Article pubs.acs.org/Langmuir

Crystallization of Confined Water Pools with Radii Greater Than 1 nm in AOT Reverse Micelles Akira Suzuki and Hiroharu Yui* Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-city, Tokyo 162-8601, Japan S Supporting Information *

ABSTRACT: Freezing of water pools inside aerosol sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles has been investigated. Previous freezing experiments suffer from collision and fusion of AOT micelles and resultant loss of water from the water pool by shedding out during the cooling process. These phenomena have restricted the formation of ice to only when the radius of the water pool (Rw) is below 1 nm, and only amorphous ice has been observed. To overcome the size limitation, a combination of rapid cooling and a custommade cell allowing thin sample loading is applied for instantaneous and homogeneous freezing. The freezing process is monitored with attenuated total reflection infrared spectroscopy (ATR-IR) measurements. A cooling rate of ca. −100 K/min and a sample thickness of ca. 50 μm overcomes the limitations mentioned above and allows the crystallization of water pools with larger radii (Rw > 1 nm). The corresponding ATR-IR spectra of the frozen water pools with Rw < 2.0 nm show similar features to the spectrum of metastable cubic ice (Ic). Further increase of the radius of the water pool (Rw > 2.0 nm), unfortunately, drastically decreased the integrated area of the ν(OH) band observed just after freezing, indicating the breakup of the micellar structure and shedding out of the water pool. In addition, it was revealed that Ic ice can also be formed in flexible organic selfassembled AOT reverse micelles for at least Rw ≤ ca. 2 nm, as well as in inorganic and solid materials with a pore radius of ca. 2 nm. The dependence of the phase transition temperature on the curvature of the reverse micelles is discussed from the viewpoint of the Gibbs−Thomson effect.

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INTRODUCTION

As well as inorganic materials, the freezing behavior of confined water in organic materials is also important for both science and technology applications, such as understanding biological activity under low temperature and the development of cell preservation techniques. In biological systems, nanospaces are formed by the self-assembly of lipids and proteins, and are ubiquitously observed. These soft organic selfassemblies with nanometer-sized pores are able to flexibly change their structures and volumes with changes in temperature and application of external forces such as pressure. Furthermore, the phase transition of confined water in organic self-assemblies sometimes drastically affects the structures and functions of the host materials themselves. Although it is an important issue, the phase transition of confined water in organic self-assembled systems is complicated and difficult to study. Here, we focus on the freezing behavior of confined water in aerosol sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles, as a representative model for nanopores formed by the self-assembly of organic molecules. The AOT molecule is an anionic surfactant with a large hydrophobic moiety that forms reverse micelles in aqueous environments.31 AOT reverse

Nanometer-sized pores are ubiquitous in both inorganic and organic materials, such as natural stones,1 clays,2 and biological cells.3 Water confined in such nanopores generally shows remarkably different thermodynamic and mechanical properties to those of bulk water at ambient conditions, such as the phase transition temperature and viscosity.4 The shape of the nanopores and their surface chemical and physical properties are considered to greatly affect the freezing and melting behavior, and the resultant ice structure of the confined water.5−30 The freezing and melting behavior of water confined in nanopores has been intensively studied using inorganic materials.5−30 This is because inorganic materials, such as MCM-41 and SBA-15, provide ideal nanopores with controllable dimensions and sizes.8−11,17−20 For example, Morishige et al. studied the freezing behavior of confined water in various pore sizes of silica materials by X-ray diffraction (XRD).8−12 They found that metastable cubic ice (Ic) formed in pores with ca. 2 nm radius instead of thermodynamically stable hexagonal ice (Ih), while both Ic and Ih formed in pores with ca. 5 nm radius.8 Interestingly, such formation of Ic has often been observed in cylindrical and interconnected cylindrical pores,5−10,16,23 and has been reported to be stable up to the melting point of ice.10 © XXXX American Chemical Society

Received: October 21, 2013 Revised: May 10, 2014

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micelles are a well-known host for confined water because they provide nanometer-sized spherical pores with well-controlled radius and narrow size distribution.31−33 The water confined in AOT reverse micelles is called the “water pool” and its radius (Rw) can be easily controlled by varying the water−surfactant molar ratio (W0 = [H2O]/[AOT]).32,33 Water pools in AOT reverse micelles provide an ideally sized model for water in biological systems, such as water at the surface of biological membranes and water at activity points in enzymes, to investigate enzymatic reactions.34,35 When investigating the role of confined water in biological systems, the phase transition behavior of such confined water is important to understand biological activity and cell preservation techniques at low temperature. However, when the phase transition of water pools has been studied, collision and fusion of the AOT micelles often occurred, resulting in shedding out of the water pools during cooling and preventing investigation of their freezing behavior. This has restricted the study of ice in AOT reverse micelles to Rw < 1 nm, and only the formation of amorphous ice has been reported.36,37 In the present study, we combined rapid cooling with small sample volumes to prevent shedding of the internal water from AOT reverse micelles, and achieved homogeneous freezing of the internal water while maintaining the structure of the micelle. We developed a custom-made cell for the attenuated total reflection infrared spectroscopy (ATR-IR) measurements that allows ca. 50-μm-thick samples and rapid cooling at ca. −100 K/min. It is expected that the combination of thin samples and rapid cooling will result in more homogeneous and instantaneous freezing of water pools in AOT reverse micelles. We also discuss the dependence of the dominant ice structure on the water−surface molar ratio.

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Table 1. Relationship between the Water−Surfactant Molar Ratio (W0) and the Corresponding Average Radius Size of the Water Pool (Rw) W0 5 10 15 20 30

Rh (nm) 2.2 2.8 3.1 3.3 5.4

± ± ± ± ±

0.1 0.1 0.02 0.1 0.3

Rwa (nm) 1.2 1.8 2.1 2.3 4.4

± ± ± ± ±

0.1 0.1 0.02 0.1 0.3

a

RW was calculated by subtracting 1.0 nm from the corresponding Rh value that was experimentally observed by DLS. that the XRD signal from ice formed in the reverse micelle will be weak because the amount of water in the system is markedly less than the surrounding organic solvent. In addition, the XRD signal from AOT reverse micelles with similar sizes will be superimposed on the signal from the water pool or ice. These factors will make it difficult to analyze the XRD spectra. Conversely, it is much easier to discriminate the signal from the water pool from that from the surrounding organic solvent by IR spectroscopy because the corresponding wavenumbers from water and organic solvent are quite different. Furthermore, IR spectroscopy has high sensitivity to water measurements because of the strong IR absorption of water molecules.46−48 These features are favorable for measuring the small amount of buried water pool in AOT micelles, and also to monitor the rapid temporal changes of the water pool. Thus, in the present study, we mainly used IR spectroscopy to monitor and investigate the structural changes of the water pools. Cooling Rates and Sample Volumes for IR Measurements. In previous studies, a cooling rate of ca. −0.50 K/min often induced the loss of internal water from the reverse micelle with relatively large micelles (W0 ≥ 5),36,40,41,49 resulting in the formation of a precipitate containing both water and AOT.42 To prevent the loss of internal water and to maintain the structure of the reverse micelle under freezing, a few studies have applied super-rapid cooling (ca. −6000 K/ min).37,50 The formation of an amorphous type of ice was observed for very small reverse micelles (W0 < 5, Rw = 1.2 nm), whereas large micelles (W0 ≥ 5) break up to form large ice clusters of size 10−500 nm, and most of the crystalline ice was found outside of the reverse micelles and coalesced into large ice crystals.37 These previous studies indicate that extremely slow or rapid cooling makes it difficult to achieve homogeneous freezing of internal water in relatively large micelles (W0 ≥ 5, Rw = 1.0 nm), and an adequate cooling rate is necessary to overcome this difficulty. In addition, to achieve homogeneous freezing of internal water, a small sample thickness is also important to prevent the generation of a large temperature gradient in the sample loaded in the chamber. This is because a large temperature gradient might induce convection in the sample solution in the sample chamber, accelerating the collision and fusion of reverse micelles and resulting in the unexpected shedding out of the water pool during the cooling process. In the present study, to reduce such unexpected convection flow in the sample chamber, the thickness of the sample was set to 50 μm, which is 10 times thinner than that in a previous study.37 Conventional Cooling (ca. −0.35 K/min) and Semi-Rapid Cooling (ca. −10 K/min). The samples were sealed between two CaF2 windows (17 × 12.5 × 0.8 mm3) with a 50 μm spacer. To produce the conventional cooling rate36,40,41,49 in our experimental system, the temperature was decreased from room temperature 298 to 193 K at 5 K intervals using a cryostat (CoolSpeK IR, Unisoku Scientific Instruments, Osaka, Japan). IR spectra were recorded 10 min after the temperature of the system was stabilized at the aimed temperature. It took almost 5 h to complete the whole cooling, waiting, and measurements, and average cooling rate was thus estimated as ca. −0.35 K/min. IR spectra were measured with Nicolet 6700 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA) with a resolution of 4 cm−1 and accumulation of 32 scans. For the semi-rapid cooling measurements, the samples were continuously cooled from 298 to 193 at −10 K/min with the cryostat.

MATERIALS AND METHODS

Sample Preparation. Sodium bis(2-ethylhexyl) sulfosuccinate (AOT) (≥96%, Sigma-Aldrich, St. Louis, MO) and n-heptane (99%, Sigma-Aldrich) were used without further purification. Water was purified by a Millipore Milli-Q system (Simpli Lab-UV, Merck Millipore, Billerica, MA). The AOT reverse micelle solution was prepared by injecting water into a 0.04 M AOT−heptane solution for transmission IR measurements and into a 0.5 M AOT−heptane solution for ATR measurements. The value of W0 in the micelles was controlled as 5, 10, 15, 20, and 30. The apparent hydrodynamic radius (Rh) of the different AOT reverse micelles was determined with dynamic light scattering (DLS) (Nano ZS, Malvern Instruments, Malvern, UK) equipped with a He− Ne laser operating at 633 nm and at a scattering angle of 90°. The AOT reverse micelle solution was prepared by injecting water into a 0.2 M AOT−heptane solution for DLS. The measurements were performed at 298 K. The observed DLS results are shown in Figure SI1. Additionally, the corresponding values of Rh and Rw are summarized in Table 1. Rw was calculated from the difference between Rh and the length of the AOT molecule (ca. 1 nm).38 IR Measurements. From the viewpoint of the phase transition of the water pools, several techniques have been applied in previous studies, including differential scanning calorimetry,39 NMR,39−41 fluorescent probes,42 and IR absorption spectroscopy.36,37 Among these probe techniques, IR absorption spectroscopy is a powerful tool that can discriminate slight changes in the hydrogen bonding network structure, namely, the band shape of the OH stretching mode (ν(OH)) to sensitively discriminate the structures of ice.43−45 For the study of the ice structure, X-ray diffraction (XRD) should also be a powerful technique to investigate the ice structure. Indeed, XRD has been frequently applied to confined water in solid inorganic materials.8−12,16,23 However, in the study of water pool in AOT reverse micelles dispersed in abundant organic solvent, it is expected B

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The IR spectra were recorded on a Nicolet 6700 FT-IR spectrometer with a resolution of 4 cm−1 and accumulation of 32 scans. Rapid Cooling (ca. −100 K/min) of Samples with ca. 50 μm Thickness. The maximum cooling rate of the cryostat used in the present study was ca. −10 K/min. To achieve a much more rapid cooling rate, we constructed a custom-made cooling unit on the ATR prism (Thermo Fisher Scientific) and cooled the sample solutions with liquid nitrogen via a thin cover glass (Matsunami, 0.12−0.17 mm thickness, C01824, Osaka, Japan) as shown in Figure 1. To set the

Figure 1. Schematic illustration of the rapid cooling cell for the ATRIR measurements. sample thickness to ca. 50 μm, 0.8 μL of the AOT reverse micelle solution was dropped on the ATR prism equipped on the Nicolet 6700 FT-IR spectrometer. The sample solution completely covered the ATR prism. The temperature of the sample solutions was monitored with a platinum resistance temperature detector (Netushin, UNR-351-100S-1-0.5-30-1000TF13-A-3-M4YS-3 mm, Saitama, Japan). The maximum cooling rate was estimated at ca. −100 K/ min and the lowest temperature was ca. 193 K in the present system. The IR spectra of the sample solutions were recorded with a resolution of 4 cm−1 and no accumulation (single scan mode). Spectral decomposition using curve fitting was performed with the equipped software (OMNIC 7.2a). The spectra were curve fitted with a Gaussian function.

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Figure 2. Comparison of the IR spectra of the water pool in an AOT reverse micelle with Rw = 1.8 ± 0.1 nm at 193 and 298 K for various cooling processes. (a) Conventional cooling (ca.−0.35 K/min), (b) semi-rapid cooling (ca. −10 K/min), and (c) rapid cooling (ca. −100 K/min). The insets indicate the corresponding temperature change in the cooling process.

RESULTS AND DISCUSSION Confined water in nanopores in inorganic materials with a radius of ca. 2 nm generally shows quite different freezing characteristics from bulk water, such as the formation of Ic rather than Ih.8,9 The reason for the different behavior is thought to be the electrostatic interaction between the polar water molecules and ionic substituent groups covering the inner surface of the nanopores.8,9 Based on these observations, we first focused on water pools confined in AOT reverse micelles with a similar radius (W0 = 10, Rw = 1.8 ± 0.1 nm) and investigated adequate freezing conditions by varying the cooling rate. At first, the water pool in the AOT reverse micelle was slowly cooled at the conventional cooling rate (−0.35 K/min), which is a similar cooling rate to a previous study.36 A structureless and broad band with a peak at about 3500 cm−1 (ν(OH)) was finally observed in the IR spectrum (Figure 2a), indicating the formation of amorphous ice in the reverse micelles, which is in good agreement with the result reported in the previous study.36 The reasons for the formation of amorphous ice under conventional cooling conditions can be considered as follows: It is widely accepted that water molecules inside reverse micelles can rapidly exchange between micelles because of fusion and subsequent dissociation of micelles at room temperature.51,52 In a slow cooling process, collision and fusion of the reverse micelles should frequently occur during the phase transition from water to ice under conventional cooling conditions. If the sample volume is large enough to induce

temperature graduations and inhomogeneity, these factors might be significant because of convection. This will prevent nucleation to the crystalline structure and lead to the formation of amorphous ice. It has also been suggested that shedding out of the internal water owing to the collapse of AOT reverse micelles is another reason for the formation of amorphous ice.36,40,41,49 To prevent the unexpected collision and fusion and/or the shedding out of the water pool, rapid freezing will be effective for reducing these phenomena by instantaneous freezing. Thus, a more rapid cooling rate of ca. −10 K/min, denoted as semirapid cooling, was applied. As shown in Figure 2b, the IR spectrum at 193 K has a band with maximum peak intensity at 3250 cm−1. Furthermore, it has two shoulder peaks at 3380 and 3150 cm−1. The appearance of the sharp peak at 3250 cm−1 and the two shoulder peaks is characteristic of the structure of ice I crystals.43−45 By comparison of the IR spectra of the conventional cooling process with the semi-rapid cooling process, the bandwidth is considerably narrower and the shape of the band is closer to that of typical crystal ice I. However, the ν(OH) band of the spectrum is still somewhat broader than the corresponding band of the ice I crystal, especially in the high wavenumber region,43−45 which may C

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indicate that a non-negligible portion of the water pool still formed amorphous ice. To achieve a much more rapid cooling rate, we used the custom-made cooling unit, which achieves a cooling rate of ca. −100 K/min and a ca. 50-μm-thick sample cell and allows simultaneous ATR-IR measurements. The IR spectrum from the rapid cooling rate is shown in Figure 2c. The IR spectrum at 193 K shows a peak at 3250 cm−1 with two shoulder peaks, and the bandwidth is narrower than that observed with semi-rapid (−10 K/min) cooling conditions. The observed ν(OH) band agrees well with that of ice I crystals.43−45 This result shows that for a 50-μm-thick sample and a rapid cooling rate of ca. −100 K/min, the water pools for Rw = 1.8 ± 0.1 nm freeze in the form of ice I, allowing us to compare the result with those observed in inorganic systems. However, before considering the ice structure and comparing the results with those observed in inorganic systems, we have to confirm whether the system remained free from the collapse of micelles and the shedding out of the water pool. In a previous study by Nucci et al. with a cooling rate of ca. −0.50 K/min, the integrated area of the observed ν(OH) band drastically decreased with decreasing temperature.36 Such a drastic decrease of the integrated area of the ν(OH) band means a remarkable change of the system, and is attributed to the local collapse of the micellar structure and shedding out of inner water or collapse of the reverse micelles. Based on these observations and assignments, we also examined the intensity changes. The changes in the integrated area of the ν(OH) band in Figure 2a,b,c are shown in Figure 3a,b,c, respectively. A drastic decrease in the integrated area of the ν(OH) band is observed in Figure 3a (the dotted arrow) from 213 to 193 K. This result indicates that local breakup of the micellar structure occurred in the freezing method with a conventional cooling rate (ca. −0.35 K/min). Conversely, drastic decreases are not observed for the semi-rapid and rapid cooling conditions (Figure 3b,c). In contrast, especially for the rapid cooling conditions, a steep increase of the integrated area of the ν(OH) band is observed at ca. 0.83 min (221 ± 3 K) (Figure 3c). Since a steep increase of the integrated area of the ν(OH) band is also observed in transition from bulk water to bulk ice I with the same cooling rate (ca. −100 K/min) (Figure 4a,b), the increase of the integrated area of the ν(OH) band, as well as the spectral shape, indicates that the phase transition of the water pool was to ice I, not amorphous ice, with the rapid cooling conditions and the micellar structure was maintained for Rw = 1.8 ± 0.1 nm. It is worth noting that in previous studies the maximum size of the ice formed in AOT reverse micelles was less than 1 nm and the ice structure was amorphous. Thus, the present experimental conditions overcome the previous limitations of the size and form of ice in AOT reverse micelles. To investigate how large micelles can retain water pools and form crystalline ice in the present conditions and what changes occur for smaller water pools, other water pools were also investigated with the same rapid-cooling conditions: one smaller water pool (W0 = 5 (Rw = 1.2 ± 0.1 nm)) and three larger water pools (W0 = 15 (Rw = 2.1 ± 0.02 nm), W0 = 20 (Rw = 2.3 ± 0.1 nm), and W0 = 30 (Rw = 4.4 ± 0.3 nm)). Figure 5 shows the IR spectra for the various Rw values. In all of the spectra, a ν(OH) band centered at 3250 cm−1 was observed with a shoulder peak on either side. Then, to examine if crystalline water successfully formed in the AOT reverse

Figure 3. Comparison of the integrated area of the ν(OH) band of the IR spectra shown in Figure 2. The area was calculated from the integration of the absorbance of the ν(OH) band in the IR spectra from 3025 cm−1 to 3800 cm−1. (a) Conventional cooling (ca.−0.35 K/ min), (b) semi-rapid cooling (ca. −10 K/min), and (c) rapid cooling (ca. −100 K/min). The plots in (c) are the average of four measurements, and the standard deviation is 31.

micelles of all of the samples, we also measured the changes in the integrated area of the ν(OH) band for all of the samples. The results are shown in Figure 6. For Rw = 1.2 nm, a gradual increase of the integrated area of the ν(OH) band is observed. Such an increase of the integrated area of the ν(OH) band is similar to the case with a cooling rate of −10 K/min, as shown in Figure 3b. Because the corresponding IR spectrum includes a broad component in the high wavenumber region (Figure 2b), the gradual increase in the integrated area of the ν(OH) band with decreasing temperature can be assigned to the coformation of crystalline and amorphous ice in inhomogeneous environments, such as the effects of the electrostatic interactions with the polar groups at the inner surface or hydration to the counterions. For Rw = 2.1 nm, a rapid increase in the integrated area of the ν(OH) band is observed at ca. 0.76 min (224 ± 2 K). The integrated area of the ν(OH) band after the rapid increase is similar to that for Rw = 1.8. This indicates that the water pool for Rw = 2.1 nm also successfully crystallized in the AOT reverse micelles. For Rw = 2.3 ± 0.1 and 4.4 ± 0.3 nm, there is also a rapid increase in the integrated area of the ν(OH) band at ca. 0.73 and ca. 0.64 min (225 ± 1 and 232 ± 2 K), D

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Figure 4. (a) IR spectra of bulk ice at 193 and 298 K for rapid cooling (ca. −100 K/min). (b) Integrated area of the ν(OH) band of the IR spectra shown in (a). The area was calculated from the integration of the absorbance of the ν(OH) band in the IR spectra from 3025 cm−1 to 3800 cm−1. The plots in (b) are the average of four measurements, and the standard deviation is 7.2.

respectively. However, unfortunately, there is a drastic decrease in the integrated area of the ν(OH) band just after the rapid increase in the integrated area of the ν(OH) band. This indicates that local breakup of the micellar structure occurred just after the phase transition of the water pool to ice. Thus, the limit for the formation of crystalline ice in AOT reverse micelles is ca. 2.0 nm for the present experimental conditions. If the radius of the water pool is greater than 3 nm, bulk water is expected to be present in the core part of the water pool. Thus, further improvement of the conditions will contribute to the study of the phase transition of confined water coexisting with bulk water. To discuss the type of ice formed in AOT reverse micelles for Rw = 1.2, 1.8, and 2.1 nm (free from collapse and shedding out), the corresponding ν(OH) bands are analyzed by multicomponent Gaussian curve fittings, which have been generally applied for the analysis of water pools.53 First, we fixed the component parameters of the ν(OH) bands for bulk ice to ice type I, as shown in Figure 4a. In general, the ν(OH) band of ice I in the bulk is analyzed by the sum of at least three components.43−45 The highest frequency component at ca. 3380 cm−1 is assigned to symmetric O−H stretching (ν1, I). The component at ca. 3250 cm−1 is assigned to asymmetric O− H stretching (ν3, II), and the component at ca. 3150 cm−1 to overtone H−O−H bending (2ν2, III).45 We first fitted the sum of three Gaussian components according to previous analyses.43−45 The fitted curve agrees well with the observed ν(OH) band (Figure SI2). The corresponding component parameters are shown in Table 2. The wavenumber of each component (Table 2) also agrees well with the previous studies based on the sum of three components.43−45 Then, according to the obtained component parameters shown in Table 2, the ν(OH) bands for Rw = 1.2, 1.8, and 2.1 nm were analyzed. However, it was difficult to fit

Figure 5. Comparison of the IR spectra of the water pool at W0 = 5 (Rw = 1.2 ± 0.1 nm), 10 (Rw = 1.8 ± 0.1 nm), 15 (Rw = 2.1 ± 0.02 nm), 20 (Rw = 2.3 ± 0.1 nm), and 30 (Rw = 4.4 ± 0.3 nm) with rapid cooling (−100 K/min).

the ν(OH) band only with the three components for ice I in Table 2, indicating that the ν(OH) bands consists not only of crystalline ice I but also of other components derived from amorphous ice. For this reason, in addition to the three components, additional ones representing amorphous ice are required. A previous study reported that amorphous ice was formed in AOT reverse micelles with Rw < 1 nm.37 As shown in Figure SI3(a), formation of amorphous ice was observed in the water pool in the reverse micelle with Rw = 0.23 ± 0.1 nm under the rapid cooling condition (ca. −100 K/min). To obtain the fitting parameters for the corresponding ν(OH) band to amorphous ice, we applied curve fitting to the ν(OH) band (Rw = 0.23 ± 0.1 nm). The fitted result and the corresponding component parameters for amorphous ice are shown in Figure SI3(b) and Table 2, respectively. The ν(OH) band for amorphous ice was fitted with two components: the main component centered at 3485 cm−1 (A) and the minor component centered at 3331 cm−1 (B). Since the component (A) dominates the ν(OH) band for amorphous ice and the center wavenumber and the bandwidth of the component (B) is similar to the component I of bulk ice, fitting with four components (A: 3485 cm−1, I: 3331 cm−1, II: 3250 cm−1, III: 3177 cm−1) was applied to the ν(OH) E

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Figure 6. Changes in the integrated area of the ν(OH) band for W0 = 5 (Rw = 1.2 ± 0.1 nm), 15 (Rw = 2.1 ± 0.02 nm), 20 (Rw = 2.3 ± 0.1 nm), and 30 (Rw = 4.4 ± 0.3 nm) in Figure 5. The area was calculated from the integration of the absorbance of the ν(OH) band in the IR spectra from 3025 cm−1 to 3800 cm−1. The plots are the average of four measurements, and the standard deviations are 5.0, 8.7, 7.4, and 6.9 for W0 = 5, 15, 20, and 30, respectively.

Figure 7. ν(OH) components determined by fitting ν(OH) in Figure 5. The solid black line, dotted gray line, and solid gray line show the experimental curve, the sum of the fitted curves, and the fitted curves, respectively.

bands for Rw = 1.2, 1.8, and 2.1 nm. In the fitting of each Rw, the full-width at half-maximum for the components A, I, II, and III were fixed at the values of 176 ± 24 cm−1, 186 ± 2.1 cm−1, 75 ± 0.80 cm−1 , and 130 ± 2.1 cm−1 , which were experimentally obtained here for the bulk ice and amorphous one, respectively. The fitting results and the corresponding component parameters are shown in Figure 7 and Table 2, respectively. The fitted curve agrees well with the ν(OH) bands for Rw = 1.2, 1.8, and 2.1 nm. The integrated area ratio of the ν(OH) band of components I, II, and II are almost the same as those of ice I, indicating that crystalline ice was formed in each sample. Interestingly, all of the samples include component A (a

component of amorphous ice), but the integrated area of the ν(OH) band (%) for Rw = 1.2 nm is about three times larger than that for Rw = 1.8 and 2.1 nm. This indicates that a nonnegligible part of the frozen water pool for Rw = 1.2 nm was amorphous ice. This might also be responsible for the decrease in the integrated area of the ν(OH) band of component III for Rw = 1.2 nm compared with those for Rw = 1.8 and 2.1 nm. In a small water pool with a radius of about 1.0 nm, all of the water molecules should electrostatically interact with the ionic surfactant head-groups and the counterions.31 Therefore, we

Table 2. Comparisons of the Peak Wavenumber and Relative Integrated Area of ν(OH) Band of Each Component in Bulk Ice, Amorphous Ice, and Iced Water Poolsa A

I (B)

II

III

component

wavenumber (cm−1)

area (%)

wavenumber (cm−1)

area (%)

wavenumber (cm−1)

area (%)

wavenumber (cm−1)

area (%)

Ice I (bulk) Amorphous ice Rw = 0.23 nm Iced WP Rw = 1.2 nm Iced WP Rw = 1.8 nm Iced WP Rw = 2.1 nm

3485 ± 8 3483 3469 3476

65 21 6 6

3331 ± 1.0 (3331 ± 29) 3334 3331 3330

45 (35) 42 43 42

3250 ± 0.6 3248 3248 3246

18 14 17 17

3177 ± 1.5 3174 3175 3172

37 23 37 35

a

WP: water pool, ice I (Figures SI2 and 7a), amorphous ice (Figures SI3(b) and 7b), and iced WPs (Figure 7c−e). F

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considered that the change in the spectrum for Rw = 1.2 nm sensitively reflects the local environment of water molecules that are prevented from forming crystalline ice because of the strong electrostatic forces. The assignment of the crystal type of ice I is also worth discussing, because many studies of ice formation in nanopores in inorganic materials have reported the formation of Ic rather than Ih, as mentioned in the Introduction. It is well-known that Ic and Ih are two representative crystal structures of ice type I.43−45 In previous studies, the intensity of the shoulder structure of the ν(OH) band at ca. 3150 cm−1 (2ν2) was higher for Ih than for Ic.43−45 As shown in Figure 5, the intensity of the shoulder structures at ca. 3150 cm−1 for Rw = 2.3 and 4.4 nm are higher than those for Rw = 1.2, 1.8, and 2.1 nm. In addition, since the ν(OH) band shapes for Rw = 2.3 and 4.4 nm agree well with that of Ih,44 the ice crystal for Rw = 2.3 and 4.4 nm can be assigned as Ih. However, since the ν(OH) band shapes for Rw = 1.2, 1.8, and 2.1 nm are similar to Ic and not Ih, the ice crystal for Rw = 1.2, 1.8, and 2.1 nm can be assigned as Ic structure.43,44 Metastable Ic is generally observed in rapid squirting of water vapor on metal surfaces at ultralow temperature45 and the freezing of water confined in MCM-41 with a ca. 2 nm pore radius.8,9 It is worth noting that the present research spectroscopically shows that such metastable Ic can also be formed in organic self-assembled AOT reverse micelles. We will also discuss the phase transition temperature for each Rw in terms of the Gibbs−Thomson equation. Steep increases of the integrated area of the ν(OH) band were observed for the phase transition from water to ice (Figures 3c, 4b, and 6). Because the drastic increase of the integrated area of the ν(OH) band was not observed for Rw = 1.2 nm, we were not able to determine the precise phase transition temperature for Rw = 1.2 nm. The phase transition temperatures for Rw = 1.8, 2.1, 2.3, and 4.4 nm are plotted in Figure 8. The phase transition

T = Tm(1 − A /R w )

(1)

A = γ /L

(2)

where T (K) is the actual nanocrystal phase transition temperature, Tm (K) is the bulk phase transition temperature, γ (J/nm2) is the surface tension of liquid, and L (J/nm3) is the volumetric latent heat of phase transition. The GT equation is generally applied to phase equilibrium rather than a situation with strong temperature gradients and supercoiling. However, it is quite difficult to exactly treat the experimental situation under rapid cooling by well-defined thermodynamic equations. On the other hand, the experimental results showed that the more the water pools’ radii (RW) were reduced, the more the freezing temperature decreased. This feature is seemingly in accord with what the GT equation predicts. Although it is not the exact way, we expected some additional quantitative information to be obtained by the analysis with GT equation. We first assumed that the phase transition temperature shown in Figure 8 followed the GT equation, and determined the values of both Tm and A by a curve fitting analysis. Then, we discussed the validity of the fitting by discussing the obtained values of Tm and A. The fitted curve reproduced the dependence of the phase transition temperature on RW well when Tm = 239 K and A = 0.17 nm (Figure 8a). It is wellknown that γ and L can be expressed as a function of temperature.55 Using a function of temperature for γ and L in bulk water, the value of A for Tm = 239 K was calculated. If the value of A for our phase transition temperature was close to 0.17 nm, our phase transition temperature followed the GT equation. As a result, the value of A is 0.31 nm, and the corresponding curve is shown as (b) in Figure 8. By comparing curves (a) and (b) in Figure 8, it is clear that the phase transition temperature does not simply follow the GT equation. We at first considered that the much smaller value of A for our phase transition temperature was caused by the high concentration of sodium ions dissolved in the water pool. However, the value of γ for highly concentrated NaCl solution is larger than that for bulk water.56 Furthermore, the value of L for highly concentrated NaCl solution is lower than that for bulk water because the value of the isobaric specific heat for highly concentrated NaCl solution is lower than that for bulk water.57 Based on these considerations, the higher concentration of sodium ions dissolved in the water pool cannot explain the decrease of A. The reason for the decrease in the A value is still unknown, but this might be because of the strong electrostatic field in the diffuse electric double layer31 at the inner surface of the reverse micelle.

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CONCLUSIONS A combination of a small sample cell volume (ca. 50 μm) and rapid cooling (ca. −100 K/min) prevented the shedding out of the water pool in AOT reverse micelle under freezing, and extended the maximum limit for the freezing of water pools in AOT reverse micelles up to Rw = 2.1 nm. The formation of metastable cubic ice was observed for water pools in micelles with a radius up to 2.1 nm. It is worth noting that such measurements of the formation of crystalline cubic ice have been limited to confined spaces in inorganic materials and interfaces. Since such confined organic nanospaces with a few nanometer dimensions are ubiquitous in biological samples, understanding such crystalline behaviors and the resultant ice structures for various cooling speeds are important for food and

Figure 8. Phase transition temperature for Rw = 1.8, 2.1, 2.3, and 4.4 nm. (a) Fitted curve by the Gibbs−Thomson equation at 239 K. (b) Gibbs−Thomson curve for bulk water at 239 K.

temperatures for all Rw were substantially lower than that for bulk water (273 K). Existence of ionic solutes in the water pool, such as sodium ions, was considered to be a major cause of the decrease of the phase transition temperature. We then investigated the relationship between the phase transition temperature and the curvature of the water pool (RW) from the viewpoint of the Gibbs−Thomson effect. The Gibbs−Thomson relationship (GT equation) predicts the decrease of phase transition temperature because of the curvature as follows54 G

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cell preservation techniques as well as the basic science of phase transition behavior. The dependence of the phase transition temperature on the curvature of the reverse micelles did not simply follow the Gibbs−Thomson equation. The mechanism for the decrease in γ/L of the water pool in AOT reverse micelles remains an issue to be solved.

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ASSOCIATED CONTENT

S Supporting Information *

Hydrodynamic radius size distributions; ν(OH) components determined by fitting ν(OH) in Figure 4a; IR spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-3-5228-8728; Fax: +81-3-5228-9060; E-mail: yui@rs. kagu.tus.ac.jp. Notes

The authors declare no competing financial interest.

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