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Influence of Water on the Interfacial Nanostructure and Wetting of [Rmim][NTf2] Ionic Liquids at Mica Surfaces Zhantao Wang Wang, Hua Li, Rob Atkin, and Craig Priest Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01790 • Publication Date (Web): 03 Aug 2016 Downloaded from http://pubs.acs.org on August 6, 2016
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Langmuir
Influence of Water on the Interfacial Nanostructure and Wetting of [Rmim][NTf2] Ionic Liquids at Mica Surfaces Zhantao Wang1#*, Hua Li2, Rob Atkin2, Craig Priest1^ 1Ian Wark Research Institute, University of South Australia, Mawson Lakes 5095, Australia 2Priority Research Centre for Advanced Fluids and Interfaces, the University of Newcastle, Callaghan, NSW 2308, Australia
ABSTRACTS: The effect of water concentration on the interfacial nanostructure and wetting behavior of a family of ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [Rmim][NTf2],
at the surface of mica was
investigated by contact angle measurement and atomic force microscopy (AFM). AFM reveals that interfacial layers of ILs observed at the surface of mica for ‘dry’ ILs are not present for water-saturated ILs. The interaction of the IL ions of [Rmim][NTf2] with water molecules through hydrogen bonding is suspected to disrupt IL ion layering and precursor film growth on mica. Without the IL precursor film, contact angle relaxation of ‘wet’ ILs on mica is less significant and ambient vapor adsorption becomes more important in determining the macroscopic wetting behavior.
ILs exhibit unusual interfacial nanostructure at interfaces
INTRODUCTION
with substrates such as mica, silica and sapphire
13,15-17
. Most
Room temperature ionic liquids (ILs) are pure salts that are
previous studies attribute such ordering at the interface to
liquids at low temperature (< 100°C). Due to their low-
the electrostatic interaction and steric forces at the IL/mica
volatility and thermal stability, ILs are good candidates as
interface
green solvents
1,2
. ILs can be designed to be ‘task-specific’
13,15,18
. We have previously reported the formation
3
of molecularly thin precursor films propagating from
because their properties can be tuned by combining
droplets
different cations and anions, which enables them to be
bis(trifluoromethylsulfonyl)imide, [Rmim][NTf2], on mica,
used
4-6
in
7,8
catalysis , 11,12
microfluidics
lubrication ,
and many other areas
solar
13,14
9,10
cells
,
of
1-alkyl-3-methylimidazolium
where R, is ethyl, butyl, or
hexyl alkyl chains on the
. Understanding
imidazolium ring . Beattie et al. have also detected wetting
the interfacial behavior of ILs is of significant importance
precursors of the same family of ILs on mica and used them
for many of the envisaged applications. Many physical and
to estimate the spreading rate of the film . These precursor
chemical
interfacial
films resemble the thin IL films produced by the solvent
uncharged
extraction approach
phenomena
interactions.
Because
are
dominated
the
charged
by and
19
20
17,21
, and surface diffusion is the
functional groups on the anions and cations lead to
dominant mechanism for precursor film growth because
repeating, correlated structure in the bulk and at interfaces
the volatility of such liquids is negligible.
on nanometer dimensions, i.e., ILs can be considered nano15
heterogeneous materials . Due to this nanoheterogeneity,
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Langmuir Most ILs are hygroscopic
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22
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and are therefore likely to
Comparing with the afore-mentioned interfacial structures
contain a significant amount of water in many cataications.
of ILs at solid surfaces, the effect of water on wetting
Water as an impurity in ILs may present a potential barrier
phenomena of ILs have received less attention . It is
for their applications, depending on its effect on the
probable that changes in IL interfacial structure on water
physical and chemical properties of ILs. Dissolving water in
addition will influence macroscopic wetting. In this paper,
an IL is known to reduce the latter’s viscosity increase
its
conductivity
29-31
at
low
to
23-29
39
and
we report that the addition of water does indeed affect
moderate
macroscopic wetting behavior, which correlates well with
concentrations. Water in ILs is thought to form hydrogen
molecular changes at the solid-liquid interface.
23
bonds with both the cation and anion of [Rmim][NTf2] . Nanoscale segregations, such as micelles and ion clusters, 32
have also been reported when water is present in the ILs . Using the surface force apparatus measurement, Horn et 33
al.
revealed that increasing the amount of water in
ethylammonium nitrate (EAN) significantly reduces the 34
solvation forces. Smith et al. also found that water notably disrupts the interfacial layers of EAN and the stability of particle suspensions in this IL. The effect of water on the interfacial layers of two imidazolium-type ILs ([Bmim][BF4] and [Emim][NTf2]) has been studied at both mica and silica surfaces, highlighting the role of hydrogen bonding and the affinity (miscibility) between water and ILs in affecting the 35
36
molecular structure of ILs . Gong et al
attributed the
layering behavior of ILs at the mica surface to the dissociation of K
+
into the adsorbed water and the +
subsequent exchange between K and the cations of ILs on the mica surface. However, recent study by McDonald et 37
al,
+
revealed that K desorbs from the mica surface and
dissolves in the dry IL phase (water content < 100 ppm), resulting in a negatively charged mica surface rather than an uncharged mica surface. Besides water, pre-adsorbed carbon has also been found to have a strong effect on the morphology of ILs on solid surfaces. By investigating the thin films of two ILs on mica using angle-resolved X-ray 38
EXPERIMENTAL SECTION Muscovite mica (ProSciTech, grade V-1, 12.5 mm disk) was chosen as the substrate in this study for its atomic flatness after cleavage. In our experiments, the mica disks were cleaved by inserting the sharp tip of a pair of tweezers at the side of the mica disk. Contact mode AFM scanning indicates that the mica surface has a RMS roughness of ~0.3 nm over a 1 µm × 1 µm scanning area. The exposed mica surface was used immediately after cleavage for surface modification by OPA and/or contact angle measurement, as described below. The
ILs
chosen
methylimidazolium
in
this
study
were
1-ethyl-3-
bis(trifluoromethylsulfonyl)imide
([emim][NTf2]),
1-butyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide ([bmim][NTf2]) and 1hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][NTf2]), which are all partially miscible with water. The solubility of water in these ILs decreases with increasing alkyl chain length 26
within the limit of ~1.5% by mass fraction . The fresh ILs, also called ‘dry’ ILs in this study, were kept in sealed bottles and stored in glass vacuum desiccators with silica gel as the desiccant. Table 1 lists the physical parameters of interest for the studied ILs.
photoelectron spectroscopy (ARXPS), Deyko et al.
revealed that on clean mica these ILs exhibit complete de-
The water-saturated (‘wet’) ILs were generated by mixing
wetting, but on a fully carbon covered surface they form a
the ‘dry’ ILs with water in a glass vial and vigorously mixing
thin film, followed by 3D growth. How water might
by hand-shaking, waiting for seven days until they have
influence this, whether by absorption in the bulk IL or
clear phase separation, and then isolating the IL phase.
adsorption at the mica surface, remains unclear. A very
Intermediate concentrations of water were obtained by
recent report
18
suggests that this molecular ordering is
mixing the ‘dry’ and ‘wet’ ILs of the same type with
largely insensitive to the substrate’s surface chemistry or
different volume ratios from 1:1 up to 50:1. The water
small amounts of absorbed water.
concentration of all ILs samples was quantified by Karl
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Fischer titration (831 KF Coulometer, Metrohm) . The
Humidity has been reported to change the surface energy
viscosity and surface tension of the ILs were measured
of mica through water vapor adsorption onto this
using the Cannon-Manning Semi Micro Viscometer
substrate . To ensure a constant water vapor adsorption on
(Cannon Instruments) and the pendant drop methods
mica surface and therefore a stable surface energy of the
(OCA 20, Data Physics Instruments), respectively.
latter, all experiments presented in this paper were
43
conducted at the same relative humidity (39 ± 2%) and Octadecylphosphonic acid (OPA) was used to pattern the
temperature (22.5 ± 1 °C) in a clean room (class 1000). As we
41,42
cleaved mica surface with nanoscale heterogeneities
.
The wettability of the nano-patterned mica surface by the ILs was altered by varying the OPA coverage on the mica
are interested in solid-IL interfacial effects, we also measured the liquid-vapor interfacial tension of the ILs for the different concentrations of water studied using pendent
19
surface . A 0.01M OPA solution was prepared in THF, as
drop tensiometry (OCA 20, Data Physics Instruments).
42
described previously . The freshly cleaved mica surface was dipped into the OPA solution for different periods of time.
Atomic force microscopy (AFM) was used for both force
Then the mica sample was taken out and blown dry using a
measurement and morphology imaging. The same AFM
high purity, dry nitrogen jet.
force-distance approach described in ref. was employed to
44
resolve the interfacial layers at the mica-[Rmim][NTf2] Contact angles were measured using the sessile drop
interface and to investigate whether the addition of water
method (OCA 20, Data Physics). The solid substrate was
(at saturation) has any influence on these interfacial layers.
firstly placed on a horizontal platform and a drop of ~2 μL
Furthermore, AFM imaging of the mica surface adjacent to
was gently deposited by a stainless steel needle from a
the IL droplet was conducted to detect the formation of a
Hamilton 500 µL Syringe. A CCD camera (Jai CV-M10BX,
precursor film at ambient laboratory conditions. Following
624 x 580 pixels) captures the image of the droplet. The
19
the same procedure used in ref. , AFM was used to scan a
contact angles were extracted from ellipse fitting on the
mica surface with ~37% coverage of octadecylphosphonic
profiles of the droplet. The experimental uncertainty of
acid (OPA). The location scanned was approximately 80 µm
Table 1 ‘Dry’ ILs used in this study and their physical properties of interest, which include the mole mass M, density 𝛒, the purity ∅, the suppliers, water concentration 𝛂, viscosity 𝛈, and surface tension 𝛄.
𝜌
[Rmim] [NTf2] ILs M (g/mol)
∅ (%)
Supplier
α (ppm)
(g/cm3 )
𝜂
𝛾
(± 0.5 mPa.s)
(± 0.3 mN/m)
[emim][NTf2]
391
1.51
≥ 99
IoLiTec
320
32.0
36.9 (mN/m)
[bmim][NTf2]
419
1.44
≥ 99
Merck
240
56.0
33.2
[hmim][NTf2]
447
1.37
≥ 99
IoLiTec
170
82.0
31.6
OCA contact angle measurement is ±2°.
away from the edge of the IL droplet. Images were recorded before and up to two hours after placing a ‘dry’ and ‘wet’
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[Rmim][NTf2] droplet on the surface, respectively. A
Figure 1 shows the time-dependent contact angles for
change in the apparent height of the OPA islands was used
droplets
as an indicator of the precursor film formation between the
[hmim][NTf2] at various water concentrations spreading
OPA islands and the bare mica, as shown in our previous
over freshly cleaved mica. We have correlated the slow
19
study .
of
[emim][NTf2],
[bmim][NTf2],
and
spreading of pure (‘dry’) ILs on mica with the formation of a precursor film that grows on the mica surface through
Sessile drop contact angle measurements were conducted
surface diffusion
on [Rmim][NTf2] droplets with different water contents. In
concentration of ILs on the macroscopic spreading. The
addition, the effect of water vapor adsorption on the
19
. Here, we consider the effect of water
spreading can be characterised by the initial contact angle
contact angle of ‘wet’ ILs was investigated through two
(θi, measured immediately after the initial viscous
steps. First, by observing the initial contact angles of the
relaxation of the droplet, i.e. several seconds), the ‘final’
‘wet’ [Rmim][NTf2] ILs on the mica surface that have been pre-exposed to ambient conditions (RH of 39 ± 2% and temperature of 22.5 ± 1 °C) for different times. And second, by equilibrating the mica surface at the aforementioned conditions for 24 hours and tracking the slow spreading of ILs on such surfaces.
contact angle (𝜃𝑓 , the contact angle that levels off with time), and a relaxation time 𝜏. 𝜏 can be determined by fitting
the
time-dependent
contact
angle
with
an
19
exponential decay function 𝜃𝑡 = 𝑎𝑒 −𝑡/𝜏 + 𝜃𝑓 , where 𝑡 is time and 𝑎 and 𝜃𝑓 are fitting parameters. The velocity of the contact line, 𝑢, can be related to the dynamic contact 46
It is worth mentioning that the mica used in our
angle by Eq. (1) as:
experiment was cleaved in a clean room laboratory and will
𝐿
𝑢 = 𝛾(𝜃𝑡3 − 𝜃𝑓3 )/(9𝜂 ln ( )) 𝑙
reasonably adsorb some adventitious carbon. Our approach differs from previous work conducted in vacuum
45
in that
our experiments reflect ambient conditions which might be expected in typical applications of ILs. . Although we cannot prevent carbonaceous adsorption on the mica
(1)
where 𝛾 is the liquid surface tension, 𝜂 is the liquid viscosity, 𝐿 and 𝑙 is the ratio of the macroscopic and 𝐿
molecular length scales (typically ~ 106), and 𝜃𝑡 and 𝜃𝑓 are 𝑙
surface, the contamination will be relatively minor, due to
the dynamic and final contact angles, respectively. The
the cleaving of the mica under controlled environment
characteristic spreading time-scale due to viscosity, 𝑡𝑠 , can
conditions (clean room of class 1000). AFM images revealed
be related to the contact line velocity 𝑢 and the radius of
no detectable contamination but it is accepted that
the droplet at equilibrium, 𝑟𝑒𝑞𝑚, through
carbonaceous contamination is ubiquitous to high-energy surfaces
19
𝑡𝑠 = 𝑟𝑒𝑞𝑚 /𝑢
(2)
RESULTS AND DISCUSSION This
section
complementary
addresses
the
measurements
results that
from
each
three
relates
to
interfacial phenomena at the solid-liquid and solid-vapor interfaces. We first consider wettability (time-dependent contact
angle
measurements),
then
AFM
force
measurements at the solid-liquid interface, and, finally, AFM imaging at the solid-vapor interface. Wettability
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the contact angle variation with time to a few degrees. Note that for ‘dry’ [hmim][NTf2] the contact angle continues to decrease beyond 4 hours until complete spreading (𝜃 < 10°) is observed at ~20 h (results not shown due to time scale) after deposition. The addition of water to ILs has a significant effect on the viscosity of the liquid (Figure 2.b) and one might expect an influence on the rate of spreading; however, this is not the case here. Both the empirical relaxation time (𝜏) and the viscous time-scale (𝑡𝑠 ), estimated using methods described above are plotted against water concentration in Figure 2 c and d. 𝜏 is an order of magnitude longer than 𝑡𝑠 suggesting that the observed relaxation is not governed by the viscous spreading regime. In addition, the viscosity of these ILs reduce with the concentration of dissolved water
27,47
, which
would be inconsistent with the trend in the spreading timescales reported.
Figure 1. Contact angle relaxation of (a) [emim][NTf2], (b) [bmim][NTf2], and (c) [hmim][NTf2] droplets on freshly cleaved mica. Concentration of water (ppm) in the ILs is given in the legend. The solid lines are the exponential fittings of the
Figure 2. Influence of water concentration on (a) surface tension, (b) viscosity, (c) empirical time-scales and (d) viscous time-scale of [Rmim][NTf2].
experimental data (see main text for discussion).
The initial contact angles are very similar in all cases (~ 40°); however, the final contact angles and 𝜏 differ for the different
concentrations
of
water.
Increasing
the
concentration of water from ~ 200 ppm (‘dry’) to saturation (‘wet’) for [emim][NTf2] and [bmim][NTf2] increases the final contact angle by ~10° from approximately 23° to 34°.
The surface tension measurements of the 3 ILs with different water concentrations showed that the liquidvapor interfacial tensions were insensitive to the water concentrations
studied
(Figure
2a),
consistent
with
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previous report that most soluble water molecules appear
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48
to be dissolved in the bulk of ILs , therefore poses a limited and non-monotonic change in the surface tension 23
of these particular ILs. . Given that neither the liquid viscosity nor the liquid-vapor interfacial
tension
is
responsible
for
the
wetting
dependence on the concentrations of water in the IL, we turn our attention to the solid-liquid and solid-vapor interfaces. Solid-Liquid Interface We used AFM to study the solid-liquid interface for ‘wet’ and ‘dry’ ILs. In these experiments an AFM tip is immersed in the IL and slowly driven towards the mica surface. The normal force is determined with respect to the separation distance as the AFM tip approaches the mica surface. Typical force-distance curves for each of the ‘wet’ and ‘dry’ ILs are shown in Figure 3. The effect of water in the ILs on the force-distance curves is significant. The ‘dry’ ILs show distinct repulsive force steps close to the solid-liquid interface similar to that reported previously
16,49-51
, while the
‘wet’ ILs show attraction at separation distances of up to ~ 2.5 nm. The widths of the steps, except for the innermost one, are in accordance with the ion pair diameters of the ILs (0.75-0.85 nm estimated by cubic packing). For dry [emim][NTf2], the innermost layer is ~ 0.18 nm, which is likely to be a compressed layer enriched in [emim]+ cations that has been pushed through. This result is consistent with previous lateral morphology study of the same system, which shows that [emim]+ cations adsorb to the mica surface in an isolated fashion, and are pushed-through from the surface at high forces
43,52
. The innermost layers for
[bmim][NTf2] and [hmim][NTf2] are ~0.30 nm, similar to 50,51
the dimensions of [bmim]+ and [hmim]+ cations
. The
force to push through the innermost layer is highest for
Figure 3. AFM force curves for ‘dry’ (blue symbols) and ‘wet’ (red
[emim][NTf2], followed by [bmim][NTf2], and lowest for
symbols) for [Rmim][NTf2] near the mica surface; the radius of the
[hmim][NTf2]. As charges are more localized for [emim]+
AFM tip (R) is ~ 20 nm.
cations, they have strongest electrical interactions with the negatively charged mica surface, thus require highest force to push through.
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Langmuir
It is clear from Figure 3 that the water-saturated (‘wet’) ILs
aforementioned
relative
show qualitatively different force-distance curves, with
temperature (22.5 ± 1 °C).
humidity
(39
±
2%)
and
respect to the dry ILs on mica. In contrast to the speculation that water adsorbed on the mica surface 36
facilitates the extended layering of ionic liquids , no clear force steps were detected for all three water-saturated ILs
Since 𝛾𝑆𝑉 and 𝛾 do not vary significantly at equilibrium, this would mean that saturating the ‘dry’ IL (~ 200 ppm) with soluble water has caused a 3~5 mN/m (~𝛾∆𝑐𝑜𝑠𝜃) increase
studied. Instead, a strong attractive van der Waals force
in the mica-IL interfacial tension (𝛾𝑚𝑖𝑐𝑎−𝑤𝑒𝑡𝐼𝐿 − 𝛾𝑚𝑖𝑐𝑎−𝑑𝑟𝑦𝐼𝐿 ,
produces a jump-in event near 2.5 nm separation, in line
c.f. Figure 4b).
35
with other studies.
This suggests that the nanostructure
(interfacial layering) of the ILs is significantly weakened or non-existent at the mica/water-saturated IL interface. As the NTf2 ILs are all hydrophobic, and the mica surface is hydrophilic, water molecules tend to move from the bulk of 35
the IL to the interface to form a water layer , thus the electrostatic interactions between the IL cations and the negatively charged mica surface are weakened and the layered interfacial structure is reduced. The interfacial water molecules are also likely to form hydrogen bonds with the IL anions and cation side alkyl chains and imidazolium rings, thus reduce the interaction between the IL cations and anions, and disrupt the order within the liquid adjacent to the solid.
29,34,53,54
.
In the discussion above, we have neglected changes to the solid-vapor interface on the basis that the ambient conditions were constant for all of the experiments (humidity and temperature were held constant in a clean room environment), therefore the change brought by water-vapor adsorption on mica in each case should be the same. Regarding the final contact angle of each type of dry and water-saturated IL, there is no significant difference in the surface tension of both mica and the ILs studied, suggesting the change of mica-IL interfacial tension is responsible for the variation of the final contact angle.
Figure 4. (a) Final contact angle after relaxation on mica and (b) the variation of mica-IL interfacial tensions of the three ILs with different water concentrations.
From a simple calculation based on the Young’s equation for the final contact angles (Figure 4a) as following Solid-Vapor Interface 𝛾𝑚𝑖𝑐𝑎−𝑑𝑟𝑦𝐼𝐿 = 𝛾𝑆𝑉 − 𝛾𝑐𝑜𝑠𝜃𝑓−𝑑𝑟𝑦𝐼𝐿 𝛾𝑚𝑖𝑐𝑎−𝑤𝑒𝑡𝐼𝐿 = 𝛾𝑆𝑉 − 𝛾𝑐𝑜𝑠𝜃𝑓−𝑤𝑒𝑡𝐼𝐿
(3) Mica is a high-energy surface that is known to absorb (4)
55
ambient water vapor . It follows that a time-dependent contact angle could be observed under certain ambient 42
where 𝜃𝑓−𝑑𝑟𝑦𝐼𝐿 and 𝜃𝑓−𝑤𝑒𝑡𝐼𝐿 represent the final contact
conditions, e.g. humidity, for freshly cleaved mica . For the
angle of ‘dry’ and ‘wet’ ionic liquids on mica, at
mica surface pre-exposed to the ambient, one might expect
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that the initial contact angle (at time zero) would also be
Figure 5. Variation of the initial contact angle (solid symbols) of
affected by the change in the solid-vapor interface. Here,
water-saturated [Rmim][NTf2] ILs on freshly cleaved mica,
we discuss our observations for ‘wet’ and ‘dry’ ILs on pre-
compared with relaxing contact angles (open symbols) of the same
exposed mica. Figure 5 shows the contact angles collected for the ‘wet’ IL on pre-exposed mica (i.e. the solid symbols represent the initial contact angle and the x-axis represents
ILs on mica surface pre-exposed to the ambient (temperature of 22.5 ± 1°C and RH of 39 ± 2 %) for 24 h. (a) [emim][NTf2] and [bmim][NTf2]; (b) [hmim][NTf2]. For the initial contact angle measurements, a new IL droplet was deposited for each data point.
the pre-exposure time of mica). In a second experiment, the mica was exposed to the ambient vapor in the laboratory
It is important to note that this time-dependence of the
for 24 hours before placing a droplet of IL on the surface
contact angle is distinct from the effect of the precursor
and measuring the relaxation of the contact angle with time
film formation previously reported by us
(i.e. the open symbols represent the relaxing contact angle
adsorption alone could not explain the magnitude of the
and the x-axis is the elapsed spreading time after droplet
decrease in the contact angle. Nonetheless, here we
deposition). These results are plotted together to show that
reconsider the possible formation of a detectable precursor
the two data sets for each IL converge at long time-scales.
film using AFM imaging. The AFM force-separation curves
This indicates that for water-saturated ILs, the observed
(Figure 3) show that the interfacial layering is disrupted,
contact angle change is more likely to be due to vapor
from which one could surmise that precursor film
adsorption on the mica surface rather than specific IL-mica
formation may be less favored. Following the methodology
interactions. The much longer time-scale for [hmim][NTf2]
reported elsewhere,
suggests that the physical picture is not as simple, and
nano-islands of OPA at surface coverage (area fraction) of
further investigation of this effect may prove helpful in
37%. We have shown that dry [Rmim][NTf2] droplets are
understanding the molecular picture.
able to develop precursor films between these islands on
19
19
. In that case,
we patterned the mica surface with
the bare mica surface and, using the height of the OPA monolayer, precursor film thicknesses can be determined 19
quantitatively .
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Langmuir and the subsequent exchange between K+ and the cations
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of ILs on the mica surface. However, the amount of adsorbed water was not clearly stated in such report, making it difficult to make a direct comparison with our 18
experimental result. Very recent study suggested that this molecular ordering process is largely insensitive to substrate surface chemistry or small amounts of absorbed water. More previous studies
34,35
suggested that water
disrupts the molecular layering of ILs at mica surface, which is in agreement with findings from this study. Different types of ILs (alkyl chain length and anion species) could results in different interfacial behavior.
20,42,50
Last but
not the least, the molecular state of water solved in the bulk of ILs and those adsorbed from the ambient should not be necessarily the same, which could also lead to different observations. Therefore, the exact mechanism of the molecular layering is not yet clear and calls for more study. Figure 6. AFM images (1 µm × 1 µm) and line scans of mica modified with a 37% coverage of OPA islands (a) before placing the IL droplet and (b) 90 min after placing the droplet of watersaturated [bmim][NTf2],(c) 90 min after placing the droplet of dry [bmim][NTf2]. Scanning position is approximately 80 µm away from the edge of the droplet. The color scale bar in the images represents 4 nm. Line scans are shown for each image.
CONCLUSIONS The effect of water concentration on the interfacial nanostructure and wetting behavior of [Rmim][NTf2] ILs on mica surface has been investigated by AFM (imaging and force) and contact angle measurements. Water is found to significantly affect the solid-liquid interfacial behavior of
In the present study, AFM images were recorded before and
ILs at mica surfaces, affecting the solid-liquid interfacial
after placing a water-saturated [bmim][NTf2] droplet on
tension. The interaction of the IL ions of [Rmim][NTf2]
the OPA-patterned mica surface, as shown in Figure 6a and
with water molecules through hydrogen bonding is the
b, respectively. The height image of placing a dry
likely cause of this difference. The time-dependence of the
[bmim][NTf2] droplet was also shown as a comparison
contact angle for the water-saturated ILs appears to be
(Figure 6c). For dry IL, the reduced OPA islands height
driven by changes in solid-vapor interfacial tension caused
clearly show the formation of a precursor film between the
by water vapor adsorption on the mica surface, rather than
OPA islands. For wet [bmim][NTf2], the measured OPA
precursor film formation. The results will be helpful in the
height is approximately the same (~ 1.9 nm) before (Figure
design of IL technologies where water adsorption will be
6a) and after (Figure 6b) placing the IL droplet, showing no
inevitable, yet the functionality of the IL must be
evidence of a precursor film between the islands for the
predictable.
water-saturated IL. It is worth mentioning that previous reports
36
■ AUTHOR INFORMATION have shown
extended layering at IL/solid interfaces where tiny amount
# Current address: Department of Science and Technology,
of water was absorbed from the ambient (RH~30%), and
University of Twente. 7522 AE Enschede, the Netherlands.
such layering behavior of ILs at the mica surface was attributed to the dissociation of K+ into the adsorbed water
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Langmuir ^Current address: Future Industries Institute, University of
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South Australia, Mawson Lakes, SA. 5095, Australia. Corresponding Author *E-mail:
[email protected]. Notes The authors declare no competing financial interest.
ACKNOWLEDGMENTS The authors want to thank Rossen Sedev and John Ralston for their valuable insight and support. This project was supported financially by the Australian Research Council Discovery Project Scheme. RA thanks the ARC for a Future Fellowship.
REFERENCES (1) Earle, M. J.; Seddon, K. R. Ionic liquids. Green solvents for the future. Pure Appl. Chem. 2000, 72, 1391-1398. (2) Earle, M. J.; Seddon, K. R. Ionic liquids: Green solvents for the future. Acs Sym Ser 2002, 819, 10-25. (3) Tang, S.; Baker, G. A.; Zhao, H. Ether-and alcoholfunctionalized task-specific ionic liquids: attractive properties and applications. Chem. Soc. Rev. 2012, 41, 4030-4066. (4) Welton, T. Ionic liquids in catalysis. Coordin. Chem. Rev. 2004, 248, 2459-2477. (5) Steinrück, H.-P.; Wasserscheid, P. Ionic Liquids in Catalysis. Catal. Lett. 2015, 145, 380-397. (6) Olivier-Bourbigou, H.; Magna, L.; Morvan, D. Ionic liquids and catalysis: Recent progress from knowledge to applications. Appl. Catal. a-Gen. 2010, 373, 1-56. (7) Ye, C. F.; Liu, W. M.; Chen, Y. X.; Yu, L. G. Roomtemperature ionic liquids: a novel versatile lubricant. Chem. Commun. 2001, 2244-2245. (8) Somers, A. E.; Howlett, P. C.; MacFarlane, D. R.; Forsyth, M. A review of ionic liquid lubricants. Lubricants 2013, 1, 3-21. (9) Kawano, R.; Matsui, H.; Matsuyama, C.; Sato, A.; Susan, M. A. B. H.; Tanabe, N.; Watanabe, M. High performance dyesensitized solar cells using ionic liquids as their electrolytes. J. Photoch. Photobio. A 2004, 164, 87-92. (10) Abate, A.; Hollman, D. J.; Teuscher, J. l.; Pathak, S.; Avolio, R.; D’Errico, G.; Vitiello, G.; Fantacci, S.; Snaith, H. J. Protic ionic liquids as p-dopant for organic hole transporting materials and their application in high efficiency hybrid solar cells. J. Am. Chem. Soc. 2013, 135, 13538-13548. (11) Abdelgawad, M.; Wheeler, A. R. The Digital Revolution: A New Paradigm for Microfluidics. Adv. Mater. 2009, 21, 920925. (12) Nanayakkara, Y. S.; Moon, H.; Payagala, T.; Wijeratne, A. B.; Crank, J. A.; Sharma, P. S.; Armstrong, D. W. A fundamental study on electrowetting by traditional and multifunctional ionic liquids: possible use in electrowetting on
Page 10 of 12
dielectric-based microfluidic applications. Anal. Chem. 2008, 80, 7690-7698. (13) Hayes, R.; Warr, G. G.; Atkin, R. At the interface: solvation and designing ionic liquids. Phys. Chem. Chem. Phys. 2010, 12, 1709-1723. (14) Lu, J. M.; Yan, F.; Texter, J. Advanced applications of ionic liquids in polymer science. Prog. Polym. Sci. 2009, 34, 431-448. (15) Hayes, R.; Warr, G. G.; Atkin, R. Structure and Nanostructure in Ionic Liquids. Chem. Rev. 2015, 115, 63576426. (16) Mezger, M.; Schroder, H.; Reichert, H.; Schramm, S.; Okasinski, J. S.; Schoder, S.; Honkimaki, V.; Deutsch, M.; Ocko, B. M.; Ralston, J.; Rohwerder, M.; Stratmann, M.; Dosch, H. Molecular layering of fluorinated ionic liquids at a charged sapphire (0001) surface. Science 2008, 322, 424-428. (17) Bovio, S.; Podesta, A.; Lenardi, C.; Milani, P. Evidence of Extended Solidlike Layering in [Bmim][NTf2] Ionic Liquid Thin Films at Room-Temperature. J. Phys. Chem. B 2009, 113, 6600-6603. (18) Anaredy, R. S.; Shaw, S. K. Long-Range Ordering of Ionic Liquid Fluid Films. Langmuir 2016, 32, 5147-5154. (19) Wang, Z. T.; Priest, C. Impact of Nanoscale Surface Heterogeneity on Precursor Film Growth and Macroscopic Spreading of [Rmim][NTf2] Ionic Liquids on Mica. Langmuir 2013, 29, 11344-11353. (20) Beattie, D. A.; Espinosa-Marzal, R. M.; Ho, T. T. M.; Popescu, M. N.; Ralston, J.; Richard, C. J. E.; Sellapperumage, P. M. F.; Krasowska, M. Molecularly-Thin Precursor Films of Imidazolium-Based Ionic Liquids on Mica. .J Phys. Chem. C 2013, 117, 23676-23684. (21) Liu, Y. D.; Zhang, Y.; Wu, G. Z.; Hu, J. Coexistence of liquid and solid phases of Bmim-PF6 ionic liquid on mica surfaces at room temperature. J. Am. Chem. Soc. 2006, 128, 7456-7457. (22) Cuadrado-Prado, S.; Dominguez-Perez, M.; Rilo, E.; Garcia-Garabal, S.; Segade, L.; Franjo, C.; Cabeza, O. Experimental measurement of the hygroscopic grade on eight imidazolium based ionic liquids. Fluid Phase Equilibr. 2009, 278, 36-40. (23) Freire, M. G.; Carvalho, P. J.; Fernandes, A. M.; Marrucho, I. M.; Queimada, A. J.; Coutinho, J. A. P. Surface tensions of imidazolium based ionic liquids: Anion, cation, temperature and water effect. J. Colloid Interf. Sci. 2007, 314, 621-630. (24) Kilaru, P.; Baker, G. A.; Scovazzo, P. Density and surface tension measurements of imidazolium-, quaternary phosphonium-, and ammonium-based room-temperature ionic liquids: Data and correlations (vol 52, pg 2306, 2007). J. Chem. Eng. Data 2008, 53, 613-613. (25) Freire, M. G.; Neves, C. M. S. S.; Carvalho, P. J.; Gardas, R. L.; Fernandes, A. M.; Marrucho, I. M.; Santos, L. M. N. B. F.; Coutinho, J. A. P. Mutual Solubilities of water and hydrophobic ionic liquids. J. Phys. Chem. B 2007, 111, 13082-13089. (26) Freire, M. G.; Santos, L. M. N. B. F.; Fernandes, A. M.; Coutinho, J. A. P.; Marrucho, I. M. An overview of the mutual solubilities of water-imidazolium-based ionic liquids systems. Fluid Phase Equilibr. 2007, 261, 449-454. (27) Jacquemin, J.; Husson, P.; Padua, A. A. H.; Majer, V. Density and viscosity of several pure and water-saturated ionic liquids. Green Chem. 2006, 8, 172-180. (28) Lauw, Y.; Horne, M. D.; Rodopoulos, T.; Webster, N. A. S.; Minofar, B.; Nelson, A. X-Ray reflectometry studies on the effect of water on the surface structure of [C(4)mpyr][NTf2] ionic liquid. Phys. Chem. Chem. Phys. 2009, 11, 11507-11514. (29) Li, W. J.; Zhang, Z. F.; Han, B. X.; Hu, S. Q.; Xie, Y.; Yang, G. Y. Effect of water and organic solvents on the ionic
10 ACS Paragon Plus Environment
Page 11 of 12
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
dissociation of ionic liquids. J. Phys. Chem. B 2007, 111, 64526456. (30) Tshibangu, P. N.; Ndwandwe, S. N.; Dikio, E. D. Density, Viscosity and Conductivity Study of 1-Butyl-3Methylimidazolium Bromide. Int. J. Electrochem. Sc. 2011, 6, 2201-2213. (31) Widegren, J. A.; Saurer, E. M.; Marsh, K. N.; Magee, J. W. Electrolytic conductivity of four imidazolium-based roomtemperature ionic liquids and the effect of a water impurity. J. Chem. Thermodyn. 2005, 37, 569-575. (32) Li, N.; Zhang, S. H.; Ma, H. C.; Zheng, L. Q. Role of Solubilized Water in Micelles Formed by Triton X-100 in 1Butyl-3-methylimidazolium Ionic Liquids. Langmuir 2010, 26, 9315-9320. (33) Horn, R. G.; Evans, D. F.; Ninham, B. W. Double-Layer and Solvation Forces Measured in a Molten-Salt and Its Mixtures with Water. J. Phys. Chem-Us 1988, 92, 3531-3537. (34) Smith, J. A.; Werzer, O.; Webber, G. B.; Warr, G. G.; Atkin, R. Surprising Particle Stability and Rapid Sedimentation Rates in an Ionic Liquid. J. Phys. Chem. Lett. 2010, 1, 64-68. (35) Sakai, K.; Okada, K.; Uka, A.; Misono, T.; Endo, T.; Sasaki, S.; Abe, M.; Sakai, H. Effects of Water on Solvation Layers of Imidazolium-Type Room Temperature Ionic Liquids on Silica and Mica. Langmuir 2015, 31, 6085-6091. (36) Gong, X.; Kozbial, A.; Li, L. What causes extended layering of ionic liquids on the mica surface? Chem. Sci. 2015, 6, 3478-3482. (37) McDonald, S.; Elbourne, A.; Warr, G. G.; Atkin, R. Metal ion adsorption at the ionic liquid-mica interface. Nanoscale 2016, 8, 906-914. (38) Deyko, A.; Cremer, T.; Rietzler, F.; Perkin, S.; Crowhurst, L.; Welton, T.; Steinruck, H. P.; Maier, F. Interfacial Behavior of Thin Ionic Liquid Films on Mica. J Phys Chem C 2013, 117, 5101-5111. (39) Delcheva, I.; Ralston, J.; Beattie, D. A.; Krasowska, M. Static and dynamic wetting behaviour of ionic liquids. Adv. Colloid Interf. Sci. 2014. (40) Tran, C. D.; De Paoli Lacerda, S. H.; Oliveira, D. Absorption of water by room-temperature ionic liquids: effect of anions on concentration and state of water. Appl. Spectrosc. 2003, 57, 152-157. (41) Woodward, J. T.; Gwin, H.; Schwartz, D. K. Contact angles on surfaces with mesoscopic chemical heterogeneity. Langmuir 2000, 16, 2957-2961. (42) Wang, Z., the University of South Australia, 2014. (43) Balmer, T. E.; Christenson, H. K.; Spencer, N. D.; Heuberger, M. The effect of surface ions on water adsorption to mica. Langmuir 2008, 24, 1566-1569. (44) Atkin, R.; Warr, G. G. Structure in confined roomtemperature ionic liquids. J. Phys. Chem. C 2007, 111, 51625168. (45) Deyko, A.; Cremer, T.; Rietzler, F.; Perkin, S.; Crowhurst, L.; Welton, T.; Steinruck, H. P.; Maier, F. Interfacial Behavior of Thin Ionic Liquid Films on Mica. J. Phys. Chem. C 2013, 117, 5101-5111. (46) Blake, T. D. The physics of moving wetting lines. J. Colloid Interf. Sci. 2006, 299, 1-13. (47) Kelkar, M. S.; Maginn, E. J. Effect of temperature and water content on the shear viscosity of the ionic liquid 1-ethyl-3methylimidazolium bis(trifluoromethanesulfonyl)imide as studied by atomistic simulations. J. Phys. Chem. B 2007, 111, 4867-4876. (48) Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Molecular states of water in room temperature ionic liquids. Phys. Chem. Chem. Phys. 2001, 3, 5192-5200. (49) Hayes, R.; Borisenko, N.; Tam, M. K.; Howlett, P. C.; Endres, F.; Atkin, R. Double Layer Structure of Ionic Liquids at
the Au(111) Electrode Interface: An Atomic Force Microscopy Investigation. J. Phys. Chem. C 2011, 115, 6855-6863. (50) Li, H.; Endres, F.; Atkin, R. Effect of alkyl chain length and anion species on the interfacial nanostructure of ionic liquids at the Au(111)-ionic liquid interface as a function of potential. Phys. Chem. Chem. Phys. 2013, 15, 14624-14633. (51) Li, H.; Wood, R. J.; Endres, F.; Atkin, R. Influence of alkyl chain length and anion species on ionic liquid structure at the graphite interface as a function of applied potential. J PhysCondens Mat 2014, 26. (52) Segura, J. J.; Elbourne, A.; Wanless, E. J.; Warr, G. G.; Voitchovsky, K.; Atkin, R. Adsorbed and near surface structure of ionic liquids at a solid interface. Phys. Chem. Chem. Phys. 2013, 15, 3320-3328. (53) Silvester, D. S.; Arrigan, D. W. M. Array of water|room temperature ionic liquid micro-interfaces. Electrochem Commun 2011, 13, 477-479. (54) Zhang, L.; Xu, Z.; Wang, Y.; Li, H. Prediction of the solvation and structural properties of ionic liquids in water by two-dimensional correlation spectroscopy. J. Phys. Chem. B 2008, 112, 6411-6419. (55) Langmuir, I. The adsorption of Gases on Plain Surfaces of Glass, Mica and Platinum. J. Am. Chem. Soc. 1918, 40, 13611403.
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