Chisato
Takahashi, Takashi Shirai, Masayoshi Fuji
先進セラミックス研究センター年報 (2012). Vol. 1,
45-52

解説

Application of Ionic Liquids on Microscopic Observation of
Hydrated Materials
Chisato Takahashi, Takashi Shirai, Masayoshi Fuji
Advanced Ceramics Research Center, Nagoya Institute of Technology,
3-101-1 Honmachi, Tajimi, Gifu 507-0033 JAPANElectron microscopic techniques that are used as a standard method characterize micro- and nano-structured materials have been
extended for observing wet materials. Most of the microstructures of solid materials, such as ceramics, metals, polymers andcomposites in their dry conditions, are characterized by electron microscopy. However, the observation methods for hydrous
materials have still been under development because the wet condition is not suitable for the electron microscopy under highvacuum conditions. This difficulty in the characterization of the hydrous materials is a barrier to the research progress related towet materials processing.
In this review paper, developed observation methods with the aid of a hydrophilic ionic liquid (IL) for various kinds of hydrated
materials are reported. By the developed observation methods, the characterization of hydrous materials, such as seaweed, agar
gel and a HAp green body, is enabled and extended to a range of related fields. Based on the interaction between water molecules
and hydrophilic IL, the mechanism for the observation of wet materials using the hydrophilic IL has been clarified and themolecular dynamics of the IL and solvents have been studied. In addition, an IL-inorganic composite material was successfully
fabricated by mixing of IL and montmorillonite clay in the proper ratio, which suggests possibility to invent novel electron
conductive transparent thin films.

1. Introduction

Electron microscopes are presently employed as
indispensable material characterization tools due to theadvancement in technologies especially in highresolution performance images, high resolution analysis
equipments and 3D observation abilities [1-5]. Among
various electron microscopy techniques, the scanningelectron microscopy (SEM) has been popularly used in
the scientific community over 40 years due to its wide
applications in metals, semiconductors, ceramics,
medical and biological field. Compared to solidmaterials, the sample preparation for biological or otherwet materials to be observed by SEM is complicated andeven difficult to maintain the morphology in a vacuum
condition. Therefore, to observe the exact morphologiesof these wet samples, new methodologies are being
searched.
Ionic liquids (ILs) are organic fused salts that consistof ions and retained fluid at room temperature (RT).
Some ILs exist below 100 ℃ [6]. These compounds
consist of an organic cation and an organic or inorganic
anion. The cation, which has a large and asymmetricstructure, prevents the formation of a crystal structure,such as NaCl, and hence it remains as a liquid at RT.
Most of the ILs are classified in seven families based on

the structures of their cation and anion parts (Figure 1).
The different combinations of cations and anions
significantly affect the IL properties, such as polarity,
melting point, hydrophilicity, hydrophobicity, etc. ILs
have unique physical properties, such as negligiblevapor pressure (up to 5 × 10-9 Torr), non-flammability,
chemical/ thermal stability, high ionic conductivity (upto 120 mS cm-1) and wide electrochemical windows (upto 5.8 V). ILs are used as solvents in various chemical
reactions [7-11], or in dispersion of carbohydrate
polymers such as cellulose and starch etc [12-14].
Furthermore, a composite gel mixed with IL is used in
drug delivery systems and fabrication of contact lenses[15-17]. Besides these applications, ILs mixed withsamples can be directly observed in the electron
microscope due to their negligible vapor pressure andhigh conductivity [18-21]. Although some reportssuggest that wet materials (seaweed, chicken tissue etc)
mixed with hydrophilic IL can be observed by SEM
without any additional conducting coating [22, 23], still
this technique needs to be further applied to other kindof wet materials.
In this review paper, an optimization method for the
microscopic observations of hydrous materials ispresented, and the study of the observation mechanism― 45 ―

Application of Ionic Liquids on Microscopic Observation of Hydrated Materials

Figure. 1. Types of cations and anions that constitute the ILs.

using a hydrophilic IL is proposed. Furthermore, thefabrication of composite materials with the aid ofvarious ILs has been described with the aim ofenhancing the conductivity of materials in various fields.
2. Observation method of hydrous materials withthe aid of IL by an electron microscope

The unique properties of RTILs, such as conductivity
and negligible vapor pressure, have opened up new
scientific methodologies under vacuum conditions. In
2005, Scherson et al. reported that RTILs can be used
under a high vacuum condition (less than 5 × 10-9 Torr)[24]. Subsequently, RTILs were the focus as solvents for
vacuum technology. This discovery is a breakthroughmethod in this scientific field, which defies the common
wisdom that liquid evaporates under vacuum conditions.
Interestingly, samples containing ILs can be observed
using an electron microscope (SEM / TEM) under highvacuum conditions. Generally, the samples for electron
microscopy need a coating of osmium, platinum or
carbon. However, ILs provide a conductive and anegligible vapor pressure properties to the samples
though they are liquids. It is reported that electrons,
which are injected into the ILs, are stable and move into
the liquid phase of the ILs under a high accelerating
voltage condition [25]. Based on this discovery,
Kuwabata et al. developed the observation method of
insulating samples using ILs [18,19,21,22,26]. The
insulating star-like sand was observed using ahydrophobic IL, [EMI][TFSI], and abrasive paper was

also observed using a hydrophobic IL, [BMI][TFSI].
The TiO2 nano-tube layers in a wet condition using
droplets of ILs were characterized in order to study thewetting behavior [20]. Surprisingly, the possibility of
microscopic observations of materials containing water
was also suggested, and many researchers reported thefine morphology and observation method of various
insulating samples using ILs. The cellular ultrastructureof a cultured human cell was characterized usinghydrophilic and hydrophobic ILs [27-29]. It has been
revealed that hydrophilic ILs are useful for observing themorphology of wet materials compared to hydrophobic
ILs. By using hydrophilic ILs, the fine structure of achromosome was observed [30]. Various treatments on
ILs have been examined, such as pre-warming, wellmixing, reduced concentration, mixing with platinum
blue staining, etc., to optimize conditions. It isconsidered that the ultrastructure of the basidiospore
ornamentation is important in the delimitation of taxa
for fungi. The use of hydrophilic ILs has enabled thedetermination of the ultrastructure of the basidiospore
ornamentation [31]. Moreover, using Choline-like ILs,
which have high penetration ability, the morphology ofseaweed and food samples was observed [23].
However, in this process, one must take note of thehygroscopic properties of the hydrophilic IL wherein theabsorption of some amount of water from theatmosphere occurs. Depending on the amount ofabsorbed water, the IL properties, such as viscosity,
conductivity and polarity, can be drastically changed.
Therefore, we need to optimize the observation method
based on the interaction between water molecules withinhydrous sample and hydrophilic ILs. In order to cover
the entire range of the inorganic and hydrous materials
research fields, biological polymers (agar gel), ceramics
green bodies (hydroxyapatite) and biological materials(seaweed) will be selected. Biological polymers,
ceramics green bodies and biological materials wereselected as the polymer materials, insulated inorganic
materials and organic materials, respectively.
2.1. Biological polymer (Agar gel)Among various materials, agar gel in its wet conditionis popularly used in biology, medicine and foodindustries. Especially in biochemistry, the agar gel iswidely used in culturing media of microbes and in
cataphoresis. Furthermore, the agar gel is also used as agelling agent for various ceramic forming processes due
to hazardousness and non-toxicity as compared to acryl― 46 ―

Chisato Takahashi, Takashi Shirai, Masayoshi Fuji

amides in traditional gel-casting method. In this method,
the agar gel enables to make complex network alongwith well distributed ceramic particles [32-34].
However, morphology of such complex network
structure in wet conditions is difficult to be observed
accurately under an electron microscope in vacuum.
Therefore, we need to develop the simple observation
method for hydrous agar gel.
With this concept, the observation method for agar gelwith the aid of hydrophilic IL; 1-butyl-3methylimidazolium tetrafluoroborate; [BMIM][BF4]
was developed. It has been noticed that addition of asmall amount of water to hydrophilic ILs changed theirproperties due to the interaction with the watermolecules [35-37]. Thereby, we optimized the IL
treatment on the basis of the interaction between watermolecules and hydrophilic IL [38-40]. Figure 2 shows
the FE-SEM images of agar gel in IL with 15-95 mol %water concentration. From the FE-SEM and size
retention before and after IL treatment results, it was
found that the exact morphology of agar gel in various
swelling condition can be achieved by adjusting thewater concentrations in the range of 15-30 mol % in thesamples. An exact morphology of swelled agar gel was
clearly observed with less than 1 % difference even in apaste condition with large amounts of liquid phase. The
surface morphology was not changed even if the sample
was kept under vacuum at 60 °C for 24 h and in FESEM chamber. This electron microscope observation
will be very important for understanding the exact

morphology of water-containing materials. In addition,
the approach on microscopic observation of wet agar gel
in different swelling condition opened up a new
methodology to observe hydrated samples in FE-SEM.
2.2. Ceramics green body (Hydroxyapatite)

Among the various types of calcium phosphate
ceramics, hydroxyapatite [Ca10(PO4)6(OH)2, HAp] is awell-known biocompatible material and is naturally
found as one of the major components of bone and tooth[41,42]. As-synthesised HAp ceramics show both
osteoconductivity and osteoinductivity and directly
bonds to the living bone through the native apatite phase[43,44]. However, for enhanced bone integration andmechanical interlocking between implanted devices andthe newly grown bone tissues, particular attention mustbe paid to the processing of the porous HAp body [45].
To establish enhanced processing conditions forceramics, it is necessary to understand the complex
structure of the green body. Thus, it is essential todevelop new techniques for microscopic observations ofceramic bodies in the hydrated form.
Therefore, our research group attempted to develop a
simple and convenient method for observing the
microstructure of the hydrated porous HAp green bodyusing a hydrophilic IL. The FE-SEM observations(Figure 3) showed that the as-prepared porous green
body had a pore diameter of approximately 300-600 mm,
which gradually decreased to approximately 200-400
mm during drying in a humid chamber from 90 to 50 %relative humidity. When HAp sample is added into IL,
it bonds with water molecules within the HAp-polymer

Figure. 3. FE-SEM images of (a) the as-prepared porous HAp
Figure. 2. FE-SEM images of agar gel in IL with (a) 15, (b)green body just after removal from the mould and of the30, (c) 60, (d) 80 and (e) 95 mol% water concentration. The

specimens treated in the humid chamber at (b) 90, (c) 80, (d)accelerating voltage for FE-SEM observation was 5.0 kV.

70, (e) 60 and (f) 50% relative humidity.
― 47 ―

Application of Ionic Liquids on Microscopic Observation of Hydrated Materials

network structure, thereby forming a weak hydrogenbond entangling with the HAp particles surrounding the
porous body. This bonding resulted in the formation of
an IL-water-HAp-polymer complex, in which the IL acts
as a conducting media; additionally, the formation of
weak hydrogen bonds between the IL and the water
further prevents the porous HAp body from dehydration
caused by evacuation of FE-SEM chamber. Further, thelimitations of observing the hydrated porous HAp green
body by micro-focused X-ray CT were revealed [46,47].
Thus, we successfully revealed the solidification
behavior of a water-HAp-polymer complex structureduring the gelcasting process. This simple method of
observation can also be applied to studies on
microstructures of a wide range of nonconductive
ceramic materials during wet processing.

tetrafluoroborate; [BMIM][BF4].
We suggested that the optimization method ofseaweed as biological materials for FE-SEM observation
was required. This is because the observation
mechanism for biological materials was different fromgel materials. It is then concluded that the osmoticpressure of biomaterials in IL solution affects its
configuration. Moreover, an excess IL around sample
disturbs acquisition of a fine morphological image;
however, a centrifuge with optimized rotation speed
enables to solve the problem [48]. Our developed
methodologies, for biological materials using the typicalcommercially available IL, are novel techniques thatallow observing the exact morphology without any
alteration.
3. Observation mechanism

2.3. Biological materials (Seaweed)

It has been reported that the microstructures of
biological materials mixed with ILs can directly beobserved by electron microscopy [18,21,22]. However,some reports suggested that wet materials, such as
seaweed, chicken tissue, and etc, mixed with acommercially available IL are not suitable forobservation by SEM [23]. Furthermore, an excess IL
around the sample is reported to be a problem for SEM
observation. Therefore, a modification in the sample
preparation method is required.
We have proposed that the exact morphology of thewet material such as agar gel or gelcast ceramics can beobserved by FE-SEM. However, microstructures of
biological materials such as seaweed could notsatisfactorily be observed by applying a similar
methodology. Therefore, we have developed a modified
method to observe the exact morphology of seaweed
using hydrophilic IL; 1-butyl-3-methylimidazolium

Figure. 4. Optical microscope and FE-SEM images of theseaweed before and after IL treatment.

We were able to establish the microscopic observation
methodology of hydrated materials (Section 2). It can beconcluded that the water concentration within thehydrous sample after the hydrophilic IL treatment isbeneficial for the optimization during the observation
method. The interaction between the water molecules in
the hydrous sample and hydrophilic IL has occurred
during the sample preparation. Therefore, we focused on
the molecular dynamics and studied the observation
mechanism of hydrous materials using hydrophilic IL
and agar gel as a sample.
Some researchers have started to point out the lack ofunderstanding of the IL structure that affects its physicalproperties when using the ILs. Therefore, the possible
presence of water in the RTILs affects their solvent
properties, such as electron conductivity, viscosity,
polarity, and etc., has been reported using a variety ofexperimental techniques and theoretical calculations [4953]. These results showed that both cations and anions of
IL played a significant role for the formation ofhydrogen bond with a water molecule, although anions
always participate dominantly [54, 55]. Also, theinteraction between OH (derived from water) and IL(both anion and cation) in different circumstances areobserved by FT-IR spectroscopy and Raman
spectroscopy [56-59].
With this concept, the observation mechanism wasstudied in terms of hydrogen bond between anionic partof IL and water molecules (also water molecules within
agar gel) by Raman spectroscopy and differential
scanning calorimeter (DSC). Based on the result of thein-situ Raman spectroscopy, DSC and FE-SEM, it is― 48 ―

Chisato Takahashi, Takashi Shirai, Masayoshi Fuji

4. Fabrication of the intercalated compounds
using various kinds of ILs

Figure. 5. The Raman spectra of IL solution after sample
preparation treated with different water concentrations (x: unitof water in mol %). (a) Keeping in a desiccator for 3 h, (b)keeping in a desiccator for 3 h + under vacuum condition at 60°C for 24 h. The arrows show the peak of the nSS(OH), nSS(CH3)and nAS(CH3) stretching vibration region.

concluded that the exact morphology of agar gel can beobserved when IL penetrate into agar gel completely.
Figure 5a shows the water molecules within the agar gel,which were taken by IL when the samples were kept ina desiccator for 3 h as a preparation condition for FESEM observation. IL and water exist as BF-4 … HOH …
BF-4 via weak hydrogen bond indicated as nSS(OH) evenin vacuum condition due to its strong bonding ability(Figure 5b). It is suggested that the water molecules
within agar gel were displaced by IL owing to the similar
weak hydrogen bond formation. However, when there is
not enough IL, water molecules are not completely taken
up by IL. Therefore, it could be understand thatmorphology of agar gel was changed due to easy
evaporation of residual water within agar gel. Fromthese results, it was found that the exact morphology ofagar gel or any gel materials could be obtained byadjusting water concentrations in the range of 15-30 mol% in the samples [38-40]. Thus, the agar gel or any othermaterials under wet condition excluding biologicalmaterials can be observed without drying using theelectron microscopy based on this anomalous
mechanism. In addition, the information on the specificinteraction between molecules can provide further
understanding for the future research on molecular
dynamics study.

Clay minerals are remarkably often used in our daily
life due to their abundant resources, high sorption
capacity, ion exchange properties and low cost. Clayshave layer structures and are used as host materials forfabricating hybrid composites [60]. Among various
applications, thermal stability of clay - organic
compound has been provided by improving physicalproperties and reducing flammability [61,62]. IL
modified clays are recently paid attention due to ILs
unique properties such as negligible vapor pressure,nonflammability and electrical conductivity for
fabricating inorganic clay - organic intercalation
compounds with high thermal stability and composite
materials with improved flame retardant property[63,64]. However, most of the cases intercalation was
carried out using polymer and IL together. In addition,some researchers studied the intercalation of clay byvarious ILs [65,66]. Thereby, we focused on IL
intercalation into clay without any additional compoundor solvent torealize a more convenient and eco-friendly
method is discovered.
In recent years, transparent electrically conducting
thin films such as Indium tin oxide (ITO) and Antimony
tin oxide (ATO) are utilized for a solar battery, liquid
crystal and organic electroluminescence (EL) display[67,68]. However, other possible materials fortransparent electrically conducting thin films are still
being researched.
Hence, the direct intercalation of an IL into themontmorillonite (M) is attempted and fabricated the ILM intercalated compounds using four kinds of ILs (MIL).
These ILs consist of the imidazolium or ammoniumtype cation with different kinds of anions. Four kinds ofIL and montmorillonite intercalated compounds was
directly fabricated by mixing IL of different salts(imidazolium and ammonium) and cation size andmontmorillonite clay [69]. The XRD results showed thatthe cation size of the ILs influenced the extent of crystal
swelling, and the TEM-EDS further confirmed theintercalation of each IL into montmorillonite interlayer.
The crystal swelling behavior as observed by XRD was
further verified by TEM-SAED. It can be seen in Figure
6 that the structure consists of the swelled-like area andit maintains the crystal structure. From the cationexchange capacity (CEC) results and mathematical
calculations, extent of intercalation and arrangement of
cations in the interlayers of montmorrillonite clay is― 49 ―

Application of Ionic Liquids on Microscopic Observation of Hydrated Materials

sample was centrifuged.
B: (1) Sample was preserved by each sampling
method for adjusting the osmotic pressure as a naturalenvironment. (2) Sample was treated by IL diluted inwater to maintain its osmotic pressure and adjusted
using a 30 mol % water concentration. (3) Sample was
kept in a desiccator for 2 h and then put under vacuumfor 24 h. (4) To remove the excess IL from aroundsample, the sample was centrifuged.

Figure. 6. TEM images of the intercalated compound inbright-field images (a)(b), high-resolution image (c), and SAED
pattern in which the Debye rings are measured (d). The
subscript M is derived from montmorillonite.

proposed. In addition, the XRD and CEC results
confirmed that IL intercalated into the montmorillonite’sinterlayer within 1 min of mixing and attained asaturation level very rapidly. The TG-DTA measurementof MIL intercalated compounds showed an improved
thermal stability as compared to IL.
This observation technique is useful to understand theintercalated behavior and swelling structure of layeredmaterials with the liquid phase. In addition, a low sheet
resistivity of montmorillonite-IL composite indicates apossibility of such a better thermally stable material asa transparent electrically conducting thin film.
5. Summary and outlook

In this review paper, it is concluded that the developed
methodology using an IL helped to observe the hydrated
materials in various fields by the electron microscopy.
The materials can be classified for observation as
follows:
A. Hydrated insulating materials, such as an agar gel, a
HAp green body and montmorillonite swelled by water,andB. Biological materials, such as seaweed, which weremaintained by osmotic pressure.
Each optimization method was described as thefollowing and shown in Figure 7.
A: (1) Sample was treated with IL and adjusted by 30
mol % water concentrations. (2) Sample was kept in a
desiccator for 2 h and then put under vacuum for 24 h.
(3) To remove the excess IL from around the sample, theFigure. 7. Observation method for (a) hydrated insulating
materials and (b) biomaterials.

These observation techniques are useful forunderstanding the exact morphology and behavior of
any kind of hydrated materials, including biologicalmaterials and inorganic materials. The understanding ofa visualization mechanism using a hydrophilic IL canalso contribute to studying the molecular dynamics ofthe IL and solvents, such as water and ethanol. In
addition, the fabrication of the montmorillonite-IL
composite materials willead production of a better
thermally stable material as a transparent electricallyconducting thin film. Thus, these applications with theaid of ILs in this review will contribute to further
development of science and technology. Although ILs
are not widely applied except for chemical scientists, it
has a great potential for scientific research undervacuum condition. Therefore, this developed
observation methodology, molecular dynamics study ofIL and solution, and fabrication of composite materials
using IL presented in this review might provide better
information for readers who start to IL research invacuum condition. Hopefully, many researchers candevelop their works related to IL through the applicationof our techniques and studies.

― 50 ―

Chisato Takahashi, Takashi Shirai, Masayoshi Fuji

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