ds 136 (2007) 147–155www.elsevier.com/locate/molliq

Journal of Molecular Liqui

Heterogeneity of acetonitrile–water mixtures in the temperature range279–307 K studied by small-angle neutron scattering technique

Toshiyuki Takamuku a,⁎, Yasukuni Noguchi a, Masaru Matsugami b, Hiroki Iwase c,Toshiya Otomo d, Michihiro Nagao e

a Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Honjo-machi, Saga 840-8502, Japanb Department of Functional Molecular Science, The Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585, Japan

c Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japand Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Oho, Tsukuba 305-0801, Japan

e Institute for Solid State Physics, The University of Tokyo, Tokai, Ibaraki 319-1106, Japan

Received 18 December 2006; accepted 26 February 2007Available online 4 April 2007

Abstract

At 279, 287, 298, and 307 K the heterogeneity of acetonitrile–water mixtures in a range of acetonitrile mole fraction xAN from 0.2 to 0.6 hasbeen evaluated in terms of Ornstein–Zernike correlation length, concentration and density fluctuations, and Kirkwood–Buff parameters obtainedfrom small-angle neutron scattering (SANS) experiments. At 298 K the correlation lengths for CH3CN–D2O (D=2H) and CD3CN–H2O mixtureshave shown that the heterogeneity of acetonitrile–water mixtures at xAN=0.3 and 0.4 is most enhanced in the mole fraction range examined. Withdecreasing temperature from 307 to 279 K, the correlation length and the fluctuations for CH3CN–D2O mixtures at xAN=0.3 and 0.4 mostsignificantly rise in the mole fraction range. The Kirkwood–Buff parameters have revealed that both acetonitrile–acetonitrile and water–waterinteractions in acetonitrile–water mixtures at the same mole fractions remarkably increase with decreasing temperature, whereas acetonitrile–water interactions decrease. According to all the parameters estimated, a change in mixing state of acetonitrile–water mixtures with loweringtemperature was discussed.© 2007 Elsevier B.V. All rights reserved.

Keywords: Acetonitrile–water; SANS; Fluctuations; Kirkwood–Buff parameters

1. Introduction

Acetonitrile is miscible with water at any ratio under ambientconditions. However, acetonitrile–water mixtures are separatedto acetonitrile-rich and water-rich phases either by cooling oraddition of salts, such as NaCl. Indeed, the upper criticalsolution temperature (UCST) of acetonitrile–water mixtures hasbeen found at 272 K when xAN=0.38 [1]. So far, manyresearchers have measured various physicochemical propertiesof acetonitrile–water mixtures, such as partial molar volumes

⁎ Corresponding author. Present address. Cyclotron Facility, Indiana Univer-sity, Bloomington, IN 47408, USA and Center for Neutron Research, NationalInstitute of Standards and Technology, Gaithersburg, MD 20899-8562, USA.Tel.: +81 952 28 8554; fax: +81 952 28 8548.

E-mail address: takamut@cc.saga-u.ac.jp (T. Takamuku).

0167-7322/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.molliq.2007.02.009

[1] and viscosities [2], and reached a conclusion that acetonitrileand water molecules are not homogeneously mixed with eachother at the molecular level even under ambient conditions.

Recently, small-angle X-ray and neutron scattering techni-ques have been often utilized to directly observe a mixing stateof binary solutions at a mesoscopic level. Nishikawa et al. havemade small-angle X-ray scattering (SAXS) measurements onacetonitrile–water mixtures at 273, 279, and 298 K andestimated the fluctuations and the Kirkwood–Buff parametersfrom the SAXS intensities [3]. It has been shown that bothacetonitrile–acetonitrile and water–water interactions in themixture at xAN=0.38 are most enhanced with decreasingtemperature in the entire xAN range. In a SANS experiment, amixture of deuterated and undeuterated solvent molecules giveshigh contrast with each other because of the largely differentneutron scattering lengths (6.67 and −3.74 fm, respectively)

148 T. Takamuku et al. / Journal of Molecular Liquids 136 (2007) 147–155

of D and H atoms. Hence, a SANS technique is a powerfultool to elucidate a mixing state of binary solutions. We havemade SANS measurements at 298 K on CH3CN–D2O andNaCl–CH3CN–D2O mixtures with varying xAN and saltconcentration [4,5] before the SAXS investigation. The resultshave revealed that the size of the clusters for CH3CN–D2Omixtures at 298 K is most enhanced between xAN=0.3 and 0.4in the entire range of the mole fraction and that the size furtherincreases when the NaCl concentration increases.

Based on both Bhatia–Thornton [6] and Kirkwood–Buff [7]theories, Almásy et al. have proposed a procedure for estimationof concentration and density fluctuations and Kirkwood–Buffparameters from SANS intensities for a binary solution [8]. Theyhave also reported the parameter values for aqueousmixtures of 3-methylpyridine [8] and pyridine [9] determined from their SANSintensities. After the studies by Almásy et al. [8,9], Bakó et al.have estimated both fluctuations and Kirkwood–Buff parametersfor CH3CN–D2O and CD3CN–H2O mixtures at 298 K as afunction of xAN [10]. It has been found that the fluctuations andthe Kirkwood–Buff parameters for CH3CN–D2O and CD3CN–H2O at xAN=0.38 are most enhanced in the entire mole fractionrange as well as the SAXS investigation [3]. However, SANSexperiments have not been made on acetonitrile–water mixturesas a function of temperature down to the UCST.

In the present investigation, to evaluate heterogeneity ofacetonitrile–water mixtures in the temperature range 279–307 K,SANS experiments have been made on CH3CN–D2O andCD3CN–H2O mixtures by using SANS-U and SWAN spectro-meters installed at a reactor and a pulsed neutron facility,respectively. First, the Ornstein–Zernike correlation lengths [11]for CD3CN–H2O mixtures at 298 K in the mole fraction range of0.2≤xAN≤0.6 have been estimated from their SANS intensitiesmeasured on SANS-U. For comparison, those for CH3CN–D2Omixtures at 298 K in the same xAN range have also beendetermined by reanalyzing the SANS intensities measured onSANS-U in the previous investigation [4,5]. Then, to clarify achange in the heterogeneity of acetonitrile–water mixtures withlowering temperature, SANS experiments using SWANhave beenmade on CH3CN–D2Omixtures at 279, 287, 298, and 307 K. TheOrnstein–Zernike correlation lengths, the fluctuations, and theKirkwood–Buff parameters for the mixtures have been estimatedand compared with those obtained from the SAXS experiments[3]. According to the present results, a change in mixing state ofacetonitrile–water mixtures with decreasing temperature towardthe UCST is discussed at the molecular level.

2. Experimental

2.1. Sample solutions

Acetonitrile (Wako Pure Chemicals, extra grade) was driedfor several days on thermally activated 4 Åmolecular sieves andthen distilled under nitrogen gas atmosphere. Heavy water,D2O, (Aldrich, D atom content of 99.9%) and acetonitrile-d3,CD3CN, (Euriso-top, D atom content of 99.8%) were usedwithout further purification. To avoid replacement of D atomsby H ones, CH3CN–D2O mixtures at xAN=0.2, 0.25, 0.3, 0.4,

0.5, and 0.6 were made up in a nitrogen-filled glove box.CD3CN–H2O mixtures were prepared by mixing acetonitrile-d3and doubly distilled water at the same mole fractions. Densitiesof the sample solutions at 279, 287, 298, and 307 K weremeasured with an electronic densimeter (ANTON Paar K.G.,DMA60 and DMA602) for analysis of SANS data.

2.2. SANS experiments

SANS experiments at 298 K were made on CD3CN–H2Omixtures at xAN=0.2, 0.25, 0.3, 0.4, 0.5, and 0.6 using SANS-Uspectrometer at a reactor JRR-3M in Japan Atomic EnergyAgency (JAEA), Tokai, Japan. In the experiments, the samplesolutions were kept into a quartz cell of 10mm inwidth, 40mm inheight, and 2 mm in sample thickness and exposed to neutronbeams (wavelength, λ=7 Å), whose size is 5 mm in diameter atthe sample position, for 1.5 h. The sample temperature wascontrolled within ±0.1 K during the measurements by circulationof thermostated water around a sample chamber. The distancebetween the sample position and the detector was set to 2 m,where the momentum transfer Q (=4πλ−1sinθ) range of 0.02–0.14 Å−1 was covered [12]. The transmission by a sample and acell was measured with a 3He detector placed at a beam stopperposition. The observed intensities were corrected for backgroundby subtraction of intensities of an empty cell, and then normalizedby dividing the intensities for each sample solution by those for astandard polyethylene sample, Lupolen [13].

When the sample temperatures were 279, 287, 298, and307 K, SANS intensities for CH3CN–D2Omixtures at xAN=0.2,0.25, 0.3, 0.4, 0.5, and 0.6 were measured on SWANspectrometer at a pulsed neutron facility (KENS) in HighEnergy Accelerator Research Organization (KEK), Tsukuba,Japan. The Q-range of 0.01–12 Å−1 was covered with SWAN[14]. The sample solutions were kept in a quartz cell of 22 mm inwidth, 25 mm in height, and 2 mm in sample thickness. Thesample temperature was controlled within ±0.5 K by circulationof temperature-controlled silicone oil in a sample jacket. Theneutron beam size at a sample position was 20×20 mm2, and thewavelength range used was λ=0.5–11.6 Å. The scatteringintensities were accumulated for 4 h per sample. The observedintensities were corrected for background, absorption, and cellscattering. The transmission by a sample and a cell wasmeasured with a 3He position sensitive detector placed at abeam stopper position. The correction for detector efficienciesand normalization to absolute units was made by dividing theintensities of each solution by those of vanadium [14].

The incoherent scattering was subtracted from the normal-ized intensities obtained on both SANS-U and SWAN. Allparameter values required for the above corrections were takenfrom the literature [15].

3. Results and discussion

3.1. CH3CN–D2O and CD3CN–H2O mixtures at 298 K

Fig. 1 shows the normalized SANS intensities Icorr(Q) for theCD3CN–H2O mixtures at 298 K obtained on SANS-U, together

Fig. 1. Normalized SANS intensities for (a) CH3CN–D2O and (b) CD3CN–H2O mixtures at 298 K measured on SANS-U as a function of xAN. The values in theparentheses are those shifted from the origin to avoid overlap of the plots.

149T. Takamuku et al. / Journal of Molecular Liquids 136 (2007) 147–155

with those for CH3CN–D2Omeasured on the same spectrometerin the previous investigation [4,5]. As seen, the SANS intensitiesfor both mixtures at xAN=0.3 and 0.4 are stronger than those atthe other mole fractions, showing that acetonitrile and watermolecules are heterogeneously mixed with each other in themixtures at xAN=0.3 and 0.4. To quantify the heterogeneousmixing of acetonitrile and water molecules in the CH3CN–D2Oand CD3CN–H2O mixtures, Ornstein–Zernike fits were per-formed on the SANS intensities for the mixtures in the Q2 rangefrom 1×10−3 to 6×10−3 Å−2 by using,

IcorrðQÞ�1 ¼ Ið0Þ�1½1þ n2Q2�; ð1Þwhere ξ gives the Ornstein–Zernike correlation length [11]. Inthe previous investigations [4,5], the heterogeneity of CH3CN–

Fig. 2. Ornstein–Zernike plots of the SANS intensities for (a) CH3CN–D2O and (b)values and dashed lines the theoretical ones. The values in the parentheses are those

D2O mixtures was evaluated in terms of the Debye correlationlength [16]. In the present analysis, however, the SANSintensities for CH3CN–D2O mixtures were reanalyzed byusing the Ornstein–Zernike equation, since the Ornstein–Zernike correlation length is more suitable to evaluate the sizefor several dimensions of clusters in highly heterogeneousmixtures than the Debye correlation length, which correspondsto the size of a single dimension of clusters [3].

Fig. 2 indicates the results of a least-squares fitting on theSANS intensities through Eq. (1); the theoretical values (dashedlines) obtained from the least-squares procedure well reproducethe observed ones (circles). The intensity at the zero-angle (Q=0)for the mixtures was determined from the intercept of thetheoretical value to estimate the concentration and density

CD3CN–H2O mixtures at various xAN and 298 K. Circles give the experimentalshifted from the origin to avoid overlap of the plots.

Table 1Ornstein–Zernike correlation lengths ξ for CH3CN–D2O and CD3CN–H2Omixtures at 298 K obtained by using SANS-U

xAN CH3CN–D2O CD3CN–H2O

ξ/Å ξ/Å

0.2 6.5 (2) 4.7 (1)0.25 8.5 (2) 6.5 (1)0.3 10.0 (2) 8.2 (1)0.4 10.1 (2) 8.6 (1)0.5 7.2 (2) 6.6 (1)0.6 3.9 (2) 4.6 (1)

The values in parentheses are standard deviations for the last figure of ξ value.

Table 2Fluctuations and Kirkwood–Buff parameters for CH3CN–D2O mixtures at298 K

150 T. Takamuku et al. / Journal of Molecular Liquids 136 (2007) 147–155

fluctuations and the Kirkwood–Buff parameters as describedbelow. The Ornstein–Zernike correlation lengths estimated forthe CH3CN–D2O andCD3CN–H2Omixtures are listed in Table 1and plotted as a function of xAN in Fig. 3. As seen in this figure,the correlation lengths for both mixtures increase with increasingxAN from 0.2 to 0.4 and decrease again with the further increase inxAN, resulting in a maximum between xAN=0.3 and 0.4. It isshown that acetonitrile and water molecules are most heteroge-neously mixed with each other in the mixtures at xAN≈0.35 in themole fraction range investigated, i.e., both acetonitrile and waterclusters are most evolved in the mixtures. The correlation lengthsfor the CH3CN–D2O mixtures estimated in the presentinvestigation are comparable with those from the previousSANS experiments by Bakó et al. [10]. However, the correlationlengths for the CD3CN–H2O mixtures are ∼2 Å smaller thanthose for the CH3CN–D2O ones, except for that at xAN=0.6. Thesmaller correlation lengths for the CD3CN–H2Omixtures may becaused by insufficient correction for incoherent and multiplescatterings due to the higher content of water protons in themixtures. Actually, the difference between the correlation lengthsfor the CH3CN–D2O and CD3CN–H2Omixtures is significant inthe range xAN≤0.4.

In accordance with the procedure proposed by Almásy et al.[8,9], the concentration fluctuation, SCC(0), the density

Fig. 3. Correlation lengths ξ for CH3CN–D2O (opened circles) and CD3CN–H2O(filled ones) mixtures at 298 K as a function of xAN. The solid lines are drawn forclarity of their variation. The standard deviations σ were indicated as error bars.

fluctuation, SNN(0), and their cross term, SNC(0), were estimatedfrom the SANS intensities at the zero-angle for the CH3CN–D2O mixtures after a conversion of the units from barns−1 intocm−1 using the number density of the mixtures. The partialmolar volumes of acetonitrile and water molecules for theirbinary mixtures at 298 K required for the analysis were obtainedfrom the excess molar volumes for CH3CN–H2O mixtures inthe literature [1]. Moreover, the isothermal compressibilities ofacetonitrile–water mixtures at 298 K were estimated byinterpolation of the values taken from Ref. [17]. However, anisotope effect of D2O on both thermodynamic parameters wasnot taken into account in the present analysis. The Sij(0)(ij=CC, NN, and NC) for the CH3CN–D2O mixtures at 298 Kestimated are summarized in Table 2, together with thoseobtained from the SAXS experiments for comparison [3], andplotted as a function of xAN in Fig. 4(a). The uncertainties ofthese values were estimated to be ∼5%. As seen in Table 2, thefluctuations obtained from the present SANS experiments arecomparable with those from the SAXS experiments [3]. In Fig.4(a), the mole fraction dependence of both concentration anddensity fluctuations shows a maximum at xAN=0.4, while thatof the cross term exhibits a minimum at the mole fraction. Thesefeatures are in agreement with the previous results from bothSAXS [3] and SANS [10] experiments. It is shown thatacetonitrile and water molecules are most heterogeneouslymixed with each other in the CH3CN–D2O mixtures atxAN≈0.4 as well as the Ornstein–Zernike correlation lengths.

The Kirkwood–Buff parameters are defined by

Gpq ¼Z l

0ðgpqðrÞ � 1Þ4kr2dr; ð2Þ

where gpq(r) represents the pair correlation function ofmolecules [7]. Although the Kirkwood–Buff parameters can

xAN SCC(0) SNN(0) SNC(0)

SANS a SAXSb SANS a SAXSb SANSa SAXSb

0.2 1.4 1.7 2.61 3.34 1.9 −2.370.25 2.63 4.51 −3.420.3 4.36 4.82 6.66 7.52 −5.37 −6.000.4 6.05 6.17 7.56 7.77 −6.74 −6.900.5 4.83 4.4 5.02 4.55 −4.90 −4.450.6 3.75 3.28 −3.48

xAN GAN,AN/Å3

molecule−1GW,W/Å

3

molecule−1GAN,W/Å

3

molecule−1

SANS a SAXSb SANS a SAXSb SANSa SAXSb

0.2 520 698 331 433 −521 −6560.25 662 739 −8120.3 739 829 1366 1527 −1127 −12470.4 478 496 2353 2397 −1212 −12420.5 159 137 2432 2206 −829 −7590.6 28 2655 −602a This work with SANS-U.b Ref. [3].

Fig. 4. Acetonitrile mole fraction dependence of (a) concentration fluctuation SCC(0) (circles), density fluctuation SNN(0) (squares), their cross term SNC(0)(diamonds), and (b) Kirkwood–Buff parameters GAN,AN (circles), GW,W (squares), and GAN,W (diamonds) for CH3CN–D2O mixtures at 298 K obtained by usingSANS-U.

151T. Takamuku et al. / Journal of Molecular Liquids 136 (2007) 147–155

be obtained from thermodynamic data [18], the parameters canalso be estimated from the concentration fluctuation SCC(0) bythe following relations:

G11 ¼ � 1x1q

þ kBTjT þ 1q

d� 1x1

� �2

SCCð0Þ; ð3Þ

G22 ¼ � 1x2q

þ kBTjT þ 1q

dþ 1x2

� �2

SCCð0Þ; ð4Þ

G12 ¼ kBTjT þ 1q

d� 1x1

� �dþ 1

x2

� �SCCð0Þ: ð5Þ

Here, x1 and x2 give the mole fractions of solvents 1 and 2,respectively, ρ is the number density for the solvent mixture,kB, T, and κT represent the Boltzmann constant, absolutetemperature, and isothermal compressibility, respectively. δ isdefined by (V1−V2) /Vmol, where V1 and V2 are the partialmolar volumes for the solvents, and Vmol is the molar volume ofthe mixture. In the present case, solvents 1 and 2 correspond toacetonitrile (AN) and water (W), respectively.

Fig. 5. (a) Normalized SANS intensities for CH3CN–D2O mixtures at 279, 287, 298Circles give the experimental values and dashed lines the theoretical ones.

The Kirkwood–Buff parameters for the CH3CN–D2Omixtures at 298 K estimated are summarized in Table 2.Those for acetonitrile–water mixtures determined from theSAXS intensities [3] are also listed in the table for com-parison. The parameter values obtained from the presentSANS data satisfactorily agree with those from the previousSAXS one [3]. In Fig. 4(b), the Kirkwood–Buff parametersfor the CH3CN–D2O mixtures at 298 K are plotted as afunction of xAN. The GAN,AN values gradually increase withdecreasing xAN from 0.6, but slightly decrease at xANb0.3,leading to a small maximum at xAN=0.3. On the other hand,the GW,W values oppositely change with the mole fraction;the values increase with increasing xAN from 0.2, and thoseare almost constant at xAN≥0.4. The GAN,W values show aminimum at xAN=0.4, suggesting that interactions betweenacetonitrile and water molecules in the mixture at xAN=0.4are the weakest in the mole fraction range investigated.Additionally, the GW,W values for the mixtures at xAN≥0.3are much larger than the GAN,AN ones. This suggests thatwater–water interaction in the mixtures is stronger thanacetonitrile–acetonitrile ones.

, and 307 K when xAN=0.4 obtained on SWAN and (b) Ornstein–Zernike plots.

Table 3Ornstein–Zernike correlation lengths ξ/Å for CH3CN–D2O mixtures at varioustemperatures obtained by using SWAN

xAN 279 K 287 K 298 K 307 K

0.2 7.88 (10) 5.93 (9) 5.05 (8)0.25 19.6 (1) 10.2 (1) 7.30 (9) 5.90 (9)0.3 37.3 (3) 12.8 (1) 8.84 (8) 6.94 (10)0.4 39.0 (3) 13.5 (1) 8.95 (9) 6.77 (9)0.5 19.1 (2) 8.76 (10) 6.65 (10)0.6 7.29 (10) 4.32 (10) 3.68 (10)

The values in parentheses are standard deviations for the last figure of ξ value.

Fig. 6. Correlation lengths ξ for CH3CN–D2O mixtures at 279, 287, 298, and307 K as a function of xAN. The solid lines are drawn for clarity of theirvariation. The standard deviations σ were indicated as error bars.

152 T. Takamuku et al. / Journal of Molecular Liquids 136 (2007) 147–155

3.2. CH3CN–D2O mixtures in the range 279–307 K

As a function of temperature from 307 to 279 K SANSintensities for the CH3CN–D2O mixtures were measured byusing SWAN. For example, Fig. 5(a) presents the correctedSANS intensities for the mixture at xAN=0.4 as a function oftemperature. The intensities gradually increase when thetemperature decreases; in particular, those at 279 K areremarkably strengthened. This shows that the heterogeneity ofthe CH3CN–D2O mixture is gradually enhanced with decreas-ing temperature. In the present investigation, the increase in theheterogeneity of the CH3CN–D2O mixtures with loweringtemperature was evaluated in terms of the Ornstein–Zernikecorrelation length, the concentration and density fluctuations,and the Kirkwood–Buff parameters.

Fig. 5(b) shows the results of Ornstein–Zernike fits using aleast-squares procedure on the reciprocal SANS intensities forthe CH3CN–D2O mixture at xAN=0.4. The theoretical valuescalculated well reproduce the observed ones over the Q2-rangefrom 1×10−3 to 6×10−3 Å−2. The SANS intensities for theCH3CN–D2O mixtures at the other mole fractions andtemperatures investigated could also be explained through theOrnstein–Zernike equation. However, the ξ values for themixtures at xAN=0.2, 0.5, and 0.6, when the temperature was307 K, could not be estimated with satisfactory reliability due tothe weak SANS intensities. The optimized correlation lengths ξfor the CH3CN–D2O mixtures are summarized in Table 3. Acomparison between Tables 1 and 3 indicates that the ξ valuesfor the CH3CN–D2O mixtures at 298 K obtained on SWAN are∼1 Å smaller than those on SANS-U. This may be due to atechnical error, e.g., the different normalization procedures forthe observed scattering intensities using vanadium and Lupolen,respectively. However, the change in the ξ values with the molefraction obtained on SWAN bears resemblance to that onSANS-U.

Fig. 6 shows the ξ values for the CH3CN–D2O mixtures at279, 287, 298, and 307 K as a function of xAN. At each tem-perature a maximum of the ξ values appears between xAN=0.3and 0.4. The ξ values for the CH3CN–D2Omixtures at xAN=0.3and 0.4 extremely rise with decreasing temperature, whereasthose at xAN=0.2 and 0.6 very slightly increase. It is thus shownthat the heterogeneity of the CH3CN–D2O mixtures betweenxAN=0.3 and 0.4 is most drastically evolved in the mole fractionrange investigated when the temperature decreases. This agreeswith the SAXS results on acetonitrile–water mixtures [3].

The fluctuations and the Kirkwood–Buff parameters for theCH3CN–D2O mixtures at the temperatures examined wereestimated from the SANS intensities at the zero-angle. Thepartial molar volumes of acetonitrile and water molecules foracetonitrile–water mixtures at 279 K were obtained from theexcess molar volumes reported in Ref. [1] as well as those at298 K. However, the excess molar volumes at 287 and 307 Kwere not available in the literature, and thus, the partial molarvolumes at the two temperatures were substituted by those at279 and 298 K, respectively. The isothermal compressibilitiesfor acetonitrile–water mixtures at 279 and 287 K which werealso necessary to the data treatment were replaced with those at298 K taken from the literature [17]. On the contrary, those at307 K were obtained from interpolation of the values at 298 and318 K [17]. The concentration and density fluctuations, theircross term, and the Kirkwood–Buff parameters estimated aresummarized in Table 4, together with those obtained from theSAXS experiments [3]. Tables 2 and 4 indicate that thefluctuations and the Kirkwood–Buff parameters for theCH3CN–D2O mixtures at 298 K obtained using SANS-U andSWAN are comparable with each other. In Table 4, theparameter values for the mixtures at 298 K obtained onSWAN satisfactorily agree with those from the previous SAXSexperiments. However, all the absolute values for theparameters obtained from the SANS experiment are largerthan those from the SAXS experiments [3]. This differencebecomes more remarkable for the CH3CN–D2O mixtures atxAN=0.3 and 0.4 when the temperature is 279 K; the absolutevalues for the mixtures at xAN=0.3 and 0.4 obtained from theSANS experiments are larger by a factor of ∼5 than those fromthe SAXS ones. This may be because the heterogeneity ofacetonitrile–water mixtures can be more clearly detected by theSANS technique than the SAXS due to highly contrasted D2Omolecules. It is likely that the advantage of the SANSexperiments becomes significant for the highly heterogeneousmixtures at xAN=0.3 and 0.4 when the temperature approachesthe UCST. Indeed, the fluctuations and the Kirkwood–Buffparameters for the acetonitrile–water mixture at xAN=0.2

Table 4Fluctuations and Kirkwood–Buff parameters for CH3CN–D2O mixtures at 279, 287, 298, and 307 K

xAN SCC(0) SNN(0) SNC(0) GAN,AN/Å3

molecule−1GW,W/Å

3

molecule−1GAN,W/Å

3

molecule−1

SANS a SAXSb SANSa SAXSb SANS a SAXSb SANS a SAXSb SANS a SAXSb SANSa SAXSb

307 K0.25 3.07 5.25 −3.99 808 879 −9560.3 4.55 6.96 −5.61 784 1438 −11840.4 6.01 7.52 −6.70 479 2361 −1216

298 K0.2 2.1 1.7 4.08 3.34 −2.92 −2.37 924 698 543 433 −814 −6560.25 3.70 6.31 −4.81 998 1059 −11390.3 5.89 4.82 9.00 7.52 −7.26 −6.00 1049 829 1861 1527 −1517 −12470.4 8.06 6.17 10.05 7.77 −8.98 −6.90 677 496 3153 2397 −1610 −12420.5 6.36 4.4 6.59 4.55 −6.45 −4.45 245 137 3226 2206 −1089 −7590.6 5.47 4.76 −5.07 87 3928 −876

287 K0.2 2.41 4.39 −3.24 1098 602 −9160.25 6.14 9.99 −7.81 1796 1739 −18760.3 10.20 14.99 −12.35 1956 3191 −26160.4 14.24 17.27 −15.66 1312 5529 −28390.5 9.19 9.30 −9.22 411 4630 −15700.6 5.58 4.76 −5.12 95 3944 −894

279 K0.2 3.47 3.4 6.29 6.2 −4.65 −4.55 1649 1634 878 869 −1305 −12950.25 17.02 27.61 −21.66 5208 4852 −51340.3 69.11 12.6 101.25 19.1 −83.63 −15.5 13,953 2440 21,717 4013 −17,523 −32480.4 97.50 18.8 117.91 23.0 −107.20 −20.7 9595 1795 37,832 7298 −19,194 −37630.5 31.78 10.3 32.05 10.3 −31.89 −10.3 1678 486 16,134 5192 −5388 −17770.6 9.94 8.44 −9.13 246 7076 −1579a This work with SWAN.b Ref. [3].

153T. Takamuku et al. / Journal of Molecular Liquids 136 (2007) 147–155

obtained from the SANS experiments are comparable withthose from the SAXS ones even at 279 K.

In Fig. 7, the concentration and density fluctuations and theircross term for the CH3CN–D2O mixtures at 279, 287, 298, and307 K are plotted as a function of xAN. Both fluctuations atxAN=0.4 most drastically increase with lowering temperature inthe present mole fraction range. When the mole fractionincreases/decreases from xAN=0.4, the increase in the fluctua-tions with decreasing temperature becomes smaller. In contrastto the fluctuations, their cross term for the mixture at xAN=0.4remarkably decreases by cooling. These are consistent with themole fraction xAN=0.38 of the UCST for acetonitrile–watermixtures.

Fig. 8 exhibits the Kirkwood–Buff parameters for theCH3CN–D2O mixtures at each temperature examined as afunction of mole fraction. At 287, 298, and 307 K the GAN,AN

values for the mixtures increase with decreasing xAN from 0.6 to0.3, but do not significantly change at xAN≤0.3. On the otherhand, the GW,W values at the same temperatures are almostconstant in the range 0.4≤xAN≤0.6, and then significantlydecrease when the mole fraction decreases from 0.4. Thefeatures are comparable with those at 298 K obtained from theSANS-U experiments as shown in the Fig. 4(b). At 279 K,however, the GAN,AN and GW,W values for the mixtures changewith mole fraction in a manner different from those at the higher

temperatures. When the xAN decreases from 0.6, the GAN,AN

and GW,W values for the mixtures increase and reach amaximum at xAN≈0.3 and 0.4, respectively. Then, thosedecrease when the xAN further decreases to 0.2. These SANSresults are in agreement with the previous SAXS ones [3],although the parameter values estimated from both experimentsdo not quantitatively agree with each other as mentioned above.

The temperature dependence of the Kirkwood–Buff para-meters can be understood as follows. For the acetonitrile–watermixtures at 287, 298, and 307 K, which are much higher thanthe UCST, the GAN,AN values suggest that acetonitrile–acetonitrile interactions in the mixtures in the water-rich rangexAN≤0.3 are stronger than those in the range xANN0.3. In thewater-rich range, the tetrahedral-like structure of water byhydrogen bonding is significantly formed in the mixtures. Thus,acetonitrile molecules may be excluded from the water structureand tightly aggregate by the dipole–dipole interaction amongthem to form acetonitrile clusters, as discussed in the previousinvestigation by a large-angle X-ray scattering technique [19].The GW,W values for the acetonitrile–water mixtures at 287,298, and 307 K indicate that water–water interactions in theacetonitrile-rich range xAN≥0.4 are more effective than thosein the water-rich range xAN≤0.4. This may arise from thatwater molecules can easily aggregate by hydrogen bonding inthe mixtures even in the acetonitrile-rich range because water

Fig. 7. Acetonitrile mole fraction dependence of concentration fluctuation SCC(0), density fluctuation SNN(0), and their cross term SNC(0) for CH3CN–D2Omixtures at 279, 287, 298, and 307 K obtained by using SWAN.

Fig. 8. Acetonitrile mole fraction dependence of Kirkwood–Buff parametersGAN,AN, GW,W, and GAN,W for CH3CN–D2O mixtures at 279, 287, 298, and307 K obtained by using SWAN.

154 T. Takamuku et al. / Journal of Molecular Liquids 136 (2007) 147–155

molecules are in the hydrophobic medium of acetonitrile. Thesemixing states are ascribed to that acetonitrile and watermolecules interact mainly by the weak dipole–dipole force,but not a strong hydrogen bond. When the temperaturedecreases to 279 K, which is only 7 K higher than the UCST,water–water interactions are strengthened in the mixtures,particularly in the water-rich range, due to enhancement ofhydrogen bonds among water molecules. Thus, the GW,W

values at 279 K in the water-rich range significantly increase aswell as those in the acetonitrile-rich range, resulting in thesignificant maximum at xAN≈0.4 in the mole fractiondependence of the GW,W values. On the other hand, acetonitrilemolecules are further excluded from the water structure

strengthened in the acetonitrile–water mixtures at 279 K.Consequently, acetonitrile–acetonitrile interactions are en-hanced in the mixtures in the water-rich range. This is alsotaking place in the mixtures in the acetonitrile-rich range due torestriction of the motion of acetonitrile molecules at the lowtemperature. Thus, the mole fraction dependence of the GAN,AN

values at 279 K shows the clear maximum at xAN≈0.3.Accompanying the enhancement of acetonitrile–acetonitrileand water–water interactions, acetonitrile–water interactions inthe mixtures become weaker when the temperature decreases. Itresults in the minimum of the GAN,W values for the mixturesbetween xAN=0.3 and 0.4, thus, the mole fraction of the UCST

155T. Takamuku et al. / Journal of Molecular Liquids 136 (2007) 147–155

is found at xAN=0.38. Hence, the self-aggregation of watermolecules and acetonitrile ones is most enhanced in the mixtureat xAN=0.38 by hydrogen bonding and dipole–dipole interac-tion, respectively, as discussed in the previous X-ray investi-gation [19].

The present results show that hydrogen bonds among watermolecules mainly govern the enhancement of heterogeneity ofthe acetonitrile–water mixtures with lowering temperature. Infact, the GW,W values for the acetonitrile–water mixtures arelarger than the GAN,AN values over the present temperaturerange, except for those at some low xAN, as shown in Table 4.

4. Conclusion

The heterogeneity of the acetonitrile–water mixtures in thexAN range 0.2–0.6 was clarified by using SANS technique withtwo spectrometers SANS-U and SWAN. At 298 K, theOrnstein–Zernike correlation lengths for the CH3CN–D2Oand CD3CN–H2O mixtures estimated by using SASN-Ushowed that the heterogeneity of the mixtures is more enhancedat xAN=0.3 and 0.4 than the other mole fractions investigated.The concentration and density fluctuations of the CH3CN–D2Omixtures at 298 K revealed the most enhancement of thefluctuations of the mixture at xAN=0.4 in the xAN rangeexamined. The mole fraction dependence of the Kirkwood–Buff parameters for the mixtures 298 K showed thatacetonitrile–acetonitrile interactions increase with decreasingxAN from 0.6 to 0.3 and very slightly decrease at xANb0.3. Onthe other hand, water–water interactions become stronger withincreasing xAN from 0.2 and are almost constant at xAN≥0.4. Itresulted in the weakest interactions of acetonitrile–water in themixtures between xAN=0.3 and 0.4.

In the temperature range from 307 to 279 K, the Ornstein–Zernike correlation lengths for the CH3CN–D2O mixturesestimated by using SWAN showed that the heterogeneity of themixtures at xAN≈0.35 is most remarkably enhanced withlowering temperature. The concentration and density fluctua-tions and their cross term also indicated the enhancement of theheterogeneity of the mixtures at xAN=0.3 and 0.4 withdecreasing temperature. The Kirkwood–Buff parameters forthe CH3CN–D2O mixtures revealed that both acetonitrile–acetonitrile and water–water interactions in the mixtures atxAN=0.3 and 0.4, respectively, are significantly strengthenedwhen the temperature is close to the UCST, whereasacetonitrile–water interactions drastically decrease in themixtures at xAN=0.3 and 0.4. Additionally, the Kirkwood–

Buff parameters for the mixtures over the present temperaturerange suggested that acetonitrile–acetonitrile interactions in themixtures are weaker than water–water ones at most of the molefractions examined. These features strongly relate to the molefraction xAN=0.38 of the UCST. It is thus concluded that phaseseparation of acetonitrile–water mixtures by cooling isgoverned mainly by enhancement of hydrogen bonds amongwater molecules.

Acknowledgements

This work was supported partly by a Grants-in-Aid (Nos.15550016 and 19550022) from the Ministry of Education,Culture, Sports, Science, and Technology, Japan. The SANSexperiments were carried out by the joint researches of theInstitute for Solid State Physics, the University of Tokyo (No.02.2523) and the pulsed neutron facility (KENS), High EnergyAccelerator Research Organization (KEK) (03-B2-019).

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