Crystallization of Confined Water Pools with Radii Greater Than 1 nmin AOT Reverse MicellesAkira 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 sodiumbis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles hasbeen investigated. Previous freezing experiments suffer fromcollision and fusion of AOT micelles and resultant loss ofwater from the water pool by shedding out during the coolingprocess. These phenomena have restricted the formation of iceto only when the radius of the water pool (Rw) is below 1 nm,and only amorphous ice has been observed. To overcome thesize limitation, a combination of rapid cooling and a custom-made cell allowing thin sample loading is applied for instantaneous and homogeneous freezing. The freezing process ismonitored with attenuated total reflection infrared spectroscopy (ATR-IR) measurements. A cooling rate of ca. −100 K/min anda sample thickness of ca. 50 μm overcomes the limitations mentioned above and allows the crystallization of water pools withlarger radii (Rw > 1 nm). The corresponding ATR-IR spectra of the frozen water pools with Rw < 2.0 nm show similar features tothe 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 micellarstructure and shedding out of the water pool. In addition, it was revealed that Ic ice can also be formed in flexible organic self-assembled AOT reverse micelles for at least Rw ≤ ca. 2 nm, as well as in inorganic and solid materials with a pore radius of ca. 2nm. The dependence of the phase transition temperature on the curvature of the reverse micelles is discussed from the viewpointof the Gibbs−Thomson effect.

■ INTRODUCTION

Nanometer-sized pores are ubiquitous in both inorganic andorganic materials, such as natural stones,1 clays,2 and biologicalcells.3 Water confined in such nanopores generally showsremarkably different thermodynamic and mechanical propertiesto those of bulk water at ambient conditions, such as the phasetransition temperature and viscosity.4 The shape of thenanopores and their surface chemical and physical propertiesare considered to greatly affect the freezing and meltingbehavior, and the resultant ice structure of the confinedwater.5−30

The freezing and melting behavior of water confined innanopores has been intensively studied using inorganicmaterials.5−30 This is because inorganic materials, such asMCM-41 and SBA-15, provide ideal nanopores with control-lable dimensions and sizes.8−11,17−20 For example, Morishige etal. studied the freezing behavior of confined water in variouspore sizes of silica materials by X-ray diffraction (XRD).8−12

They found that metastable cubic ice (Ic) formed in pores withca. 2 nm radius instead of thermodynamically stable hexagonalice (Ih), while both Ic and Ih formed in pores with ca. 5 nmradius.8 Interestingly, such formation of Ic has often beenobserved in cylindrical and interconnected cylindricalpores,5−10,16,23 and has been reported to be stable up to themelting point of ice.10

As well as inorganic materials, the freezing behavior ofconfined water in organic materials is also important for bothscience and technology applications, such as understandingbiological activity under low temperature and the developmentof cell preservation techniques. In biological systems, nano-spaces are formed by the self-assembly of lipids and proteins,and are ubiquitously observed. These soft organic self-assemblies with nanometer-sized pores are able to flexiblychange their structures and volumes with changes in temper-ature and application of external forces such as pressure.Furthermore, the phase transition of confined water in organicself-assemblies sometimes drastically affects the structures andfunctions of the host materials themselves. Although it is animportant issue, the phase transition of confined water inorganic self-assembled systems is complicated and difficult tostudy.Here, we focus on the freezing behavior of confined water in

aerosol sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reversemicelles, as a representative model for nanopores formed by theself-assembly of organic molecules. The AOT molecule is ananionic surfactant with a large hydrophobic moiety that formsreverse micelles in aqueous environments.31 AOT reverse

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

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micelles are a well-known host for confined water because theyprovide nanometer-sized spherical pores with well-controlledradius and narrow size distribution.31−33 The water confined inAOT reverse micelles is called the “water pool” and its radius(Rw) can be easily controlled by varying the water−surfactantmolar ratio (W0 = [H2O]/[AOT]).

32,33 Water pools in AOTreverse micelles provide an ideally sized model for water inbiological systems, such as water at the surface of biologicalmembranes and water at activity points in enzymes, toinvestigate enzymatic reactions.34,35

When investigating the role of confined water in biologicalsystems, the phase transition behavior of such confined water isimportant to understand biological activity and cell preservationtechniques at low temperature. However, when the phasetransition of water pools has been studied, collision and fusionof the AOT micelles often occurred, resulting in shedding outof the water pools during cooling and preventing investigationof their freezing behavior. This has restricted the study of ice inAOT reverse micelles to Rw < 1 nm, and only the formation ofamorphous ice has been reported.36,37

In the present study, we combined rapid cooling with smallsample volumes to prevent shedding of the internal water fromAOT reverse micelles, and achieved homogeneous freezing ofthe internal water while maintaining the structure of themicelle. We developed a custom-made cell for the attenuatedtotal reflection infrared spectroscopy (ATR-IR) measurementsthat allows ca. 50-μm-thick samples and rapid cooling at ca.−100 K/min. It is expected that the combination of thinsamples and rapid cooling will result in more homogeneous andinstantaneous freezing of water pools in AOT reverse micelles.We also discuss the dependence of the dominant ice structureon the water−surface molar ratio.

■ MATERIALS AND METHODSSample 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 waspurified by a Millipore Milli-Q system (Simpli Lab-UV, MerckMillipore, Billerica, MA). The AOT reverse micelle solution wasprepared by injecting water into a 0.04 M AOT−heptane solution fortransmission IR measurements and into a 0.5 M AOT−heptanesolution for ATR measurements. The value of W0 in the micelles wascontrolled 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°. TheAOT reverse micelle solution was prepared by injecting water into a0.2 M AOT−heptane solution for DLS. The measurements wereperformed at 298 K. The observed DLS results are shown in FigureSI1. Additionally, the corresponding values of Rh and Rw aresummarized in Table 1. Rw was calculated from the difference betweenRh and the length of the AOT molecule (ca. 1 nm).38

IR Measurements. From the viewpoint of the phase transition ofthe water pools, several techniques have been applied in previousstudies, including differential scanning calorimetry,39 NMR,39−41

fluorescent probes,42 and IR absorption spectroscopy.36,37 Amongthese probe techniques, IR absorption spectroscopy is a powerful toolthat can discriminate slight changes in the hydrogen bonding networkstructure, namely, the band shape of the OH stretching mode(ν(OH)) to sensitively discriminate the structures of ice.43−45 For thestudy of the ice structure, X-ray diffraction (XRD) should also be apowerful technique to investigate the ice structure. Indeed, XRD hasbeen frequently applied to confined water in solid inorganicmaterials.8−12,16,23 However, in the study of water pool in AOTreverse micelles dispersed in abundant organic solvent, it is expected

that the XRD signal from ice formed in the reverse micelle will beweak because the amount of water in the system is markedly less thanthe surrounding organic solvent. In addition, the XRD signal fromAOT reverse micelles with similar sizes will be superimposed on thesignal from the water pool or ice. These factors will make it difficult toanalyze the XRD spectra. Conversely, it is much easier to discriminatethe signal from the water pool from that from the surrounding organicsolvent by IR spectroscopy because the corresponding wavenumbersfrom water and organic solvent are quite different. Furthermore, IRspectroscopy has high sensitivity to water measurements because ofthe strong IR absorption of water molecules.46−48 These features arefavorable for measuring the small amount of buried water pool in AOTmicelles, and also to monitor the rapid temporal changes of the waterpool. Thus, in the present study, we mainly used IR spectroscopy tomonitor and investigate the structural changes of the water pools.

Cooling Rates and Sample Volumes for IR Measurements. Inprevious studies, a cooling rate of ca. −0.50 K/min often induced theloss of internal water from the reverse micelle with relatively largemicelles (W0 ≥ 5),36,40,41,49 resulting in the formation of a precipitatecontaining both water and AOT.42 To prevent the loss of internalwater and to maintain the structure of the reverse micelle underfreezing, a few studies have applied super-rapid cooling (ca. −6000 K/min).37,50 The formation of an amorphous type of ice was observed forvery small reverse micelles (W0 < 5, Rw = 1.2 nm), whereas largemicelles (W0 ≥ 5) break up to form large ice clusters of size 10−500nm, and most of the crystalline ice was found outside of the reversemicelles and coalesced into large ice crystals.37 These previous studiesindicate that extremely slow or rapid cooling makes it difficult toachieve homogeneous freezing of internal water in relatively largemicelles (W0 ≥ 5, Rw = 1.0 nm), and an adequate cooling rate isnecessary to overcome this difficulty. In addition, to achievehomogeneous freezing of internal water, a small sample thickness isalso important to prevent the generation of a large temperaturegradient in the sample loaded in the chamber. This is because a largetemperature gradient might induce convection in the sample solutionin the sample chamber, accelerating the collision and fusion of reversemicelles and resulting in the unexpected shedding out of the waterpool during the cooling process. In the present study, to reduce suchunexpected convection flow in the sample chamber, the thickness ofthe sample was set to 50 μm, which is 10 times thinner than that in aprevious study.37

Conventional Cooling (ca. −0.35 K/min) and Semi-RapidCooling (ca. −10 K/min). The samples were sealed between twoCaF2 windows (17 × 12.5 × 0.8 mm3) with a 50 μm spacer. Toproduce the conventional cooling rate36,40,41,49 in our experimentalsystem, the temperature was decreased from room temperature 298 to193 K at 5 K intervals using a cryostat (CoolSpeK IR, UnisokuScientific Instruments, Osaka, Japan). IR spectra were recorded 10 minafter the temperature of the system was stabilized at the aimedtemperature. It took almost 5 h to complete the whole cooling,waiting, and measurements, and average cooling rate was thusestimated as ca. −0.35 K/min. IR spectra were measured with Nicolet6700 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 werecontinuously cooled from 298 to 193 at −10 K/min with the cryostat.

Table 1. Relationship between the Water−Surfactant MolarRatio (W0) and the Corresponding Average Radius Size ofthe Water Pool (Rw)

W0 Rh (nm) Rwa (nm)

5 2.2 ± 0.1 1.2 ± 0.110 2.8 ± 0.1 1.8 ± 0.115 3.1 ± 0.02 2.1 ± 0.0220 3.3 ± 0.1 2.3 ± 0.130 5.4 ± 0.3 4.4 ± 0.3

aRW was calculated by subtracting 1.0 nm from the corresponding Rhvalue that was experimentally observed by DLS.

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The IR spectra were recorded on a Nicolet 6700 FT-IR spectrometerwith 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 thepresent study was ca. −10 K/min. To achieve a much more rapidcooling rate, we constructed a custom-made cooling unit on the ATRprism (Thermo Fisher Scientific) and cooled the sample solutions withliquid nitrogen via a thin cover glass (Matsunami, 0.12−0.17 mmthickness, C01824, Osaka, Japan) as shown in Figure 1. To set the

sample thickness to ca. 50 μm, 0.8 μL of the AOT reverse micellesolution was dropped on the ATR prism equipped on the Nicolet6700 FT-IR spectrometer. The sample solution completely coveredthe ATR prism. The temperature of the sample solutions wasmonitored 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 resolutionof 4 cm−1 and no accumulation (single scan mode). Spectraldecomposition using curve fitting was performed with the equippedsoftware (OMNIC 7.2a). The spectra were curve fitted with aGaussian function.

■ RESULTS AND DISCUSSIONConfined water in nanopores in inorganic materials with aradius of ca. 2 nm generally shows quite different freezingcharacteristics from bulk water, such as the formation of Icrather than Ih.

8,9 The reason for the different behavior isthought to be the electrostatic interaction between the polarwater molecules and ionic substituent groups covering the innersurface of the nanopores.8,9 Based on these observations, wefirst focused on water pools confined in AOT reverse micelleswith a similar radius (W0 = 10, Rw = 1.8 ± 0.1 nm) andinvestigated adequate freezing conditions by varying the coolingrate.At first, the water pool in the AOT reverse micelle was slowly

cooled at the conventional cooling rate (−0.35 K/min), whichis a similar cooling rate to a previous study.36 A structurelessand broad band with a peak at about 3500 cm−1 (ν(OH)) wasfinally observed in the IR spectrum (Figure 2a), indicating theformation of amorphous ice in the reverse micelles, which is ingood agreement with the result reported in the previousstudy.36

The reasons for the formation of amorphous ice underconventional cooling conditions can be considered as follows:It is widely accepted that water molecules inside reversemicelles can rapidly exchange between micelles because offusion and subsequent dissociation of micelles at roomtemperature.51,52 In a slow cooling process, collision and fusionof the reverse micelles should frequently occur during the phasetransition from water to ice under conventional coolingconditions. If the sample volume is large enough to induce

temperature graduations and inhomogeneity, these factorsmight be significant because of convection. This will preventnucleation to the crystalline structure and lead to the formationof amorphous ice. It has also been suggested that shedding outof the internal water owing to the collapse of AOT reversemicelles is another reason for the formation of amorphousice.36,40,41,49

To prevent the unexpected collision and fusion and/or theshedding out of the water pool, rapid freezing will be effectivefor reducing these phenomena by instantaneous freezing. Thus,a more rapid cooling rate of ca. −10 K/min, denoted as semi-rapid cooling, was applied. As shown in Figure 2b, the IRspectrum at 193 K has a band with maximum peak intensity at3250 cm−1. Furthermore, it has two shoulder peaks at 3380 and3150 cm−1. The appearance of the sharp peak at 3250 cm−1 andthe two shoulder peaks is characteristic of the structure of ice Icrystals.43−45 By comparison of the IR spectra of theconventional cooling process with the semi-rapid coolingprocess, the bandwidth is considerably narrower and theshape of the band is closer to that of typical crystal ice I.However, the ν(OH) band of the spectrum is still somewhatbroader than the corresponding band of the ice I crystal,especially in the high wavenumber region,43−45 which may

Figure 1. Schematic illustration of the rapid cooling cell for the ATR-IR measurements.

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

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indicate that a non-negligible portion of the water pool stillformed 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 allowssimultaneous ATR-IR measurements. The IR spectrum fromthe rapid cooling rate is shown in Figure 2c. The IR spectrum at193 K shows a peak at 3250 cm−1 with two shoulder peaks, andthe bandwidth is narrower than that observed with semi-rapid(−10 K/min) cooling conditions. The observed ν(OH) bandagrees well with that of ice I crystals.43−45 This result showsthat 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 inthe form of ice I, allowing us to compare the result with thoseobserved in inorganic systems. However, before considering theice structure and comparing the results with those observed ininorganic systems, we have to confirm whether the systemremained free from the collapse of micelles and the sheddingout 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) banddrastically decreased with decreasing temperature.36 Such adrastic decrease of the integrated area of the ν(OH) bandmeans a remarkable change of the system, and is attributed tothe local collapse of the micellar structure and shedding out ofinner water or collapse of the reverse micelles. Based on theseobservations and assignments, we also examined the intensitychanges.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 drasticdecrease in the integrated area of the ν(OH) band is observedin Figure 3a (the dotted arrow) from 213 to 193 K. This resultindicates that local breakup of the micellar structure occurred inthe freezing method with a conventional cooling rate (ca. −0.35K/min). Conversely, drastic decreases are not observed for thesemi-rapid and rapid cooling conditions (Figure 3b,c). Incontrast, especially for the rapid cooling conditions, a steepincrease of the integrated area of the ν(OH) band is observedat 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 Iwith the same cooling rate (ca. −100 K/min) (Figure 4a,b), theincrease of the integrated area of the ν(OH) band, as well asthe spectral shape, indicates that the phase transition of thewater pool was to ice I, not amorphous ice, with the rapidcooling conditions and the micellar structure was maintainedfor Rw = 1.8 ± 0.1 nm. It is worth noting that in previousstudies the maximum size of the ice formed in AOT reversemicelles was less than 1 nm and the ice structure wasamorphous. Thus, the present experimental conditions over-come the previous limitations of the size and form of ice inAOT reverse micelles.To investigate how large micelles can retain water pools and

form crystalline ice in the present conditions and what changesoccur for smaller water pools, other water pools were alsoinvestigated with the same rapid-cooling conditions: onesmaller water pool (W0 = 5 (Rw = 1.2 ± 0.1 nm)) and threelarger 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 wasobserved with a shoulder peak on either side. Then, to examineif crystalline water successfully formed in the AOT reverse

micelles of all of the samples, we also measured the changes inthe 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 gradualincrease of the integrated area of the ν(OH) band is observed.Such an increase of the integrated area of the ν(OH) band issimilar to the case with a cooling rate of −10 K/min, as shownin Figure 3b. Because the corresponding IR spectrum includes abroad component in the high wavenumber region (Figure 2b),the gradual increase in the integrated area of the ν(OH) bandwith decreasing temperature can be assigned to the coformationof crystalline and amorphous ice in inhomogeneous environ-ments, such as the effects of the electrostatic interactions withthe polar groups at the inner surface or hydration to thecounterions.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). Theintegrated area of the ν(OH) band after the rapid increase issimilar to that for Rw = 1.8. This indicates that the water poolfor Rw = 2.1 nm also successfully crystallized in the AOTreverse micelles. For Rw = 2.3 ± 0.1 and 4.4 ± 0.3 nm, there isalso a rapid increase in the integrated area of the ν(OH) bandat ca. 0.73 and ca. 0.64 min (225 ± 1 and 232 ± 2 K),

Figure 3. Comparison of the integrated area of the ν(OH) band of theIR spectra shown in Figure 2. The area was calculated from theintegration of the absorbance of the ν(OH) band in the IR spectrafrom 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 fourmeasurements, and the standard deviation is 31.

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respectively. However, unfortunately, there is a drastic decreasein the integrated area of the ν(OH) band just after the rapidincrease in the integrated area of the ν(OH) band. Thisindicates that local breakup of the micellar structure occurredjust after the phase transition of the water pool to ice. Thus, thelimit for the formation of crystalline ice in AOT reversemicelles is ca. 2.0 nm for the present experimental conditions. Ifthe radius of the water pool is greater than 3 nm, bulk water isexpected to be present in the core part of the water pool. Thus,further improvement of the conditions will contribute to thestudy of the phase transition of confined water coexisting withbulk 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 sheddingout), the corresponding ν(OH) bands are analyzed bymulticomponent Gaussian curve fittings, which have beengenerally applied for the analysis of water pools.53 First, wefixed the component parameters of the ν(OH) bands for bulkice 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 threecomponents.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 toovertone H−O−H bending (2ν2, III).

45 We first fitted the sumof three Gaussian components according to previousanalyses.43−45 The fitted curve agrees well with the observedν(OH) band (Figure SI2). The corresponding componentparameters are shown in Table 2.The wavenumber of each component (Table 2) also agrees

well with the previous studies based on the sum of threecomponents.43−45 Then, according to the obtained componentparameters 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

the ν(OH) band only with the three components for ice I inTable 2, indicating that the ν(OH) bands consists not only ofcrystalline ice I but also of other components derived fromamorphous ice. For this reason, in addition to the threecomponents, additional ones representing amorphous ice arerequired.A previous study reported that amorphous ice was formed in

AOT reverse micelles with Rw < 1 nm.37 As shown in FigureSI3(a), formation of amorphous ice was observed in the waterpool in the reverse micelle with Rw = 0.23 ± 0.1 nm under therapid cooling condition (ca. −100 K/min). To obtain the fittingparameters for the corresponding ν(OH) band to amorphousice, we applied curve fitting to the ν(OH) band (Rw = 0.23 ±0.1 nm). The fitted result and the corresponding componentparameters for amorphous ice are shown in Figure SI3(b) andTable 2, respectively. The ν(OH) band for amorphous ice wasfitted with two components: the main component centered at3485 cm−1 (A) and the minor component centered at 3331cm−1 (B). Since the component (A) dominates the ν(OH)band for amorphous ice and the center wavenumber and thebandwidth of the component (B) is similar to the component Iof bulk ice, fitting with four components (A: 3485 cm−1, I: 3331cm−1, II: 3250 cm−1, III: 3177 cm−1) was applied to the ν(OH)

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 IRspectra shown in (a). The area was calculated from the integration ofthe 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.

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 ± 0.02nm), 20 (Rw = 2.3 ± 0.1 nm), and 30 (Rw = 4.4 ± 0.3 nm) with rapidcooling (−100 K/min).

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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, andIII 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 wereexperimentally obtained here for the bulk ice and amorphousone, 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 ofcomponents 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 theν(OH) band (%) for Rw = 1.2 nm is about three times largerthan that for Rw = 1.8 and 2.1 nm. This indicates that a non-negligible part of the frozen water pool for Rw = 1.2 nm wasamorphous ice. This might also be responsible for the decreasein the integrated area of the ν(OH) band of component III forRw = 1.2 nm compared with those for Rw = 1.8 and 2.1 nm. In asmall water pool with a radius of about 1.0 nm, all of the watermolecules should electrostatically interact with the ionicsurfactant head-groups and the counterions.31 Therefore, we

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 ± 0.1nm), and 30 (Rw = 4.4 ± 0.3 nm) in Figure 5. The area was calculatedfrom the integration of the absorbance of the ν(OH) band in the IRspectra from 3025 cm−1 to 3800 cm−1. The plots are the average offour measurements, and the standard deviations are 5.0, 8.7, 7.4, and6.9 for W0 = 5, 15, 20, and 30, respectively.

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

componentwavenumber

(cm−1)area(%)

wavenumber(cm−1)

area(%)

wavenumber(cm−1)

area(%)

wavenumber(cm−1)

area(%)

Ice I (bulk) - - 3331 ± 1.0 45 3250 ± 0.6 18 3177 ± 1.5 37Amorphous ice Rw = 0.23 nm 3485 ± 8 65 (3331 ± 29) (35) - - - -Iced WP Rw = 1.2 nm 3483 21 3334 42 3248 14 3174 23Iced WP Rw = 1.8 nm 3469 6 3331 43 3248 17 3175 37Iced WP Rw = 2.1 nm 3476 6 3330 42 3246 17 3172 35

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

Figure 7. ν(OH) components determined by fitting ν(OH) in Figure5. The solid black line, dotted gray line, and solid gray line show theexperimental curve, the sum of the fitted curves, and the fitted curves,respectively.

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considered that the change in the spectrum for Rw = 1.2 nmsensitively reflects the local environment of water moleculesthat are prevented from forming crystalline ice because of thestrong electrostatic forces.The assignment of the crystal type of ice I is also worth

discussing, because many studies of ice formation in nanoporesin inorganic materials have reported the formation of Ic ratherthan Ih, as mentioned in the Introduction. It is well-known thatIc and Ih are two representative crystal structures of ice typeI.43−45 In previous studies, the intensity of the shoulderstructure of the ν(OH) band at ca. 3150 cm−1 (2ν2) was higherfor Ih than for Ic.

43−45 As shown in Figure 5, the intensity of theshoulder structures at ca. 3150 cm−1 for Rw = 2.3 and 4.4 nmare 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 agreewell with that of Ih,

44 the ice crystal for Rw = 2.3 and 4.4 nm canbe assigned as Ih. However, since the ν(OH) band shapes forRw = 1.2, 1.8, and 2.1 nm are similar to Ic and not Ih, the icecrystal for Rw = 1.2, 1.8, and 2.1 nm can be assigned as Icstructure.43,44 Metastable Ic is generally observed in rapidsquirting of water vapor on metal surfaces at ultralowtemperature45 and the freezing of water confined in MCM-41with a ca. 2 nm pore radius.8,9 It is worth noting that thepresent research spectroscopically shows that such metastable Iccan also be formed in organic self-assembled AOT reversemicelles.We will also discuss the phase transition temperature for each

Rw in terms of the Gibbs−Thomson equation. Steep increasesof the integrated area of the ν(OH) band were observed for thephase 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 todetermine the precise phase transition temperature for Rw = 1.2nm. The phase transition temperatures for Rw = 1.8, 2.1, 2.3,and 4.4 nm are plotted in Figure 8. The phase transition

temperatures for all Rw were substantially lower than that forbulk water (273 K). Existence of ionic solutes in the water pool,such as sodium ions, was considered to be a major cause of thedecrease 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. TheGibbs−Thomson relationship (GT equation) predicts thedecrease of phase transition temperature because of thecurvature as follows54

= −T T A R(1 / )m w (1)

γ=A L/ (2)

where T (K) is the actual nanocrystal phase transitiontemperature, Tm (K) is the bulk phase transition temperature,γ (J/nm2) is the surface tension of liquid, and L (J/nm3) is thevolumetric latent heat of phase transition. The GT equation isgenerally applied to phase equilibrium rather than a situationwith strong temperature gradients and supercoiling. However, itis quite difficult to exactly treat the experimental situation underrapid cooling by well-defined thermodynamic equations. Onthe other hand, the experimental results showed that the morethe water pools’ radii (RW) were reduced, the more the freezingtemperature decreased. This feature is seemingly in accord withwhat the GT equation predicts. Although it is not the exactway, we expected some additional quantitative information tobe obtained by the analysis with GT equation.We first assumed that the phase transition temperature

shown in Figure 8 followed the GT equation, and determinedthe values of both Tm and A by a curve fitting analysis. Then,we discussed the validity of the fitting by discussing theobtained values of Tm and A. The fitted curve reproduced thedependence of the phase transition temperature on RW wellwhen Tm = 239 K and A = 0.17 nm (Figure 8a). It is well-known that γ and L can be expressed as a function oftemperature.55 Using a function of temperature for γ and L inbulk water, the value of A for Tm = 239 K was calculated. If thevalue of A for our phase transition temperature was close to0.17 nm, our phase transition temperature followed the GTequation. As a result, the value of A is 0.31 nm, and thecorresponding curve is shown as (b) in Figure 8. By comparingcurves (a) and (b) in Figure 8, it is clear that the phasetransition temperature does not simply follow the GT equation.We at first considered that the much smaller value of A for ourphase transition temperature was caused by the highconcentration of sodium ions dissolved in the water pool.However, the value of γ for highly concentrated NaCl solutionis larger than that for bulk water.56 Furthermore, the value of Lfor highly concentrated NaCl solution is lower than that forbulk water because the value of the isobaric specific heat forhighly concentrated NaCl solution is lower than that for bulkwater.57 Based on these considerations, the higher concen-tration of sodium ions dissolved in the water pool cannotexplain the decrease of A. The reason for the decrease in the Avalue is still unknown, but this might be because of the strongelectrostatic field in the diffuse electric double layer31 at theinner surface of the reverse micelle.

■ CONCLUSIONSA combination of a small sample cell volume (ca. 50 μm) andrapid cooling (ca. −100 K/min) prevented the shedding out ofthe water pool in AOT reverse micelle under freezing, andextended the maximum limit for the freezing of water pools inAOT reverse micelles up to Rw = 2.1 nm. The formation ofmetastable cubic ice was observed for water pools in micelleswith a radius up to 2.1 nm. It is worth noting that suchmeasurements of the formation of crystalline cubic ice havebeen limited to confined spaces in inorganic materials andinterfaces. Since such confined organic nanospaces with a fewnanometer dimensions are ubiquitous in biological samples,understanding such crystalline behaviors and the resultant icestructures for various cooling speeds are important for food and

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

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cell preservation techniques as well as the basic science of phasetransition behavior. The dependence of the phase transitiontemperature on the curvature of the reverse micelles did notsimply follow the Gibbs−Thomson equation. The mechanismfor the decrease in γ/L of the water pool in AOT reversemicelles remains an issue to be solved.

■ ASSOCIATED CONTENT*S Supporting InformationHydrodynamic radius size distributions; ν(OH) componentsdetermined by fitting ν(OH) in Figure 4a; IR spectra. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Tel: +81-3-5228-8728; Fax: +81-3-5228-9060; E-mail: yui@rs.kagu.tus.ac.jp.NotesThe authors declare no competing financial interest.

■ REFERENCES(1) Katsube, T. J.; Williamson, M. A. Effects of Diagenesis on ShaleNano-Pore Structure and Implications for Sealing Capacity. ClayMiner. 1994, 29, 451−461. Katsube, T. J. Shale Permeability and Pore-Structure Evolution Characteristics. Geological Survey of Canada 2000,E15, 1−9.(2) Swenson, J.; Bergman, R.; Longeville, S. A Neutron Spin-EchoStudy of Confined Water. J. Chem. Phys. 2001, 115, 11299−11305.Watanabe, K.; Mizoguchi, M. Amount of Unfrozen Water in FrozenPorous Media Saturated with Solution. Cold Reg. Sci. Technol. 2002,34, 103−110.(3) Mazur, P. Cryobiology: The Freezing of Biological Systems.Science 1970, 168, 939−949. Asahina, E.; Emura, M. Types of CellFreezing and the Post-Thawing Survival of Mammalian AscitesSarcoma Cells. Cryobiology 1966, 2, 256−262. Mazur, P. Freezing ofLiving Cells: Mechanisms and Implications. Am. J. Physiol. Cell Physiol.1984, 247, C125−C142.(4) Derjaguin, B. V.; Churaev, N. V. Structure of Water in ThinLayers. Langmuir 1987, 3, 607−612. Churaev, N. V.; Bardasov, S. A.;Sobolev, V. D. On the Non-Freezing Water Interlayers between Iceand a Silica Surface. Colloids Surf., A 1993, 79, 11−24.(5) Steytler, D. C.; Dore, J. C.; Wright, C. J. Neutron DiffractionStudy of Cubic Ice Nucleation in a Porous Silica Network. J. Phys.Chem. 1983, 87, 2458−2459.(6) Dunn, M.; Dore, J. C.; Chieux, P. Structural Studies of IceFormation in Porous Silicas by Neutron Diffraction. J. Cryst. Growth1988, 92, 233−238.(7) Baker, J. M.; Dore, J. C.; Behrens, P. Nucleation of Ice inConfined Geometry. J. Phys. Chem. B 1997, 101, 6226−6229.(8) Morishige, K.; Uematsu, H. The Proper Structure of Cubic IceConfined in Mesopores. J. Chem. Phys. 2005, 122, 044711−1-4.(9) Morishige, K.; Nobuoka, K. X-ray Diffraction Studies of Freezingand Melting of Water Confined in a Mesoporous Adsorbent (MCM-41). J. Chem. Phys. 1997, 107, 6965−6969.(10) Morishige, K.; Yasunaga, H.; Uematsu, H. Stability of Cubic Icein Mesopores. J. Phys. Chem. C 2009, 113, 3056−3061.(11) Morishige, K.; Yasunaga, H.; Matsutani, Y. Effect of Pore Shapeon Freezing and Melting Temperatures of Water. J. Phys. Chem. C2010, 114, 4028−4035.(12) Morishige, K.; Yasunaga, H.; Denoyel, R.; Wernert, V. Pore-Blocking-Controlled Freezing of Water in Cagelike Pores of KIT-5. J.Phys. Chem. C 2007, 111, 9488−9495.(13) Watanabe, K.; Oguni, M.; Tadokoro, M.; Oohata, Y.; Nakamura,R. Glass Transition on the Development of a Hydrogen-BondNetwork in Nano-Channel Ice, And Subsequent Phase Transitions of

the Ordering of Hydrogen Atom Positions within the Network in[Co(H2bim)3](TMA)·20H2O. J. Phys.: Condens. Matter 2006, 18,8427−8436.(14) Tadokoro, M.; Ohhara, T.; Ohhata, Y.; Suda, T.; Miyasato, Y.;Yamada, T.; Kikuchi, T.; Tanaka, I.; Kurihara, K.; Oguni, M.; Nakasuji,K.; Yamamuro, O.; Kuroki, R. Anomalous Water Molecules andMechanistic Effects of Water Nanotube Clusters Confined toMolecular Porous Crystals. J. Phys. Chem. B 2010, 114, 2091−2099.(15) Tadokoro, M.; Iida, C.; Saitoh, T.; Suda, T.; Miyazato, Y. One-Dimensional Tube-like {51262}n Water Clusters Stabilized in aMolecular Nanoporous Framework. Chem. Lett. 2010, 39, 186−187.(16) Yoshizawa, M.; Kusukawa, T.; Kawano, M.; Ohhara, T.; Tanaka,I.; Kurihara, K.; Niimura, N.; Fujita, M. Endohedral Clusterization ofTen Water Molecules into a ‘‘Molecular Ice’’ within the HydrophobicPocket of a Self-Assembled Cage. J. Am. Chem. Soc. 2005, 127, 2798−2799.(17) Erko, M.; Wallacher, D.; Hoell, A.; HauB, T.; Zizak, I.; Paris, O.Density Minimum of Confined Water at Low Temperatures: ACombined Study by Small-Angle Scattering of X-rays and Neutrons.Phys. Chem. Chem. Phys. 2012, 14, 3852−3858.(18) Jahnert, S.; Vaca Chavez, F.; Schaumann, G. E.; Schreiber, A.;Schonhoff, M.; Findenegg, G. H. Melting and Freezing of Water inCylindrical Silica Nanopores. Phys. Chem. Chem. Phys. 2008, 10,6039−6051.(19) Erko, M.; Findenegg, G. H.; Cade, N.; Michette, A. G.; Paris, O.Confinement-Induced Structural Changes of Water Studied by RamanScattering. Phys. Rev. B 2011, 84, 104205−1−8.(20) Liu, D.; Zhang, Y.; Chen, C.-C.; Mou, C.-Y.; Poole, P. H.; Chen,S.-H. Observation of the Density Minimum in Deeply SupercooledConfined Water. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 9570−9574.(21) Iiyama, T.; Nishikawa, K.; Suzuki, T.; Kaneko, K. Study of theStructure of a Water Molecular Assembly in a HydrophobicNanospace at Low Temperature with in Situ X-ray Diffraction.Chem. Phys. Lett. 1997, 274, 152−158.(22) Crupi, V.; Majolino, D.; Stroscio, E.; Venuti, V. Dependence ofWater Vibrational Dynamics upon Different Confining Matrices.Philos. Mag. 2004, 84, 1405−1412.(23) Fouzri, A.; Dorbez-Sridi, R.; Oumezzine, M. Water Confined inSilica Gel and in Vycor Glass at Low and Room Temperature, X-rayDiffraction Study. J. Chem. Phys. 2002, 116, 791−797.(24) Tadokoro, M.; Fukui, S.; Kitajima, T.; Nagao, Y.; Ishimaru, S.;Kitagawa, H.; Isobe, K.; Nakasuji, K. Structures and Phase Transitionof Multi-Layered Water Nanotube Confined to Nanochannels. Chem.Commun. 2006, 1274−1276.(25) Kittaka, S.; Sou, K.; Yamaguchi, T.; Tozaki, K. Thermodynamicand FTIR Studies of Supercooled Water Confined to Exterior andInterior of Mesoporous MCM-41. Phys. Chem. Chem. Phys. 2009, 11,8538−8543.(26) Takamuku, T.; Yamagami, M.; Wakita, H.; Masuda, Y.;Yamaguchi, T. Thermal Property, Structure, and Dynamics ofSupercooled Water in Porous Silica by Calorimetry, NeutronScattering, and NMR Relaxation. J. Phys. Chem. B 1997, 101, 5730−5739.(27) Jelassi, J.; Castricum, H. L.; Bellissent-Funel, M.-C.; Dore, J.;Webber, J. B. W.; Sridi-Dorbez, R. Studies of Water and Ice inHydrophilic and Hydrophobic Mesoporous Silicas: Pore Character-isation and Phase Transformations. Phys. Chem. Chem. Phys. 2010, 12,2838−2849.(28) Schreiber, A.; Ketelsen, I.; Findenegg, G. H. Melting andFreezing of Water in Ordered Mesoporous Silica Materials. Phys.Chem. Chem. Phys. 2001, 3, 1185−1195.(29) Findenegg, G. H.; Jahnert, S.; Akcakayiran, D.; Schreiber, A.Freezing and Melting of Water Confined in Silica Nanopores.ChemPhysChem 2008, 9, 2651−2659.(30) Drost-Hansen, W.; Etzler, F. M. Melting of Ice in Silica Pores.Langmuir 1989, 5, 1439−1441.(31) Silber, J. J.; Biasutti, A.; Abuin, E.; Lissi, E. Interactions of smallmolecules with reverse micelles. Adv. Colloid Interface Sci. 1999, 82,189−252.

Langmuir Article

dx.doi.org/10.1021/la501210t | Langmuir XXXX, XXX, XXX−XXXH

(32) Bhattacharyya, K.; Bagchi, B. Slow Dynamics of ConstrainedWater in Complex Geometries. J. Phys. Chem. A 2000, 104, 10603−10613.(33) Eastoe, J.; Young, W. K.; Robinson, B. H.; Steytler, D. C.Scattering Studies of Microemulsions in Low-Density Alkanes. J.Chem. Soc., Faraday Trans. 1990, 86, 2883−2889.(34) Martinek, K.; Levashov, A. V.; Klyachko, N. L.; Pantin, V. I.;Berezin, I. V. the Principles of Enzyme Stabilization Vi. Catalysis byWater-Soluble Enzymes Entrapped into Reversed Micelles ofSurfactants in Organic Solvents. Biochim. Biophys. Acta 1981, 657,277−294.(35) Luthi, P.; Luisi, P. L. Enzymatic Synthesis of Hydrocarbon-Soluble Peptides with Reverse Micelles. J. Am. Chem. Soc. 1984, 106,7285−7286.(36) Nucci, N. V.; Vanderkooi, J. M. Temperature Dependence ofHydrogen Bonding and Freezing Behavior of Water in ReverseMicelles. J. Phys. Chem. B 2005, 109, 18301−18309.(37) Dokter, A. M.; Petersen, C.; Woutersen, S.; Bakker, H. J.Vibrational Dynamics of Ice in Reverse Micelles. J. Chem. Phys. 2008,128, 044509−1−7.(38) Dijk, M. A. v.; Joosten, J. G. H.; Levine, Y. K.; Bedeaux, D.Dielectric Study of Temperature-Dependent Aerosol OT/Water/Isooctane Microemulsion Structure. J. Phys. Chem. 1989, 93, 2506−2512.(39) Hauser, H.; Haering, G.; Pande, A.; Luisi, P. L. Interaction ofWater with Sodium Bis(2-ethyl-1-hexyl) Sulfosuccinate in ReversedMicelles. J. Phys. Chem. 1989, 93, 7869−7876.(40) Simorellis, A. K.; Van Horn, W. D.; Flynn, P. F. Dynamics ofLow Temperature Induced Water Shedding from AOT ReverseMicelles. J. Am. Chem. Soc. 2006, 128, 5082−5090.(41) Van Hom, W. D.; Simorellis, A. K.; Flynn, P. F. Low-Temperature Studies of Encapsulated Proteins. J. Am. Chem. Soc. 2005,127, 13553−13560.(42) Munson, C. A.; Baker, G. A.; Baker, S. N.; Bright, F. V. Effects ofSubzero Temperatures on Fluorescent Probes Sequestered withinAerosol-OT Reverse Micelles. Langmuir 2004, 20, 1551−1557.(43) Rowland, B.; Kadagathur, N. S.; Devlin, J. P.; Buch, V.; Feldman,T.; Wojcik, M. J. Infrared Spectra of Ice Surfaces and Assignment ofSurface-Localized Modes from Simulated Spectra of Cubic Ice. J.Chem. Phys. 1995, 102, 8328−8341.(44) Whalley, E. A Detailed Assignment of the O-H Stretching Bandsof Ice I. Can. J. Chem. 1977, 55, 3429−3441.(45) Hornig, D. F.; White, H. F.; Reding, F. P. The Infrared Spectraof Crystalline H2O, D2O, and HDO. Spectrochim. Acta 1958, 12, 338−349.(46) Woutersen, S.; Emmerichs, U.; Bakker, H. J. Femtosecond Mid-IR Pump-Probe Spectroscopy of Liquid Water: Evidence for a Two-Component Structure. Science 1997, 278, 658−660. Du, Q.; Freysz, E.;Shen, Y. R. Surface Vibrational Spectroscopic Studies of HydrogenBonding and Hydrophobicity. Science 1994, 264, 826−828. Graener,H.; Seifert, G.; Laubereau, A. New Spectroscopy of Water UsingTunable Picosecond Pulses in the Infrared. Phys. Rev. Lett. 1991, 66,2092−2095.(47) Dekkers, H. F. W.; Gallo, A.; Van Elshocht, S. Infrared MolarAbsorption Coefficient of H2O Stretching Modes in SiO2. Thin SolidFilms 2013, 542, 8−13.(48) Shi, L.; Skinner, J. L. Proton Disorder in Ice Ih andInhomogeneous Broadening in Two-Dimensional Infrared Spectros-copy. J. Phys. Chem. B 2013, 117, 15536−15544. Perakis, F.; Widmer,S.; Hamm, P. Two-Dimensional Infrared Spectroscopy of Isotope-Diluted Ice Ih. J. Chem. Phys. 2011, 134, 204505−1−9.(49) Zulauf, M.; Eicke, H.-F. Inverted Micelles and Microemulsionsin the Ternary System H2O/Aerosol-OT/Isooctane as Studied byPhoton Correlation Spectroscopy. J. Phys. Chem. 1979, 83, 480−486.(50) Baglioni, P.; Nakamura, H.; Kevan, L. Electron Spin EchoModulation Study of AOT Reverse Micelles. J. Phys. Chem. 1991, 95,3856−3859.

(51) Eicke, H.-F.; Shepherd, J. C. W.; Steinemann, A. Exchange ofSolubilized Water and Aqueous Electrolyte Solutions between Micellesin Apolar Media. J. Colloid Interface Sci. 1976, 56, 168−176.(52) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. The Kinetics ofSolubilisate Exchange between Water Droplets of a Water-in-OilMicroemulsion. J. Chem. Soc., Faraday Trans. 1 1987, 83, 985−1006.(53) Zhou, G.-W.; Li, G.-Z.; Chen, W.-J. Fourier Transform InfraredInvestigation on Water States and the Conformations of Aerosol-OTin Reverse Microemulsions. Langmuir 2002, 18, 4566−4571.Gonzalez-Blanco, C.; Rodriguez, L. J.; Velazquez, M. M. Effect ofthe Addition of Water-Soluble Polymers on the Structure of AerosolOT Water-in-Oil Microemulsions: A Fourier Transform InfraredSpectroscopy Study. Langmuir 1997, 13, 1938−1945. Onori, G.;Santucci, A. IR Investigations of Water Structure in Aerosol OTReverse Micellar Aggregates. J. Phys. Chem. 1993, 97, 5430−5434. Jain,T. K.; Varshney, M.; Maitra, A. Structural Studies of Aerosol OTReverse Micellar Aggregates by FT-IR Spectroscopy. J. Phys. Chem.1989, 93, 7409−7416. MacDonald, H.; Bedwell, B.; Gulari, E. FTIRSpectroscopy of Microemulsion Structure. Langmuir 1986, 2, 704−708.(54) Pan, D.; Liu, L.-M.; Slater, B.; Michaelides, A.; Wang, E. Meltingthe Ice: On the Relation between Melting Temperature and Size forNanoscale Ice Crystals. ACS Nano 2011, 5, 4562−4569. Nada, H.;Furukawa, Y. Growth Inhibition at the Ice Prismatic Plane Induced bya Spruce Budworm Antifreeze Protein: A Molecular DynamicsSimulation Study. Phys. Chem. Chem. Phys. 2011, 13, 19936−19942.(55) Yongjun, L.; Bingbo, W. A Molecular Dynamics Study onSurface Properties of Supercooled Water. Sci. China-Phys. Mech. Astron.2006, 49, 616−625. Kostinski, A.; Cantrell, W. Entropic Aspects ofSupercooled Droplet Freezing. J. Atmos. Sci. 2008, 65, 2961−2971.Leyendekkers, J. V.; Hunter, R. J. Thermodynamic Properties of Waterin the Subcooled Region. I. J. Chem. Phys. 1985, 82, 1440−1446.(56) Weissenborn, P. K.; Pugh, R. J. Surface Tension of AqueousSolutions of Electrolytes: Relationship with Ion Hydration, OxygenSolubility, and Bubble Coalescence. J. Colloid Interface Sci. 1996, 184,550−563.(57) Archer, D. G.; Carter, R. W. Thermodynamic Properties of theNaCl + H2O System. 4. Heat Capacities of H2O and NaCl(aq) inCold-Stable and Supercooled States. J. Phys. Chem. B 2000, 104,8563−8584.

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