Nanoconfined ionic liquids under electric fieldsGuoxin Xie, Jianbin Luo, Dan Guo, and Shuhai Liu Citation: Applied Physics Letters 96, 043112 (2010); doi: 10.1063/1.3292213 View online: http://dx.doi.org/10.1063/1.3292213 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/96/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Ionic liquids as oxidic media for electron transfer studies J. Chem. Phys. 136, 244501 (2012); 10.1063/1.4729306 Nuclear magnetic resonance studies on the rotational and translational motions of ionic liquids composed of1-ethyl-3-methylimidazolium cation and bis(trifluoromethanesulfonyl)amide and bis(fluorosulfonyl)amideanions and their binary systems including lithium salts J. Chem. Phys. 135, 084505 (2011); 10.1063/1.3625923 Communication: X-ray scattering from ionic liquids with pyrrolidinium cations J. Chem. Phys. 134, 121101 (2011); 10.1063/1.3569131 Erratum: “Dip-coated films of volatile liquids” [Phys. Fluids 14, 1154 (2002)] Phys. Fluids 14, 2026 (2002); 10.1063/1.1476916 Dip-coated films of volatile liquids Phys. Fluids 14, 1154 (2002); 10.1063/1.1449467

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Nanoconfined ionic liquids under electric fieldsGuoxin Xie,a� Jianbin Luo,a� Dan Guo, and Shuhai LiuState Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China

�Received 9 December 2009; accepted 21 December 2009; published online 27 January 2010�

The effect of external electric fields �EEFs� on ionic liquid films confined within a nanogap has beeninvestigated by measuring the film thickness with the thin film interferometry and calculating theeffective viscosity. Experimental results indicated that the film thickness of ionic liquids could beincreased obviously by the application of EEFs with strengths weaker than the electric interactionsbetween cationic head groups and anions. The effect of EEFs on the confined ionic liquid film witha shorter alkyl side chain is more noticeable. It is thought that the charged anions and headgroupsof the cations are structured near electrified walls to form ordered layers and short alkyl side chainsat the interfaces are aligned along the EEF direction due to induced dipoles. © 2010 AmericanInstitute of Physics. �doi:10.1063/1.3292213�

Ionic liquids are recently-developed smart materialsshowing many distinct physicochemical properties fromcommon organic liquids.1 The application of external electricfields �EEFs� on ionic liquids has attracted increasing atten-tion in many fields such as field effect transistors,2 andelectrowetting.3,4 Hence, the microscopic view of the struc-tural properties of ionic liquids in nonequilibrium states be-comes important.5,6 It was suggested by simulation that thestructure of bulk ionic liquids could experience from spa-tially heterogeneous to spatially homogeneous, and then tonematiclike reordering when EEFs were applied and in-creased to critical values.7,8

On the other side, the understanding of the structures anddynamics of confined ionic liquids is very important to theirapplications in such areas as solar cells9 and lubrication innarrow conjunctions.10,11 Theoretical results reveal that ionicliquids narrowly confined between two flat parallel surfaceswould result in a structure of layers parallel to the surfaces.12

It has been also demonstrated that phase change from liquidto high melting point perfect crystal occurred in ionic liquidsconfined in a carbon nanotube.13 Then, it is of great interestto ask whether the layers of ionic liquids near confined sur-faces could be increased after the application of EEFs. Froma tribological point of view, it is also of practical importanceto quest for the possibility that the structured regions nearsolid surfaces in the lubricated region can be expanded byEEFs, and then better lubrication properties could be reason-ably expected. However, the work on this point has beenrarely done.

In the present letter, the thickness of confined ionic liq-uid films under EEFs is detected by applying the relativeoptical interference intensity method which has been provenbeing an effective tool for investigating the confined liquidfilm at the nanoscale. The scheme of the experimental setupcan be seen in Ref. 14. Detailed measuring mechanism of thefilm thickness can be found in Refs. 15 and 16. During theexperiments, external voltages Ue were varied to get variousEEF strengths Efilm on the liquid film.17 The experimentswere conducted at a temperature of 25�1 °C.

Three representative ionic liquids �1-butyl-3-methy-limidazolium hexafluorophosphate ��BMIm�PF6�, 1-hexyl-3-methylimidazolium hexafluorophosphate ��HMIm�PF6�, and1-octyl-3-methylimidazolium hexafluorophosphate��OMIm�PF6�� �Purity�99%� with a fixed anionic structureand cationic backbone with different alkyl side chain lengthshave been selected. Some physical properties of these ionicliquids are summarized in Table I. In addition, three siliconoils with different viscosities �192, 450, and 718 mPa s�25 °C�� were used for comparison.

The variations of the thickness of �BMIm�PF6 film withspeed under no EEF and EEFs of 150 and 300 MV/m areshown in Fig. 1�a�. It can be found that the film thickness of�BMIm�PF6 under no EEF in the central contact region islarger than that of the silicon oil with a close viscosity of 192mPa·s. The film thickness of �BMIm�PF6 under no EEF be-tween two surfaces with relative motions could be predictedby using the Hamrock–Dowson �H–D� equation19 �as indi-cated by the solid line in Fig. 1�a��

Hc� = 2.69

G�0.53u�0.67

W�0.067 �1 − 0.61e−0.73k� , �1�

where Hc�=hc /R, G�=�E�, u�=�0u /E�R, W�=W /E�R2, hc is

the film thickness, R is the radius of the ball, � is thepressure-viscosity coefficient of the ionic liquid, �0 is thebulk viscosity of the ionic liquid, u is the rolling speed, W isthe pressure, and E� is the reduced Young’s modulus of the

a�Authors to whom correspondence should be addressed. Electronic ad-dresses: luojb@tsinghua.edu.cn and xie-gx@163.com.

TABLE I. Some physical parameters of the ionic liquids �25 °C�.

aReference 18.

APPLIED PHYSICS LETTERS 96, 043112 �2010�

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two contacting solids defined by E�=2��1−v12� /E1+ �1

−v22� /E2� where vi is Poisson’s ratio for material i, Ei is the

elasticity modulus of material i. k is a coefficient ��1�.When a liquid is confined in a narrow gap, the effectiveviscosity is enhanced compared to the bulk due to the pres-ence of layered structures near solid surfaces.20 Hence, theeffective viscosity is important to determine the film thick-ness of a confined liquid, and the normalized effective vis-cosity by the bulk can be obtained by rewriting Eq. �1� as

�ef f/�0 �E�R

u�0�0.52Hc

�W�0.067

G�0.53 �1.49

. �2�

The measured film thickness hc can be used to calculatethe effective viscosity �ef f by using Eq. �2�, and the obtainedresult of the effective viscosity of �BMIm�PF6 versus filmthickness is shown in Fig. 1�b�.

At lower speeds, the measured film thickness withoutEEFs is larger than that predicted with the H–D equation.With the increase of speed to a critical point of about 20mm/s, the film thickness tends to agree with the calculatedone, as seen in the inset figure of Fig. 1�a�. The effectiveviscosity of the confined ionic liquid in Fig. 1�b� is greaterthan the bulk one at low speeds. When an EEF of 150 MV/mwas applied, the film thickness increases slightly by a maxi-mum value of about 5 nm. The increase of the film thicknessbecomes more significant when the EEF was raised to 300MV/m, and a maximum increase by about 16 nm can beseen. The effective viscosity increases more obviously underEEF of 300 MV/m compared to that without EEFs with thedecrease of the film thickness. An increase of the effectiveviscosity by nearly threefold can be observed when the filmthickness is about 30 nm.

Such an increase in the effective viscosity of a lubricantfilm with the thickness decreasing to a critical point is char-acteristics of thin film lubrication �TFL�. It is a lubricationregime which is different from elastohydrodynamic lubrica-tion and boundary lubrication.15 The electrostatic interactionsbetween the ionic liquid and solid metal surfaces becomeimportant to determine the film properties in the TFL regime.Generally, one or two ordered ionic liquid layers should bepresent near confined solid surfaces since positive chargesare easily formed during sliding11 when no EEF is applied.Although we were not able to apply an EEF of as high as

�104 MV /m at which most of bulk ions were thought to bealigned along the direction of the EEF with simulation,7 rea-sonable increase in the film thickness and the effective vis-cosity of confined ionic liquids under EEFs of �100 MV /mhave been observed. It is primarily due to the fact that elec-tric charges on solid surfaces after the application of EEFsbuild up more intensively to produce stronger surface elec-tric fields, inducing more ions to be attracted to form athicker electrical double layer. Besides, the alkyl side chaincan be polarized and oriented. In contrast, the alignment ofthe alkyl chain is the main cause for the enhancement of theeffective viscosity for pure organic oils without ions.16 Withthe increasing of the film thickness, the intermediate part ofthe ionic liquid away from the solid surfaces tends to show adisorder equilibrium structure, and the effective viscositygradually approaches the bulk one, as shown in Fig. 1�b�.

The enhancement in the film thickness by EEFs can bealso seen in the other two ionic liquids ��HMIm�PF6 and�OMIm�PF6� with longer alkyl side chains, as shown in Fig.2. In the case of �HMIm�PF6, the film thickness can be mod-erately increased by 5–10 nm under EEFs compared to thatwithout EEFs, and a double increase in the effective viscos-ity can be seen when the film thickness is about 25 nm, asshown in Fig. 2�a�. In the case of �OMIm�PF6, when an EEFof 240 MV/m was applied, the film thickness increasesslightly by a maximum value of about 4 nm. The increase ofthe film thickness becomes more significant when the EEFwas raised to 480 MV/m and then to 960 MV/m, and amaximum increase by about 8 nm can be seen under the EEFof 960 MV/m, as shown in Fig. 2�b�.

A close observation of Fig. 1�b� and the inset figures inFig. 2 reveals that the effect of EEFs on the change of theeffective viscosity is most significant for �BMIm�PF6 and theleast for �OMIm�PF6 at small gap distances. The normalizedeffective viscosity of around 2.0 is present at a film thicknessof about 40 nm under the EEF of 300 MV/m for�BMIm�PF6, at 25–30 nm under the EEF of 800 MV/m for�HMIm�PF6, and at 15–20 nm under the EEF of 960 MV/mfor �OMIm�PF6.

How to explain the difference in the EEF effect on thefilm forming property and the effective viscosity when thealkyl side chain length was changed? Simulation shows thatcations near confined solid surfaces orient tilted with respect

FIG. 1. �Color online� �a� Film thickness vs rolling speed of �BMIm�PF6 under no EEF and EEFs with different strengths. Inset figure is the log-log form ofrelationship between the film thickness and the rolling speed; �b� the normalized effective viscosity �Eq. �2�� against the film thickness under no EEF and EEFswith different strengths. The applied pressure was 0.53 GPa.

043112-2 Xie et al. Appl. Phys. Lett. 96, 043112 �2010�

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to the surfaces.12 The nonpolar tail group of an alkyl sidechain is with little charge, especially for that with a longlength21 when no EEF was applied. It was also suggestedthat the nonpolar alkyl cationic tail groups of ionic liquidscan aggregate and form nanoscale spatial heterogeneity un-der an EEF weaker than the electric interactions betweencationic head groups and anions ��1000 MV /m�.8 In thecase of confinement, the tail group near the electrified solidsurfaces may be an effective nonzero partial charge when theside chain is short due to electric polarization by EEFs. Then,the tail group with a shorter side chain has a larger probabil-ity to be aligned along the EEF direction, as schematicallyshown in Fig. 3, giving rise to increases in film thickness andeffective viscosity.

In conclusion, the film thickness of confined ionic liq-uids under EEFs makes clear that the interfacial ionic liquidfilm possesses a relatively large effective viscosity so that thethicker film is formed at low rolling speed. The “structured”layers of ions could be increased near electrified solid sur-faces. Shorter side alkyl chains at the interfaces have a larger

probability to be aligned along the EEF direction due to di-electric polarization, resulting in its film forming propertyand effective viscosity at low speeds having a slightly stron-ger response to EEFs.

The work is financially supported by the National Natu-ral Science Foundation of China �Grant Nos. 50721004 and50823003�, the International Science &Technology Coopera-tion Project, and the National Key Basic Research Programof China �Grant No. 2007CB607604�.

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FIG. 2. �Color online� Film thickness vs speed of �HMIm�PF6 �a� and �OMIm�PF6 �b� under no EEF and EEFs with different strengths. Inset figure is thenormalized effective viscosity �Eq. �2�� against film thickness under no EEF and EEFs with different strengths. The applied load was 0.53 GPa.

FIG. 3. �Color online� Schematic diagrams of ionic liquids with shorteralkyl chains �a� and longer alkyl chains �b� confined in narrow gaps withoutEEFs �left� and with EEFs �right�.

043112-3 Xie et al. Appl. Phys. Lett. 96, 043112 �2010�

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