Journal of Molecular Structure 892 (2008) 47–52

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Journal of Molecular Structure

journal homepage: www.elsevier .com/ locate /molst ruc

Neutron scattering experiments on aqueous sodium chloride solutionsand heavy water. Comparison to molecular dynamics and X-ray results

Salah Bouazizi a, Férid Hammami a, Salah Nasr a,*, Marie-Claire Bellissent-Funel b

a Laboratoire Physico-Chimie des Matériaux, Département de Physique, Faculté des Sciences de Monastir, 5019 Monastir, Tunisiab Laboratoire Léon Brillouin (CEA-CNRS) CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France

a r t i c l e i n f o

Article history:Received 10 March 2008Received in revised form 21 April 2008Accepted 28 April 2008Available online 23 May 2008

Keywords:NaCl aqueous solutionLocal orderH-bondingX-ray and neutron scattering

0022-2860/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.molstruc.2008.04.062

* Corresponding author.E-mail address: salah_nasr1@yahoo.fr (S. Nasr).

a b s t r a c t

A neutron diffraction study has been carried out on pure water and sodium chloride aqueous solutionsranging from 0.5 to 3 m under ambient conditions. The present contribution extends our earlier studyby means of X-ray scattering and molecular dynamics simulations. The recorded data are analyzed toyield the total structure factor and the pair correlation function of each system. It was shown that theD–D and O–D intramolecular distances remain independent on concentration. Whereas, the degree ofhydrogen bonding in solutions decreases in comparison with that of pure water. To describe the localorder of solutions, simulated results using a rigid SPC water model are used to propose a model of inter-molecular arrangement. Good agreement is found between computed structure factors and experimentalones.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Salt sodium chloride dissolved in water can be regarded asthe standard aqueous electrolyte solution. It is commonplace inthe natural environment and an essential component in manybiological systems [1]. Water is the most important liquid andsolvent in a variety of ways, including the biological one. Underthe influence of electrostatic and steric effects of ions, waterchanges its original bulk structure forming solvation shells ofions. These structural changes give rise to the complex dynamicsof hydrogen bonds, ion–water and ion–ion association. In partic-ular, the manner in which water solvated alkali cations is rele-vant to problems such as the mechanism of enzymaticcatalysis [2]. Due to the extraordinary importance of aqueouselectrolyte solutions, several attempts have focused on theirproperties by means of experimental techniques or/and molecu-lar dynamics simulations (MD).

The NaCl aqueous solutions have been widely studied by MDand Monte Carlo (MC) simulations [3–10], mainly due to theirrole in the behavior of chemical, biochemical and geological sys-tems. In their MD calculations performed on aqueous NaCl solu-tions at different salt concentrations [3–5], the authors suggestthat the average number of hydrogen bond per molecule de-creases with increasing salt concentration. Chowdhuri et al. [6]have used MD simulations to investigate the properties of aque-

ll rights reserved.

ous NaCl at different salt concentrations. Their results clearlyshow a reorganization of the water-molecules whereas the intra-molecular frequencies are slightly affected by the presence ofions.

The experimental study of aqueous NaCl solutions is not tooextended in the literature [9–13]. This is why we have firstlyperformed, by means of X-ray scattering and MD simulation, astructural investigation of pure water and aqueous NaCl solu-tions at different salt concentrations ranging from 0.5 to 4 m.We have particularly highlighted that the simulated structurefactors are compatible with the observed changes in the experi-mental data.

To get further insight into ions effect on the water structure andto give a clear idea of the structural properties of aqueous NaClsolutions, we report in this paper a neutron diffraction study onthe same systems described above. We particularly aim to showthe possibility of constructing, via MD predictions, a model ofintermolecular arrangement able to describe the local order ofour systems as deduced from neutron data, and to underline thecomplementarities between X-ray and neutron information. InSection 2, we describe the experimental procedure. In Section 3,we present the succinct neutron scattering formalism and the datatreatment. Experimental results are discussed in Section 4 in termsof pair correlation functions and in relation with previous X-ray re-sults. Particularly, the weighted structure factors and the radialdistribution ones as deduced from the proposed model are com-pared to experimental data. Conclusion and final remarks are pre-sented in Section 5.

48 S. Bouazizi et al. / Journal of Molecular Structure 892 (2008) 47–52

2. Experiments

Sodium chloride was supplied by Pharmacopée Européenne(purity >99%) and the solutions were prepared with heavywater. The measurements of the neutron scattering spectra ofaqueous NaCl solutions were performed on the 7C2 spectrom-eter of the Orphée reactor of the Laboratoire Léon Brillouin inSaclay (France) [14]. The spectrometer investigates the wideangle (up to 126�) to collect the diffraction pattern which cov-ers a neutron momentum transfer Q ¼ 4p sinðhÞ

k from 0.5 to15.90 Å with a monochromatic neutron beam of wavelength0.701 Å. The sample is contained inside a null matrix Ti67Zr33

cell for which the intensity is principally incoherent. The cellis a cylinder with an internal diameter of 5 mm and a wellthickness of 6 mm. The usual measurements of the vanadiumrod, the cadmium rod and the empty cryostat were alsoperformed.

3. Neutron formalism and data treatment

The basic theory of neutron scattering by liquids is well known[15,16], and only a brief outline is given here to describe the localorder in our systems. The total structure factor of each solutionmay be defined as:

SMðQÞ ¼drdX

� �coh

ðP

ixibiÞ2ð1Þ

where Q stands for the modulus of the wave vector Q in the case ofelastic scattering, ðdr

dX Þcoh is the contribution of the differential scat-tering cross section, xi and bi are, respectively, the atomic fractionand the scattering length of atom i. At large Q, SM(Q) goes to theasymptotic value

0 2

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

C = 0.5 m

C = 3 m

C = 1.5 m

heavy water

Q[S

M(Q

)-S M

(∞)]

Q

4 6

Fig. 1. The neutron Q-weighted structure factors of heavy water

SMð1Þ ¼P

xib2i

ðP

ixibiÞ2ð2Þ

and for Q ? 0, limQ!0

SMðQÞ ¼ qKBTvT, where q is the molecular num-

ber density; KB, the Boltzmann’s constant; T, the absolute tempera-ture; and XT, the isothermal compressibility.

The pair correlation function of each system may be deduced asthe Fourier transformation of the structure factor SM, (Q)

gðrÞ ¼ 1þ 12p2qr

Z 1

0Q ½SMðQÞ � SMð1Þ� sinðQrÞdQ ð3Þ

We should also use the radial distribution functiondðrÞ ¼ 4pqrbgðrÞ � 1c.

Many terms contribute to the measured intensity. We sum-marize here the procedure previously applied [17,18] in orderto go from the experimental intensity Iexp to the differentialcross section dr

dX

� �. The measured intensity was corrected from

the background contribution, the cell contribution, and the mul-tiple scattering contribution involving the sample. The variousintensities were calculated by a standard numerical program[17].

The differential cross section of the sample a was obtained byusing the vanadium measurements

drdX

� �a¼ dr

dX

� �V

qV

IcorrV

ISaPa

Aaqa; ð4Þ

where (dr/dX)V is the differential incoherent cross section of vana-dium; ICorr

V was obtained from the vanadium intensity after correc-tions of multiple scattering, absorption and inelastic scatteringaccording to the standard procedure [17]. Aa is the attenuation fac-tor for neutrons scattered from the sample and attenuated by thecell and the sample itself; qV and qa are the number densities of

8 10 12 14 16

( Å-1

)

and aqueous NaCl solutions at various salt concentrations.

S. Bouazizi et al. / Journal of Molecular Structure 892 (2008) 47–52 49

vanadium and sample, respectively. Pa is the factor which takes intoaccount the inelasticity effects. The inelasticity correction was per-formed using a polynomial procedure described elsewhere [18].

4. Results and discussion

The neutron Q-weighted structure factor of each system as de-duced from Eq. (1) is plotted in Fig. 1. For clearer view, the curvesare translated by 0.25 in amplitude. At high Q-values, the curvesare well superimposed which indicates that there is no significantvariation of the intramolecular structure of water in aqueous NaClsolutions compared with the pure liquid. The salt effect on waterstructure is easily deduced from neutron pair correlation functionsgn(r) (Fig. 2a) which are derived from the Fourier transformationsof the structure factors. In order to compare such effect with X-ray diffraction results [19], the gx(r) curves are also plotted inFig. 2b.

The scattering amplitudes ensure that interactions involving Ddominate the neutron curves while those involving O dominatethe X-ray ones. The main conclusions for the latter curves [19]are summarized here. The X-ray pair correlation functions arecharacterized, at ambient conditions, by two dominant peaks at2.81 and 4.45 Å, respectively, assigned to the first and the secondoxygen–oxygen interactions. As the ions concentration increases,the amplitude of these peaks decreases, leading to the rearrange-ment of the water tetrahedral structure. Moreover, the gx(r) curvesshow, at a weak concentration (1.5 m), a hump at about 2.37 Åwhich grows gradually to become a well pronounced peak at high-

0r ( Å )

g n (r)

C = 1.5 m

00.0

0.5

1.0

1.5

2.0

2.5

a

2 4 6

heavy water

C = 3 m

C = 0.5 m

Fig. 2. The experimental pair correlation functions of aqueous NaCl solutions at var

er concentrations. This distance is ascribed to the Na+���O interac-tion. We also notice the presence of a second ion–waterseparation distance at about 3.17 Å, related to the Cl����Ocorrelation.

The curves derived from neutron scattering, gn(r), show a dom-inant peak at 0.96 Å and a second one at 1.51 Å, respectively, as-signed to the intramolecular OD and DD interactions. One cannotice the good concordance between these values and those de-duced by Narten et al. [20] who have, respectively, found 0.94and 1.5 Å in their study of aqueous LiCl at concentrations to satu-ration. The observation of concentration-independent intramolec-ular distances is in accordance with previous data obtained byTromp et al. [21], who used neutron diffraction on aqueous LiClsolution at 1 and 10 m. A constant intramolecular D–D distancewas also found for a LiCl concentration between 12 and 26 m[22]. The location of the third peak in gn(r) at 1.94 Å, assigned tothe intermolecular O���D interaction, slightly decreases in ampli-tude from 0.93 to 0.89 with increasing salt concentration. A similareffect can be pointed out about the fourth peak at 2.36 Å. Accordingto earlier studies [19], the latter position is related to D���D interac-tion and probably the Cl����D correlation. In fact, in the detailedstudy of Narten et al. [20] of aqueous LiCl solutions at higher ionsconcentrations, the authors found a prominent increasing peak at2.3 Å which was assigned to the hydrogen bond Cl����D–O. Thegn(r) curves also show a hump at 2.85 Å characteristic of waterintermolecular O���O correlation [17]. It is interesting to notice thatthe effect of ions concentration on water structure is more pro-nounced in X-ray data. In fact, the gx(r) curves (Fig. 2b) clearly

4

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

g x (

r)

C = 3 m

C = 1.5 m

C = 0.5 m

pure water

r ( Å )

b

2 6 8

ious salt concentrations as deduced from neutron (a) and X-ray diffraction (b).

0.00

0.75

1.50

2.25 a

0.00

0.75

1.50

r ( Å )

g HH

(r)

gO

H(r

)g O

O(r

)

b

0 6 100.00

0.75

1.50

2.25

3.00

c

pure waterC = 0.5 mC = 1.5 mC = 3 m

pure water

C = 0.5 m

C = 1.5 m

C = 3 m

pure water

C = 3 m

C = 0.5m

C = 1.5 m

82 4

Fig. 3. The oxygen–hydrogen, hydrogen–hydrogen and oxygen–oxygen pair corre-lation functions of aqueous NaCl solutions, compared to the pure water curves asdeduced from MD simulations.

Table 1The rms vibrational amplitudes (lij) and the interatomic distances rij as deduced fromthe fitting procedure (see text)

r1;i j l1;i jr2;ij l2;i j

C = 3 mNa+���D 2.881 (0.047) 0.011 5.271 (0.019) 0.125Cl-���D 2.283 (0.005) 0.025 3.520 (0.040) 0.345O���O 2.810 (0.014) 0.162 4.522 (0.015) 0.212O���D 1.851 (0.045) 0.192 3.239 (0.009) 0.184D���D 2.390 (0.012) 0.220 3.767 (0.008) 0.353O–D (intra) 0.980 (0.020) 0.066 – –D–D (intra) 1.601 (0.017) 0.332 – –

Heavy waterO���O 2.810 (0.014) 0.149 4.522 (0.015) 0.160O���D 1.851 (0.045) 0.129 3.239 (0.009) 0.276D���D 2.390 (0.012) 0.210 3.767 (0.008) 0.212O–D (intra) 0.980 (0.020) 0.059 – –D–D (intra) 1.601 (0.017) 0.331 – –

The values in parenthesis represent the rij variation in comparison with initialsimulated ones.

-0.4

-0.2

0.0

0.2

heavy water

-0.4

-0.2

0.0

0.2

-0.4

-0.2

0.0

0.2

Q (Å-1

)

SM

(Q)-

SM

(∞)

0 2 4 6 8 10 12 14 16

-0.4

-0.2

0.0

0.2

C = 3 m

C = 1.5 m

C = 0.5 m

Fig. 4. Experimental and theoretical structure factors (solid and dotted lines,respectively) of heavy water and aqueous NaCl solutions.

50 S. Bouazizi et al. / Journal of Molecular Structure 892 (2008) 47–52

show two real peaks at 2.8 and 4.5 Å, respectively, assigned to thefirst and second oxygen–oxygen interactions, and whose ampli-tudes gradually decrease with increasing salt concentrations.Moreover, the sodium–oxygen and chloride–oxygen interactionsare more highlighted particularly at higher concentration by twoother peaks, respectively, at 2.24 and 3.17 Å. As expected, thegn(r) curves show another peak at 3.39 Å assigned to a second O���Dinteraction [19]. The amplitude of this peak gradually increaseswhile its shape becomes largest with increasing salt concentration.This may be due to the contribution of the Cl����D and Cl����O inter-action which occur, respectively, at 3.67 and 3.22 Å [17]. The ob-served effect of ions on O���D and D���D interactions in gn(r) andmore again on O���O correlation in gx(r) may be regarded as a de-crease of the degree of hydrogen bonding in solutions when com-pared with pure water.

In what follows, we compare the experimental results withthose of the computer simulation. The calculations were performedwith rigid SPC water model as a solvent, while the ions were trea-ted as charged Lennard–Jones particles (for more details, see Refs.[19,23,24]). In Fig. 3, we plot, respectively, the oxygen–oxygen,oxygen–hydrogen and hydrogen–hydrogen pair correlation func-tions for pure water and 0.5, 1.5 and 3 m solutions. In the hydro-gen–hydrogen figure, the first and the second peaks, respectively,at 2.42 and 3.8 Å are at the same position for all systems. The

height of the second one appears irrelevant to the presence of ions,whereas the amplitude of the first one decreases from 1.35 for purewater to 1.22 for 3 m solution; such variations are also observed ingOH(r). The behavior of the curves is on the one hand in concor-dance with the observed feature of the neutron curves, discussed

-0.2

0.0

0.2

0.4

-0.2

0.0

0.2

04

-0.2

0.0

0.2

0.4

0 2 4 6 8

-0.2

0.0

0.2

0.4

r(Å)

C = 1.5m

C = 3 m

d(r)

C = 0.5m

heavy water

Fig. 5. Experimental and theoretical radial distribution functions (solid and dottedlines, respectively) of heavy water and aqueous NaCl solutions.

S. Bouazizi et al. / Journal of Molecular Structure 892 (2008) 47–52 51

above and on the other hand in agreement with earlier studies[7,19]. Otherwise, the analysis of the oxygen–oxygen pair correla-tion functions clearly show a disruption of the water structuresince the amplitude of the first peak dramatically decreases from2.89 in pure water to 2.17 at 3 m solution. The curves also showa gradual decrease of the second maximum amplitude whichmoves outwards with increasing salt concentration. This displace-ment, previously mentioned in earlier studies [19,25], is similar tothat observed in water at elevated temperature [26]. However, inthe recent study of Mancinelli et al. [10], the authors found thatthe second maximum position moves inwards as the concentrationincreases; in an analogous manner to the pressure effect in purewater. The final step of our investigation is to construct a theoret-ical model able to reproduce experimental data. It is interesting tonotice here that in our previous study related to X-ray investiga-tion, all the interactions involving H atoms were ignored sincethe contribution of the X���H correlation (X = Na+, Cl�, O) is weakparticularly at high Q-values. In contrast, by using the isotopic sub-stitution technique, neutron data allows us to locate with moreprecision the deuterium atom positions. Admitting this andaccording the previous analysis of the neutron data, we can write

SMðQÞ ¼ SW ðQÞ þ SW�IðQÞ ð5Þ

where SWðQÞ and SW�IðQÞ represent, respectively, the total structurefactor of water and the solvent–solute interactions. The partialstructure factor, SijðQÞ, of the (i, j) pair particles is expressed as:

SijðQÞ ¼ 2xixjbibj

ðP

ixibiÞ2

"sinðQr1;ijÞ

Qr1;ijexp �

ðl1;ijÞ2

2Q 2

!

þsinðQr2;ijÞ

Qr2;ijexp �

ðl2;ijÞ2

2Q 2

!#ð6Þ

where r1;ij and r2;ij are, respectively, the first and the second sep-aration distances between the particles i–j, l1;ij and l2;ij are thecorresponding root-mean-square (rms) vibrational amplitudes orthe Debye–Waller (DW) parameters. The computed patterns areestimated according to the two following criteria: (i) by a least-square fitting of Eq. (5) to the experimental neutron structurefactors; (ii) by reducing the spurious oscillations of the pair cor-relation functions at short distances. In this calculation, only theDW parameters are fitted and the bond length intramolecular andintermolecular variations are, respectively, fixed within 2.5% and5% from MD simulated values [19]. In Table 1, we have reportedsome values of the optimized rij distances and the correspondinglij parameters.

Despite the crude model used, good agreement is observed be-tween theoretical and experimental structure factors (Fig. 4, dottedand solid lines, respectively). Simultaneously, the main features ofthe experimental radial distribution functions, (Fig. 5, solid line)are reproduced by the computed ones (Fig. 5, dotted line). Particu-larly, the first four peaks in the experimental curves, respectively,assigned to the intramolecular O–D, D–D and first intermolecularO���D, D���D and Cl����D interactions and second O���D one are wellaccounted for by the proposed model.

5. Conclusion

In this paper, we have presented a structural investigation ofheavy water and aqueous NaCl solutions at salt concentrationranging from 0.5 to 3 m. The neutron scattering data performedto our knowledge for the first time on these solutions, associatedto X-ray scattering ones recently published, give some comple-mentary information about the ion-induced disruption of waterstructure and the solute–solvent interactions

The main results of our investigations can be summarized asfollows:

(a) Neutron data give more details about the intramolecularconformation of water and intermolecular correlationsinvolving D atoms. Although, because of the lower X-rayscattering of H atoms, only the intermolecular O���O interac-tion can be pointed in the X-ray data. Neutron results alsoshow that the intramolecular water structure is not affectedby the presence of ions; this result is supported by more pre-vious studies on similar systems at large concentrations.

(b) The analysis of the intermolecular O���O X-ray correlations aswell as the D���D and D���O neutron ones show, in conformitywith MD predictions, a decrease in the orientational order-ing of a pair of hydrogen-bonded molecules with increasingsalt concentration. In parallel, MD results using rigid SPCwater model clearly show a decrease of the number ofhydrogen bond per molecule.

(c) The analysis of neutron pair correlation functions, and moreagain the X-ray ones highlights the existence of an electro-static interaction whose ‘‘signature” is, respectively, assertedby the Cl����D, Na+���O and Cl����O interactions. The neutronscattering data presented here as well as X-ray ones recentlypublished and MD investigations clearly show a disruption ofwater structure as the salt concentration increases. The num-ber of H-bonds per molecule gradually decreases whereas the

52 S. Bouazizi et al. / Journal of Molecular Structure 892 (2008) 47–52

oxygen–oxygen coordination numbers increase. Both cationsand anions are dissolved by water-molecules at the expenseof the break down of the water H-bond network.

(d) According to the MD results, a theoretical model principallybased on solvent–solvent, solvent–solute and solute–soluteinteractions is proposed to describe the experimental data.A computed structure factors which take into account themore scattering element in each case reproduce well theexperimental ones as deduced from X-ray or neutronexperiment.

(e) The advantage of complementary data from X-ray [19], neu-tron scattering and also MD simulation is clearly pointed outacross our investigation.

To get further insight into NaCl aqueous solution properties,neutron scattering experiments and MD simulation studies undernon-ambient conditions are in progress.

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