Electron Microscopic Observation of Fine Morphology of Wet Agar Gel Using a Typical Hydrophilic Ionic Liquid, 1-Butyl-3-Methylimidazolium Tetrafluoroborate

Chisato Takahashi, Takashi Shirai and Masayoshi Fuji

Submitted: 28 September 2011, Accepted: 24 November 2011

Topics Covered

Abstract

Ionic liquid (IL) enables wet samples to be observed using an electron microscope due to their negligible vapor pressure and high conductivity. In the present study, we successfully observed the fine morphology of wet agar (organic polymer) swelled by water in presence of IL [1-butyl-3-methylimidazolium tetrafluoroborate; [BMIM][BF₄]] using a field emission scanning electron microscope. The displacement rate of IL to the agar gel, the behavior of ethanol and the presence of water in the agar gel were also investigated using Raman spectroscopy from the ratio of the CH and OH stretch vibrations at 2800 to 3800 cm^-1. The technique and analysis allowed the exact morphology to be observed and understand the mechanism of various water-containing samples such as polymer, plant and food substance.

Keywords

Ionic Liquid, Gel, Water, Hydrogen Bonding, FE-SEM, Raman Microscopy

Introduction

Room-temperature ionic liquids (RTILs) are organic fused salts that consist of ions and remained as fluid below 100 °C. The first RTIL (1-ethyl-3-methylimidazolium tetrafluoroborate; [emim][BF₄]) was reported in 1992. This finding accelerated the research involving ionic liquids [1-3] and attracted significant attention as an environment-friendly solvent and lubricant. This is due to their unique properties such as non-volatile, non-flammable, chemical/ thermal stability and high conductivity.

The different combinations of cations and anions significantly affect the IL properties such as polarity, melting point, hydrophilic and hydrophobic. Due to its negligible vapor pressure and high conductivity, samples containing IL can be observed using an electron microscope (SEM / TEM) under high vacuum conditions. [4, 5] Based on this approach, an in-situ electrochemical SEM observation including electron dispersive X-ray fluorescence (XRF) analysis have been developed [6]. Recently, sputter deposition onto the ionic liquid, a breakthrough method, improved the dispersion of metal nano particles without additional chemical species. [5, 7-10] Interestingly, the IL was used in biomaterials and agro materials owing to improve their properties such as compatibility and thermal stability etc [11-13]. In addition, studies involving the formation of conductive polymer using IL showed positive results especially in the field of life science [14-19]. However, in this process, one must take note of the hygroscopic properties of the hydrophilic IL wherein the absorption of some amount of water from the atmosphere occurs. Depending on the amount of absorbed water, the IL property such as viscosity, conductivity and polarity can be drastically changed [20]. Many researchers have reported the interaction between IL and water in various experimental and simulation methods [21-26].

Recently, agar gel is popularly used in the forming ceramics as an organic monomer to make a complex network containing ceramics particles. However such a wet complex structure is difficult to be observed accurately under vacuum condition.

In the present study, we report the fine morphology of water-containing agar gel using typical imidazolium IL by FE-SEM under high vacuum. Also we examined the effect of water content in the agar gel and the presence of ethanol in agar gel using Raman spectroscopy. Furthermore the displacement rate of IL into the water-containing agar gel was also investigated in order to understand the displacement mechanism of IL.

Experimental Methods

Materials

The hydrophilic 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]) used as the IL (Kanto Chemical Co., Japan) was dried in a vacuum desiccator. The water content of [BMIM][BF₄] was 142 ppm as measured by Karl-Fischer titrations (Kyoto Electronics Co., Japan. MKA-610).

In the present study, commercially available agar S-10 (Ina Food Industry Co., Japan) was used. The solidification and melting points were 30 °C and 90 °C, respectively. Ultrapure water (ADVANTEC Co, Japan) and agar were mixed and heated at 150 °C for 5 min. The water content of agar gel was determined by Karl-Fischer titrations. The ethanol was used after removal of the water molecules using molecular sieve (Wako Co., Japan).

Morphological Observations by Laser Microscope and FE-SEM

The morphology of agar gel was observed by a laser microscope (SHIMADZU, OLS 4000) and a JEOL JEM 7600F FE-SEM (operated at 5.0 kV). We used an agar gel with 81. 27 wt % water content (determined by Karl-Fischer titrations). The untreated agar gel was placed on a Cu (3mm ϕ) mesh in a Petri dish (150 mm ϕ x 10 mm in height) and the morphology was observed using a laser microscope in air. After observation by laser microscope, the agar gel was cut (sample size: 2.5 x 2.5 x 2.0mm³) and treated with IL diluted by ethanol which was already mixed in the ultrasonic bath for 30 minutes. It was set in a Petri dish for 2 h in the desiccator. The sample was picked up and mounted on the SEM mounting with a small amount of the IL solution. After vacuum drying over 24 h, the same region of agar gel was observed by FE-SEM. In order to obtain a thin coat of IL, the agar gel was treated with IL diluted by ethanol at several concentrations as trial experiments [6]. When concentration of IL was high, the surface of the agar gel coated with IL become chunky that caused the surface to be unidentifiable. At low IL concentration, a thin IL coating was produced and then dried sufficiently before putting into the vacuum dryer. The agar gel treated with various concentrations of the IL solution was observed by a FE-SEM (used the proportion of materials as 1 : 10 (IL : ethanol on weight basis).The agar gel was prepared using agar, water, IL and ethanol (amount weight ratio; 1: 10 : 293 : 2930) so that the sample contain 30 mol % H₂O. The agar gel without IL was prepared under the vacuum at 60 °C for over 24 h, and coated with osmium using sputter coater (Filgen Co., Japan. OPC 60 A) as a reference. SEM images were taken in the same area for 1 h at 15-min intervals in high magnification to check the stability for long period. The IMAGE PRO-PLUS^TM analysis software was used for analyzing the morphology.

Effect of Water Content in Agar Gel on Displacement of IL

The different water-containing agar gels were prepared by mixing agar and water in the weight ratio; 1: 10 and 1: 20 and abbreviated as A and B, respectively. The agar gels were prepared at the pyrex tubes of 5.0 mm ϕ, 0.8 mm wall thickness. (See the Supporting Information Figure S1) Moreover, the total water content of the IL and the agar gel was kept 30 mol % in this study. Raman spectrum was measured at room temperature by a JASCO NRS-3100 laser Raman spectrophotometer equipped with a single monochromator and a CCD detector. The Raman spectrum of the IL + agar gel was taken at the agar side 2.0 mm away from the IL / agar gel interface and 1.0 mm depth from the pyrex surface from 0 to 40 minutes at 5 minutes interval in the range of 2800 to 3800 cm^-1 using the pyrex tubes and averaged 5 times. The Raman spectrum was taken at 2.5 mW of the 514.5 nm line using an Ar ion laser as the excitation source.

Effect of Ethanol in Agar Gel on Displacement of IL

The agar gel was cut into 2.5 x 2.5 x 2.0 mm³ in size and treated with IL + ethanol and kept in a desiccator from 0 to 60 minutes. The Raman spectrum was taken on the surface of the agar gel as described above in the range of 2800 to 3800 cm^-1.

The agar gel 2.5 x 2.5 x 4.0 mm³ in size was treated with IL + ethanol for 2 h and kept in a desiccator. After keeping in vacuum for over 24 h (same as SEM sample preparation), the agar gel sample was cut (2.0 mm in height) using a razor blade. The Raman spectra were taken on the surface of agar gel at 0.0, 1.0 and 2.0 mm across the centre from one end to the other end.

Results and Discussion

Morphology of Agar Gel Using Laser Microscope and FE-SEM

Figure 1a and b shows laser microscope and FE-SEM image of agar gel treated with IL + ethanol. For comparison, FE-SEM of agar gel without IL was observed and is shown in Figure 1c.

It can be seen from Figure 1a that most of the sample parts were flat with no asperities even on the surface. Although the agar gel with IL was exposed to the vacuum, the morphology of the agar gel could be clearly observed (as seen in Figure 1b). The SEM image of the agar gel using osmium coating (Figure 1c) showed the aggregates to be significantly different from that of Figure 1b. Though the surface of the agar gel treated with the IL was flat, the morphology of agar gel using this method was a layered structure with wrinkles. From this result, it can be concluded that the conventional method was not suitable to observe the accurate morphology of water-containing materials using FE-SEM due to the vaporization of the water molecules from the agar gel inside the chamber. On the other hand, there was no difference in the morphology of agar gel using laser microscope or FE-SEM without IL or with IL, respectively. Then, morphology of the agar gel treated with IL (30 mol % H₂O) was remained stable and could be accurately observed without any charging at a low accelerating voltage. Furthermore, the agar gel and IL did not show any interaction. This result showed that the morphology of the agar gel was not drastically changed by the IL solution even if the IL was diluted by ethanol.

Figure 2 shows FE-SEM images of the agar gel using IL at different time interval. For comparison, morphology of same region as that of Figure 1b at a high magnification was chosen. Because the sample was observed using various magnifications at different time, there were some movements in comparison to the images with respect to the starting time. Results showed that the water-containing agar gel could be clearly observed at high magnification and the fine structure of the surface morphology remained unchanged.

There were no significant differences in the form of the individual images from the point of view of visibility, although the water and IL interact with each other [26]. The agar gel with IL was stable enough to be observed except for the cases when high accelerating voltage and high intensity beam was used. The morphology of the agar gel was clearly observed even at high magnification by this method (see in the Supporting Information Figures S2, S3 and Table S1).

Figure 1. Comparison of (a) laser microscope image of agar gel, (b) FE-SEM image of an agar gel + IL diluted by ethanol and (c) FE-SEM image of an agar gel using osmium coating. The accelerating voltage for FE-SEM observation was 5.0 kV.

Figure 2. Magnified FE-SEM images (Accelerating voltage, 5.0 kV) of agar gel + IL diluted by ethanol at (a) 0, (b) 15, (c) 30, (d) 45 and (e) 60 min.

Effect of Water Content in Agar Gel on Displacement of IL

Figure 3 shows the Raman spectra of IL + agar gel prepared with different water content at different time interval. In this study, we consider the CH and OH stretching vibration in the region of 2800 to 3800 cm^-1 due to interaction between the water and IL from 0 to 40 min. The symmetric and asymmetric CH stretching mode of the [BMIM]⁺ cation appeared in the region of 2800 to 3200 cm^-1. The broad peak in the range of 3000 to 3800 cm^-1 of the bulk water (OH group) represents the non homogeneous environment of water molecules due to hydrogen bonding. The observed CH stretching vibrations region of 2800 to 3050 cm^-1 was mainly attributed to the agar gel. From Figure 3a, it can be seen that, when IL was mixed with A mainly showed the OH spectrum of water molecule in the initial period. Interestingly, the intensity of CH vibration peak of the [BMIM]⁺ cation gradually increased up to 40 minutes. On the other hand, the spectrum of Figure 3b was mainly formed by the OH spectral range of water molecule even after 40 minutes, although a small intense CH peak appeared around 3163 cm^-1 from the CH normal mode vibrations of hydrogen attached directly to the imidazolium ring. The hydrophilic IL is known as highly hygroscopic, and interacts even with a small amount of water. Previous studies reported the IL + water interaction [20, 22 and 26] wherein hydrogen bonding had an important role in the interactions during the IL and water. Cammarata and co-workers obtained OH and CH peaks of the IL + water mixtures at various concentrations using Raman spectroscopy [20]. In the present study, the OH spectral range of the water molecule was much wider than the results published in the literature, even though we have used the same condition (30 mol % H₂O in the agar gel and IL) [27, 28]. Hence, the displacement of IL to the agar gel was confirmed to be occurred at a slow rate.

Figure 3. Raman spectra of IL + agar gel consist of 30 mol % H₂O in the region of 2800 to 3800 cm^-1 at different time (t express in minute). The arrows show the strong peaks of the OH and CH stretching vibration region in the range of 2800 to 3700 cm^-1. (a) Water and agar in 10: 1 ratio (b) Water and agar is in 20: 1 ratio.

Figure 4a shows the Raman spectra of IL + water with varying water content. In order to understand the displacement of the IL into the agar gel, the peak height ratio ν_ASCH₃ to ν_LiqOH of IL + different water content as obtained from Raman spectroscopy (see in Figure 4b). Figure 4a represents 2966 cm^-1 as the IL peak and 3450 cm^-1 as the water peak. The stretching mode peak at 2966 cm^-1 was assigned to ?asCH3 (as represents the antisymmetric stretch mode) of butyl chain from cation and stretching mode peak at 3450 cm^-1 was assigned to ν_LiqOH (Liq represents liquidlike peak) of hydrogen bonding [20, 28-30]. The water concentration (IL concentration) can be obtained using the peak height ratio ?ASCH3 to ?LiqOH of spectra of the IL + water mixtures and is represented as

     y = (42.647 - x) / 0.426      (1)

where, y is the water concentration; x is the peak height ratio ν_ASCH₃ to ν_LiqOH. Based on the equation (1), we can obtain the water concentration during the mixtures of agar gel + IL. After the CH stretch vibration regions of 2800 to 3050 cm^-1 derives from the agar gel were completely removed, the peak height ratio ν_ASCH₃ to ν_LiqOH of the spectra of agar gel + IL was calculated. (See in Figure 5a) Using this results, the water concentration during the agar gel + IL was obtained from formula (1) (see the Supporting Information Table S2). Therefore, amounts of displaced IL can be calculated from the IL concentration and water content within agar gel. The approximate graph of the IL displacement rate is shown in Figure 5b. The IL displacement rate of A (Figure 3a) was 4.56 x 10^-4 cm³ / min, while the B (Figure 3b) was 7. 78 x 10^-5 cm³ / min. Up to 5 minutes, the displacement rate of IL to B was much faster compared to the agar swelled by A, however, after 15 minutes, the IL displacement rate was reversed. Based on the result of the in-situ Raman spectroscopy the IL displacement rate was quite slow and the displacement rate of agar gel using a large amount of water was slower than using small amount of water. We suggested that the IL first interacts with the water molecules on the surface of the agar gel, but the IL started to interact with water molecules that were brought into the three dimensional network of the double helix structure after 15 minutes. Based on these results, the agar gel using small amount of water easily interacts with IL by the water molecules that produced the three dimensional network of the double helix structure.

Behavior of Ethanol in IL Diluted by Ethanol + Agar Gel

The Raman spectra in the CH-stretch region of IL diluted by ethanol + agar gel at different time interval in air are shown in Figure 6. The agar gel was prepared with agar and water in the amount ratio of 1: 10, and treated with IL diluted by ethanol. In order to understand the behavior of ethanol used in the SEM observation, the Raman spectra were measured during treatment with the IL solution. The surface of the agar gel (which directly contacts the IL solution) was selected for the Raman analysis due to not paying attention to the displacement of the IL.

In this study, we considered the region of 2800 to 3200 cm^-1 derived from the CH stretch vibration region of agar, ethanol and IL. At the beginning of the IL treatment (0 to 10 minutes), the ethanol peaks appeared in the CH stretch vibration region and after 20 minutes, the IL peak appeared clearly. As observed, the peak at 3163 cm^-1 from the CH normal mode vibrations of hydrogen attached directly to the imidazolium ring starts to significantly increase and the peak at 0 minute shows OH stretching vibrations derived from the water in the agar gel. The spectrum of ethanol was not observed beyond 30 minutes. Contrary to expectations, it appeared that ethanol into the agar gel volatilized during the 2-h treatment of the IL solution without vacuum condition.

Figure 4. (a) Raman spectra of IL with different H₂O concentrations (x express in mol % H₂O) in the region of 2800 to 3800 cm^-1. The arrows show the strong peaks of the OH and CH stretching vibration region in the range of 2800 to 3700 cm^-1. (b) Ratio of CH vibration peaks to OH vibration peaks in IL + water with increasing water concentration.

Figure 5. (a) Ratio of CH vibration peaks to OH vibration peaks in the IL + agar gel at different time. (b) Approximate graph of displacement rate of IL to agar gel.

Figure 6. Raman spectra of IL diluted by ethanol + agar gel consist of 30 mol % H₂O at different time (t express in minute). The topmost line represents spectrum of agar + IL.

Figure 7 compares the Raman spectra of IL diluted by ethanol + agar gel and IL + agar gel (both containing 30 mol % H₂O) kept in vacuum for over 24 h. The observation of the agar gel in both condition showed similar spectra at three different positions defined as 0, 1.0, and 2.0 mm. As observed, the behavior of ethanol and water into the agar gel explains the existence of ethanol and water in the SEM observations. In the spectrum of the CH stretch vibration regions, 2800 to 3200 cm^-1, the [BMIM]⁺ cation showed a similar peak both in Figure 7a and b at different positions. Although the agar-derived spectrum was included in the CH peak, the peak intensity from the [BMIM]⁺ cation was too strong to confirm the agar peak. The spectrum of the OH stretch vibration region, 3000 to 3800 cm^-1, derived from the water and agar gel had a similar peak at each position. From this result, it was revealed that ethanol had no effect on changing the OH stretch vibration region derived from water and agar. The displacement of the IL clearly occurred at the 2.0 mm depth of the agar gel, because the CH peaks of the [BMIM]⁺ cation could be mainly observed. Still the results of the OH stretch vibration region spectra were not appreciably accurate, because it can be assumed that some of the water in the agar gel vaporized during the vacuum treatment and contained an OH peak derived from the agar. The state of the water and the displacement rate of IL inside the agar gel cannot be totally confirmed by the present study.

Figure 7. Raman spectra of (a) IL diluted by ethanol + agar gel, (b) IL + agar gel consist of 30 mol % H₂O at different positions (x express in mm) after keeping in vacuum for over 24 h.

Conclusions

The fine morphology of an agar gel (polymer gel) could be successfully observed using a typical IL by FE-SEM and there was no interaction between the agar, IL and ethanol. Based on the result of the in-situ Raman spectroscopy, the displacement rate of agar gel using a large amount of water was slower than using a small amount of water. It is expected that Raman observations of agar gel will provide information for observing samples containing water. From the results, it is concluded that the exact morphology of agar gel can be observed when IL penetrate into agar gel completely. We consider that the interaction of the IL and water-containing gel was more complicated and the interaction of IL with water must be taken into consideration when observed under the SEM. Furthermore, it was found that the ethanol had no effect on the interaction of the IL, water and agar by Raman spectra and from the point of view of the SEM observations compared to the laser microscope. This observation is useful in order to realize the interaction of the water-containing sample, IL and ethanol. Moreover, this electron microscope observation will be very important in understanding the exact morphology of water-containing materials.

Acknowledgement

The authors are grateful to Dr. R. Kawano, of Kanagawa Academy of Science and Technology (KAST) and Prof. H. Saka, of Eco Topia Science Institute, Nagoya University for useful discussions.

Supporting Information

(1) Sample preparation for Raman experimental, (2) Differential ratio of line length and area of FE-SEM images of agar gel using analysis software, (3) Water concentration during the mixtures of agar gel + IL obtained from formula (1). This material is available free of charge via the Internet.

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Contact Details

Chisato Takahashi, Takashi Shirai and Masayoshi Fuji
Ceramics Research Laboratory, Nagoya Institute of Technology, 3-101-1, Honmachi, Tajimi, Gifu 507-0033, Japan

E-mail: [email protected]

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 13[1] (2011) 39-47.