Who’s in charge? Electro-responsive QCM Studies of Ionic Liquid as an

Additive in Lubricant Oils Degree Project in Molecular Science and Engineering

Author:

Erik BERGENDAL

Supervisors:

Nicklas HJALMARSSON

Prof. Mark RUTLAND

Examiner:

Prof. Mark RUTLAND

KTH ROYAL INSTITUTE OF TECHNOLOGY

i

Abstract Electrochemical quartz crystal microbalance has been employed to investigate electro-responsiveness

of an ionic liquid as an additive in lubricant oils on a gold surface. Polarisation of the surface reveals

changes in frequency where an increase in magnitude amplified the observed response, corresponding

to a controllable alternation of the ionic liquid configuration on the surface as a function of applied

potential. The frequency changes are due to different packing of the anion and cation, respectively, on

the surface as their mass densities and geometries are different. Relaxation of the system was

reversible to the application of a potential and it was also found to be diffusion dependent, where the

ratio between the ion diffusivities could be extracted from the results. Measurement of the system

relaxation reveals a potential decay of that of a discharging capacitor, with an internal resistance

inducing an initial potential drop due to the resistivity of the oil medium. The discharge behaviour was

also proven to show high internal reproducibility validity within experiments. This newly discovered

insight in responsive differences of ion packing is of importance, not only for ionic liquid additives in

tribology, but for understanding and exploiting ionic liquids in an array of electrochemical

applications.

ii

Sammanfattning En kvartskristallmikrovåg (engelska: quartz crystal microbalance), modifierad för att utföra

elektrokemiska experiment, har använts för att undersöka hur en jonvätska, som additiv i två olika

smörjmedel, svarar mot elektrisk påverkan av en guldyta. Polarisering av ytan inducerade

frekvensändringar vars utslag ökade med stigande applicerad spänning. Detta indikerar hur

förändringar i jonvätskans ytsammansättning kan justeras med varierad pålagd spänning.

Frekvensändringarna beror på olika packningsbeteenden för an- och katjonen på guldytan, grundat i

deras inbördes avvikande densitet och geometri. Systemets återgång till normaltillstånd var reversibel

jämfört med förändringarna observerade vid pålagt spänning. Denna relaxation visade sig vara

diffusionsberoende; förhållandet mellan an- och katjonens diffusionsförmåga kunde beräknas från

resultaten. Systemets restpotential vid avstängning visade en minskande potential över tid likt

urladdningen av en kondensator med en inre resistans som påtvingade ett omedelbart potentialfall på

grund av oljans resistivitet. En hög intern validitet inom experimenten kunde även påvisas för

potentialens urladdningsbeteende. De nyligen funna upptäckterna beträffande skillnader i packning av

joner svarande mot polarisation är av intresse, inte bara för jonväskor som additiv inom tribologi, men

även för en mer fundamental förståelse för jonvätskor och dess applikationer inom elektrokemi.

iii

Acknowledgements I would like to sincerely thank my supervisor Prof. Mark Rutland for giving me the opportunity to

write my thesis under his supervision as well as for his inspiring lectures during my time at KTH. I

would also like to extend my gratitude to Nicklas Hjalmarsson for his supervision and countless

interesting discussions.

Thank you Daniel Wallinder and Teodor Aastrup at Attana AB for all the help with the QCM.

Lastly, I would like to thank everyone at the division of Surface and Corrosion Science for a warm

working environment.

Stockholm, February 2015

Erik Bergendal

iv

Table of contents Abstract .................................................................................................................................................... i

Sammanfattning....................................................................................................................................... ii

Acknowledgements ................................................................................................................................ iii

Table of contents .................................................................................................................................... iv

Introduction ............................................................................................................................................. 1

Background ......................................................................................................................................... 1

Ionic Liquids........................................................................................................................................ 1

Overview ......................................................................................................................................... 1

Classification ................................................................................................................................... 2

General and electrochemical properties .......................................................................................... 3

Applications ..................................................................................................................................... 5

Quartz Crystal Microbalance (QCM) .................................................................................................. 7

Addition of electrodes ..................................................................................................................... 8

Karl-Fischer titration ........................................................................................................................... 8

Experimental section ............................................................................................................................... 9

Materials .............................................................................................................................................. 9

QCM .................................................................................................................................................... 9

Attana .............................................................................................................................................. 9

Q-Sense ......................................................................................................................................... 10

Cleaning procedure ............................................................................................................................ 11

Attana QCM .................................................................................................................................. 11

Q-Sense QCM ............................................................................................................................... 11

Results and discussion ........................................................................................................................... 12

Mass change as a function of applied potential ................................................................................. 12

OCP decay and capacitance .............................................................................................................. 20

Conclusions ........................................................................................................................................... 23

Future work ........................................................................................................................................... 23

Appendix ............................................................................................................................................... 24

References ............................................................................................................................................. 25

1

Introduction

Background As early as 1914 the German chemist Paul Walden reported on the synthesis of an ionic liquid (IL);

ethylammonium nitrate (EAN): a salt that, without any solvent, was liquid at room temperature [1].

This was one of the first reported ILs but it is only in the last three decades that research on ILs has

advanced and the number of publications has grown rapidly [2]. Historically, IL research focused on

organic synthesis and electrochemistry but is today an interdisciplinary field with interest ranging

from engineering and green chemistry to physics and biology [2, 3].

Be it in large scale factories or in gardening equipment for everyday life, parts moving in relative

motion usually need some kind of lubrication to reduce friction and mechanical wear. Striving for

reduced fuel consumption and material loss, looking towards the field of tribology should be an

integral part of environmental investments [4]. Even though advancements in IL research has proven

the molten salts to show great potential as lubricants their still high costs prevents their use in a larger

scale [5]. When looking towards using ILs as additives in already applied lubricant mineral oils, it is

important to understand their behaviour at surfaces in order to develop, choose from, and tweak this

versatile group of chemicals.

Ionic Liquids

Overview An IL is a salt with a melting point below 100 °C, while liquid salts at higher temperatures are denoted

molten salts. Sometimes the distinction between room temperature ionic liquids (RTIL) with Tm < 25

°C and ILs is made [2]. However, as this distinction is not fully established, and might cause confusion

[6], only the term IL (as defined above) will be used in this study, and if relevant, actual melting

temperatures will be presented. ILs usually consist of a bulky, organic cation and an inorganic anion

(see Figure 1). The bulkiness of the cation makes structuring into a crystal lattice difficult, which is in

contrast to spherical ions, such as Na+ and Cl- in NaCl that orders well into a crystal lattice. The non-

uniform charge density on the cation also contributes in destabilising the crystal formation. Together,

these properties make ILs liquid at room temperature [2, 6, 7].

2

Figure 1 Some common ions used in ILs. Reproduced from ref [4].

The well-known maxim “like dissolves like” is put upside down for ILs since they can incorporate

both polar and non-polar elements, as well as other ILs. This makes them into impressive tuneable

solvents [8-10]. The tunability, in addition to their very low vapour pressure [11, 12] and thermal

stability [13], increases the interest of using them as solvents in green chemistry. That said, one should

be careful about generalising ILs as green solvents, since some are toxic [14, 15], hazardous [16] or in

other ways not living up to the requirements of being green solvents [17, 18].

Classification Classification of ILs is no easy task, however, a general division between protic and aprotic ILs (PILs

and AILs, respectively) can be made. Protic, or proton donating ILs, are formed through an acid-base

reaction between a Brønsted acid and a Brønsted base [19]. This simple production method allows for

cheap production of PILs free of by-products. AILs are generally more cumbersome to synthesise;

requiring breaking and formation of covalent bonds through multistep reactions [20, 21]. They are thus

seldom completely by-product-free and often contain traces of halides from reaction precursors.

However, AILs are thermally and electrochemically more stable than their protic counterparts because

of their formation through covalent bonding of functional groups [2]. A few other subclasses of ILs

3

have been mentioned in the literature [2], such as ILs with a chiral centre, a paramagnetic atom/group,

and polymeric ILs. Polymeric ILs could be used as polymer electrolytes in electrochemical

applications and as building blocks in material science [22]. Magnetic ionic liquids can also find

applications in the fields of electrochemistry and material science [23].

General and electrochemical properties Charge transport, and the ability to withstand applied potential without decomposing are important

parameters when designing solvents for electrochemical applications. ILs differentiate themselves in a

number of aspects from expected behaviour (that of molecular solvents). Many ILs have been reported

to deviate from the Nernst-Einstein relation regarding diffusion and from Arrhenius behaviour

regarding kinetic coefficients [24-27]. A recent review [6] highlighted how small anions of ILs have

lower mobility compared to their much larger cation counter-ions [24, 28]. A suggested explanation

for this is stronger solvation of the anions by their counter-ions than vice versa [29]. IL viscosity does,

in general, decrease with increased ion size and temperature [30].

Conductivity The low conductivity in ILs can be attributed to their high viscosity and low ion mobility.

Conductivities are usually in the range of 0.1-18 mS cm-1, whereas it typically lies between 200-800

mS cm-1 for conventional aqueous electrolytes used in electrochemistry [31, 32]. Increased

temperature greatly improves conductivity in ILs [24, 33]. For example, [EMIM][BF4] increases its

conductivity from ~16 mS cm-1 at 25 °C to 70 mS cm-1 at 100 °C [34]. Adding Li+ to an IL might be of

interest in electrochemical applications because of the high reduction potential in relation to mass of

the lithium ion, however, this addition typically increases the solution viscosity and lowers its

conductivity [35]. Water can be added to increase the IL conductivity [25] but it will reduce the

electrochemical stability of the solution [36]. More electrochemically stable organic solvents could

also be added to increase conductivity and decrease viscosity [37]. Two examples are acetonitrile and

propylene carbonate, they are, however, volatile and not thermally stable. This is discussed more in

the section Energy applications.

Electrochemical stability As higher demand is put on batteries and as solar power advances, an electrochemically stable solvent

(a solvent that has a large potential-gap between its oxidation and reduction potentials) is needed for

electrochemical applications. This range of stability is referred to as the electrochemical window (EW)

and one of the main advantages of ILs is their large (typically 2-4.5 V, but measured up to 6 V) EW

compared to regular organic solvents [6, 38]. The range of electrochemical stability often becomes

narrower as other molecules are added to a neat IL. It has been suggested that ILs could be

catalytically oxidized or reduced by these additives and that this was the cause of the EW reduction

[39]. Small amounts of water, that could be absorbed from the atmosphere, will considerably decrease

4

the EW [36]. Somewhat logical, but not intuitively obvious, is that the EW of an IL not only depends

on the constituting ions but also on the particular combination of ions [40, 41].

The electrical double layer (EDL) The surface behaviour of a charged surface in an electrolyte was described by Helmholtz as a layer of

immobilized counter ions cancelling a naturally or artificially occurring charge of a surface.

Considering Brownian motion, Chapman and Gouy suggested that the charged layer cancelling the

surface charge must instead decrease over a larger distance. German physicist Stern, could not find

experimental evidence for either the Gouy-Chapman theory or for the Helmholtz model alone. Stern

therefore suggested a combination of the both, a small layer of tightly bound counter ions at the

surface (Helmholtz layer) and a diffuse layer (Gouy layer) to neutralise excess charge. In calculations

and simulation for moderate electrode polarisations, the Gouy-Chapman (GCh) theory is often used

[6]. However, for ILs this model has been proven to not work well [42], one of the main reasons being

its neglect of the finite size of ions, an assumption that plays an important role for ILs, even at

potentials below that of electrochemical reactions; where it otherwise only would manifest itself in

dilute electrolytes [6, 43]. The recent review “Ionic Liquids at Electrified Interfaces” [6] goes in-depth

into models and formulae to quantitatively evaluate the EDL. Here, only a model for ILs in the closest

proximity to charged surfaces will be explained with the introduction of the terms “overscreening” and

“crowding” [43-45]. Figure 2a) shows the effect of overscreening: when a surface with low charge

attracts a layer of counterions that overcompensate for the charge of the surface and overscreens the

surface with excess charge. This overcompensation results in an excess of co-ions in the next layer,

with a total absolute charge greater than the absolute difference in charge between the first layer and

the surface. Eventually this overcompensation decays and the EDL-structure of the IL fades into its

bulk properties. When the surface is highly charged (as shown in Figure 2b), the first layer of

counterions is not enough to neutralize the charge on the surface. This leads to a crowding of

counterions in the second layer of ions to cancel out the charge of the surface.

Figure 2 A schematic illustration of overscreening and crowding at low and high surface charge respectively.

Bulk structure Ionic liquid bulk structure was initially thought to be similar to that of a molecular solvent: an ordered

solid crystal structure that melts into a disorganised, random liquid [2]. However, this is not the case as

5

ILs usually has a liquid quasi- or pseudo-structure [46, 47], related to a collapsed crystal structure.

One might think that a more local structure of only two oppositely charged ions is reasonable, since

many ILs have been shown to evaporate in pairs [48, 49]. However, a lot of controversy surrounds the

subject of viewing ILs as either dilute electrolytes with ion pairs and few “free” ions, or as more

concentrated electrolytes of mostly “free” ions.

A study of bulky cations, trapping their smaller anion counterparts, has suggested that this leads to a

larger degree of ion-pairing [50]. Another study, conducted with Fourier Transform IR (FTIR) and

density functional theory (DFT) calculations on imidazolium and a fluoro-substituted sulfimide, also

recognised the existence of ion pairs in neat ILs [51]. It has also been claimed that ions in ILs are only

dissociated to 0.1% and thus behave as dilute electrolytes [52]. This claim was stated as debunked in

association with simulations, favouring the view of ILs as concentrated ionic melts [53].

Networks of hydrogen-bonding in bulk PIL (EAN, PAN [propylammonium nitrate], DMAN

[dimethylammonium nitrate]) was suggested in 1981 and was generally accepted until convincingly

demonstrated in 2009 [54]. AILs with chiral imidazolium cations can form H-bond networks in the

same way as PILs can [55, 56]. Large, positively charged ion clusters have been proven to exist in

PILs like EAN and PAN where the most abundant was C8A7+ (C=cation, A= anion).

Applications

Tribology Owing to their low vapour pressure, non-flammability, and thermal stability, ILs has since the first

publication on the subject at beginning of the millennium received considerable attention in the field

of tribology [57, 58]. Neat ILs have been shown to increase lubricity and wear resistance compared to

regularly used mineral oils [58]. However, employing neat ILs as lubricants in large industrial

applications would be very expensive. ILs have been added to mineral oils to successfully, due to their

affinity for surfaces, increase lubricity and wear resistance [59-61].

Altering surface potential of an Au(111) surface has been used to tune lubricity of a series of neat

imidazolium ILs [62].

Organic synthesis ILs have been used as “green solvents” (see above) in both organic synthesis and catalysis [8, 63],

with varying results in terms of yields, products/by-products, and reactant solubility, when compared

with molecular solvents or other ILs. Not surprisingly, some ILs are not always suitable as solvents as

they react with dissolved reactants [64].

Medical research After the Gordon Research conference in 2014 (Ionic Liquids: Solvents, Materials, or Medicines?) the

view on ILs changed in medical research. What was previously seen as toxic and possibly bio-

accumulating is now regarded as a potential tool in the field [2]. To increase thermal stability,

6

dissolution rate, and solubility (bioavailability), around fifty perfect of the currently administered

drugs are salts [65], often with the active substance as a cation and a counter-ion as sulphate,

phosphate, or of the like. If this inert counter-ion could instead be replaced by an active one, a new

field of specifically tailored drugs would be within reach. The counter-ion could be chosen to change

for example thermal stability, solubility, or bioavailability. One could in principle choose a counter-

ion to enhance the properties or quench the side-effects of its counterpart [65].

Energy applications Given their low vapour pressure, high thermal stability, and conductivity, the interest for ILs in

electrochemical advices has steadily increased. These properties make ILs attractive from both a

technical point of view and a safety perspective [66-69]. To allow for higher working temperatures

and thus higher efficiencies in intermediate-temperature polymer electrolyte fuel cells (PEFC), the

possibility to use PILs as solvents instead of water has been investigated. Proton conductivity and fuel

cell activity was reported to be similar between a normal hydrated acidic Nafion® membrane and an

IL-soaked Nafion® membrane. The apK (the difference in dissociation constants between the

Brønsted acid and base) for the PILs was seen to be important for the thermal stability of the IL and

for the open circuit potential (OCP) of fuel cells. Proton conductivity appears as either diffusion of a

single charge carrier through the solution, or as a proton-hopping (Grotthus) mechanism, where the

charge (proton) is donated from molecule to molecule through the solution but the molecules

themselves stay relatively motionless [69, 70].

In the battery field, specifically Li-ion batteries, ILs are also seen as a potentially potent solvent due to

their large EWs. However, the cathodic instability of the very common imidazolium ion versus a

Li+/Li0-electrode is impeding this process and prevents the use of both carbon and lithium metal

electrodes [66, 68]. Some quaternary ammonium ILs, with the most promising one being N-Methyl-N-

propylpiperidinium, has been shown to be resistant to reduction at a lithium metal cathode [67].

Another important aspect, hindering the development, is still the high price of ILs [68]. With

continued growing demand and large scale production, one could expect to see a continued decline in

prices. For those inclined to survey this further, ref. [71] contains an extensive table of tested ILs as

electrolytes for different anode/cathode pair in Li-ion batteries at the time (2009).

The use of ILs as electrolytes in dye-sensitized solar cells (DSSC) was summarised in a review from

2008 [72]. As in other electrochemical devices, ILs could offer a thermally stable, non-flammable,

highly conductive electrolyte with low vapour pressure and large EW compared to the presently used

volatile organic solvents (such as acetonitrile and propylene carbonate) [37, 73, 74]. One of the main

problems concerning the use of IL in DSSC is their inherently high viscosity; this leads to mass

transport limitations of the commonly used redox couple I-/I3- at low temperatures [6, 75]. On the other

hand, the importance of the possible long term stability when employing IL electrolytes, has been

7

stressed [72, 73], as this is one of the most limiting factors for industrialisation of DSSC. It has also

been concluded that ILs play an important role in gel-like or quasi-solid electrolytes [72].

Quartz Crystal Microbalance (QCM) Materials which crystallise into non-centrosymmetric space groups have the possibility to generate a

surface charge when exposed to mechanical stress. If the mechanical stress is applied across an

appropriate direction, the atomic displacements skew the dipoles in the acentric crystal lattice,

resulting in a change in net dipole in the entire crystal, which induces a charge on the crystal surface.

Materials possessing these traits, such as quartz, are called piezoelectric. The converse piezoelectric

effect (when an applied voltage over a material results in mechanical stress within a crystal) is the

basis of the QCM, where an alternating voltage induces an alternating mechanical deformation

(vibration) of the exposed crystal [76].

The main component of a QCM is a quartz crystal together with one electrode on either side of the

crystal. The oscillation of the crystal at applied voltage is partly dependent on the thickness of the

crystal [77]. Very small changes in the thickness of the crystal, and thus the crystal’s resonance

frequency, can be detected, which makes QCM an attractive instrument for measuring mass changes at

the nanoscale [78, 79]. To assign a mass to an observed change in resonance frequency, the Sauerbrey

equation is applied [80]:

2

02 f mf

A

(1)

where 0f is the resonance frequency, m the mass change, A the electrode area, the density of

quartz, and the shear wave velocity in quartz. This equation is only applicable when the adsorbed

mass is small in relation to the mass of the crystal. It is also assumed that the adsorbed specie forms an

even and rigid layer; seen as an extension of crystal thickness [78]. Sauerbrey’s equation will not be

valid for adsorbed viscoelastic bio-molecules adsorbing water [78]. However, it has been shown that

the mass-frequency relationship for such systems is kept linear and that the addition of a suitable

coefficient allows Sauerbrey’s equation to be kept valid [81]. It has later also been shown that the

Sauerbrey equation is valid even for non-rigid films as long as the films are thin enough [82].

If the power circuit, resonating the QCM crystal, is periodically disconnected, an exponential decay of

crystal oscillation can be measured. The characteristic of this decay is denoted as the dissipation in a

system and provides information about energy losses (damping) of the crystal to the surroundings

[83]. Dissipation is usually expressed as the viscoelastic properties of an adsorbed layer on the crystal

surface, where a low and high measured dissipation indicates a rigid and a more viscoelastic adsorbed

layer, respectively[84].

8

Addition of electrodes When doing electrochemical experiments in aqueous solution, a regular reference electrode such as

Ag/AgCl is employed. This is not convenient in experiments with ILs since they would easily be

contaminated by the water solution. Even though contamination by solvent would still occur, a

reference electrode in an aprotic solvent (for example Ag/Ag(I) in acetonitrile) could instead be

employed to avoid water and thus preserve the EW of the IL [85]. It would, however, be preferred to

use a reference electrode based on the IL of interest. Regardless of the media in which the reference

electrode is employed it is essential that its potential is stable with time and that it should return to the

starting potential after polarisation. Applying a metal wire as counter as well as reference electrode has

been common practice in voltammetric experiments in non-aqueous solvents, then calling it a pseudo-

or quasi reference electrode [21]. The potential of the electrode is then assumed to not change (or

return to its previous value) during the course of the experiment. To secure its actual potential the

electrode should be tested against a known redox couple such as the IUPAC suggested

ferroscene/ferroscenium (Cp2Fe/Cp2Fe+, Cp=C5H5 or Fc/Fc+) which is regarded to have a stable EW

regardless of solvent [85]. The use of a redox couple thus allows for reliable inter-experimental

comparisons.

Karl-Fischer titration Even though an organic solvent or an IL might be hydrophobic, it will still absorb a small amount of

water from the atmosphere if not stored specifically to avoid this. The amount of absorbed water in an

IL depends on the basicity of the anion; a more basic anion will increase the solubility of water [9, 86].

Karl-Fischer titration is commonly applied to measure traces of water in solvents. A so called Karl-

Fischer reagent quenches water accordingly

5 5 2 5 5 2 5 5 2 5 5 5 5 32C H N I C H N SO C H N H O C H N HI C H N SO (2)

furthermore the pyridine sulphur trioxide complex reacts quickly with methanol

5 5 3 3 5 5 4 3(H)SOC H N SO CH OH C H N CH (3)

to inhibit the consumption of water by the sulphur trioxide

5 5 3 2 5 5 4C H N SO H O C H N SO H (4)

[87]. This is important since the principle of coulometric Karl-Fischer titration relies on

electrochemical re-oxidation of I through an applied current. The required current for re-oxidation

corresponds to the amount of quenched water according to (4) i.e. two moles of electrons are

consumed for each mole of water.

9

Experimental section

Materials The IL and oils used were provided by Luleå Tekniska Universitet (LTU) and the Department of

Machine Design at KTH, respectively.

Figure 3 PBMB or Trihexyl(tetradecyl)phosphonium cation (left) and bis(mandelato)borate anion (right).

Figure 4 Oil 203 or 2-((hexanoyloxy)methyl)-2-methylpropane-1,3-diyl dioctanoate (left). Oil 211 or OSP-46, a polyalkylene

(PAG) random copolymer from propylene oxide (m) and butylene oxide (n). Figure reproduced from [88].

Karl-Fischer titration was used to determine the water content of the IL-oil solutions to around 600

and 3700 ppm for Oil 203 and Oil 211, respectively. Upon first visual inspection of the IL-oil

solutions, threadlike strings of IL were seen in the oils. To completely solvate the ILs in the Oils, the

solutions were sonicated for 15 min at 40 °C. The threadlike behaviour was then not observed and the

IL were regarded as solvated in the oils.

QCM

Attana One of the QCMs used was an Attana A100® from Attana AB (Stockholm, Sweden). The quartz

crystals, with an area of 15.9 mm2 and thickness of 150 nm, were sputtered with gold and were

provided by Attana AB. The QCM flow cell (multi part PTFE chips sealed with a PTFE o-ring and

metal screws) had to be rebuilt to allow for measuring of potentials. A 0.15 mm platinum wire was

included in the flow cell and acting as a pseudo reference electrode running across, but not in direct

contact with, the gold surface. Applied potentials were adjusted using a home-built potentiostat,

10

powered by one or two 9 V batteries, including several resistors and operational amplifiers to

minimise the current running through the cell. Potentials and OCP-decays were monitored with

FLUKE 3000 FC wireless multimeters. ILs were inserted via a syringe as no flow of liquid was used

in this QCM. Crystals were set to stabilise in the QCM overnight prior to experiments as this process

was slow, owing to the viscous nature of the ILs. An internal temperature regulator fixed the

temperature to 22.0 °C during experiments.

Each experiment started with the application of the lowest potential in a series of equal polarity. That

potential was then held constant for 5 minutes. At shut-off, the OCP, as well as the frequency change

was monitored for 10 minutes. Before the application of the next potential in the series, the OCP was

allowed to equilibrate close to its original value to ensure a correct value of the pseudo reference

electrode. After potentials in one series had been applied the polarity was changed and a new series

started with the lowest potential in that series.

Q-Sense The QCMD used was a Q-Sense E4 from Biolin Scientific AB. Crystals were gold plated AT-cut

crystals with an area of 69,1 mm2. The flow cell was a Q-Sense Flow module, QFM 401 with an

aluminium shell. Parts in contact with sample liquid and O-rings are titanium and Vitron®,

respectively.

It was of interest to investigate the affinity of PBMB to gold surfaces to elucidate if the IL was present

on the surface already before a potential was applied or not. If not, the IL would have to migrate to the

surface at applied potentials giving rise to density and viscosity changes near the gold surface,

affecting the observed QCM frequency. To settle this, a so called adsorption isotherm was performed.

An isotherm is, as the name indicates, performed at constant temperature and with varying pressure, or

concentration, which was the case here. Essentially, increased adsorption (i.e. frequency response in

QCM) is monitored against increased concentration of adsorbing substance. The characteristics of an

adsorption isotherm can also indicate the mode of adsorption.

An adsorption isotherm was only made with Oil 203, since its lower viscosity enabled an isotherm to

be performed at room temperature.

The flow rate over the cell was 5 µL/min and the base medium was Oil 203. Six IL-solutions of

increasing concentration (0.5, 1, 2, 3, 4, and 5 wt% in oil 203) made up the isotherm. After an

injection of IL-solution the frequency was allowed to stabilise, the system was then washed with neat

Oil 203 and the next injection of IL-solution took place only after the base line with neat Oil 203 was

once again stable at its original value.

11

Cleaning procedure

Attana QCM The plastic chip and electrodes were ultra-sonicated for 30 minutes in a 2 vol% solution of Deconex in

MilliQ (18.2 MΩ∙cm) water. Thereafter they were rinsed generously with MilliQ water, rinsed with

pure ethanol and dried under filtered nitrogen gas. The syringe used for inserting fluid into the chip

was rinsed 20 times with 2 vol% Deconex solution, 20 times with MilliQ water, and finally 20 times

with pure ethanol. It was then left to dry in laminar flow cabinet.

The crystal, O-ring, and platinum quasi reference electrode were left in a solution of 2 vol% Deconex

in MilliQ water for 30 minutes. They were thereafter rinsed generously with MilliQ water, rinsed with

pure ethanol and dried under filtered nitrogen gas.

Q-Sense QCM The metal cells, stored in 2 vol% Deconex solution were rinsed thoroughly with MilliQ water, rinsed

with pure ethanol and then left to dry in laminar flow cabinet. The tubing was run through (using a

syringe) 20 times with 2 vol% Deconex solution, 20 times with MilliQ water, and finally 10 times

with pure ethanol. The tubing was then dried under filtered nitrogen gas. O-rings and protective mats

were left in 2 vol% Deconex solution for 30 minutes, then thoroughly rinsed with MilliQ water and

pure ethanol before being dried under filtered nitrogen gas.

The crystals were put under high intensity UV-light for 20 minutes, then left in BIC (Chromosulphuric

acid: 2-5% Na2Cr2O7 in circa 90% H2SO4) for 3 minutes and thoroughly rinsed with MilliQ water, and

finally rinsed with pure ethanol before being stored in ethanol until use the same day.

12

Results and discussion

Mass change as a function of applied potential A QCM measures the change of resonance frequency of the quartz crystal. As previously mentioned,

this change in frequency does not necessarily result from a build-up of a film on the quartz surface, but

can also be a function of density and viscosity changes in the close vicinity of the crystal, as described

by (5) [83],

0

2f f

q q

ff

t

(5)

where 0f is the resonance frequency of the crystal, qt and q are the thickness and density of the

crystal,

respectively, and f and f are the density and viscosity of the fluid on the crystal,

respectively. This results in a frequency change versus vacuum. The dissipation varies as a function of

fluid density and viscosity accordingly [83]:

0

1f f

q q

Df t

(6)

It should be noted that throughout the report, positive frequency changes are defined to correspond to

a decrease in crystal resonance frequency (i.e. a supposed increase in mass or density and viscosity).

The exception to this is Figure 5 treating an adsorption isotherm done on a QCM-system where a

decrease in crystal frequency is represented by a negative frequency change.

The raw data from an adsorption isotherm, performed with Oil 203 as a base line, is presented in

Figure 5. At the injection of an IL-oil solution, the frequency decreases and the dissipation increases.

After each injection, when the frequency and dissipation were stabilised, the system was rinsed with

neat Oil 203; stabilising the frequency and dissipation at the baseline before another injection was

made. The frequency and dissipation changes increased with increased concentration IL in the injected

solutions as to be expected from equations (5) and (6), since a higher concentration of IL increases the

density and viscosity of the IL-oil solutions. When the IL-oil solution first comes in contact with the

sensor, there is a steep frequency and dissipation change which eventually levels out. A similar

behaviour is observed when the IL-oil solution is switched to neat Oil 203. These changes are similar

in appearance for the first three injections (0.5, 1, and 2 wt% PBMB) but then alter in character after

injection for the higher concentrations to a more gradual levelling out of the frequency and dissipation

changes.

13

After accounting for viscosity and density changes between neat Oil 203 and the injected samples,

according to measured values of the viscosity and using equation (5) above, the resulting frequency

changes corresponding to adsorbed mass were positive; indicating a mass loss on the sensor surface.

For this to hold true, a region of lower density than Oil 203 would have to occupy the surface upon

injection. As any combination of the injected sample with Oil 203 has higher density than neat Oil

203, a mass loss on the sensor surface cannot be a valid interpretation of the acquired data. Equation

(5) gives a frequency change between vacuum and an observed medium, on a perfectly flat sensor

surface. In order to approximate the frequency change between interchanging fluids, a linear

relationship between the frequency change and the square root of the changing density and viscosity is

assumed. Analysing a very high viscosity system, where miniscule changes in viscosity correspond to

significant frequency change, puts equation (5) at, or possibly even outside, the verge of its

applicability.

ILs have a tendency to order on surfaces [2, 89-92] and have also been shown adsorb to silica from oil

solution [61].

The sample distribution in a circular flow cell is far from instant considering flow velocity, convection

and diffusion of species on the solution. Simulations and experimental results presented in a study

from 2006 [93], on concentration profiles in a circular QCM-sensor showed responses corresponding

to what is observed in Figure 5. Although the time scale of the results presented here is several orders

of magnitude larger, the very low diffusion coefficient of the PBMB, 1.2x10-11 and 2.5x10-11 m2 s-1 for

5 wt% PBMB in Oil 203 and 211, respectively [94], the solution viscosity, and system flow rate

largely influences the flow profile. This could thus be a possible explanation for the reversible flow

profiles observed for the first three injections in Figure 5.

14

Figure 5 Raw data plot from one sensor of an adsorption isotherm with Oil 203 as base line, showing frequency and

dissipation responses for the third overtone as a function of time. Response peaks labelled from “a” to “f” and starting with

the lowest concentration 0.5, 1, 2, 3, 4, and 5 wt% PBMB.

Following the isotherm, the next step was to investigate what happens on the gold surface as a

potential is applied over the IL-oil solution. Figure 6 depicts the frequency response as a function of

time for ±1500 [mV] when the potential was applied (bold lines) as well as shut-off (thin lines) At

applied negative potentials the frequency response is negative and at applied positive potentials the

frequency response is positive. Applying potentials of both polarisations to neat Oil 203 and 211 did

not result in any change in frequency, leading to the conclusion that the oils can be considered inactive

to applied potentials and thus that the observed frequency changes stem from IL at the surface and in

the bulk responding to the applied potential. Furthermore, in Figure 6 there is a steep initial change in

frequency levelling out after 5 minutes, similar behavior is observed for both applied potentials as well

as in reverse after the potential was shut off.

a b

c

d

e

f

15

Figure 6 Oil 211 3wt% PBMB, frequency changes as a function of time after applied potentials (ON) and after shut-offs

(OFF) for -1500 [mV] and +1500 [mV]. The drift was very low and was not accounted for.

The frequency change monitored with the QCM was opposite when the potential was turned off. The

absolute response was, however, very similar indicating a reversible system on the sensor surface. The

rate of frequency change is also comparable between the applied potentials and shut-offs. Frequency

changes observed for Oil 211 and Oil 203 are summarised in Figure 7 and Figure 8, respectively,

which shows the correlation between adsorbed mass and applied potential. With an increase in applied

potential, of either polarisation, the frequency response is increased. The trend in frequency change for

3 and 5 wt% follow each other for negative applied potentials, which is especially clear for Oil 211.

There is a steeper increase in frequency change observed for positive potentials as compared to

negative for Oil 211. The reversibility of the observed frequency changes in Figure 7 and Figure 8 are

consistent with what is observed in Figure 6.

Frequency changes observed previously in a similar QCM system, where potentials were applied over

a neat IL, were hypothesised to depend on the mass difference of the ions composing the IL [95].

There, applying a negative potential in a neat IL system with a heavy anion and a lighter cation was

proposed to attract lighter cations to the surface, expelling the heavier anions and thus giving a

frequency response corresponding to a decrease in absolute mass on the surface. Such an interpretation

cannot be applied to the data collected in this work, presented in Figure 6 and Figure 7, and thus an

alternative explanation has been developed and will be expounded below. It was not possible to

conduct a control experiment with neat PBMB because its very large viscosity (7.18 [Pa s] at 20 °C)

prevented the quartz crystal from oscillating. A useful comparison would have been to perform these

-8

-6

-4

-2

0

2

4

6

8

-8

-6

-4

-2

0

2

4

6

8

0 50 100 150 200 250 300

freq

uen

cy c

han

ge [

Hz]

time [s]

-1500 [mV] ON -1500 [mV] OFF +1500 [mV] ON +1500 [mV] OFF

16

measurements using the IL employed in the previous study, however, this IL was found to not dissolve

in Oil 203 nor in Oil 211, most likely because of its very non-polar nature [95].

Figure 7 Oil 211 with varying wt% PBMB, frequency changes after applied potentials (ON) and after shut-offs (OFF). Each

data point represents the frequency change 100 seconds after applied potential or shut-off. After 100 seconds the major

frequency change was reached before the frequency to time dependence levelled out or perturbed due to drift.

-8

-6

-4

-2

0

2

4

6

8

-1500 -1250 -1000 -750 -500 -250 0 250 500 750 1000 1250 1500

freq

uen

cy c

han

ge [

Hz]

potential before shut-off [mV]

3 wt% ON 5 wt% ON 3 wt% OFF 5 wt% OFF

17

Figure 8 Oil 203 with varying wt% PBMB, frequency changes after applied potentials (ON) and after shut-offs (OFF). Each

data point represents the frequency change after 100 seconds after applied potential or shut-off. After 100 seconds the major

frequency change was reached before the frequency to time dependence levelled out or perturbed due to drift. Note that the

value for +250 [mV] at shut-off has been deleted because the capture of data at that potential failed.

Simulations shown in Figure 9 have shown that the anion in PBMB packs more densely than the

cation at charged interfaces. Calculations extracted from the simulations have provided estimated

densities and corresponding molar volumes for the species in the system [96]. Polarisation of a

surface crowded with IL would induce an expulsion of co-ions from the surface in order to maintain

charge neutrality in the adsorbed layer. Expelling ions from the surface would create a gap that would

need to be filled with either counter-ions, or charge-neutral ion pairs. Introducing a statistically equal

amount of cations and anions would conserve charge neutrality as well as the thickness of the

adsorbed layer. In the case of PBMB, the cation is larger than the anion and also has a lower density.

A positively polarised surface would thus expel cations and fill the void with a combination of cations

and anions answering to their presence in the bulk, and as this would introduce a certain amount of the

comparably dense anions, this would correspond to a mass increase observed on the QCM sensor. If

this explanation is correct, then its robustness would be demonstrated if it also explained the trends

observed in the previous QCM on the IL [EMIM][FAP] [95]. In fact, the relative densities of the

anion, cation, and bulk are such that an identical treatment will also explain the (opposite) mass trends

with potential observed in that study. The simulations presented in Figure 9 were conducted on neat IL

and not with IL as additives in the oils investigated in this work. Under the assumption that there is

mostly IL occupying the surface at applied potentials, these simulation should still give an indication

of the behaviour of the investigated IL-oil system. Since the exact amount of IL on the surface cannot

-4

-3

-2

-1

0

1

2

3

4

-1500 -1250 -1000 -750 -500 -250 0 250 500 750 1000 1250 1500

freq

uen

cy c

han

ge [

Hz]

potential before shut-off [mV]

3 wt% ON 5 wt% ON 3 wt% OFF 5 wt% OFF

18

be determined from the isotherms (Figure 5) it is difficult say whether the density difference argument

should be applied to an essentially pure layer of IL on the surfaces, or to a more concentrated solution

of IL in oil. In either case a density difference argument can be used.

Figure 9 Simulation of neat IL PBMB, where the purple spheres represent the charge density of the anion and the smaller

orange spheres represent the charge density of the anion. The top figure (A) shows the random distribution of the IL between

two uncharged surfaces. In the bottom picture (B) the blue surface (left) and the red surface (right) are accommodating a

surface charge of plus and minus 40 µC cm-2, respectively. Figure courtesy of Yonglei Wang, SU, unpublished results.

Frequency changes at higher positive potentials appear to increase steeply for experiments on 5 wt%

PBMB (Figure 7) compared to the results for 3 wt% PBMB. If significant, it is not clear how this

should be interpreted. A concern would be if this proved to be the signs of anodic breakdown.

Generally, Cyclic Voltammetry (CV) should be performed on a system prior to electrochemical

experiments to determine stability under applied potentials. A CV would provide a voltage span where

the system is inert to oxidative and reductive breakdown. It would, however, be difficult to apply to

this work as only a pseudo reference electrode was employed and its potential is predominantly

internally valid [21, 97]. Regarding breakdown due to faradic currents, it could be argued that this

would prevent, or somewhat skew the now observed reversibility of the system frequency change at

shut-off; a hypothesis that could be tested through experiments with applied potentials clearly outside

the electrochemical window of the IL, controlled with the presence of an internal reference electrode.

At shut-off, the accumulated, capacitive charge from the imbalance in adsorbed IL species at the

interface has to be dissipated to achieve charge neutrality. This displacement of mass is manifested in

A

B

19

the frequency change at shut-off. During the initial period after shut-off (up to one minute), the

frequency changes linearly with respect to t1/2 (inset of Figure 10), giving rise to the gradients (k-

values) presented in Figure 10. These k-values were calculated as the slope of the linear regions of

frequency change versus t1/2 for each applied potential. This frequency change can be attributed to

diffusion according to Fick’s law of diffusion [98] and the slope of the curve is related to the diffusion

coefficient. It should, however, be stressed that no absolute value for diffusion coefficients can be

drawn for this information without solving the diffusion equation for the complex geometry of the

system. Nevertheless, the relative slope of the different polarisations indicates the ratio between their

diffusion coefficients. This ratio is consistent with diffusion NMR data on PBMB in Oil 203 and Oil

211 performed by collaborators in the research program “I-LEAP” [94]. The rate of diffusion

increases with increasing potential in Figure 10, also strengthening the hypothesis that the observed

frequency changes are due to adsorbed mass on the surface, as a high concentration gradient (large

amount of IL on the surface) would increase the rate of diffusion. A similar t1/2-dependence and

increasing slopes for increasing potentials were also seen when potentials were applied. This process

must be due to migration of charged species to the polarised electrodes. The steep initial increase in

frequency leading to its t1/2-dependence is consistent with the assumption that IL is present on the

surface prior to the application of a potential, as attraction and expulsion of nearby anions would result

in a rapid frequency response. This increase decays over time as the induced surface charge of the

electrode is eventually cancelled out by migrated ions.

Figure 10 Oil 211 3 wt%, slopes of linear regions of frequency vs time1/2 at shut-off plotted against varying starting

potentials. Inset shows frequency change vs time1/2 for one potential.

20

OCP decay and capacitance In a previous study [95], the OCP decay of an IL system observed in QCM was characterised as a

capacitor during discharge according to the following relation

0

0

0

exptV

V VRQ

(7)

which was derived from

0 expt

V VRC

(8)

where the capacitance C relates to the initial charge density 0Q and potential

0V in a t manner. R

represents the resistance. Figure 11A shows how the OCP decays as a function of the square root of

time after shut-off for three different starting potentials. The dotted lines show exponential fits were

the corresponding R2-values are shown in the top right of the figure. This shows how well the decay

functions correspond to equation (7), which can be used to extract the capacitive charge of the gold

surface. A discontinuity from the equation (7) at t>0 is observed in Figure 11A, which is not consistent

with the discharge of an ideal capacitor. This non-ideal behaviour was not observed in the previous

study where a neat IL was used and the decay only depended on diffusion [95]. However,

electrochemical capacitors (also known as supercapacitors) are known to have an internal resistance

(IR-drop), displayed as an immediate voltage drop upon discharge. This drop in potential is due to the

conductivity of the electrolyte being lower than that of the added IL and thus increasing the resistance

of the capacitive current [99-102]. This is a non-trivial quantity governed by ion concentration and

diffusion coefficients for ion-ion and ion-solvent interactions [103]. Assigning the first points to this

internal resistance instead of intrinsic capacitance charge, and thus discarding them it is possible to

receive an excellent exponential fit for a capacitor discharge, as shown in Figure 11B. The potential

drops were similar between 3 and 5 wt% PBMB (see Fel! Hittar inte referenskälla.A and B), which

is to be expected from solutions with closely comparable bulk resistivity. With a known bulk

resistivity, a surface charge could be extracted from equation (7) and related to an applied voltage.

21

Figure 11 Oil 211 3 wt% PBMB, OCP decay as a function of time1/2. The left figure (A) includes the potential drop due to

internal resistance. The first data point from figure (A) has been subtracted to yield figure (B) on the right, showing an

exponential decay of potential as expected from a capacitor.

A B

3 wt%

PBMB

3 wt%

PBMB

A B

5 wt%

PBMB

5 wt%

PBMB

Figure 12 Oil 211 5 wt% PBMB, OCP decay as a function of time1/2. The left figure (A) includes the potential drop

due to internal resistance. The first data point from figure (A) has been subtracted to yield figure (B) on the right,

showing an exponential decay of potential as expected from a capacitor.

22

In order to investigate if the decay functions were coherent throughout the experiment, the potentials

at arbitrary time stamps after shut-off were plotted against the applied potentials before shut-off. For

two example time stamps, 30 and 240 seconds, this yields Figure 13 where the absolute value of the

potential decreases linearly with decreasing applied potential before shut-off. The slope of the lines are

notably steeper for the shorter time stamps. The excellent linear fits indicate a strong internal validity

within the experiment as the time constant is here proven constant throughout the experiment.

Figure 13 Oil 211 3 wt% PBMB, absolute potentials at an arbitrary time value plotted against starting potentials before

shut-off.

0

50

100

150

200

250

300

-1500 -1250 -1000 -750 -500 -250 0 250 500 750 1000 1250 1500

|po

ten

tial

| [m

V]

starting potential [mV]

t=30s neg t=30s pos

t=240s neg t=240s pos

23

Conclusions QCM can be used to study electro-responsiveness of ILs in oil. There is no response in the

absence of IL, revealing that oil (and any water) have no intrinsic effect.

The change in mass measured depends on the magnitude of the potential, allowing control

over the IL surface configuration.

A new understanding of the electro-response has been developed. Rather than a direct

exchange of mass as in the neat IL, the density of the species need to be considered and in this

case the mass exchange is opposite to what is expected from the primitive model.

The relaxation process after potential shut-off is diffusion limited, where both species could

contribute to this process. This capacitive, diffusion limited current indicates ion transport

through the lubricant oil.

Once again, the relative diffusion coefficients can be obtained from the asymmetry of the

frequency relaxation gradients.

The system cannot be treated as an ideal capacitor as there is an extra internal resistance

involved. Nonetheless, once this internal resistance is overcome, a capacitance decay can be

measured and used to estimate surface charge.

For highly viscous systems it is impossible to extract an adsorption isotherms without first

performing a viscosity calibration using known standards.

Future work The hypothesis suggesting IL preference to, and presence on uncharged gold surfaces needs to be

strengthened. IL layering could be detected with atomic force microscopy (AFM) which could provide

information about surface composition with and without surface polarisation. Leading into

measurements of friction force with AFM would also be a natural continuation of the project.

Employing advanced surface specific spectroscopic methods as sum frequency generation

spectroscopy (SFG) would give insight into the orientation of species on the surface.

Even though no QCM data could be acquired on neat PBMB, it would be of interest to observe the

OCP decay for this system. Furthermore an electrochemical study, including CV of the IL-oil system

with an internal reference electrode is necessary in order to provide means of strengthening external

validity in addition to relating the system to other electrochemical studies on ILs.

24

Appendix The observed potential decays at arbitrary time stamps, as previously presented in Figure 13, are

below presented in Figure 14A. Figure 14B presents a replicate of the experiment in Figure 14A,

where the only difference is the order of applied potentials. In the experiment presented in figure A, all

the negative polarisations were applied in succession, starting with -250 [mV], and after the potential

decay was observed for -1500 [mV], the positive potentials were applied, starting from +250 [mV]. In

the experiment presented in figure B, the potentials were applied in a similar manner as in figure A,

but positive polarisations initiated the experiment, starting with +250 [mV]. The decays presented in

figure B show an equally linear behaviour as in figure A, indicating a comparable internal validity of

the decay. There is, however, a significant difference in observed absolute potentials between the two

experiments, where the experiment presented in figure B shows much lower values, stemming from a

larger initial drop in potential at shut-off. This behaviour, connected to the order of applied potential,

was observed for replicates of Oil 203 as well as for Oil 211 with 5 wt% PBMB.

A feasible explanation for this is yet to be presented, having in mind that a hysteresis effect would be

assumed to influence at least one polarisation in each experiment.

Figure 14 Oil 211 3 wt% PBMB, absolute potentials at an arbitrary time value plotted against starting potentials before

shut-off. For the experiment presented in figure A, the negative polarisations were applied in succession followed by the

positive polarisations. The opposite order of applied polarisation was performed for the experiment presented in figure B;

starting with the positive polarisations.

A B

25

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