Wetting Films of Two Ionic Liquids: [C(8)mim][BF4] and [C(2)OHmim][BF(4)]

Full text

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Published:July 08, 2011
r2011 American Chemical Society
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pubs.acs.org/JPCC
WettingFilmsofTwoIonicLiquids:[C8mim][BF4]and[C2OHmim][BF4]
Jos? e Restolho,†,‡Jos? e L. Mata,†,§Karina Shimizu,†Jos? e N. Canongia Lopes,†and Benilde Saramago*,†
†Centro de Química Estrutural, Instituto Superior T? ecnico, T U Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
‡Research Institute for Medicines and Pharmaceutical Sciences, Faculty of Pharmacy, University of Lisbon,
Av. Prof. Gama Pinto, 1649-019 Lisbon, Portugal
§Academia Militar, Pac -o da Rainha, 29, 1150-244 Lisbon, Portugal
’INTRODUCTION
Thinliquidwettingfilmshavebeenunderinvestigationduring
the past decades because of their importance from both practical
and theoretical points of view.1,2A number of authors have
studied films of aqueous solutions of electrolytes and surfactants
with the objective of understanding the nature of forces within the
film.Liquidfilmsobtainedfromsolutionsofbiomoleculesdeserved
special attention because of their relevance in biomedical and
pharmacological applications. The problem of thin film stability
involving liquid crystals and lubricants has also been addressed.
Wetting films can provide information on the equilibrium
surface forces in the liquid film bounded by two different bulk
phases, a solid substrate and a gas phase. The force of interaction
of film interfaces equals the disjoining pressure acting within the
film. The disjoining pressure derives from several contributions,
namely, London dispersion forces, electrostatic double layer
forces, and structural forces. The relative importance of each
componentdependsonthenatureofthesystem.Mostinvestiga-
tions have been carried out with aqueous systems where the
dispersion forces are negligible in comparison with the electrical
double layer forces. However, for nonaqueous systems, the
dispersion forces are known to control the stability of the liquid
film. The confinement of a liquid between two walls induces a
molecular layering that is responsible for oscillatory forces
designated by structural forces. In a recent review, Boinovich3
summarized different theoretical approaches to explain the
nature of hydration, solvation, or structural repulsive forces as
well as hydrophobic attractive forces as a result of deviations of
various structural parameters of the thin interlayer from the
corresponding values in the bulk.
Room-temperature ionic liquids (RTILs) present a rare
opportunity to study the interplay among a wide range of
molecularinteractionssuchasCoulombic,vanderWaals,dipole?
dipole,hydrogen-bonding,andsolvationforces.Furthermore,they
are strongly promising materials in applied science because of the
possibilityoftuningtheirphysicochemicalpropertiesaccordingto
thepretendedapplication.Althoughthevastmajorityofprocesses
inapplicationsofRTILsinvolvesolid?liquidinterfaces,theamount
of research in this area is still scarce.
Previous studies were based on sum-frequency vibrational spec-
troscopy (SFVS),4X-ray reflectivity,5?7atomic force microscopy
(AFM),8?12andalsomolecularsimulations.13?15Thesestudieshad
as a common objective the determination of the structure of the
ionic liquid near the solid surface. To our knowledge, there are no
studies in the literature concerning the interactions in thin ionic
liquid films and the kinetics of thinning and rupture.
The objective of the present work was to investigate the stability
of wetting films of two well-known ionic liquids, 1-octyl-3-methy-
limidazolium tetrafluoroborate, [C8mim][BF4], and 1-ethanol-3-
methylimidazoliumtetrafluoroborate,[C2OHmim][BF4],inawide
Received:
Revised:
May 26, 2011
July 4, 2011
ABSTRACT: The stability of wetting films of two ionic liquids, 1-octyl-3-
methylimidazolium tetrafluoroborate, [C8mim][BF4], and 1-ethanol-3-
methylimidazolium tetrafluoroborate, [C2OHmim][BF4], on alumina was
assessed through the measurement of disjoining pressure isotherms, which
represent the dependence of the disjoining pressure on the film thickness.
Two experimental techniques were used at low and high disjoining pressure,
respectively: captive bubble and modified thin film pressure balance.
Theoretical predictions of the disjoining pressure isotherms were made
using the microscopic approach of London and Hamaker based on the van
der Waals contribution to the disjoining pressure. Good agreement was
found between the experimental and the theoretical isotherms for
[C8mim][BF4], whereas in the case of [C2OHmim][BF4], the experimental
dataareslightlyshiftedtowardlargerthickness,especiallyathigherdisjoining
pressures. An interpretation of these results is given in terms of a corresponding-states principle argument applied to the surface
tensionandtheuseofauxiliarymoleculardynamicsdata.TheconclusionisthatalthoughbothCoulombanddispersioninteractions
contribute to determine the bulk properties of these ionic liquids the surface properties are mainly related to the dispersive forces.

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range of disjoining pressures. The choice of these ionic liquids was
based on the fact that the former has a long side chain in the
imidazoliumring,whereasthelatterhasthealcohol(CH2)2OHasa
functional group. [C2OHmim][BF4] has high surface tension and
contactangleonhydrophilicsubstratescomparedwiththevaluesof
[C8mim][BF4].16,17
Thestabilityoftheliquidfilmsmaybeinferredfromtheshape
of the disjoining pressure isotherms that represent the depen-
dence of the disjoining pressure on the film thickness.
Different experimental techniques have been used to measure
thedisjoiningpressureandthefilmthickness.18Inthisstudy,two
methods were chosen to assess different ranges of disjoining
pressures. The method of the captive bubble was adopted to
obtain the disjoining pressure isotherm in the range of low
pressures. With this method, an air bubble is pressed against a
solid surface immersed in a liquid until a liquid film with
equilibrium thickness is attained. The equilibrium situation
occurs when the pressure in the film equals the pressure of the
gas inside the bubble. The disjoining pressure Π, which opposes
film thinning, is equal to ΔP, the pressure difference across the
bubble/liquid interface, which is given by the Laplace equation
ΔP = 2γ/rb(where γ is the surface tension of the liquid and rbis
the radius of curvature of the bubble). Thus, the range of
disjoining pressures available for the measurements is deter-
mined by the radius of the bubble holder. The determination of
film thickness by interferometry is based on the observation of
the interference pattern upon reflection of light by the film.
Further details of this method may be found in our previous
publications.19,20
Amodificationofthethinfilmpressurebalance(TFPB)using
the porous-plate technique was used to extend the disjoining
pressure range to higher values. This method that has been
applied by several authors21,22was recently implemented in our
laboratory. Details of our apparatus and the experimental tech-
nique are given in the Materials and Methods.
The experimental isotherms were compared with those calcu-
latedusingasimplemodelbasedonthemicroscopicapproachof
London and Hamaker, and the results of this comparison were
interpreted at the light of the corresponding states principle
applied to the surface tension. To our knowledge, this is the first
time that disjoining pressure isotherms for films of RTILs were
measured.
’MATERIALS AND METHODS
Materials. The ionic liquids were purchased from Solchemar
andhaveapurity>98%.Beforetheexperiments,theliquidswere
vacuum-dried at 80 ?C for at least 3 days; after that, they were
manipulated inside a glovebox filled with dried nitrogen. The
water content, checked by Karl Fischer, is ∼340 ppm for
[C8mim][BF4] and ∼188 ppm for [C2OHmim][BF4]. The
water used for cleaning was distilled and deionized. The tem-
perature dependence of the density and the surface tension was
determined ina previous work16inthe temperature range of298
to 470 K: F (g3cm?3) = 1.3244?7.0 ? 10?4T (K) and γ
(mJ3m?2) = 84.3?0.065T (K) for [C2OHmim][BF4]; F
(g3cm?3) = 1.1171?7.0 ? 10?4T (K) and γ (mJ3m?2) =
49.3?0.055T (K) for [C8mim][BF4].
The solid substrates used in the captive bubble and TFPB
methods were R-alumina plates (Melles Griot, 1 mm thickness
and 10 mm diameter). The contact angles of [C2OHmim][BF4]
and [C8mim][BF4] on alumina, at room temperature, measured
according to the procedure previously explained,17were 53 and
30?, respectively.
The refractive indexes of the ionic liquids were measured with
an ABBE 60 refractometer from Bellingham Stanley Limited as a
function of the temperature. To get values at three different
wavelengths, λ = 546.1, 589.3, and 650.0 nm, we used a 100 W
halogen lamp with three interchangeable 10 nm band-pass
interference filters (Melles Griot). The temperature dependence
of the refractive indexes of the ionic liquids and alumina,
measured at different wavelengths, was fitted to a linearequation
(n = a + b3T (K)), whose parameters are given in Table 1. The
refractive index of nitrogen was taken to be 1.0.
Experimental Methods. The disjoining pressure isotherms
were determined using two methods: the captive bubble in the
low-pressure range and a modified thin film pressure balance in
the high-pressure range. In both techniques, the film thickness is
determined interferometrically by measuring the reflection of
monochromatic light. The intensity of the reflected light may be
measured with a photosensor (TFPB) or calculated through the
analysis of the interference pattern (captive bubble). The film
thicknessisrelatedwiththereflectivity,definedastheratioofthe
intensity of the light reflected by the film, If, and the intensity of
the incident light, I0, assuming the incident beam to be perpen-
dicular to the film through the so-called Rayleigh equation
If
I0
¼
ðr1 þ r2Þ2? 4r1r2sin2δ
ð1 þ r1r2Þ2? 4r1r2sin2δ
where r1and r2are the normal incidence Fresnel coefficients
ð1Þ
r1¼
ns? nf
nsþ nfr2¼
nf? ng
nfþ ng
ð2Þ
where ns, nf, and ngare the refractive indices of the substrate, the
liquid film, and the gas, respectively. The phase difference δ is
Table1. ParametersaandboftheLinearEquation(n=a+b3T(K))fortheTemperatureDependenceoftheRefractiveIndex,n,
of Alumina, [C8mim][BF4], and [C2OHmim][BF4] at Several Wavelengths, λ
aluminaa
[C8mim][BF4][C2OHmim][BF4]
a
λ = 546.1 nm
589.3 nm
650.0 nm
λ = 546.1 nm
589.3 nm
650.0 nm
1.7669
1.7643
1.7613
1.3 ? 10?5
1.3 ? 10?5
1.3 ? 10?5
(298?348) K
1.5323
1.5228
1.4923
0.0003
0.0003
0.0002
(298?328) K
1.5385
1.5375
1.5331
0.0002
0.0003
0.0002
(328?348) K
b
temperature range
aFrom ref 23, where the reported values are ordinary refractive indices.

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defined as
δ ¼2πnfl
where l is the film thickness and λ is the wavelength of
themonochromaticlight.IntroductionofthequantityΔ,defined
as
If? Imin
Imax? Imin
whereImaxandIminarethemaximumandminimumintensitiesof
reflected light that correspond to optical thicknesses multiples,
respectively, of half wavelengths and quarter wavelengths, leads
to the following equation for the relation between Δ and the
thickness

1 ?
λ
ð3Þ
Δ ¼
ð4Þ
sin2 2πnfl
λ
!
¼
1 ?Δ
4r1r2
ð1 þ r1r2Þ2Δ
ð5Þ
In the captive bubble method, the liquid film is formed through
the introduction of a nitrogen bubble inside a bubble holder
usingagassyringe.Thisprocedure,whichisbasedontheworkof
Blake and Kitchener,24has been intensively used in our labora-
tory. The apparatus and the operating procedure were described
in detail in a previous work.19The disjoining pressure, Π, which
is equal to the capillary pressure at equilibrium is given by Π =
2γ/rB, where γ is the surface tension of the liquid and rBis the
radius of curvature of the bubble, assuming that its base is
spherical, and it may be calculated from the interferometric
pattern obtained when the bubble touches the solid. We present
here a brief description of the cleaning method. All parts of the
cell were submitted to the following procedure: (1) washing and
sonication for 15 min in Extran aqueous solution (2% v/v); (2)
rinsing and sonication with water for 3 ? 10 min; and (3) drying
with nitrogen, followed by drying overnight inside an oven at
60 ?C. When dried, all components were transferred into a
glovebox filled with nitrogen, where the bottom and the cover of
the cell were assembled separately. Both parts were plasma-
cleaned for 5 min.
The modified TFPB was recently implemented in our labora-
tory,anditisbasedontheinterferometricmethodofMyselslater
extendedby Exerowa and Scheludko.25The liquid filmisformed
in ahole (diameter of 1mm)drilledthrough a porous glass plate
(poresize3μm)connectedtotheatmospherebyacapillaryglass
tube fused laterally on the plate. The plate, enclosed inside a
hermetically sealed AISI 316 stainless steel cell, supports an
alumina substrate, and the film is formed on this substrate by
adjusting the gas pressure in the cell using a homemade
compressor. The cell is enclosed in a double-walled aluminum
chamber that allows circulation of water through a temperature-
controlled bath. A scheme of the cell is shown in Figure 1.
Thefilmisilluminatedfrombelowwithwhitelightthroughan
optical window of BK7 glass (Melles Griot) located at the
bottom of the cell. This window is tilted so that the reflected
lightfromitssurfacesdoesnotimpingeonthemeasuringsystem.
Thereflectedlightiscollectedbyafiberopticcableandmeasured
with a photosensor module (H5784 Hamamatsu). Before enter-
ing the photosensor, the light beam crosses a 10 nm bandpass
interference filter (Melles Griot), and the working wavelength
(λ = 578.28 nm) is isolated. The maximum and minimum
intensities of reflected light are determined, respectively, when
the light is reflected by the alumina plate (film of null thickness)
andbythebulkliquidfillingthehole,previoustothefilmformation.
The disjoining pressure is given by the following expression
Π ¼ Pg? Pr? ΔFghc þ 2γ=rc
ð6Þ
where (Pg? Pr) is the difference in pressure inside and outside
the cell measured with a differential pressure transducer
(Paroscientific model 5306D-101, 0?6 psid), ΔFghc is the
hydrostatic pressure of the liquid column in the glass tube (ΔF
isthedifferenceindensitybetweentheliquidandthegas,gisthe
gravitation acceleration, and hc is the height of the liquid
column), and 2γ/rcis the capillary pressure in the glass tube of
radius rc. The height of the liquid column was measured with a
cathetometer as a function of the applied pressure.
Before filling thecell with the ionic liquid, all parts werecarefully
cleaned.Bothstainlesssteelcellandaluminaplateweresubmittedto
thefollowingcleaningprocedure:(1)15minofsonicationinExtran
aqueous solution (2% v/v), (2) 10 min of sonication in water
(repeatedtwice), (3) rinsing withdistilledand deonized water, and
(4) drying under nitrogen flow, followed by drying at 80 ?C for at
least 2 h. Immediately before use, the alumina plate was plasma-
cleaned for 5 min.
The porous glass plate was boiled 30 min in ethanol, followed
by30minofboilinginExtranaqueoussolution(2%v/v)and3?
30minboilinginwaterand, finally,driedat80?Cforatleast2h.
Periodically, the porous plate was heated in the oven at 600 ?C
during 12 h to remove any trace of contaminants.
Thecellwas filled withthe sample through thecapillarywitha
constant volume of liquid (∼0.2 mL). When equilibrium was
achieved, the experiment started by increasing the pressure
stepwise and allowing the film to equilibrate for at least 1 h at
each chosen pressure until the reflected intensity remained
constant. After equilibration, 200 intensity points were recorded
during 400 s, where the Ifvalues were obtained from the average
of all data points.
Themeasurementswerecarriedoutat55?Cfor[C8mim][BF4]
and at 65 ?C for [C2OHmim][BF4]. These temperatures were
chosen to ensure that the viscosities of the ionic liquids (86 mPa3s
for [C8mim][BF4] and 85 mPa3s for [C2OHmim][BF4]16) were
compatiblewithnotverylongequilibrationtimes.Temperaturehad
no effect on the disjoining pressure isotherms, as demonstrated by
thesimilarityofΠversuslmeasurementsatdifferenttemperatures,
in the range (35?75) ?C (data not shown).
Simulation Methods. All simulations used to complement the
interpretation of the experimental data were performed using
moleculardynamicsalgorithms,implementedusingtheDL_POLY
program.26The molecular force field used in the simulations of the
Figure 1. (1) Removable cover, (2) cell body, (3) alumina substrate,
(4) porous glass plate, (5) 1 mm hole, (6) capillary glass, (7) tilted BK7
glass window, (8) pressure inlet, and (9) outlet to pressure transducer.

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two ionic liquids studied in this work is based on the CLaP force-
field.27For both ionic liquids [C8mim][BF4] and [C2OHmim]
[BF4], we started from low-density initial configurations composed
of150ionpairs.Theboxeswereequilibratedinisothermal?isobaric
ensemble conditions for 400 ps at 300 K using the Nos? e?Hoover
thermostat and isotropic barostat with time constants of 0.5 and
2 ps, respectively. Electrostatic interactions were treated using the
Ewald summation method considering six reciprocal-space vectors,
and repulsive?dispersive interactions were explicitly calculated
below a cutoff distance of 1.6 nm. (All long-range corrections were
applied, assuming the system has an uniform density beyond that
cutoff radius.) Details concerning this type of simulation can be
found elsewhere.27Furthermore, simulations of the vapor phase
werecarriedout,consideringittobeformedbyisolatedneutralion
pairs. These were equilibrated in canonical ensemble (N?V?T)
conditions for 40 ns at 300 K using the Nos? e?Hoover thermostat
withatimeconstantof1.0ps.Electrostaticinteractionsweretreated
using the usual Ewald summation method, and a cutoff distance of
500 Åwas appliedhere.Becausethe statistics arepoor as aresultof
the small number of atoms, each MD production run took 40 ns,
and 32 such runs were used to calculate the average gas-phase
properties. The cohesive energy of the bulk liquid (which can be
related to the enthalpy of vaporization) was calculated as the
difference between the configurational energies of the simulation
boxesrepresentingtheliquidandgasphase.ThevanderWaalsand
Coulomb contributions to the total cohesive energy are calculated
based on the interaction energies emerging from the implemented
short-range (van der Waals) and long-range (Coulomb) potential
functions, respectively. The former is based on the Lennard-Jones
12?6 potential, with repulsive and dispersive terms.
’RESULTS AND DISCUSSION
Figures 2 and 3 report the disjoining pressure isotherms
(disjoiningpressure,Π,vsthickness,l)measuredinawiderange
of pressures for [C8mim][BF4] and [C2OHmim][BF4],
respectively. The open symbols represent data obtained at low
disjoiningpressureswiththecaptivebubblemethod,whereasthe
closed symbols refer to the values obtained at higher disjoining
pressures with the porous plate technique. The lines on these
graphs correspond to a theory discussed later.
The first observation is the very good agreement exhibited by
the two sets of data points. The disjoining pressure isotherms
obtained with the modified TFPB prove to be reversible because
the same data were measured by increasing or decreasing the
pressure. The films were stable in the whole range of pressures
being the maximum pressure limited by the features of the
compressor. Filmrupture wasrarelyobservedandwasattributed
to the presence of impurities in the porous glass. Figure 4a,b
shows images of a stable film and of unstable film, a few seconds
before rupture, of [C8mim][BF4]. The colors in Figure 4b
indicate the instabilities that precede the film rupture.
Comparisonoftheexperimentaldisjoiningpressureisotherms
with the theoretical predictions allows the assessment of the
relative importance of the various contributions for the disjoining
pressure.Accordingtothewell-knowntheoryofcolloidalstability,
Figure 2. Disjoining pressure, Π, versus thickness, l, for films of
[C8mim][BF4] at 55 ?C. The open symbols represent data obtained
withthecaptive bubblemethod,whereastheclosedsymbolsrefertothe
values obtained with the modified TFPB. The line represents the
theoretical predictions of London?Hamaker.
Figure 3. Disjoining pressure, Π, versus thickness, l, for films of
[C2OHmim][BF4]at65?C.Theopensymbolsrepresentdataobtained
with thecaptivebubblemethod,whereas theclosedsymbolsreferto the
values obtained with the modified TFPB. The line represents the
theoretical predictions of London?Hamaker.
Figure 4. Images of films of [C8mim][BF4]: (a) stable film with
thickness = 18 nm and (b) unstable film before rupture at Π ≈ 500 Pa.

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DLVO theory after Derjaguin and Landau,28both van der Waals
and double-layer contributions have to be considered in the
calculation of the disjoining pressure. Later, a contribution from
structural forces was also considered.29The van der Waals
contribution may be either positive or negative depending on
the dielectric properties of the three phases. A positive contribu-
tion exists when the dielectric properties of the liquid are inter-
mediate between those of the two limiting media. This is the case
for the ionic liquid between alumina and air. The electrostatic
contribution derives from the electrostatic repulsion between the
diffusepartsoftheelectricdoublelayersofbothfilmsurfaces.The
origin of the steric and structural forces is less clear. However, for
films of polyelectrolyte solutions,30aqueous foam films31and
emulsion films,32oscillatory forces have been measured between
thefilmsurfacesandinterpretedasderivingfromstructuraleffects.
The van der Waals disjoining pressure may be calculated
through the following equation33
ΠvdWðlÞ ¼Asl3f ? All3f0
6πl3
ð7Þ
where Asland Allare the Hamaker constants for the solid/liquid
and the liquid/liquid interactions and f and f0are retardation
correction functions.34These Hamaker constants may be calcu-
lated from the characteristic frequencies, υic, and the limiting
values of the dielectric constants of the liquid and the solid, εi0,
accordingtotheproceduredescribedinapreviouswork,35where
the index i stands for the solid substrate (s) or the liquid (l)
?
All¼27
εl0 þ 2
whereas the retardation correction functions are
Asl¼27
32
hνlcνsc
ðνlc þ νscÞ
εl0?1
εl0 þ 2
εl0?1
?
εs0? 1
εs0 þ 2
?2
??
64hυlc
?
ð8Þ
l< 3λc=2π
f ¼ 1:01 ? 0:28pslþ 0:0143ðpslÞ3? 0:00193ðpslÞ4
f0¼ 1:01 ?0:28pllþ 0:0143ðpllÞ3? 0:00193ðpllÞ4
ð9Þ
l >3λc=2π
f ¼ 1:47ðpslÞ?1? 0:816ðpslÞ?2
f0¼ 1:47ðpllÞ?1?0:816ðpllÞ?2
slandpll=2πl/λc
ð10Þ
wherepsl=2πl/λc
the arithmetic average of the characteristic frequencies of the
solid and the liquid.
Calculation of the electrostatic component for electrolyte
solutions is based on the Gouy?Chapman theory that relies
onthedilute-solutionapproximation,whichcannotbeappliedto
an ionic liquid. Modern statistical mechanics of dense Coulomb
systems, or density functional theory, would be needed to deal
with properties of the interfacial double layer of ionic liquids.36
Application of these theories is beyond the scope of our work.
Furthermore, it is now known that although ionic liquids are
madeofions,thelargesizeoftheseions(someofthemwithlong
alkyl side chains) is responsible for important dispersion forces,
in particular, if compared with inorganic molten salts.27MD
simulationsoftheenergeticsofvaporizationofbothionicliquids
under study were used to estimate the dispersiveand Coulombic
contributions to the cohesive energy at room temperature.
Dispersive forces were found to account for 31% of the total
ll,whereλc
slwascalculatedfrom
forces in [C2OHmim][BF4], whereas this percentage increased
to 43% in the case of [C8mim][BF4]. Important contributions
from dispersion forces are the norm for ionic liquids, with some
ionic liquid families showing contributions above 50%.27
In a recent work,17we determined the polarity of ionic liquids
defined as the ratio between the nondispersive component (that
is often called “polar” component) and the total surface tension
using the Fowkes approach. Polarity fractions of 0.38 and 0.32,
respectively, for [C2OHmim][BF4] and [C8mim][BF4] were
calculated. It is interesting to point out that whereas calculations
based on the surface behavior (polarity fractions) indicated a
predominance of dispersive interactions, the values obtained for
the bulk (MD simulations) suggest the opposite situation. This
discrepancy is only apparent because both methods refer to
different situations (the bulk in the case of the MD simulations
and the liquid?vacuum interface in the case of the Fowkes
approach) and also to two different ways to tally the contribu-
tions(short-range(vanderWaals)versuslong-range(Coulomb)
interactions in the case of the MD simulations and dispersive
versus nondispersive in the case of the Fowkes approach).
Itmust bestressedthat oneof thedistinctivecharacteristics of
ionic liquids is their highly complex, nonisotropic structure. This
lack of fluid isotropy that is already dominant in the bulk is also
present (and enhanced) at the surface.37The reorganization of
the 3D structure of the ionic liquid imposed by the 2D nature of
the surface boundary implies a further segregation of the polar
and nonpolar domains of the ionic liquid. The latter domains
(alkyl side chains attachedto the polar heads of the ions) tend to
concentrate at the surface.37,38This fact justifies the dominance
of the nonpolar interactions at the surface, estimated by the
Fowkes approach. In both situations (bulk and surface), the
contribution of Coulombic (nondispersive) forces is higher, as
expected, for [C2OHmim][BF4] than for [C8mim][BF4].
To investigate further the role of the various types of inter-
molecular forces on the surface behavior of the ionic liquids, we
decided to apply a simple, two-parameter form of the corre-
sponding-states principle to the surface tension data. According
to Weiss et al.,39the reduced surface tension may be defined by
γR¼ c
γ
TcF2=3
c M?2=3
ð11Þ
wherecisaconstant∼1.016ifγismeasuredinmN3m?1,Misthe
molarmassing3mol?1,TcisthecriticaltemperatureinK,andFcis
the critical density in g3cm?3. The critical temperatures were
obtained from the fitting of Guggenheim equation (γ = Const-
(1?T/TC)11/9) to the surface tension versus temperature data.16
Thecriticaldensitieswereestimatedfromtheaveragedensities,F ̅ ,
in the temperature range of the surface tension measurements
using the empirical correlation Fc≈ 0.333F ̅ . This correlation was
found from the comparison of experimental values of liquid
densities40and critical densities41for a series of molecular liquids.
Figure5showsplotsofthereducedsurfacetensionasafunctionof
reducedtemperature(TR=T/Tc)forseveralfamiliesofmolecular
liquidsandformoltensalts.Theexperimentalvaluesofthesurface
tension and the critical parameters Tcand Fcfor these molecular
liquids were compiled from the data given in the two references
cited above. The reduced surface tensions of NaCl and KCl were
calculated from surface tension data reported by Jaeger,42and the
critical parameters were taken from Kirshenbauml et al.43The
experimental data are compared with Guggenheim’s universal
curve applied to argon, a purely dispersive fluid.

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Analysis of the Figure 5a?e clearly indicates that apart from
theexceptionsofheliumandhydrogenthemolecularliquidsthat
deviate from Guggenheim’s curve toward lower values of the
reduced surface tension (negative deviations) have small mol-
ecules and strong hydrogen bonds. As the size of the molecules
increases, the reduced surface tension also increases, and the
deviations become positive. A case in point is the series of n-
alkanols (Figure 5e) that shows reduced surface tensions below
the line for methanol, “ideal” behavior for propanol, and positive
deviations for longer alkanols. Figure 5f shows the results for the
ionic liquids, [C8mim][BF4] and [C2OHmim][BF4], and two
inorganic molten salts. The most striking result is that the
reduced surface tensions of both ionic liquids deviate strongly
from the data of fluids dominated by Coulomb interactions
(molten salts) and lie close to the master curve for dispersive
liquids. The negative deviation observed for [C2OHmim][BF4]
is consistent with the small size of the ions and the presence of
hydrogen bonds. Similar results were reported by Leroy et al.,44
who found that the corresponding states surface-tension data of
[bmim][BF4] and [bmim][NTf2] are similar to typical values of
liquefied noble gases.
Our main point here is that although Coulomb interactions
are important in the bulk behavior of [C8mim][BF4] and
[C2OHmim][BF4], the application of the corresponding states
Figure5. Reducedsurfacetension,γR,(yaxis)asafunctionofthereducedtemperature,TR,(xaxis)forseveralrepresentative familiesofliquids:noble
gases (a), diatomic and triatomic molecules (b), alkanes (c), haloalkanes (d), alcohols (e), and low- (this work) and high-temperature molten salts (f).

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principle to reduced surface tension clearly indicates that the
behavior of their liquid/vapor interface is mainly determined by
the London dispersive interactions. Coincidentally, a very interest-
ing case is that of [C8mim][BF4] that lies closer to the dispersive
mastercurveofArgonthanmanyofthemolecularfluidsdepictedin
Figure 5a?e. This does not mean that the interactions in this
particular ionic liquid are purely dispersive; it means that the
negative deviations that would be created by the Coulomb interac-
tions present at the surface (subdued but not negligible) are
compensated by the positive deviations caused by the dispersive
interactionsbetweenthelongalkylchainofthecationsegregatedat
the surface. As in the case of n-alkanols, where the negative
deviations caused by hydrogen bonding tend to be canceled-out
by positive deviations produced by longer alkyl chains and a break-
evenpointis achievedaroundC3,inthe caseofimidazolium-based
ionicliquids,suchabreak-evenpointisreachedonlyforlongeralkyl-
side chains (C8) (and more important dispersive contributions)
due to the presence of the coulomb interactions.
However, we must be aware that this is a simplified version of
the corresponding states principle where no shape effects were
considered. Furthermore, uncertainties in the determination of
the critical temperature and critical density from long-range
extrapolations may also affect our conclusions.
The reasoning presented above led us to try a comparison
between the experimental disjoining pressure isotherms and
those based only on the van der Waals contribution. The
isotherms (Π vs l) calculated according to eq 7 are represented
bythelinesinFigures2and3.Thedeviationbetweentheoryand
experiment is almost negligible for [C8mim][BF4], whereas the
datafor[C2OHmim][BF4]shiftstowardlargervaluesofthefilm
thickness, mainly at higher disjoining pressures. These results
confirm that the main forces that are responsible for the stability
ofthinfilmsofionicliquidsaredispersiveinteractions.Thedeviation
found for [C2OHmim][BF4] is in agreement with the existence of
strong hydrogenbonds. We may alsospeculatethat other repulsive
interactions, namely, structural forces, could be responsible for this
film thickening. These forces are known to depend on the orienta-
tional ordering at the interface between the film and the solid.45
Recent MD simulations of the behavior of [C2OHmim][BF4] and
[C8mim][BF4] at solidinterfaces12,15showedthat the stratification
process is more effective for the former ionic liquid, supporting the
above speculation on the role of structural forces.
Further systematic investigations on a series of ionic liquids
wouldbenecessaryforabetterunderstandingofthenatureofthe
interactions within the liquid films.
’CONCLUSIONS
Themainoutcomeofthisworkisthefindingthatthebehavior
of thin wetting films of two ionic liquids, [C8mim][BF4] and
[C2OHmim][BF4], is mainly determined by the London disper-
sion forces. This conclusion is in agreement with the results of
the application of the corresponding states principle to these
ionic liquids. The reduced surface tension of [C8mim][BF4] is
very well described by Guggenheim’s universal curve for simple,
purelydispersivefluids.Inthecaseof[C2OHmim][BF4],thereis
a deviation of the reduced surface tension toward lower values,
which is the typical behavior of molecular fluids with small
molecules and strong hydrogen bonds. This deviation should
be related to the small shift toward larger thickness of the
disjoining pressure isotherm of [C2OHmim][BF4] from the
theoretical van der Waals contribution.
’ACKNOWLEDGMENT
WeareindebtedtoRumenKrastevandRegineKlitzingforthe
valuable advice relative to the implementation of the modified
ThinFilmPressureBalance.Thisstudywasfinanciallysupported
by the projects PCDT/QUI/66211/2006 and PTDC/QUI-
QUI/101794/2008. J.R. was awarded with a research grant of
the former project, and K.S. acknowledges grant SFRH/BPD/
38339/2007.
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