Effect of Electric Field and Trace Water on Confined Undecanol and

Jan 24, 2018 - *(S.J.O.) E-mail: [email protected]. ... The physics of nanoconfined liquids can be very different from the bulk.(1) For insta...
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Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Effect of Electric Field and Trace Water on Confined Undecanol and Tetradecane Eugene J. H. Soh and Sean J. O’Shea* Agency for Science Technology and Research, Institute of Materials Research and Engineering, 2 Fusionopolis Way, Innovis 08-03, 138634, Singapore, Singapore ABSTRACT: The effect of electric field on the liquids undecanol and tetradecanol is studied in an atomic force microscope (AFM). A strong electric field is applied by biasing a gold-coated AFM tip up to 3 V across approximately 5 nm or less of liquid confined between the tip and a graphite surface. Oscillatory forces are observed when no electric field is applied, thus indicating molecular ordering of the liquids at the interface. These oscillatory forces are observed far less frequently when a strong electric field is applied across undecanol, whereas only a slight decrease in observation frequency is found in tetradecane. Our interpretation is that trace amounts of water within the liquids is attracted into the tip−sample contact region under high electric field, thus disrupting the molecular layering. This hypothesis is supported by experiments in dried undecanol in which the probability of observing oscillatory forces only decreases slightly up to 3 V. The results highlight the importance of considering the effect of water in nonaqueous liquids under confinement in addition to changes in the molecular dipole moment under high electric field.

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temperature.10 However, both of these approaches have clear limitations in any practical application, whereas the use of a voltage/electric field across the confining surfaces is both simple and viable.11,12 Previous studies on the effect of electric field on nanoconfined liquids have principally been undertaken on water,13,14 and more recently on ionic liquids,15,16 using both AFM and scanning tunneling microscopy (STM). Ionic liquids in particular appear promising for electrically controlled lubrication at surfaces17 because the highly polar nature of the liquids suggest that electric fields can strongly modify the fluid properties near a surface. However, ionic liquids are prohibitively expensive and the use of familiar long chain hydrocarbons is preferable for practical application, and some initial tribometer experiments have demonstrated the effective viscosity of confined liquid hydrocarbons can be modified by external electric fields.12,18 Hence, we were motivated to further study the effects of high electric field using AFM in hydrocarbon liquids, in our case 1-undecanol and ntetradecane. In our work the AFM confines liquid between the tip and surface and allows the mechanical response of the confined liquid to be measured with Angstrom level resolution in the separation distance. With regards our experiments, closely related work has been reported by Luo and co-workers who have extensively studied the effect of electric field on the viscosity of various hydrocarbons (e.g., heptane, hexadecane, heptanol, decanol) confined between a steel ball and Chrome-on-glass discs.12,18

INTRODUCTION The physics of nanoconfined liquids can be very different from the bulk.1 For instance, when confined between two closely spaced surfaces, the density of the liquid near the surface oscillates about its bulk density with periodicity approximately equivalent to its molecular diameter, implying the formation of ordered layers near the surface.2 The understanding of the mechanical properties of these ordered liquid layers has importance and broad implications in many areas including nanofluidics,3,4 nanotribology and lubrication.5−7 The ordered liquid layers may give rise to a periodic oscillation in the force acting between two surfaces immersed in the liquid. In order to observe and measure these forces with high sensitivity, techniques such as the surface force apparatus (SFA)6,7 and atomic force microscope (AFM) have been applied.8 In both techniques, two surfaces are approached toward each other and the forces acting between the surfaces are measured as a function of their separation distance. The major difference between AFM and SFA is the AFM measures with greater spatial resolution (nanometers) compared to SFA. The AFM also allows facile measurements of other experimental parameters to be simultaneously measured with the force data, e.g., magnetic, electrical, and optical phenomena. For example, in the AFM work reported here, the effect of electric field on the mechanical properties of nanoconfined liquids is studied. Our interest in electric field effects stems from finding ways to tune mechanical properties of nanoconfined liquids at interfaces for applications such as controlled adhesion and lubrication. Alternative methods of controllably changing the mechanical properties of confined fluids include adjusting the rate of approach of the confining surfaces9 or changing the © XXXX American Chemical Society

Received: October 1, 2017 Revised: January 2, 2018 Published: January 24, 2018 A

DOI: 10.1021/acs.jpcc.7b09752 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Briefly, they find that for sufficiently thin films (less than ∼15 nm) strong electric fields (30 V over ∼6 nm, or ∼5 V/nm) increase the effective film viscosity in pure heptanol by a factor of ∼2,18 with the effect becoming less pronounced for longer chain alcohols because the molecule−molecule interactions are stronger, making it more difficult to orientate the molecular dipole with an external field. There was no significant change in effective viscosity with applied field for the alkanes. Interestingly, from a practical viewpoint, they find much bigger increases in viscosity (a factor of ∼10) with applied electric field for alcohol−alkane (decanol−heptane) mixtures at low alcohol fraction12 because the alcohol can move and reorientate more freely compared to the pure liquid. In the interpretation of Luo and co-workers, the creation or disruption of molecularly ordered layers at a surface is a fundamental mechanism causing the observed changes in effective viscosity, with more ordered layers creating a larger film thickness and hence an increase in measured viscosity. They hypothesize that for polar molecules an applied electric field can create thicker ordered layers because of increased adsorption and easier molecular reorientation.12 Our AFM measurement of the surface layering in undecanol and tetradecane under electric field can therefore be linked to the studies of Luo and co-workers. We directly observe oscillatory surface forces arising from the formation of ordered liquid layers under applied electric fields up to ∼1 to ∼6 V/nm. We find no significant change in the number of layers observed with and without an electric field, although some caution is required in this assertion because of the statistical nature of the AFM observation of solvation layering19 and the surfaces investigated are very different (graphite compared to metals). Our data suggest an additional mechanism must also be considered, arising from the presence of trace amounts of water in the liquids. In this mechanism, water is attracted to the tip− sample contact region by the applied electric field and disrupts the liquid layering.20

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MATERIALS AND METHODS We carried out our experiments in a modified AFM (Molecular Imaging Co.) operating in contact mode at room temperature (22 ± 2 °C). We apply the sample modulation method in taking force spectroscopy data as it can be more sensitive, within chosen force ranges, as compared to static measurements of the force.21 The liquids used are 1-undecanol (Aldrich, 99%) and n-tetradecane (Aldrich, 99+%). Highly orientated pyrolytic graphite (HOPG) is used as the substrate because it is electrically conducting and pronounced ordered liquid layers of long chain hydrocarbons can be observed on the surface in AFM.8,22 The HOPG and AFM cantilever are placed in a liquid cell and fully immersed in the liquid. The conductive AFM tips used are 7 N/m levers coated with ∼30 nm of platinum (Mikromash, HQ:XSC11/Pt). An additional 5 nm of chromium followed by 60 nm of gold is coated on the tips by thermal evaporation to ensure robust electrical performance. The experimental setup is illustrated in Figure 1a. For sample modulation, the HOPG substrate is oscillated by placing it on top of a piezoelectric plate. The plate is oscillated using a lockin amplifier (EG&G 7265) at an amplitude of approximately 1 Å peak-to-peak at 1 kHz, well below the cantilever resonance frequency (∼100 kHz). The HOPG is electrically grounded to one electrode of the piezoelectric plate, but if required, the current flow between the tip and HOPG can be measured by connecting this electrode to a current amplifier. The other

Figure 1. (a) Schematic of AFM experimental set up. (b) Spring and dashpot model to describe motion of the cantilever.

electrode of the piezoelectric plate is connected to the lock-in voltage modulation and is isolated from the AFM liquid cell by a piece of Macor. A 1 MΩ resistor is placed in series to prevent a high current from damaging the tip. The entire stack is held together by an inert conductive adhesive. Because of tip− sample interactions, the motion of the cantilever is coupled to the sample motion via the interaction stiffness of the confined liquid when the tip is close to the HOPG surface. There is no modulation signal detected when the cantilever is far from the HOPG. A typical experiment consists of taking a force curve at approach speeds of ∼1 nm/s and recording the cantilever deflection signal, oscillation magnitude and phase. The cantilever oscillation magnitude and phase is measured with the lock-in amplifier. An electric field is applied by biasing the gold coated tip from 0 to 3 V. Given that the long chain alcohols and alkanes lie flat on a graphite surface,8,23 forming ordered layers approximately equal to the molecular thickness (∼5 Å), the maximum electric field applied ranges up to ∼6 V/ nm for one confined layer to ∼1 V/nm for the outer 5−6 confined layers. In order to investigate the effect of trace water on the layering, we also undertook experiments with reduced water B

DOI: 10.1021/acs.jpcc.7b09752 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C content in undecanol. The boiling point of undecanol and water is 243 and 100 °C respectively. Therefore, boiling is a good method to reduce the amount of water in the undecanol. Previous analysis of n-dodecanol using Fourier Transform Infrared Spectroscopy and Gas Chromatography−Mass Spectrometry showed that the boiling procedure does not chemically modify the liquid and increases the purity of the sample.24 In our experiments, the undecanol is boiled at 140 °C in a nitrogen filled chamber for 3 h. Subsequently, it is allowed to cool in a vacuum chamber (base pressure ∼0.1 Torr) before being transferred to the AFM liquid cell for experiments. The entire AFM is located in an isolation chamber purged with nitrogen gas to reduce the relative humidity of the experimental environment to less than 5%. Mechanical Model. The sample modulation system can be modeled using springs and dashpots,25 as illustrated in Figure 1b. The dynamics of the cantilever is described by mz ̈ + γcz ̇ + kcz = ki(d − z) + γi(d ̇ − z)̇

(1)

where z(t) is the response of the cantilever and d(t) is the driving motion of the sample. Here, m is the effective mass of the cantilever, kc is the cantilever stiffness, ki is the interaction stiffness and γ are damping terms. Substituting d = Ad cos(ωt) and z = A cos(ωt − ϕ), where A is the measured amplitude, ϕ is the measured phase, Ad is the drive amplitude and ω is the drive frequency, eq 1 can be solved for the measured amplitude A and phase ϕ to obtain26 A = Ad tan ϕ =

ki 1 +

ωγi

Figure 2. Force curve (approach) taken in undecanol with no electric field showing the raw static cantilever deflection and sample modulation signals (normalized amplitude A/Ad and phase). Note that attractive forces, corresponding to decreasing values in the deflection curve, give rise to pronounced peaks in A/Ad, one of which is greater than 1. The cantilever stiffness is 7 N/m, the approach speed is 1 nm/s, the drive amplitude is 1 Å peak-to-peak, and the drive frequency is 1 kHz.

2

( )

2 2

Hence, in the deflection signal of Figure 2, we observe no force i.e. no deflection of the cantilever, far from the surface, and a linear increase in the force beginning around −4 nm piezoelectric displacement as the tip enters hard repulsive contact with the surface. The sharp “jumps” in the repulsive part of the force curve show the occurrence of solvation layering.22 Briefly, on approach the undecanol confined between the tip and surface is compressed, and at sufficiently large repulsive force one layer of undecanol is squeezed out of the contact region and the tip “jumps” to the next solvation layer. In Figure 2, five solvation layers labeled n = 1−5 are observed. The label n = 0 refers to the tip being in contact with the graphite surface. The distance the tip jumps is approximately ∼5 Å for both undecanol and tetradecane, corresponding to the molecular width of linear chain alcohols and alkanes, and this indicates that these molecules align in parallel layers on graphite.8,28 The solvation oscillations are clear in the amplitude and phase sample modulation data. Oscillations in the magnitude arise from the changing stiffness of the confined liquid due to molecular layering (see eq 3). In the amplitude data we observe the oscillations are superimposed upon an increasing interaction stiffness i.e. increasing amplitude, as the surface is approached. The normalized amplitude signal loses sensitivity and approaches A/Ao ∼ 1 when the interaction stiffness becomes much larger than the cantilever stiffness, and in the example of Figure 2 this occurs when the tip contacts the n = 1 layer closest to the surface. The phase signal is zero on the surface as the tip and surface move in phase, and is simply noise far from the surface because there is no signal for the lockin amplifier to measure. For these reasons it is difficult in sample modulation to quantify the phase data to extract the viscous damping terms, and hence we only use to highlight the solvation oscillation signals that occur.

ki

2

2

(ki + kc − mω ) + ω (γi + γc)

(2a)

ω(γi(mω 2 − kc) + γcki) ki(ki + kc − mω 2) + ω 2γi(γi + γc)

(2b)

Note that kC = mωO2, where ωO is the fundamental resonance frequency of the cantilever. Hence, in the low frequency limit ω ≪ ωO as in our experiments, eq 2a reduces to

ki A = Ad ki + kc

(3)

Equation 3 is used in the calculation of interaction stiffness (ki) from experimental data. Theoretically, the viscosity data (γi) can also be extracted from eq 2b at low frequency. However, this is difficult in practice as the stiffness ki strongly couples into the phase data. Decoupling the viscosity signal requires very careful calibration of the low frequency phase response19,27 which is not possible in our experiment due to instrumental limitations. However, the phase data is still useful due to the ease of observing the oscillatory forces in the raw phase signal.

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RESULTS AND DISCUSSION Figure 2 shows an example of raw force curve data in undecanol with no applied electric field. The cantilever deflection, amplitude and phase are measured as the piezoelectric controlling the tip-to-sample separation moves the tip toward the surface. Similar curves can be taken on tip retraction from the surface. The magnitude of the piezoelectric distance is arbitrary, but in our experiments zero distance corresponds to the approximate contact mode control location, and large negative distances indicate the tip is far from the surface. C

DOI: 10.1021/acs.jpcc.7b09752 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Note the amplitude maximum in Figure 2 where the normalized amplitude is greater than unity, which occurs in the attractive force region −kc < ki < −kc/2.21 In effect the spring constant of the overall system softens and the amplitude increases dramatically for a given driving force. Although a rare occurrence, this observation serves as a good indication that the mechanical model accurately represents the dynamic behavior of the system. If ki < −kc the tip motion becomes unstable and “jumps” to the attractive force minima, giving rise to the “jumps” in the static deflection curve noted above. Similar observations can be made regarding the phase data, in which the phase becomes very sensitive to changes in interaction stiffness ki as ki → −kc, giving rise to the pronounced peaks in the phase data near the oscillatory force minima. A full discussion of the analysis of cantilever dynamics can be found in the literature.26 An important aspect experimentally is to verify an electric field is actually applied across the confined liquid when voltage is applied to the tip. This is because mechanical wear and high electrical stress can lead to substantial degradation and removal of the metal film coating the apex of the tip. Two observations lead us to conclude the tip apex is satisfactorily conducting and a field is generated across the tip−sample gap when a voltage is applied. First, we note that the adhesion is consistently larger when a bias is applied due to the additional attractive force provided by the electric field. A second, and more direct method, is to measure the current flow under bias. Figure 3

Figure 3. Typical current−voltage (I−V) curve taken at the contact mode control distance, indicating both the presence of an electric field during the experiments and that the tip is in contact with the graphite. Data taken with a 7 N/m, gold-coated cantilever in undecanol.

Figure 4. Fraction of curves in which oscillatory forces were observed plotted as a function of bias voltage for (a) as-received undecanol, (b) tetradecane, and (c) dried undecanol. For each bias voltage, at least 50 curves were taken. Five different cantilevers (Au coated, stiffness 7 N/ m) were used for each liquid.

shows a representative current−voltage (I−V) curve taken at the contact mode control point, i.e., around the n = 0 region of the force curve. A large current is observed, with the linear I−V slope arising from the 1 MΩ resistor placed in series. The relatively low resistance of the contact leads us to conclude that (a) the tip is able to punch through all the solvation layers and contact the underlying graphite, i.e., the hard-repulsion at n = 0 is due to the HOPG and not a solvation layer, and (b) sufficient gold coating remains on or near the tip apex, implying that an electric field must exist across the tip−sample gap when voltage is applied. Our main findings of the influence of electric field on confined undecanol and tetradecane are summarized in Figure 4, showing the fraction of force curves in which oscillations are observed as the bias voltage is changed. Note that the observation of oscillatory forces with AFM is statistical in nature. This is possibly due to the changing geometry of the tip, stochastic behavior of the squeeze-out phenomena, or

contamination.19 Therefore, we take at least 50 curves for each bias from 0 to 3 V and five different cantilevers are used for each liquid. If a particular tip is not able to observe the solvation structure, the tip is discarded and replaced with a new tip. For completeness, Figure 5 shows representative approach and retract force curves indicating the general appearance of curves labeled as “oscillatory forces observed” (Figure 5a) and “no oscillatory forces observed” (Figure 5b). Figure 4a shows that in as-received undecanol the fraction of curves with oscillation decreases markedly from 0.80 to 0.36 as the applied bias increases from 1 to 3 V. In tetradecane, a nonpolar liquid, it is expected that the applied electric field will have negligible effect. Indeed, oscillations are observed almost independent of voltage bias up to 3 V (Figure 4b). However, there is a slight but clear decrease in the fraction of oscillatory curves observed at high bias, although the effect is far less pronounced than in the as-received undecanol. Lastly, Figure 4c presents the fraction of curves where oscillatory forces are D

DOI: 10.1021/acs.jpcc.7b09752 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

voltage is applied, leading to a disruption of the solvation layering. The two key aspects underlying this hypothesis, the presence of sufficient water in the liquids and the drawing of water toward the tip apex, will now be discussed. Trace amount of water is certainly present in both the liquids. Undecanol can readily dissolve water, up to ∼3.2% by weight at room temperature,29 so it is not surprising that the asreceived and dried undecanol can exhibit different surface interactions. In this regard, note the fraction of curves with oscillations at zero bias is also higher in dried undecanol than in as-received undecanol. The slight decrease in observation frequency in tetradecane (Figure 4b) with increasing voltage is also probably due to trace water disrupting the solvation layering. The solubility of water in tetradecane at room temperature is small but significant; around ∼55 ppm by weight.30 Hersam and co-workers31 calculate that similar amounts of trace water in hexadecane leads to a relative concentration of water at a surface which is essentially equivalent to the water concentration at the surface in air. This is significant because they further show there is sufficient water even at low humidity in dry (anhydrous) hexadecane to form a meniscus at the AFM tip apex under an applied electric field. The influence of electric fields on trace amounts of water, either in solution or vapor phase, has been most studied within the context of AFM Nanoscale lithography.31−36 Here, voltage is applied to a STM or conducting AFM tip and features are written on the underlying surface. Field Induced Oxidation (FIO) is the most common nanolithography31 and it is found that water must be present at the tip for oxidation to occur. Hence, the substrates typically used for AFM lithography are hydrophilic. In humid air, water is either present as a film on the hydrophilic surface or may spontaneously condense from the vapor phase to form a water meniscus at the AFM tip.32 Alternatively, and of most relevance to our discussion, a water bridge can be formed or enhanced between the tip and surface by application of a high electric field in noncontact AFM operation.33,34 Further, although most Nanolithography studies are undertaken in air, identical arguments for meniscus or water bridge formation on hydrophilic surfaces hold for waterhydrocarbon systems,31,35,36 the difference being that the dielectric medium is now a hydrocarbon liquid (e.g., relative permittivity ε ≈ 2 for alkanes) rather than air (ε = 1). It is unlikely that these well-known mechanisms of water bridge and meniscus formation occur in our experiment as the HOPG surface is hydrophobic. However, an alternative and more general process is outlined below. The preponderance of hydrophilic surfaces in nanolithography studies stems from the importance of using Silicon for applications in microelectronics. However, water-dependent AFM nanolithography is also observed on hydrophobic substrates, including HOPG.37 This observation led Butt and co-workers to re-evaluate how water concentrates under high fields in AFM, leading them to propose an additional possible mechanism called electric-field-induced condensation.38 Their analysis begins with the general consideration that material with large permittivity e.g. water, is drawn toward high field locations. From this they show that the saturation vapor pressure is lowered because of an external electric field, and under very high electric fields (∼1 V/nm) condensation can occur. The result is general and applies, for example, to hydrophobic surfaces and liquid−liquid systems. In our experiments, the estimated fields are ∼1 to ∼6 V/nm, and

Figure 5. Representative force curves taken in as-received undecanol with (a) oscillatory forces observed, taken with no electric field and (b) no oscillatory forces observed, taken at 3 V bias. In both cases, a gold-coated cantilever of stiffness 7 N/m is used, the approach speed is 1 nm/s, the drive amplitude is 1 Å peak-to-peak, and the drive frequency is 1 kHz.

observed for undecanol subjected to drying, and shows little change in observation frequency as the voltage is increased up to 3 V; a result very different from the as-received undecanol (compare with Figure 4a). The major difference between the as-received (Figure 4a) and dried undecanol (Figure 4c) suggests that trace water present in undecanol is a major influence underlying the electric field effects. Previous studies with the surface force apparatus (SFA) have shown that trace amounts of water in organic liquids disrupt the oscillatory forces20 and we propose a similar effect is occurring under high electric field. That is, trace water is attracted into the confined tip−sample region when a E

DOI: 10.1021/acs.jpcc.7b09752 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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the tip−sample gap to modify the solvation layers. However, in analyzing all force curves which show oscillatory forces, we do not observe any significant difference in the periodicity of the solvation oscillation or the average number of solvation layers as the bias voltage is changed. The solvation periodicity remains at ∼5 Å, suggesting no change in the orientation of the molecules which could arise, for example, from reorientation of the undecanol dipole under an electric field. Small changes in the solvation periodicity due to water incorporated in the layers may be difficult to observe because of limitations in the resolution of contact mode AFM. However, one would expect weakly bound layers furthest from the surface to be disrupted by water and hence the average number of observable layers to change; but this is not the case. Clearly computer simulations are required to clarify the interplay between water, solvation layering, and electric field. At present we speculate that perhaps the drawing of water into the tip−sample gap by the electric field is kinetically limited, such that the amount of water available to disrupt the layering is variable, resulting in force curves showing either no change in layering at low water content or entire layer disruption at high water content. We now consider the results in the context of the work of Luo and co-workers.12,18 A conclusion from their research is that very high electric fields disrupt the hydrogen bonding between alcohol molecules, resulting in modification of the solvation layers near the surface, and hence increasing the effective viscosity of the confined liquid. While not ruling out this mechanism, and considering that our confined liquid film is thinner (5 nm), our data suggests that trace water in the undecanol is an additional cause of solvation layer modification. Importantly, the disruption of solvation layers with increasing field arising from water can, in principle, also give rise to an increase in effective viscosity because fluidlike molecular layers show higher energy dissipation than crystalline layers.39 Thus, our results are not necessarily in contradiction. Indeed, in a comment closely related to the discussion above on the effect of water, Luo and co-workers also note “ions in the liquid film may move and concentrate near solid surfaces especially when an external electric field is applied” and this may give rise to additional mechanisms explaining their measured viscosity changes.18 Another uncertainty is our work is undertaken on HOPG whereas Luo and co-workers study metal surfaces. Long chain hydrocarbons form strongly bound layers on HOPG and one may speculate this is one reason no significant change is observed in the solvation layers per se as the electric field is changed. The energy available from the external field is too small to disrupt the molecular ordering or orientation within the layer. Conversely, metal surfaces are not atomically smooth and will certainly exhibit much weaker liquid layering.1 Again one can speculate that in the case of metals an external electrical field may have a larger influence on the confined liquid because lower energy barriers are encountered to reorientate the molecules.

hence, within the electric-field-induced condensation theory, water in a hydrocarbon liquid can be attracted to, and possibly condense, at the tip. There are several additional observations supporting the strong influence of water in our experiments, namely, the following: (a) In dried undecanol, the oscillatory forces at high electric field are observed with less frequency over 1 to 2 days, suggesting that water from the environment returns back into the undecanol. (b) In as-received undecanol, the fraction of oscillations switches between ∼0.8 and ∼0.36 as the voltage bias cycles between 0 and 3 V, showing the observed effects are reversible and do not appear to arise from a buildup of contaminants or modification to the tip under high electric field. We do not observe any major qualitative difference in the magnitude of the interaction stiffness in undecanol as the bias voltage is changed (see Figures 5 and 6). The stiffness increases

Figure 6. Examples of force curves converted into Force and interaction stiffness (ki) taken in as-received undecanol with (a) oscillatory forces observed, at 0 V, and (b) no oscillatory forces observed, at 2 V bias. In both cases, a 7 N/m gold coated cantilever is used, the approach speed is 1 nm/s, the drive amplitude is 1 Å peak-topeak, and the drive frequency is 1 kHz. The interaction stiffness is found using eq 3 and the force calculated as simply kc × deflection.

to ∼40 N/m (the maximum value we can measure using a 7 N/ m cantilever) over a relatively large distance of ∼3 nm irrespective of whether solvation oscillations are observed or a strong electric field is present. Notably, we were able to compare the stiffness for the layer closest to the surface (n = 1) and no significant change in stiffness was observed with or without high electric field (3 V bias). The difference between data with and without solvation oscillations appears to be simply that the oscillation signal is superimposed upon a gradually increasing stiffness signal, with the range of interaction remaining the same. Although no significant change in stiffness with electric field is observed, one would anticipate the presence of water within

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CONCLUSIONS We use AFM in contact mode with sample modulation to study the effect of electric field on the oscillatory solvation forces of confined undecanol and tetradecane. We find that a high electric field (>1 V/nm) reduces the fraction of oscillations observed substantially in as-received undecanol and slightly in as-received tetradecane. A major reason underlying this result is the presence of trace water in the liquids and we hypothesize F

DOI: 10.1021/acs.jpcc.7b09752 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C the high electric field attracts water into the tip−sample contact region38 thus disrupting the ordered solvation layering.20 This is an alternative mechanism to be considered in addition to previously proposed electric field effects, such as molecular dipole reorientation and changes in the number of solvation layers.12,18 The results add to the increasing realization of the importance of water in confined hydrocarbons and ionic liquids,40,41 especially for proposed applications using electric field effects at surfaces to control lubrication and flow. For hydrocarbon systems, several experiments can be suggested to build on the preliminary AFM results above. It is known that long chain alkanes, as used in this study, are strongly bound to HOPG and hence it may be very difficult to change the molecular response using an applied electric field. Larger changes in molecular order should be observed with the use of both shorter chain molecules and different substrates which allow easier motion of the liquid molecules, e.g., metals, silicon. Finally, using noncontact AFM methods one could measure the tip−sample energy dissipation in force curves39 and observe how the effective viscosity changes with electric field. Such data could be tied directly to the measurements of Luo and coworkers,18 thus linking high spatial resolution AFM insight with more application orientated ball-on-disk results.

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AUTHOR INFORMATION

Corresponding Author

*(S.J.O.) E-mail: [email protected]. ORCID

Sean J. O’Shea: 0000-0003-0385-6245 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

E.J.H.S. was funded through a National Science Scholarship awarded by the Agency for Science, Technology, and Research (A*STAR), Singapore.

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