Journal of Molecular Liquids 190 (2014) 30–33

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Journal of Molecular Liquids

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Properties of LiOH and LiCl at sub and supercritical water conditions

Ruth A. Carvajal-Ortiz ⁎, Igor M. SvishchevSupercritical Water Research Facility, Department of Chemistry, Trent University, 1600 West Bank Dr., Peterborough, ON K9J 7B8, Canada

⁎ Corresponding author. Tel.: +1 705 748 1011x7163;E-mail address: rcarvajalorti@trentu.ca (R.A. Carvajal-

0167-7322/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.molliq.2013.10.018

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 September 2013Received in revised form 16 October 2013Accepted 23 October 2013Available online 31 October 2013

Keywords:Electrolyte infinite dilutionsHydration structureDiffusion coefficientLimiting molar conductivity

The hydration structures and dynamics of Li+, Cl− and OH− ions in aqueous infinite dilute solutions are investi-gated via molecular dynamics simulations at sub- and supercritical water conditions. Radial distribution func-tions of these ions are examined along the coexistence curve of water at 298, 373, 473, 573 K and above thecritical point at 666 and 673 K in the range of densities from 0.998 to 0.276 g/cm3. The coordination numbersof the ions at infinite dilution are calculated. Tracer diffusion coefficients of the ions are also evaluated, as wellas the limiting molar conductivity for LiOH and LiCl solutions.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

In nature and technology, aqueous systems at high temperaturesand pressures contain electrolytes that interact with water, modifyingthe solution properties. As water reaches the critical point, the densityand dielectric constant decrease and extended H-bonded structure issignificantly perturbed, enhancing the diffusivity of solutes. As a result,the association of electrolytes at these conditions is greatly increased.The ionization product of water exhibits a maximum at 259 °C and0.81 g/cm3 (pKw ≈ 11), and decreases sharply as the critical tempera-ture is approached [1]. At the supercritical conditions, water behavesas a low-density non-polar solvent, with low affinity for inorganic com-pounds. These modifications in solvent properties create a significantchallenge for the development of new hydrothermal technologiessuch as materials processing, hazardous waste utilization, and powergeneration. As an example, LiOH(aq) is added in some nuclear and fossilfuel-fired power reactors in order to control the pH of themedia. Duringthis process some inorganic impurities, can also interfere and form un-desired by-products (e.g. LiCl). Hydration structure and conductivitymeasurements of ionic aqueous solutions can provide indirect informa-tion on how ions and ion-pairs behave in the solution. However, there isa scarcity of experimental studies [2–5] of thermodynamic and trans-port properties of ionic solutions at temperatures above 100 °C, espe-cially for Li+ containing solutions, due to the difficulty of performingmeasurements at high temperatures and pressures. An alternative forthese experimental studies is the computer simulation of electrolyteaqueous solutions that can give a direct insight into the structure andtransport properties.

fax: +1 705 748 1625.Ortiz).

ghts reserved.

The aim of our simulation work is to examine the interaction of Li+,Cl− and OH− ions with the water structure at different temperatures.We compute the coordination andH-bond numbers. The diffusion coef-ficients are also calculated and are used to compute the limiting conduc-tance of LiOH and LiCl via the Nernst–Einstein relationship. Thesimulation results are compared to experimental conductivitymeasure-mentsmadepreviously [2,6–9]. In particular, these results help estimatequantum effect contribution in OH− diffusion below the critical point.

The remainder of this paper is organized as follows. The simulationdetails are described in Section 2. The simulation results are discussedin Section 3. The conclusions are given in Section 4.

2. Simulation details

In our simulations the Simple Point Charged Extended (SPC/E) [10]water model was used; this model provides a reasonably good predic-tion of the structure and dielectric constant for water at the conditionsof interest [11]. The Lorentz–Berthelot mixing rules were applied to cal-culate the Lennard–Jones interaction parameters for the ion–water andion–ion interactions. The equations of motion were solved by usingVerlet leap-frog algorithm in a cubic simulation cell with a time stepof 1 fs. The SHAKE constraint scheme [12] was applied to keep mole-cules rigid. The spherical cut-off radius was set at the half of the simula-tion cell. Simulations have been performed in the NVT ensemble withtemperature controlled by the Nosé–Hoover thermostat [13]. Long-range electrostatic interactions were handled by the Ewald method[14], subject to the “conducting” periodic boundary conditions.

In the presentworkwehave employed parallel dynamics code for anarbitrarymolecularmixture (M.Dynamix) by Lyubartsev and Laaksonen[15], which has beenmodified to perform the analysis of diffusion coef-ficients. Simulations have been performed on a Linux cluster built on theTyan/Opteron 64 platform. Parameters from Dang [16], and Smith and

Table 1Interaction parameters of the employed models.

Atom ε (kJ/mol) σ (nm) q (e)

SPC/E [10] H 0 0 0.4238O 0.6507 3.166 −0.8476

Dang, L.X. [16] Li 0.69 1.506 1Smith D.E. and Dang L.X. [17] Cl 0.419 4.4 −1Wiener et al. [18] O 0.6320 3.233 −1.3

H 0.2310 2.083 0.3

Fig. 1. Radial distribution function,g Li−OWð Þ of Li+ in aqueous solution at temperatures and

densities of: 298, 373, 473, 573 and 673 K, and 0.99, 0.94, 0.83, 0.65 and 0.28 g/cm3

respectively.

31R.A. Carvajal-Ortiz, I.M. Svishchev / Journal of Molecular Liquids 190 (2014) 30–33

Dang [17] were employed to model Li+–water and Cl−–water interac-tions respectively; for OH−–water interactions parameters fromWienerand Kollman [18] were used. Table 1 lists the site charges, q, andLennard–Jones parameters, ε and σ. The total length of each simulationrunwas 500 ps. Simulationswere run over a wide range of state points.MD simulations of single-ion properties were performed with 342watermolecules and the ion (Li+, Cl−, OH−). The state points examinedwere: 0.99 g/cm3, 298 K; 0.94 g/cm3, 373 K; 0.83 g/cm3, 473 K;0.65 g/cm3, 573 K; and 0.28 g/cm3, 673 K. For each temperature, 3 sim-ulation runswere performed in order to obtain an average value of eachproperty evaluated. The uncertainty in the simulated diffusion coeffi-cients is estimated to be less than 10%.

The radial distribution function (RDF) of ion–oxygen, gi–o(r), wasemployed to compute the average number of water molecules thatwere in the primary shell of the ion–water structure, n (see Eq. (1)),

n ¼ N=Vð Þ � 4π �Zr1

0

gi−o rð Þ � r2dr; ð1Þ

whereN is the number of particles in the cell and V is the total volume ofthe cell, r1 is the radius of the first hydration sphere which correspondsto the first minimum in gi–o(r). The integral is evaluated over the dis-tance of solvent molecules to the ion. The H-bond numbers were com-puted using the RDFs of ion–hydrogen pairs, gi–H(r), for the twohydrogen atoms present in the water.

The diffusion coefficient at infinite dilution,Do (tracer diffusion coef-ficient) was evaluated using the linear velocity autocorrelation function(LVACF) technique [14].

3. Results and discussion

3.1. First hydration shell of Li+, Cl− and OH− in water at infinite dilution

Table 2 shows the coordination numbers and number of H-bonds forLi+, Cl− andOH− ions inwater. Fig. 1 shows the radial distribution func-tion, RDF of Li+ in water at 298, 373, 473, 573 and 673 K. The first peakof the Li+–O interaction (g Li−OWð Þ) has its maximum at 1.85 Å, a goodmatch with the experimental results reported by Howell and Neilson[19], giving a coordination number for Li+ in water at infinite dilutionin the range of 4 and 4.7, slightly decreasing as the temperature in-creases. MD simulation results at ambient temperature reported byZhang and Duan [20], Varanasi et al. [21], and Lee and Rasaiah [22]agree with the tetrahedral hydration structure obtained in this study.

Table 2Total coordination numbers for Li+, Cl− and H-bond numbers for OH− ions in aqueous solutio

State point Coordination number

Temperature, K Density, g/cm3 Cl–Ow

298 0.99 7.2373 0.94 7.5473 0.83 7.5573 0.65 7.7673 0.28 7.7

At supercritical conditions, the 4.3 coordination number obtained inthis study is in accordance with the 4.18 hydration number reportedby Lee and Cummings [23] at the low density of 0.31 g/cm3. The resultsin the current study also agree with the results showed by Rempe et al.[24] and Szász and Heinzinger [25] using different simulation tech-niques (ab initio molecular dynamics and quasi-chemical theory com-putations for Li+ in water at infinite dilution, respectively). Neutrondiffraction experiments carried out by Howell and Neilson [19] regard-ing the hydration of Li+ ion in concentrated solutions showed a coordi-nation number of 6.5 ± 1 for a LiCl salt concentration of 1 m in aqueoussolution, with the first maximum at 1.96 Å, noting that there is an im-portant effect of the ion concentration in the coordination structure ofthe lithium ion in aqueous solutions.

The Cl−–Ow radial distribution function is shown in Fig. 2. Thefirst maximum is located at 3.32 Å at ambient temperature, with acoordination number of 7.2 increasing slightly in the supercriticalregion. In a neutron diffraction study using hydrogen isotope substi-tution, Mancinelli et al. [26] reported coordination numbers in therange of 6.9 to 7 for Cl− in NaCl aqueous solutions of concentrationsas low as 0.66 M. Even though, the experiments of Mancinelli et al.were performed at the finite dilution, the concentrations reportedare rather low and experimental data appear to be in good standingwith the coordination numbers found in this study. It is noticeablethat at room temperature, a sharp and tall peak in the gCl–o(r) corre-sponds to the first solvation shell, followed by a shorter and broadersecond peak. At supercritical conditions, this second peak has almostdisappeared. We have also noticed that the Cl−–O coordinationnumber progressively exceeds the Cl−–H coordination number(see Table 2), which implies the presence of an extra water moleculein the hydration shell that is not forming an H-bond with the chlo-ride. The H-bonds computed Cl− showed a decrease as the temperatureincreased. It is worthwhile mentioning that some experimental studies[27–30] performed at ambient conditions reported the effect of the

ns at 298, 373, 473, 573 and 673 K.

HO−

H-bondsCl−

H-bondsLi–Ow HO–Ow

4.8 6.1 5.8 7.04.6 5.9 5.8 6.84.4 5.8 4.8 6.34.4 5.9 5.2 5.84.3 5.9 4.7 4.9

Fig. 2. Radial distribution function, g Cl−OWð Þ of Cl− in aqueous solution at temperatures and

densities of: 298, 373, 473, 573 and 673 K, and 0.99, 0.94, 0.83, 0.65 and 0.28 g/cm3

respectively.

32 R.A. Carvajal-Ortiz, I.M. Svishchev / Journal of Molecular Liquids 190 (2014) 30–33

increase in concentration on the coordination numbers for Li+ and Cl−,particularly at high concentrations, noticing a slight increase as the con-centration rises, and a slight shift in the first maximum peak of the ion-water RDF to smaller separations.

Fig. 3 compares the RDFs of HO−–Hw at 298 K and 673 K. Thisstudy has found coordination numbers from 5.8 to 6 for OH−, theg HW−Oð Þ having the first maximum at about 2.6 Å at all temperaturesfrom 298 to 673 K (see Fig. 2). Balbuena et al. [31] report similarvalues for OH− ion in sub- and supercritical water. Coordinationnumber of OH− was found to decrease very slightly as the systemapproaches critical water conditions. The vanishing of the firstminimum at high temperatures is related to the H-bonding natureloss in the structure as the temperature reaches the supercritical re-gion. The nature of hydroxide ion solvation environment in high-temperature and supercritical water has been subject of discussionand several studies [32–38]. The hydration of OH− ion is complex,as the O atom acts as acceptor of four H-bonds in a square-planar ac-commodation, and at the same time the hydrogen atom in the OH−

ion can be an H-bond donor attracting one or two more water mol-ecules in the first hydration shell. Tuckerman et al. [38,39] havereported this ambiguity as “hypercoordination” structure. (H9O5)−

Fig. 3. Radial distribution function, g OOH− −HWð Þ of OH− in aqueous solution at ambienttemperature and density of 0.99 g/cm3, and supercritical temperature and density of0.28 g/cm3.

complexes were found to coordinate additional water molecules toform (H9O5)−·H2O complexes at infinite dilution.

3.2. Tracer diffusion coefficient and limiting molar conductivity

Fig. 4 illustrates the tracer diffusion coefficient (Do) for Li+, Cl− andOH− at 298, 373, 473, 573, 666 and 673 K. The decrease in thewater den-sity as the temperature gets close to and above the critical region, leads toan increase in the values of the tracer diffusion coefficient for the threeions. The values of Do for Li+ at 298 and 673 K in this study match thevalues from Lee and Rasaiah [22] (Do = 1.18 × 10−9 m2 s−1) andChakraborty and Chandra [40] (Do = 37.8 × 10−9 m2 s−1), respective-ly. Chloride ion diffusion coefficient at 298 K is in good agreementwith the values of 1.65 × 10−9 m2 s−1 and 1.71 × 10−9 m2 s−1 re-ported by Koneshan et al. [41] and Lee and Rasaiah [22]. The diffu-sion coefficient at 673 K computed in this study matches the valueof 32.08 × 10−9 m2 s−1 reported by Chakraborty and Chandra[40]. We may note that classical simulations used did not considerquantum effects of the ion dynamics.

The diffusion coefficients obtained from the simulations were usedto calculate the limiting molar conductivity for an electrolyte (Λo)from single ion mobilities (λ+/−) using a variation of the Nernst–Einstein relationship:

Λþ=− S � cm2=mol

h i¼ Fe−DMD

=KBT ; ð2Þ

where F is the Faraday constant, e− is the electron charge, DMD is thediffusion coefficient from the simulations, and KB and T are theBoltzmann constant and the temperature respectively [42]. Limitingmolar conductivity of LiOH solutions was calculated using theKohlrausch's law (Eq. (3)):

Λo ¼ λþ þ λ−: ð3Þ

Table 3 shows the experimental values reported by Corti et al. [43]and Ho et al. [9] as well as the simulated ΛLiOHo and ΛLiClo values obtainedin this study. The obtained values at supercritical conditions are veryclose to the experimental values which attest to the quality of themodels used (at supercritical conditions the diffusion of ions is expectedto be purely classical). Below the critical point of thewater, lithium chlo-ride simulated values are fairly close to the experimental results. A largesystematic difference between the simulated and the experimental data

Fig. 4. Diffusion coefficients of Li+, Cl− and OH− in water at 298, 373, 473, 573 and 673 K,and 0.99, 0.94, 0.83, 0.65 and 0.28 g/cm3 respectively.

Table 3Limiting molar conductivity, Λo of LiOH and LiCl.

Temperature, K Density, g/cm3 LiOH LiCl

ΛoMD, S·cm2·mol−1 Λoexp, S·cm2·mol−1 ΛoMD, S·cm2·mol−1 Λoexp, S·cm2·mol−1

298 0.99 46.5 237.9 [43] 71.61 115.1 [45]373 0.94 178.06 540.8 [8] 229.64 326.6 [9]473 0.83 361.59 1017.4 [8] 461.81 645.8 [9]573 0.65 657.88 1166.6 [8] 667.11 943.8 [9]673 0.28 936.48 948.05 [8] 1087.71 1229.2 [9]666.028a 0.37 953.65 1101.55 1344.7 [9]

a Temperature at the corresponding state.

33R.A. Carvajal-Ortiz, I.M. Svishchev / Journal of Molecular Liquids 190 (2014) 30–33

is seen in the conductivities of hydroxide, which is related to the quan-tum effects in themobility of this ion [44] (proton tunneling) that are ig-nored in the classical simulations used in this study.

4. Conclusions

Hydration and diffusivity of single ions (Li+, Cl− and OH−) in sub-and supercritical water (at 298, 373, 473, 573 and 673 K)were analyzedusing molecular dynamics simulations. The calculated coordinationnumbers are in a good agreement with literature data, showing a de-crease in the hydration structure as the H-bond nature is affected bythe high-temperature environment. The H-bond loss and the highmass-transport of supercritical water were evident from the MD simu-lations. The simulated high-temperature limiting molar conductivities,Λo of LiOH and LiCl appear to be in agreement with experimental mea-surements, where such comparisons can be made. Molecular-basedcomputer simulations can thus provide good quality reference data forexperimental measurements of transport properties of Li-containingelectrolytes in low density supercritical water and the details of theirspeciation, as well as an estimate of quantum effect contribution inOH− diffusion below the critical point.

Acknowledgments

The authors are grateful for the financial support of the NSERC/NRCan/AECL Generation IV Energy Technologies Program.

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