Computer Simulation of the Effect of Wetting Conditions on the

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Computer Simulation of the Effect of Wetting Conditions on the Solvation Force and Pull-Off Force of Water Confined between Two Flat Substrates Gerson E. Valenzuela*,†,‡ †

Chemical Engineering Department, Universidad de La Frontera, Av. Francisco Salazar, Temuco 01145, Chile Centro de Excelencia de Modelación y Computación Científica, CEMCC, Universidad de La Frontera, Temuco 01145, Casilla 54-D, Chile

‡

J. Phys. Chem. C Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/16/19. For personal use only.

S Supporting Information *

ABSTRACT: Modeling experimental results of pull-off versus relative humidity obtained by atomic force microscopy (AFM) has suggested that the classic interaction force models (van der Waals and capillarity) do not capture the water behavior confined between two surfaces at a distance D ≈ 1 nm. In this paper, pull-off generated by bridges of water confined between two flat substrates is studied using molecular dynamics simulations, varying the wetting conditions and the number of water molecules of the bridge. The method used imitates the approach and retraction curves in AFM. The confined water exhibits an oscillatory solvation force for D < 1.3 nm for both hydrophilic and hydrophobic surfaces. For hydrophilic surfaces, the pull-off corresponds precisely to the maximum of attractive force produced at D ≈ 0.7 nm, where the bridge is composed of two water layers. These results are applicable to the tip−sample nanocontact of experimental systems for substrates at a contact angle >40°.

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forms on contact with the surfaces.10 The effect of humidity on the interaction force between two surfaces in air is expressed in the pull-off measurement, the maximum force obtained in force spectroscopy during retraction of the tip.7 The capillary theory has been recently tested for water drops and bridges between planar substrates at different wetting conditions11 and for the instability of such bridges,12 finding that it works in the length from 2 to 10 nm. However, several studies have shown that the classic force models (van der Waals and capillary theory) do not qualitatively describe the pull-off force in the presence of humidity. In particular, experimental and computer simulation results have suggested that the classic models do not capture confined water behavior between surfaces at distances less than 1 nm.6,13 There are currently different proposals that could describe the contribution of water bridges to adhesion force. Asay and Kim14 showed that the tendency to pull-off with relative humidity for SiO2 surfaces is correctly described when the van der Waals

INTRODUCTION

Adequate understanding of the interaction forces between particles and surfaces is crucial to technological development.1 These determine the phenomena of aggregation and particle scattering, and they are particularly relevant in every process that includes nanoparticles.2 The interaction between particles and surfaces is composed essentially of the van der Waals forces and electrostatics; the capillary and solvation forces are a result of the interactions of the molecules of the medium with particles and surfaces.3 With the introduction of atomic force microscopy (AFM),4 interaction forces have been measured for a variety of particles and surfaces in different media. The first measurement of force between two surfaces in a liquid medium by Horn and Israelachvili5 with the surface force apparatus provided indications that water molecules, even in low concentration, can be adsorbed on surfaces by forming capillary bridges, the force of which controls adhesion. This phenomenon has been observed frequently in the AFM measurements in humid air, where water can condense between the surfaces, forming a bridge that controls the adhesion;1,6−9 if condensation is not favored, the surfaces can adsorb monolayers of water, in which case, the capillary bridge © XXXX American Chemical Society

Received: October 10, 2018 Revised: November 25, 2018 Published: December 20, 2018 A

DOI: 10.1021/acs.jpcc.8b09907 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C force between the surfaces, the capillary force, and the force required to separate two layers of ice is incorporated; this is based on the experimental results that show that water adsorbed on a SiO2 surface acquires an icelike state for film thicknesses less than 1 nm.14,15 Bartošik et al.16 showed that it is possible to model results of pull-off as a function of relative humidity for SiO2 surfaces using classic capillary force models where the surface tension is a function of the curvature of the capillary following Tolman’s equation.17 Salameh et al.18 incorporated the solvation force into the van der Waals and capillary forces to model the tendency to pull-off with relative humidity for oxidized metallic particles (TiO2 and Al2O3). The clear differences in these approaches demonstrate that the behavior of confined water at a length ∼1 nm is currently a subject of debate. The capillary bridges have been studied for various systems using Monte Carlo and molecular dynamics (MD) simulations.2,19−26 Among other results, these studies have shown that the force obtained using computer simulation can be quantitatively compared to the experimental results. As an example, Salameh et al.27 calculated the force between two TiO2 nanoparticles connected by a water bridge using MD simulation with excellent agreement with experimental AFM measurements. The present work seeks to study the relation between the pull-off and the force spectroscopy of water bridges in hydrophilic and hydrophobic surfaces with differing degrees of wetting by means of MD simulations. In the next section, the systems and methods used are presented, followed by force spectroscopy for water bridges between two surfaces at different wetting conditions. Finally, the pull-off force is studied for hydrophilic surfaces by varying the size of the capillary bridge. The scope of the results is discussed with respect to the experimental background.

Figure 1. (A) Initial configuration. (B) Sessile drops obtained at the end of the simulation (for ϵ/kB = 125).

wetting hysteresis, and contact angle, which are presented as Supporting Information. The capillary force curves F versus D are obtained according to a method developed in previous works.13,31 Figure 2 shows a

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METHODS The simulations consist of water and solid substrates with an FCC structure made up of copper atoms. The interactions of water and copper are defined by the TIP4P/200528 potential and the Lennard-Jones29 potential, respectively. The water− substrate cross interaction is defined by the interaction between the oxygen sites of the water model and the copper atoms according to the Lennard-Jones potential. The parameter σO−Cu = (σO + σCu)/2 is fixed, whereas the parameter ϵO−Cu = ϵ is varied in the range 55 ≤ ϵ/kB ≤ 250 to generate different wetting conditions. Specific interaction parameters can be reviewed in ref 13. The simulations are performed with a program developed by the author that has been used in previous works.13,30,31 The water temperature is controlled with a Berendsen thermostat at 298 K. The integration step in the simulations is 4 fs. The effect of ϵ on wetting is obtained from simulations of droplets. The simulations begin with a water cube ∼0.3 nm above a molecularly smooth substrate. As the simulation progresses, the substrate is wetted by water according to the value of ϵ. The initial and final configuration for ϵ/kB = 125 is shown as an example in Figure 1. The simulations consist of 864 water molecules and 4232 copper atoms. Each simulation lasts 2 ns: the wetting occurs for 0.5 ns, and in the following 1.5 ns, 1500 configurations are extracted for analysis. Then, the contact angle is measured by density profile analysis. A complete description of this type of analysis is given by Giovambattista et al.11 Similar methods were used in this work to analyze the wetting dynamics,

Figure 2. System used for the measurement of both force vs distance curves and pull-off.

general diagram of the system used. It is made up of a bridge of 500 water molecules, 4232 copper atoms for each substrate, and 576 springs for the cantilever (SP). The cross-interaction parameter ϵ is the same between water and each solid. The force exerted by the capillary bridge is measured with the deflection of the SP; the interactions between the two substrates (S1−S2) are suppressed in the simulation so that the force of the water bridge is exposed for analysis. The tip of this system is formed by S2. The tip is moved by displacing the top sites of the SP with constant speed; the dynamics of this system has been studied previously.31 The pull-off simulation is analogous to an AFM experiment.

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RESULTS The results of the contact angle versus ϵ/kB are presented graphically in Figure 3, where the horizontal line separates the hydrophobic and hydrophilic areas. Values of the contact angles are available in Table S1 of the Supporting Information. Contact angles were checked in simulations with 500 water molecules, without appreciable differences. Using these results for contact angle versus ϵ, the force F as a function of separation D was measured for six values of ϵ. Four B


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The Journal of Physical Chemistry C

Table 1. First and Second Maximum of the Attractive Force for N = 500, Varying the Wettability ϵ/kB

Fmax,1, nN

Fmax,2, nN

Fmax,1/Fmax,2

250 225 200 175

5.4 3.9 2.6 1.4

1.5 1.1 0.7 0.4

3.6 3.6 3.7 3.5

From the previous data, an approximately constant Fmax,1/ Fmax,2 ratio is observed for each case. Theoretically, the contribution of solvation to the total interaction energy can be described by an exponential decay of the shape W = W(0) cos(2πD/σ)exp(D/σ), where the decay length of the oscillation is associated with the molecular diameter σ.3,18 From this, the ratio between two successive minimums in the force curve is exp(D1/σ)/exp(D2/σ), regardless of the degree of wetting. Obtaining data from D1 and D2 directly from Figure 4 and with σ = 0.3 nm, ratios of 4.0 and 3.2 for ϵ/kB = 225 and 175 are obtained, respectively. These theoretical values correspond adequately to the force ratios indicated in Table 1. Images of the water bridge at different separation distances D show that the oscillations in the force respond to structural changes in the water bridges, as has been shown previously for the system ϵ/kB = 250.13 For example, for ϵ/kB = 225, the bridge is composed of two water layers (one adhered to S1 and the other to S2) between 0.55 nm < D < 0.7 nm. When the separation distance is increased, the transition of the water bridge between 0.7 nm < D < 0.85 nm is observed until three well-defined water layers form at 0.85 nm < D < 1 nm. From this configuration, a new transition is observed when the separation distance increases, forming a bridge consisting of four water layers at D ≈ 1.2 nm. The same observations are valid for ϵ/kB = 175, 125, and 75. The transition areas between two and three water layers are of pronounced slope in the force curve, which corresponds to areas associated with mechanical instability in AFM.3,32 Pull-off simulations analogous to AFM experiments were carried out following the procedure outlined in Figure 2; the approach and retraction of the tip (S2 substrate in Figure 2) are carried out by displacing the cantilever (SP in Figure 2) at constant velocity. During the approach, the simulation has to run as long as FS reaches a repulsive value FS*; during retraction, the simulation has to run until detecting a jump in FS or until reaching a certain value D* of the separation distance D. Figure 5 illustrates the pull-off simulation where the behavior of water during the approach and retraction of the tip can be seen. The simulation begins with water adsorbed on “the sample” (S1 substrate in Figure 2). Sequence I illustrates the approach and sequence II the retraction. Figure 6 shows the result of the pull-off simulation for the system ϵ/kB = 225, where the force of the SP (FS) is recorded against the position of the cantilever, Δz, defined as the distance between the sample and the top end of the SP (see Figure 2). In Figure 6, the approach curve at a compression speed of va = 10 m/s, red series, shows oscillations around FS = 0 (point A in Figures 5 and 6) because of the conservative potential imposed on the SP. When Δz ≈ 5.5 nm, the contact between surface S2 and water forms a bridge whose attractive force produces the deflection of the SP at Δz ≈ 5 nm (point B in Figures 5 and 6). The displacement continues, causing water to be compressed between the surfaces. The force increases to F*S

Figure 3. Contact angle (α) vs water−solid interaction parameter (ϵ/ kB). The horizontal line separates the hydrophobicity ranges: lower section, hydrophilic systems; upper section, hydrophobic systems.

hydrophilic cases (ϵ/kB = 250, 225, 200, and 175) and two hydrophobic cases (ϵ/kB = 125 and 75). Figure 4 shows the

Figure 4. Force vs distance curves for hydrophilic and hydrophobic systems.

force curves obtained for four of these systems. The results shown correspond exclusively to the force exerted by the water bridge because the interactions between the substrates (S1 and S2) are suppressed in the simulations. For D < 1.3 nm, oscillations are observed for both hydrophilic and hydrophobic systems. For the hydrophilic systems, two minimums are noted in the attractive force range, thus being the maximums of attractive force. For ϵ/kB = 225 and 175, the first attractive maximum is at D ≈ 0.7 nm. In both hydrophilic cases, this force can be interpreted as the contribution of the water bridge to the adhesion between S1 and S2, the pull-off in AFM. By contrast, for the hydrophobic cases (ϵ/kB = 125 and 75), the oscillation occurs in the repulsive force range, which is why the minimums of these curves cannot contribute to pull-off. In Table 1 the strength of the two attractive maximums are shown (Fmax,1 and Fmax,2) for each hydrophilic system. C


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Figure 5. Screenshots of the pull-off simulation for ϵ/kB = 225. I: approach. II: retraction. Points A, B, C, and D correspond to those in Figure 6.

This effect may be explained by the structural nature of the oscillatory force, which produces the pull-off in this case. Different approach speeds might drive to slightly different orders inside of the bridge, and so to different pull-off forces. The statistical discrepancy of the pull-off force with the initial configuration was analyzed for N = 500 with ϵ/kB = 225 and 175, using va = 10 m/s and vr = 5 m/s. After 20 simulations for each case, the mean values of pull-off force are 4.1 ± 0.18 and 1.64 ± 0.09 nN, which match with the maximum adhesion forces in Figure 4. The details of the analysis are presented in the Supporting Information. The spring hardness k will also affect the simulation time in both approach and retraction curves. A low value of k would require longer simulation times because a longer elongation of the cantilever will be needed to get FS* during the approach. In retraction, a longer elongation will be needed to exert a force strong enough to separate the two water layers which were formed during the approach; too high values of k would be insensitive to measure the pull-off. Thus, an intermediate value of k is needed, as reported in Figure 6. In Figure 7 the pull-off simulation of hydrophobic surfaces is compared with an hydrophilic case. The hydrophobic surfaces do not produce a pull-off on the force, as expected from the

Figure 6. Determination of pull-off for ϵ/kB = 225. Points A, B, C, and D correspond to those in Figure 5. The retraction curves after pull-off oscillate around FS ≈ 0. These oscillations are not shown in order to keep the figure clear.

≈ 2 nN (point C in Figures 5 and 6) where the bridge is formed by two water layers. All retraction results start from this state. Three speed conditions (vr) were applied to the SP. The force obtained with vr = 10 m/s shows the pull-off at point F where FS = −4.2 nN. This value is close to the first minimum of the force in Figure 4 (F = −3.9 nN). The difference is attributable to the attractive force because of the viscosity of the liquid when separating two bodies within a short time, which is proportional to the separation speed of the surfaces.33 Indeed, the retraction curves with vr = 5 and 0.2 m/s show that the pull-off decreases at points E and D, respectively. At point D, the pull-off is FS ≈ −3.9 nN. The previous results show that the measured pull-off is the maximum attractive force in Figure 4 between two water layers right before initiating the transition to a configuration with three water layers. Regarding to the effect of the approach speed (va) on the pull-off for hydrophilic surfaces, this speed produces a random effect on the pull-off. It is shown in Table 2. Table 2. Effect of va on the Pull-Off Forcea va, m/s (A) pull-off, nN (B) pull-off, nN

2 3.99 2.39

5 3.92 2.12

10 3.93 2.26

15 4.05 2.14

20 4.19 2.13

Figure 7. Pull-off simulation for ϵ/kB = 225, 125, and 75. Simulation of 500 water molecules. For each case, solid lines are for approach while dashed lines are for retraction. Pull-off is not detected for the hydrophobic surfaces.

In all of the cases, the retraction speed was set in vr = 5 m/s and ϵ/kB = 225. (A) 500 water molecules and (B) 256 water molecules. a

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force versus distance curves in Figure 4 because the oscillation develops in the repulsive force range (F > 0). It is highly probable that these oscillations respond to a different mechanism comparing with the oscillations for hydrophilic surfaces. With the described method, the pull-off can be determined for the hydrophilic systems (ϵ/kB = 225 and 175), varying the number of water molecules (N) in the bridge. For both systems, the pull-off simulation was performed with N = 32, 256, 380, 500, 650, and 864 using a tip displacement speed of vr = 5 m/s. This speed presents a good compromise between the computational output of the simulations and the precision of the pull-off with the attractive maximum on the force curve. Two approach speeds were used (va = 10 m/s and va = 15 m/ s) to incorporate the random effect because of slightly different structures of the bridges at the beginning of the retraction. The results are shown in Figure 8.

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DISCUSSION

Salameh et al.27 measured the interaction force between nanoparticles of TiO2 (radius ∼10 nm) at 50% relative humidity, where the nanoparticles form a film on a mica substrate. In the measurement of pull-off, the nanoparticles adhere to the tip on approach in such a way that on retraction, it is possible to register the contact force between two nanoparticles. When successive pull-off measurements are taken, a log-normal distribution is obtained in the range 1.2−8 nN with a median at 2.5 nN. The contact angle of water on a TiO2 wafer [root mean square (rms) roughness of 0.4 nm] has been reported34 in the range of 50−70°. The contact angle for ϵ/kB = 225 (57 ± 2° in Figure 3) is in this range; thus, the results in Salameh et al. could be compared to the results of the pull-off simulations for ϵ/kB = 225. Indeed, according to Figure 8 for 200−900 water molecules, the pull-off is in the range of 2−7 nN approximately, which is quantitatively comparable to the distribution obtained by Salameh et al. This suggests that the pull-off distribution in Salameh et al. could be explained at different sizes of the water bridge between two nanoparticles at different pull-off measurements. It is reasonable to consider water bridges between 200 and 900 water molecules in the contact between two nanoparticles when estimating, according to the results in Salameh et al., that each nanoparticle with a radius of 10 nm would be covered by some 13800 water molecules on average. Zarate et al.10 analyzed the effect of relative humidity on pull-off for stainless steel and Perspex substrates with a silicon nitride tip (radius ∼20 nm). For these substrates, contact angles of 50 ± 5° and 90 ± 5° and rms roughness 2 and 1.75 nm, respectively, are reported, comparable to the cases ϵ/kB = 225 and 175 in this paper (57 ± 2° and 85 ± 2°, respectively, in Figure 3). Zarate et al. noted an important effect of humidity on pull-off in stainless steel. Capillary condensation in the pores and/or cracks of the substrate at 65% relative humidity (RH) is ruled out because their dimensions are less than the critical length, Dc = 2rk cos ϕ, on which the capillary condensation is favored as described by the Kelvin equation rk = γVm cos ϕ/(RT ln(RH)). For this reason, Zarate et al. associate the effect of humidity with water layers adsorbed on the surface of the stainless steel and the tip. The maximum pull-off obtained by Zarate et al. is ∼30 nN in the humidity range between 25 and 35% which drops to ∼20 nN in the 55% humidity range and reaches ∼5 nN at 15% RH. In the last case, pull-off could be associated with the van der Waals interactions between the tip and the substrate because of the low water adsorption at 15% RH. Thus, as a rough approximation, the contribution of humidity to pull-off could be estimated by difference: ∼25 nN for RH between 25 and 35% and ∼15 nN at 55% RH. Zarate et al. assume that during the approach, the tip contacts the surface of the topographic peaks of the substrate at three contact points, the moment when it can no longer continue, forming interstitial spaces 0.4 nm in length (a layer of confined water) or 0.9 nm (three layers of confined water), depending on the humidity of the environment. Under the previous assumptions, the results of the present study can be related to those in Zarate et al. in the nanocontact, although considering that two water layers are confined as a minimum in the nanocontact (because both the stainless steel and the tip are hydrophilic). The pull-off in Figure 8 for ϵ/kB = 225 and N = 864 is 6.5 nN, which, for an assumption of four contact points, shows a pull-off of 26 nN. This is similar to the

Figure 8. Pull-off versus number of water molecules for ϵ/kB 175 and 225. For all of the cases, vr = 5.

For ϵ/kB = 225, the pull-off increases with the number of particles in a nonlinear continuously increasing relation. The snapshot observation of the simulations for N = 256, 380, 500, 650, and 864 during the retraction curve shows in the pull-off that water remains adhered in both S1 and S2, forming a capillary bridge when the surfaces are separated (similar to Figure 5(II)). Thus, the pull-off in these cases responds to the force limit that the two water layers can withstand before bulk water forms between them. Therefore, the recorded pull-off is a measurement of the adhesion between two water layers and not of water−substrate adhesion. By contrast, the snapshot observation of the simulations for N = 32 in Figure 9 shows that the pull-off for A1, A2, and B1 coincides with the separation of water regarding S1 (or S2), being a direct measurement of the water−substrate adhesion. For simulation B2, during the approach, the bridge was compressed till it formed one layer between the surfaces, and then, the pull-off corresponds to the formation of two layers between the surfaces. The previous results show that different phenomena are related to the pull-off force measurement in this simple system. Although 32 water molecules seem to be irrelevant for experiments, Zarate et al.10 have estimated that only 60 water molecules entering the hydrogen bonding interaction would be necessary to explain the increase of the pull-off force between the tip and surface. E


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Figure 9. Retraction simulation for N = 32. Cases A1, A2, B1, and B2 correspond with those in Figure 8. Simulation B1 ends with the bridge separated from S1. In the snapshots, pull-of f points out the configuration with the maximum attractive force during the retraction.

humidity contribution to the pull-off estimated from the results in Zarate et al. in the humidity range between 25 and 35% only considering one additional contact point. If on the approach the interstitial space is limited to 0.9 nm because of the greater humidity, on retraction the pull-off should detect the second minimum of the oscillating profile of the force curve, thus leading to a smaller pull-off. With respect to the measurements on Perspex, Zarate et al. do not observe any effect of humidity in the pull-off; however, the force curve for ϵ/kB = 175 (wetting comparable to Perspex) in Figure 4 presents an oscillation that should contribute to the pull-off. In this case, it could be speculated that the contribution of water to the pulloff in Perspex is less than the tip−Perspex interaction force, with an imperceptible effect in pull-off experiments. Bartošiḱ et al.16 measured pull-off versus relative humidity for the SiO2 substrate (contact angle of 40° and rms roughness of 0.14 nm) with a silica tip (radius of 10 nm). In the measurement, the approach was made up to a repulsive force of 1 nN. In the results, a maximum pull-off in the order of 9.5 nN with 60% RH is observed. From the results, it can be estimated that the substrate-tip van der Waals interactions contribute ∼3.5 nN at 60% RH. With this, in a rough approximation, the contribution of water to the pull-off would be in the order of ∼5.5 nN. These results are comparable to the system with ϵ/kB = 250 (contact angle of 41 ± 1° in Figure 3), where the first minimum of the force curve is −5.4 ± 0.4 nN for N = 500 (force curve not shown in Figure 4). Owing to the low roughness of the substrate, the system in Bartošiḱ et al. could be modeled (in the nanocontact) by two flat surfaces, which could explain the remarkable similarity between the contribution of the humidity to the experimental pull-off and the minimum of the force curve obtained in the simulations in this paper.

The previous comparisons suggest that the similarity between the contact angle measured on a flat substrate (simulation) and a wafer (experimental) is sufficient for the pull-off simulations to lead to comparable results with nanocontact in the experimental system. However, this conclusion could be valid only in the wetting range addressed, that is, for a contact angle between 40° and 90°. In systems with greater wetting, the structure of the substrate must be considered, as simulations35−38 and experiments14,15 suggest. Computer simulations have shown that slight differences in the lattice constant for a hydrophilic substrate made of LennardJones atoms produce a stable water droplet forming on the first-ordered water layer.37 The phenomenon was observed for Pd (lattice constant 0.389) but not for Cu (lattice constant 0.362 nm) when the crystal face 100 is in contact with water. The effect of relative humidity on the thickness and structure of water adsorbed on a silicon ATR crystal was studied by Asay and Kim.15 The surface of the crystal assumes saturated by hydroxyl groups, which is why it can form hydrogen bonds with water molecules. Asay and Kim observed that at RH < 30% from 1 to 3 water monolayers with a structure completely connected by hydrogen bonds (called icelike) are adsorbed. The adsorption of up to three icelike monolayers on the crystal is explained by the hydroxyl groups of the surface that force water to adopt an ordered structure in the first layer, the one propagated in the following water layers by the hydrogen bonds. The icelike structure would be presented up to 0.75 nm high. For 30−60% RH, a fourth water layer is added with a transitional structure, reaching a height of ∼1.2 nm. For RH > 60%, the liquid water adsorption is observed over the four previous layers. Later, Asay and Kim14 studied the effect of relative humidity on pull-off in this system. Their results are adequately modeled using the combination of van der Waals force, capillarity force, and the force required to separate two F


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The Journal of Physical Chemistry C ice layers where a surface energy (103.3 mN/m) greater than the bulk water (72.8 mN/m) is considered when the relative humidity decreases. By contrast, Bartošiḱ et al.16 model their pull-off results considering van der Waals and capillarity forces, where the surface energy decreases (when the RH is reduced) from 72 to 9 mN/m. It is clear that with the substrate in the present study, the adsorption in Asay and Kim14 cannot be reproduced as it does not have the capacity to form hydrogen bonds with water molecules; however, it has been observed in this substrate that water reduces the number of hydrogen bonds when the confinement is increased,13 which could explain that the results here are quantitatively similar to those in Bartošiḱ et al.16 Another aspect regarding the effect of the substrate lies with the possibility of an effect of the approaching speed on the pull-off. For solids where the interaction water−solid is mediated by hydrogen bonds, it could be possible that different structures (icelike) of water could be obtained at different approaching speeds. Thus, as a hypothesis, during the retraction, different structures of the bridge will produce different values of pull-off.

results in the literature that suggests that the surface tension of a water bridge decreases when the confinement increases between two partial wetting substrates. These two phenomena (increase and decrease of hydrogen bonds) in which water would undergo as a result of substrates of different structures could explain that in the modeling of pull-off versus humidity in the literature different laws of force have been used in order to come close to describe these phenomena.

CONCLUSIONS In this paper, pull-off due to water confined between two flat substrates was measured using a nonequilibrium MD simulation following a direct procedure that imitates an experiment of approach and retraction in AFM. In this method, the retraction speed affects the measurement of the pull-off, likely because of viscous effects on the bridge. With a speed of 0.2 m/s, the pull-off measured corresponds to the minimum observed in the force curve (adhesion force) of water (curve obtained from equilibrium simulations). A speed of 5 m/s represents a good compromise between the pull-off measured and the computational cost of the simulations. On the other hand, the approach speed produces a random effect on the pull-off. It may be explained by the structural nature of the oscillatory force, which produces the pull-off in this case. Different approach speeds might drive to slightly different orders inside of the bridge and so to different pull-off forces. The results for a hydrophilic system (contact angle ∼50°) and one of intermediate hydrophobicity (contact angle ∼90°) show that the pull-off responds to the water solvation force that manifests as an oscillating profile on the force curve. This oscillation contains two minimums: the first at 0.7 nm of separation (two water layers) and the second at 0.9 nm of separation (three water layers) for the hydrophilic system. The oscillation is also developed in hydrophobic systems in the repulsive force range where it cannot contribute to adhesion. For the intermediate hydrophobicity system, the results show that the first minimum of the force curve generates pull-off, although three times less than that in the hydrophilic system. The comparison of the results obtained in the simulations with experimental background from different systems indicates that the pull-off simulations can represent the nanocontact in experimental systems simply if there is similarity of the contact angle measured on the flat substrate (simulation) and a wafer (experimental). Within the scope of this study, the hypothesis is put forward that this is valid for systems with a contact angle >40° but not for perfectly wetted systems where it has been observed experimentally that water is structured in the confinement because of the formation of water−substrate hydrogen bonds. By contrast, the substrates in this study cause the reduction of hydrogen bonds in water when the confinement increases, a phenomenon consistent with the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b09907. Analysis of wetting versus water−substrate interaction parameter and the statistical analysis of the effect of the initial configuration on the pull-off force (PDF)

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AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected].

Gerson E. Valenzuela: 0000-0003-3013-4299 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS Gerson E. Valenzuela thanks CEMCC UFRO for computational resources and the Dirección de Investigación of the Universidad de La Frontera for financial support. Powered@ NLHPC: this research was partially supported by the supercomputing infrastructure of the NLHPC(ECM-02).

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REFERENCES

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Computer Simulation of the Effect of Wetting Conditions on the

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