are due to Siepmann et al.,30 and are mentioned in our previous works.31,32 The initial configurations of the SAM systems are taken from these prior works which have studied dry SAM systems.33,34 All the interactions between the atoms of different molecules and also interactions between the atoms which belong to the same molecule, and are not interacting by any of the bond potentials, and interactions between alkyl chain atoms and gold surface, are governed by a shifted 612 Lennard−Jones potential which is truncated at a distance of rc = 1 nm. For the interaction of unlike groups Lorentz−Berthelot’s combining rules are used so εij = (εiεj)1/2 and σij = (σi + σj)/2. Both implicit and explicit models of gold surface are often used in molecular dynamics simulations. The Au(111) substrate is modeled by a closed packed fcc atomic structure with four layers of fixed gold atoms in the Z direction (surface normal). The lattice constant of gold is 0.408 nm, and the nearest neighbor distance is 0.288 nm. The Au(111) substrate, shown in Figure 1a, is used for simulation of C14 and C15, and its dimensions are 7.790 × 7.495 nm2 in the lateral (XY) directions. In our system, as shown in Figure 1b, for a C15 SAM, a single chain consists of head, tail, and end groups. The initial configuration of the molecules was started as upright, all trans, with each sulfur group attached to a threefold lattice site on the gold surface via a harmonic potential with an equilibrium distance of r0 = 0.244 nm. The monolayer molecules on the gold substrate had their sulfur head groups arranged in a hexagonal (√3 × √3)R30° structure relative to the underlying Au(111) lattice. This produces a packing density of 21.6 Å2/chain and a distance of ∼0.5 nm between the grafting positions. This structure corresponds to a fully saturated coverage and maximum packing density, and has been reported by using several different experimental methods. Following the original work of Mar and Klein,35 in our model, Au−S−CH2 was not subjected to the bending potential. Simulations36 have been shown, using an explicit atomic model for the gold surface, to mitigate the effect of using a flexible or rigid Au−S−CH2 interaction. Periodic boundary conditions were applied to the system in the X and Y directions. A vacuum space, 1.2 times the extended length of the molecule in height, was allowed on top of the monolayer during equilibration. For hydrated systems, this vacuum space was then filled by water molecules before the loading stage. The interaction parameters of Au are chosen by fitting the calculated and experimental desorption data of alkanes from metal surfaces. These values are εw/kB = 990 K and σw = 0.2655 nm, which yield energy and length parameters for the interaction of CH2 and Au of εwf = 1.795 kJ/mol = 4.59, εCH2 and σwf =0.328 nm. Parameters for the potential model used here are adopted from the literature,28,30 and have been used by us in our earlier works.34 Initial random velocities for individual atoms were assigned to give a Maxwell−Boltzmann distribution corresponding to the target temperature which was kept constant, by rescaling the velocities, at 300 K unless it was stated otherwise. Berro et al.37 have shown that the choice of thermostat is only relevant at very high shear rates (>5 × 1010 s−1), and our results are mostly below this range of shear rates. The equations of motion were integrated using the velocity Verlet algorithm with a time step of 2.35 fs (0.001 in reduced units, based on the energy, diameter, and mass of the CH2 united atoms). The number of gold atoms on each surface was 3240, and the total number of SAM united atoms on each substrate for C14 and C15, respectively, were 4050 and 4320. The constant normal pressure was applied using Nose-Hoover method described elsewhere,38,39 Under this scheme, the normal (to wall) component of the pressure is kept constant by allowing the confining gold walls to contract and expand. The properties of the monolayers were studied under these conditions.

Table 1. Parameters for Intramolecular and Intermolecular Interaction Potentials parameters

value

O mass H mass qH qO σOO εOO εOH, εHH stiffness K for bond potential (OH bond) r0 of OH bond stiffness Kθ for angular potential (HOH angle) equilibrium angle for HOH angle

15.9994 amu 1.008 amu 0.415 −0.830 0.3188 nm 0.102 kcal/mol 0.0 450 kcal/mol Å2 0.9572 nm 55 kcal/mol 104.52°

The simulations were conducted under NPNT (PN, normal pressure and T, temperature) conditions. For hydrated SAM−Au contacts, the lower layer of gold was covered by a SAM (C14 or C15). We used an equilibrated unconfined33 system of SAMs as the start-up configuration for our simulations. The volume of the water part is calculated from the distance between the top gold surface and the location of the average position of the first peak in the SAM density profile. The initial gap between the SAM and top gold surface was filled with 1656 water molecules so that the average density for the water layer between the SAM and gold surfaces before the application of pressure was 1 g/cm3. The surface area of the simulated substrate was A = 58.38 nm2, so that the number of water molecules per unit of surface area was 28.36 (water molecules/nm2). This is comparable with the Lorenz et al.26 simulation work of alkylsilane SAM on silica with 27 water molecules/nm2. Each simulation was carried out for 2 ns to yield equilibrium. The equilibrated configuration was then used as the starting point and sampling was performed for another 2 ns. 2.2. Calculations of Shear Stress and Friction Coefficient. The sliding shear runs for various SAMs were started from an equilibrated configuration obtained from simulation of the same system under a given normal pressure. The shear was applied by translating the lower and upper Au surfaces at equal velocities in opposite directions parallel to the X axis. The shear stress was calculated by summing the lateral forces applied by the SAM molecules on either gold surface atoms and using eq 1. NW NS

σxz =

∑∑ i

j

FX , ij A

(1)

where NW and NS are the number of top (or bottom) Au and SAM atoms, respectively, Fx,ij is the force between atoms i and j, in the x direction, and A is the surface area of the gold substrate. The friction coefficient then was obtained from the slope of the linear fit of the shear stress against the normal pressure. This assumes the normal and friction forces are proportional according to Amonton’s laws. 13369


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Figure 2. Normalized density profiles of water (ρ/ρw) as a function of location in the Z direction for hydrated (a) C14 and (b) C15, at pressures of 300 and 1100 MPa. The numbers show different peaks in the density profiles which are described in text. The double arrow line shows the region where water molecules have penetrated into SAMs.

3. RESULTS

To study the extent of water penetration into our simulated SAM systems, we have calculated the normalized density profile of water molecules and SAMs and show them in Figure 2a and b for hydrated C14 and C15 SAMs, at normal pressures of 300 and 1100 MPa. We can see that the depth of water penetration is small and concentrated at the tips of molecules. At lower pressures, the overlap between the water and SAMs densities is small, reflecting the fact that the water molecules do not penetrate strongly into the SAMs region. At higher pressures, the water−SAM overlap region increases in size, implying that more water molecules have penetrated into the SAM. In Figure 2a, we have labeled four distinct peaks. Peak 1 represents the region occupied by S atoms (neighboring the Au layer), and peak 2 represents a high density region occupied by the first CH2 group attached to the S group. Peak 3 is a signature of the terminal CH3 groups; this density peak overlaps the first density peak of the water molecules labeled by 4. Peak 4 indicates relatively low concentration of water molecules next to the end group of SAM chains. We define the penetrated water molecules as those in the region where water and SAM coexist. The fraction of penetrated water molecules was calculated from the overlap of density profiles of water and SAM regions. Since the water penetrated volume is defined as the volume where both SAM and water molecules coexist (shown in Figure 2a), this is presented as the penetrated volume. So from the number density of water for this volume, one can extract the total number of penetrated

3.1. Compression Simulations. In the loading stage, the gold substrates were compressed along the surface normal direction by applying a constant normal pressure in the range of 300 MPa to 2 GPa. The system was then allowed to equilibrate for 2 ns after loading and effect of compression on the structural properties and in penetration of water molecules into the monolayer. 3.1.1. Effect of Compression on Water Penetration into SAMs. Examining the snapshots of the compressed hydrated C14 and C15 systems showed that some water molecules have penetrated between the SAM chains due to the applied pressure. Water penetration into SAM has been reported before for other confined hydrated SAM systems.16,44 The terminal CH3 group makes the SAM surface strongly hydrophobic. Despite this, some water molecules make their way into the SAM’s ordered structure. Lane et al.17 have studied the behavior of water in confinement and its interaction with carboxyl-terminated alkanethiol SAM system on gold and silica. They have reported water penetration in SAMs under confinement. Ismail et al.45 have reported that the water penetration is a result of hydrophilic characteristic of OEG (ethylene oxide) SAMs. This penetration however was very small, and mainly limited to the terminal region of the SAM. 13370


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water is not crystallized. The difference in gOO for C14 and C15 seems to be insignificant, especially at higher pressures. Now we turn our attention to comparing the water penetrations for C14 and C15. To quantify this and also the effect of compression on water penetration in SAM−water systems, we have calculated the percentage of total water molecules penetrated into SAM. Figure 4 shows the result as a

molecules and hence the fraction of those penetrated. The majority of the penetrated water molecules coexist at the interfacial region with SAM terminal group. Some water molecules whose location is in the Z direction are below that shown by peak 3 and have deeply penetrated into the SAM chains. As it can be seen in Figure 2, there are two density peaks of water molecules near the bare Au layer (peaks 5 and 6) which have the highest concentration of the water molecules. The sharp density peaks which increase in size at the higher pressure (1100 MPa) indicate strong layering of water molecules. The layering is stronger next to the bare Au substrate and becomes weaker next to the terminal methyl groups. Lane et al.17 have reported a monolayer−bilayer transition due to surface-induced effects on confined water in their simulation work. They have also noted highly ordered water configuration for confined hydrated SAMs. They however noted that unconstrained lateral diffusion indicated absence of water solidification. To explore the structure of water oxygen−oxygen, the radial distribution function gOO(r) of water was calculated for C14 and C15 for pressures in the range of 300−2000 MPa. The results are shown in Figure 3. We can see from this figure the structure of

Figure 4. Percentage of water molecule penetration and the thickness of the SAM−water film as a function of normal pressure shown for C14 and C15 SAMs under stationary conditions (no shear).

function of applied normal pressure for C14 and C15 SAMs. Here we can see that, for both systems, by increasing the pressure, water penetration also increases. Comparing C14 and C15 at the same pressure, we clearly see larger water penetration into the C15 (an odd SAM). The results show that water penetration increases by up to 30% in going from even to odd SAM. Weaker water penetration into C14 is accompanied by stronger layering effect seen by sharper peaks in the density profile of water region. To understand structural origins of larger water penetration of C15, the fraction of gauche defects in the dihedral angels along the backbone of SAM molecule for C14 and C15 systems under 300 MPa has been calculated and plotted in Figure 5a. The calculation method is described in our earlier works,.34 It can be clearly seen that the gauche defects of the last dihedral for C15 is 4 to 5 times larger than that for C14. The results provide evidence of larger conformational disorder for C15 under compression compared with C14, and an explanation for the difference in water penetration. Such odd−even differences in the conformation of dihedral angle of dry SAMs have been observed in our previous works,.31,33 The geometrical origins of increased water penetration for C15 is depicted in schematic pictures of hydrated C14 and C15 under pressure in Figure 5b. Here, we can see that the terminal end groups in odd systems stand upright whereas in even systems they are tilted. These tilted terminal groups in even system produce a blocking effect on water penetration and result in lower water penetration compared with that seen for the odd system. We will explore how this increase in water penetration might affect frictional properties of hydrated odd and even systems. The average film thickness against the normal pressure is also plotted in Figure 4 for hydrated C14 and C15 SAM−Au contacts. The thickness of the SAM−water film is measured from the

Figure 3. Oxygen−oxygen radial distribution functions (gOO(r)) of water layer calculated for various pressures 300−2000 MPa, shown shifted for clarity. The results are shown for C14 (black lines) and C15 (dashed blue lines, for P = 500, 1100, 2000 MPa). The inset shows the calculated gOO(r) from empirical potential structure refinement (ESPR) method based on neutron diffraction data of water (ice) at 220 K and 0.1 bar.46

water remains liquid up to pressures of 1100 MPa. The position of the first peak in the intensity does not change much with increasing the pressure, but it becomes more distinct. As the pressure increases, a second peak starts to emerge at a shell distance of ∼5 Ǻ . At the highest pressure of 2000 MPa, we can see strong evidence of icelike structure for the water layer. To make comparison, calculated gOO(r) from empirical potential structure refinement (ESPR) method based on neutron diffraction data of ice water at 220 K and 0.1 bar extracted from ref 46 are shown in the inset of Figure 3. The results at 2000 MPa show most of the intermolecular structure in the region r = 2−6 Ǻ is similar to that calculated from ESPR for ice. The nonzero intensity of gOO(r) at sections between the distinct peaks however suggests that the 13371


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normal pressures. For each pressure, the hydrated SAM−Au systems were subjected to shear at various rates (108, 109, and 1010 s−1). All shear rates are nominal average shear rate calculated by γ̇ = 2UW/H, where UW is substrate velocity and H is the thickness of SAM−water system (1.7−2.4 nm). We have calculated friction coefficients (μ) for some cases. In Figure 6a, we have plotted shear stress versus the applied normal pressure for hydrated C14 for three nominal shear rates.

Figure 5. (a) Fraction of gauche defects of hydrated SAMs of C14 and C15 (P = 300 MPa); (b) exaggerated schematic pictures of hydrated C14 (left) and C15 (right) SAM−Au contacts, under load. Water molecules penetrate less into the even alkanethiol due to the tilted terminal groups. The black arrows in the snapshots represent the direction of the methyl end group.

location of gold atoms at the interface. Zero film thickness is defined as the distance between the gold surfaces in the absence of any SAM and water molecule. Here we can see a continuous decrease in the film thickness with increasing the applied normal pressure. The calculated film thickness for hydrated C14 and C15 at PN = 300 MPa, were respectively, 2.32 and 2.35 nm, and then were reduced by ∼23% at PN = 2 GPa for both systems. Comparing dry33 and hydrated C14 systems, we can see similar trends in the film thickness reduction. For dry SAM−Au contacts, most of the change in the film thickness is accommodated by tilting or by deformation of individual chains during compression.33 The average film thickness for C15 and C14 is close. This can be explained by larger penetration of water and structural deformation for C15, which makes up for the slightly thicker (∼0.1 nm) film expected for C15. 3.2. Shear Simulations of Hydrated C14 and C15 SAM− Au Contacts. 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 nominal shear rates in the range of 108− 1010 s−1. 3.2.1. Friction: Effect of Shear Rate and Water Penetration. To study the frictional properties of hydrated C14 and C15 SAMs, we have performed various simulations at different

Figure 6. (a) Shear stress and (b) percentage of water penetration of hydrated C14 as a function of normal pressure at rest (no shear) and under various shear rates. (Error bars were smaller than the symbol, so they are not shown in the plot).

At pressures below 700 MPa, we can see almost for all systems that shear stress increases with the shear rate. However, for systems under normal pressure of 900 MPa and higher, this increase with the shear rate is either not as pronounced or completely reversed. Stronger dependence of the shear stress on the shear rate at lower pressures (PN < 700 MPa) has also been observed in our simulations of dry SAM−Au contacts,.33 This pressure, PN =700 MPa, appears to be the critical pressure for both SAM−Au and hydrated SAM−Au contacts where the effect of sliding velocity becomes smaller or reversed. The reversal of the effect in hydrated SAM−Au contacts where the friction becomes smaller at higher sliding velocities perhaps stem from the level of water penetration. We will investigate this correlation shortly. For the hydrated C14 in Figure 6a, we can see that shear stress is almost linearly dependent on the applied load. Shear stress 13372


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increases when PN < 700 MPa with the applied load, then slightly decreases for 700 < PN < 900 MPa, and subsequently increases up to 2 GPa. To investigate dependence of shear stress on water penetration, the percentage of penetrated water molecules, calculated from the overlap of density profiles of water and SAM, are plotted as a function of pressure in Figure 6b. The results for water penetration in the system without shear rate are also plotted in Figure 6b for comparison. Here we can see that for systems under shear in most cases the water penetration is lower; this is more obvious at higher pressures. Here, four regimes can be detected where the water penetration has a direct correlation with the friction. In regime I (PN < 700 MPa), with increasing the pressure, the water penetration also increases. In regime II (700 < PN < 900 MPa), with decreasing the amount of water molecules within SAMs, shear stress also decreases. In regime III (900 < PN < 1100 MPa), the percentage of water penetration and shear stress both increase. This is consistent with simulation results of Major et al.,25 who have shown increased friction and viscosity for water confined between S(CH2)15COOH monolayers. Simulations of alkylsilane SAM−SAM contacts on silica substrate have also shown a decreased friction by increasing the number of water molecules.26 In regime IV, although the water penetration decreases, the shear stress increases and hence the shear stress loses its dependency on water penetration. The reason perhaps arises from the structural defects of SAMs occurring under very high pressures. From these results, we may conclude that for C14 hydrated SAMs, PN = 1100 MPa is a critical pressure below which; shear stress shows a direct relationship with water penetration. However, at higher pressures, there is no direct relation between shear stress and water penetration. Figure 7a shows the shear stress as a function of normal pressure for hydrated C15 SAM−Au contacts. Interestingly, only at PN = 300 MPa do higher shear rates result in larger shear stress. For P ≥ 700 MPa, for shear rates > 109 s−1, there is no dependence on the sliding velocity. Considering the effect of normal pressure on shear stress and water penetration, three regimes can be detected in Figure 7a. At regime I (PN < 700 MPa) and regime II (700 < PN < 900 MPa), we can see that the shear stress increases with the normal load. In regime III (PN > 900 MPa), the shear stress decreases dramatically with the load. In Figure 7b, the percentage of water penetration is plotted against pressure for the three different shear rates. Water penetration as a function of pressure for C15 systems under no applied shear is also shown in Figure 7b for comparison. As it can be seen, with no sliding, water penetration continuously increases with pressure. However, when the shear is applied, we find some correlation between the shear stress and water penetration. Comparing regime II and III in Figure 7, we can see that for PN >700 MPa, shear stress and water penetration follow a similar trend and show direct correlation, whereas for regime I (PN < 700 MPa) this cannot be seen. At regime I, the water penetration and shear stress show opposite trends. Comparing the no-shear case with systems under shear again, we find that shearing reduces water penetration, and that reduction is more pronounced at higher pressures. 3.2.1. Odd−Even Effect in Friction Coefficient μ. To investigate the effect of shear rate and odd−even effects on the frictional properties of hydrated-SAM systems, the friction coefficients of hydrated C14 and C15 were calculated. Our results show that up to a pressure of 1100 MPa for all three shear rates of 108−1010 s−1, there is almost a linear relationship between the shear stress and the applied pressure, suggesting the friction obeys the Amonton’s law. Extrapolation of the lines to

Figure 7. Same as in Figure 6 shown for hydrated C15 SAM.

zero-load shows nonzero shear stress. This is expected due to adhesion forces which are much more appreciable at these length scales, a phenomenon that has also been observed in our simulations of SAM−SAM contacts.31 We have calculated the friction coefficient from the slope of linear fit of σxz versus PN in Figures 6a and 7a for PN ≤ 1100 MPa and for shear rates of 108, 109, and 1010 s−1. The calculated friction coefficients are listed in the Table 2. We have also listed the friction coefficients for dry SAM−SAM contact obtained under similar conditions.31 Table 2. Calculated Friction Coefficients for Hydrated and Dry C14 and C15 Systems for PN ≤ 1100 MPa contact type hydrated SAM shear rate, s‑1 μC15 μC14

108 0.098 0.071

109 0.064 0.053

dry SAM−SAM 1010 0.056 0.023

108 0.096 0.045

109 0.124 0.062

1010 0.111 0.064

Our calculated friction coefficients for hydrated systems are comparable with those reported in the simulation work of Lorenz et al.26 (μ = 0.06 at v = 20 m/s). For hydrated systems, here we also detect an odd−even effect similar to that we have previously reported for dry SAMs,.33,34,31 Here we can see the even SAM (C14) shows lower friction coefficient compared to that of the odd SAM (C15) for all the shear rates we examined. However, in comparison with dry SAM−SAM systems, the odd−even effect for hydrated systems is less pronounced. Lorenz et al.26 have 13373


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friction to enhanced viscosity of water between the monolayer and the surface. The bulk viscosity of water at 300 K is ∼0.8 mPa S. Our calculated viscosity in the range of pressures 300−1100 MPa and the lowest shear rate (108 s−1) is 30−638 mPa S for C14 and 40−800 mPa S for C15. Here we can see ∼1−3 orders of magnitude enhancement in the shear viscosity of water. At higher shear rates, however, there is significant shear thinning. At a shear rate of 1010 s−1, the shear viscosity drops to ∼1.74−4.2 mPaS for C14 and ∼2−7 mPaS for C15. Here the viscosity shows enhancement only by a factor of 2−7. Lorenz et al.26 at similar water coverage values for alkylsilane (Si(OH)3(CH2)10COOH) monolayers and at PN = 268 MPa and γ̇ = 2.57 × 1011 s−1 have reported η = 0.22 mPa S. Extrapolating our results for C14 shown in Figure 8, for PN =300 MPa, to this shear rate, γ̇ = 2.57 × 1011 s−1, we obtain a very close value of η = 0.237 mPa S for water shear viscosity. The water layer in ref 26 is confined between two SAM layers, and the density of water remains mostly homogeneous. This is in contrast to our work where the water density profiles show strong layering effect. In our work, water is confined in one side by a smooth solid wall, and this has made density of water strongly inhomogeneous. The viscosity of confined water between amorphous silica substrates have been shown to increase only slightly at pressures up to 1100 MPa.27 Also for confined alkanes the rheology of the film changes from solidlike to liquidlike when the confining substrates are changed from crystalline to amorphous,.47 In some cases (with model mica) at higher shear rates however the ordered structure, which causes the increased viscosity, is destroyed. However, with gold surface, the shear thinning is observed without the film ordered structure being destroyed.41 The density profiles of water shown in Figure 2 provide strong evidence of layering and indication of possible solidlike behavior. Our inspection of density profiles showed that, at higher shear rates, a similar layering remains intact. The calculated radial distribution function shows however the ice like structure form at pressures larger than 1100 MPa (Figure 3). Comparing the apparent shear viscosity for hydrated C14 and C15 confined layers (see Figure 8); we find for P = 300 MPa the apparent viscosity is larger by a factor of ∼2−3 for the water layer confined by the odd system. This persists for pressures up to 1100 MPa which has been shown here. For P ≤ 1100 MPa, one may attribute the larger apparent viscosity for C15 to slightly larger penetration of water molecules inside the SAM. We note, as the pressure is increased to 2000 MPa, the effect is reversed and the calculated viscosity for C14 shows higher values. We have not shown this case here as there is a strong evidence of solidification into ice-like structures at this higher pressure. This reversal in effect can be attributed to ice-like structures at P = 2000 MPa (Figure 3). 3.2.4. Comparing Friction for Hydrated and Dry SAM Contacts. Here we will investigate the effects of introduction of water molecules on friction by comparing the results for dry and hydrated SAM−Au and also SAM−SAM systems at the same range of normal pressures and shear rates. In Figure 9, for C14 SAMs, the shear stress (σxz) against shear rate is plotted for hydrated and dry SAM−Au, and also SAM−SAM contacts. Here the results are shown for three normal pressures, 300, 700, and 1100 MPa. The results for C15 under the same conditions and for the same contact assemblies are shown in Figure 10. The trends in dependence of σxz on the type of contacts are summarized in Table 3. If we compare dry and hydrated SAM− Au contacts, we can see that, at a normal pressure of 300 MPa, while for C14 the lubricating effect of hydration is small, for C15

shown the friction coefficient for hydrated SAM−SAM contacts increases with the sliding velocity. We have also observed a similar trend for dry SAM contacts. Here however for hydrated SAM−Au contacts we can see a decrease in the friction coefficient by increasing the sliding velocity. This may be due to shear melting of the ordered water layers, and subsequent thinning of the film viscosity. Our inspection of the density profiles at the highest shear rate showed water retained strongly layered structure. We will investigate the shear thinning of the viscosity of the film in the next section. The mechanism of odd− even effect for hydrated system may be similar to that reported before for the dry SAM−SAM contacts. Here however, this odd−even phenomenon may also be attributed to the amount of water penetration into SAMs. As shown in Figure 4, due to increased penetration, there are smaller amounts of water molecules for C15 compared with C14. Having a larger amount of water molecules in the space between SAMs and the upper gold surface may enhance the lubrication properties of the sliding contacts leading to reduction of friction coefficient. 3.2.3. Water Apparent Shear Viscosity. Figure 8 shows the log−log plot of apparent shear viscosity (σxz/γ̇) versus the

Figure 8. Comparison of apparent shear viscosity of C14 (filled symbols) and C15 (open symbols) hydrated SAM−Au contacts for PN = 300, 900, and 1100 MPa normal pressures. The arrow shows the experimental zero shear viscosity of bulk water at atmospheric pressure.

applied shear rate (γ̇) for hydrated SAM systems of C14 and C15. For all ranges of normal pressures, by increasing the shear rate, shear viscosity decreases. This represents a shear thinning behavior whose onset is below the minimum shear rate of 108 s−1 used in our simulations. A similar shear thinning behavior is reported in the work of Lorenz et al.26 for shear viscosity of confined and bulk water in the range of 3 × 1010−2 × 1011 s−1. In our results, shear viscosity increases with the pressure, similar to that reported by Lorenz et al.26 for confined systems; however, their work has covered pressures up to 268 MPa and higher shear rate of ∼1011 s−1. Simulations by Lorenz et al.26 have shown that viscosity of bulk water also increases by up to 70%, as the pressure is increased from 0.1 to 480 MPa. Qian et al.15 have reported lower friction coefficient at higher humidity due to the lubrication characteristic of thin layer of water on the hydrophobic SAM surface. However, at low humidity, they have seen higher friction and attributed this increase in the 13374


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Figure 9. Shear stress (σxz) against applied shear rate for hydrated and dry SAM−Au, and dry SAM−SAM contacts; the results are shown for C14 at normal pressures of (a) 300 MPa, (b) 700 MPa, and (c) 1100 MPa.

adding some water to the monolayer results in significant reduction of friction. For C15, this lubricating effect is even more significant at the higher shear rates. Hydrated systems have the lowest shear stress at PN = 300 MPa. Introduction of water to SAM−Au systems enhances the lubrication capacity by reducing the shear stress as the water penetration is not large. This is more obvious at higher shear rates, perhaps due to more fluid nature of water at this higher shear rates. This is consistent with results of Lorenz et al.26 simulation work at low pressures. For C14, increasing the pressure to 700 MPa causes a dramatic reversal in lubricating capacity of hydrated SAMs. Here we can see hydrated SAM−Au contacts show consistently larger shear stress than that of dry SAM−Au contacts at all shear rates. For this even SAM, the friction of the dry contact is much lower than that of the hydrated contact. We can attribute higher water penetration at this pressure to this major reversal in frictional characteristic of C14. At a pressure of 1100 MPa, interestingly, increasing the shear rate leads to a reduction in shear stress for hydrated SAMs, perhaps due to decrease in water penetration (see Figure 6b). For C15, the loss of water lubricity happens at a higher pressure of 1100 MPa, where at the lower pressure of 700 MPa, except at the lowest shear rate, the hydrated SAM−Au

systems show lower shear stress than that of dry SAM−Au system. It is interesting to also make comparison of the hydration versus covering both surfaces with SAM (SAM−SAM contact). For C15, the results suggest that at all ranges of pressures and shear rates examined here SAM−SAM contacts show larger friction than hydrated SAM−Au system. For C14 however the results are mixed, and while at 300 and 1100 MPa, in most cases, SAM−SAM systems show higher friction than hydrated systems, at intermediate pressure of 700 MPa the friction is larger for hydrated system. Here also we can see some evidence of possible the odd−even effect in frictional properties of hydrated systems. C14, the even hydrated SAM, produces lower friction perhaps due to lower water penetration. However, the effect of water on friction depends on the applied pressure. At lower pressure of 300 MPa, friction for hydrated systems is lower than that of dry systems due to improved lubrication by water molecules. However, for even alkanethiols with increasing the pressure to PN > 700 MPa, hydrated systems show larger friction compared with dry systems which have very low friction coefficient.32 Lorenz et al.26 also found lower friction for hydrated systems compared with dry 13375


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Figure 10. Shear stress (σxz) against applied shear rate for hydrated and dry SAM−Au, and dry SAM−SAM contacts; the results are shown for C15 at normal pressures of (a) 300 MPa, (b) 700 MPa, and (c) 1100 MPa.

Table 3. Comparison of the Intensity of Shear Stress of Hydrated and Dry SAM−Au (HSA), SAM-AM(SS), and SAM−Au(SA) Systems for C14 and C15 Systems at Different Pressures and Shear Rates γ̇ 108 s−1 higher

middle

109 s−1

1010 s−1

lower

higher

middle

lower

higher

middle

lower

C14 PN (MPa)

300 700 1100

SS HSA HSA

SA SS SS

HAS SA SA

SS HSA SS

SA SS HSA

HSA SA SA

SS HSA SS

SA SS HSA

HSA SA SA

C15 PN (MPa)

300 700 1100

SS SS SS

SA HSA HSA

HAS SA SA

SS SS SS

SA SA HSA

HSA HSA SA

SS SS SS

SA SA HSA

HSA HSA SA

SAM and gold surface shows strongly layered structure, in contrast to bulklike structure reported for water confined by two SAMs.26 We find some dependency between the friction and the amount of water penetration. However, this dependency of friction on water penetration vanishes at pressures larger than 1100 MPa. This may be attributed to solidlike behavior observed for water layer at higher pressures. We have compared the friction for dry and hydrated SAMs and have found lubrication dependency on the applied load. At lower

systems at low pressures and their results show an increasing trend of friction with pressure.

4. CONCLUSIONS We have studied the effect of hydration on frictional and compression properties of SAMs. We have investigated the effect of loading, sliding velocity, and SAM being odd or even on water penetration into the SAMs and friction. Structural properties were also examined in the presence of water. Water confined by a 13376


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(5) Zhang, L.; Goddard, W. A., III. Molecular simulation study of the c(4 × 2) superlattice structure of alkanethiol self-assembled monolayers on Au(111). J. Chem. Phys. 2002, 117, 7342−7350. (6) Bhushan, B., Ed. Nanotribology and Nanomechanics; Springer: Berlin, Heidelberg, 2005. (7) Dubois, L. H.; Nuzzo, R. G. Synthesis, structure, and properties of model organic surfaces. Annu. Rev. Phys. Chem. 1992, 43, 437−463. (8) Tao, F.; Bernasek, S. L. Understanding odd−even effects in organic self-assembled monolayers. Chem. Rev. 2007, 107, 1408−1453. (9) Choi, J.; Kato, T. Self-assembled monolayers as lubricants for magnetic disk drives. IEEE Trans. Magn. 2005, 41, 599−603. (10) Wang, J., Ou, J., Ren, S., Yang, Sh. Construction of various selfassembled films and their application as lubricant coatings. In New Tribological Ways; Ghrib, T., Ed.; InTech: Rijeka, Croatia, 2011; pp 403−418. (11) Hsu, S. Nano-lubrication: concept and design. Tribol. Int. 2004, 37, 537−545. (12) McDermott, M.; Green, J.; Porter, M. Scanning force microscopic exploration of the lubrication capabilities of n-alkanethiolate monolayers chemisorbed at gold structural basis of microscopic friction and wear. Langmuir 1997, 13, 2504−2510. (13) Tsukruk, V. Molecular lubricants and glues for micro- and nanodevices. Adv. Mater. 2001, 13, 95−108. (14) Patton, S. T.; Eapen, C. K.; Jeffrey, Z. S.; Sanders, J. H.; Voevodin, A. A. Lubrication of microelectromechanical systems radio frequency switch contacts using self-assembled monolayers. J. Appl. Phys. 2007, 102, 024903. (15) Qian, L.; Tian, F.; Xiao, X. Tribological properties of selfassembled monolayers and their substrates under various humid environments. Tribol. Lett. 2003, 15, 169−176. (16) Henda, R.; Grunze, M.; Pertsin, A. J. Static energy calculations of stress-strain behavior of self-assembled monolayers. Tribol. Lett. 1998, 5, 191−195. (17) Lane, J. M. D.; Chandross, M.; Stevens, M. J.; Grest, G. S. Water in nanoconfinement between hydrophilic self-assembled monolayers. Langmuir 2008, 24, 5209−5212. (18) Marsalek, O.; Uhlig, F.; Vandevondele, J.; Jungwirth, P. Structure, dynamics, and reactivity of hydrated electrons by Ab initio molecular dynamics. Acc. Chem. Res. 2012, 45, 23−32. (19) Liu, H.; Ahmed, S. I. U.; Scherge, M. Microtribological properties of silicon and silicon coated with diamond like carbon, octadecyltrichlorosilane and stearic acid cadmium salt films: A comparative study. Thin Solid Films 2001, 381, 135−142. (20) Khatri, P.; Biswas, S. K. Friction of Octadecyltrichlorosilane monolayer self assembled on silicon wafer in 0% relative humidity. J. Phys. Chem. C 2007, 111, 2696−2701. (21) Xiao, X.; Qian, L. Investigation of humidity-dependent capillary force. Langmuir 2000, 16, 8153−8158. (22) Fujihira, M.; Aoki, D.; Okabe, Y.; Takano, H.; Hokari, H.; Frommer, J.; Nagatani, Y.; Sakai, F. Effect of capillary force on friction force microscopy: A scanning hydrophilicity microscope. Chem. Lett. 1996, 23, 499−500. (23) Binggeli, M.; Mate, C. M Influence of water vapor on nanotribology studied by friction force microscopy. J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct.Process., Meas., Phenom. 1995, 13, 1312−1315. (24) Li, L. Y.; Chen, S. F.; Jiang, S. Y. Protein adsorption on alkanethiolate self-assembled monolayers: Nanoscale surface structural and chemical effects. Langmuir 2003, 19, 2974−2982. (25) Major, R. C.; Houston, J. E.; McGrath, M. J.; Siepmann, J. I.; Zhu, X.-Y. Viscous water meniscus under nanoconfinement. Phys. Rev. Lett. 2006, 96, 117803−117807. (26) Lorenz, C. D.; Chandross, M.; Lane, J. M. D.; Grest, G. S. Nanotribology of water confined between hydrophilic alkylsilane selfassembled monolayers. Modell. Simul. Mater. Sci. Eng. 2010, 18, 034005−043018. (27) Lorenz, C. D.; Chandross, M.; Grest, G. S. Large scale molecular dynamics simulations of vapor phase lubrication for MEMS. J. Adhes. Sci. Technol. 2010, 24, 2453−2469.

pressures, hydrated SAMs show lower friction than their dry counterpart; however, this is more pronounced for the odd SAM (C15). At higher pressures, the effect maybe reversed. We find that the critical pressure of losing lubrication efficiency may depend on the SAM being odd or even; this for C14 is PN > 300 MPa and for C15 is PN > 700 MPa. At a pressure of 1100 MPa, the SAM−SAM systems had the highest friction. Despite this, up to PN = 1100 MPa, hydrated systems show almost a linear dependence between friction and load. At this range of pressure, we find an odd−even effect on the friction coefficient where the even hydrated SAMs show lower friction coefficient. This may be attributed to lower water penetration seen for the even system compared with the odd system. This would enhance lubrication capacity and reduce friction coefficient. For the odd system, we found the friction is always higher when both surfaces are covered by SAM than when one surface is covered by SAM and a thin layer of water is present between the SAM and the bare gold surface. For dry systems, such odd−even effects have been reported in the range of C12−C15. Here we examined only one odd and one even system due to larger computational cost for hydrated SAMs. Perhaps future calculations for other odd−even systems (e.g., C12, C13) would reveal the extent of this odd− even effect. At lower sliding velocity, the calculated water viscosity showed enhancement by up to 3 orders of magnitude, suggesting ordered structure. This is in contrast to the small enhancement of viscosity reported in some of works in the literature for water when confined by two SAMs. However, we attribute this to using a smooth gold surface as one of the confining surfaces in our study. This clearly has caused strong layering of the water molecules and at extreme pressures has led to icelike structures. Reductions in shear viscosity and friction coefficients at higher sliding velocities suggest strong structural order for water when confined between gold substrate and SAM.

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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 The support of these studies by an Australian Research Council Discovery Project grant, a University of Sydney Grant, an Australian Postgraduate Award (APA) scholarship for the first author, and computational time grants by Intersect Australia Ltd and also Australian National Computational Infrastructure Facility is thankfully acknowledged.

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REFERENCES

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