pressure dependence on the single-particle dynamics and hydrogen-bond structural relaxation of...

This article was downloaded by: [Umeå University Library]On: 08 September 2014, At: 05:34Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Molecular Physics: An International Journal at theInterface Between Chemistry and PhysicsPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tmph20

Pressure dependence on the single-particledynamics and hydrogen-bond structural relaxationof water–DMSO mixtures under ambient and coldconditionsSnehasis Chowdhuri a & Subrat Kumar Pattanayak aa School of Basic Sciences , Indian Institute of Technology , Bhubaneswar 751013 , IndiaAccepted author version posted online: 02 Jul 2012.Published online: 30 Jul 2012.

To cite this article: Snehasis Chowdhuri & Subrat Kumar Pattanayak (2013) Pressure dependence on the single-particledynamics and hydrogen-bond structural relaxation of water–DMSO mixtures under ambient and cold conditions,Molecular Physics: An International Journal at the Interface Between Chemistry and Physics, 111:1, 135-146, DOI:10.1080/00268976.2012.707692

To link to this article: http://dx.doi.org/10.1080/00268976.2012.707692

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information. Taylor and Francis shall not be liable forany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

http://www.tandfonline.com/loi/tmph20

http://www.tandfonline.com/action/showCitFormats?doi=10.1080/00268976.2012.707692

http://dx.doi.org/10.1080/00268976.2012.707692

http://www.tandfonline.com/page/terms-and-conditions

http://www.tandfonline.com/page/terms-and-conditions

© 2013 Taylor & Francis

Molecular Physics, 2013Vol. 111, No. 1, 135–146, http://dx.doi.org/10.1080/00268976.2012.707692

RESEARCH ARTICLE

Pressure dependence on the single-particle dynamics and hydrogen-bond structuralrelaxation of water–DMSO mixtures under ambient and cold conditions

Snehasis Chowdhuri* and Subrat Kumar Pattanayak

School of Basic Sciences, Indian Institute of Technology, Bhubaneswar 751013, India

(Received 7 June 2012; final version received 21 June 2012)

The effects of pressure on the single-particle translational and rotational dynamics and the hydrogen-bondstructural relaxation of water–dimethyl sulfoxide (DMSO) mixtures were investigated under ambient and coldconditions using classical molecular dynamics simulations. We considered five different concentrations of DMSOwith mole fractions XDMSO¼ 0.012, 0.09, 0.20, 0.35, and 0.50 and seven different pressures ranging from 0.1to 500MPa. It was found that the addition of DMSO to water leads to slow dynamics for both water and DMSOmolecules under pressure. At very low concentrations of DMSO, water exhibits anomalous behavior withthe application of pressure at 258K, whereas DMSO molecules exhibit average pressure dependencies. Theformation of strong hydrogen bonds between water and DMSO molecules occurs when DMSO is added to water,but the extended network is altered due to the absence of any hydrogen-bond donating ability of DMSO.Our calculated water–water and DMSO–water hydrogen-bond structural relaxation times show an initialdecrease with the application of pressure at 258K for solutions with low DMSO concentrations. However,at higher concentrations of DMSO, the relaxation times increase, on average, and no anomalous behavior of thedynamical properties is found on the application of pressure.

Keywords: water–DMSO mixture; self-diffusion coefficients; orientational relaxation times; hydrogen bondsstructural relaxation times

1. Introduction

Dimethyl sulfoxide (DMSO) and its mixtures withwater have attracted much interest in the past fewdecades both chemically and biologically. DMSOacts as a cryoprotective agent in a variety of cells andtissues, allowing prolonged storage at subzero temper-atures. It has the unique property of penetrating intoliving tissues and the ability to replace water molecules,or can affect the water structure in cellular constituentswithout causing significant damage [1,2]. DMSO exertsits effect on the hydrophilic region of membrane lipidsand prevents a decrease of membrane fluidity at tem-peratures where cells otherwise sustain freezing injury.Recently, the phase behavior of aqueous lysozymesolutions in the presence of additives, namely glycerol

and DMSO, was investigated in a combined theoreticaland experimental study [3], and it was found thatDMSO seems to affect the lysozyme interactions bymainly lowering the solvent dielectric constant. Sinceaqueous DMSO is an important solvent medium bothin chemistry and biochemistry, the structural, dynam-ical and dielectric properties of water–DMSO mixtureshave been the subject of extensive experimental andtheoretical investigations [4–28]. Earlier computer

simulation studies attributed the highly non-ideal

behavior of these mixtures to the strong hydrogen-

bond acceptor capability of DMSO, which leads to the

formation of stable aggregates between water and

DMSO molecules [4–8]. Two different kinds of

complexes are mainly found in the mixtures, i.e. one

DMSO–two water for water-rich mixtures and two

DMSO–one water for DMSO-rich mixtures [6,7]. The

formation of one DMSO–three water hydrogen-

bonded aggregates has been observed experimentally

in the solid phase [9] and also from computer simula-

tion in the liquid state [6,8,10]. It is also pointed out

that the hemispherical nature of the electrostatic

potential for gas-phase DMSO means that it is likely

that the hydrogen bonds would have a much greater

range of angular orientation, and the charge density is

consistent with up to three water molecules donating

hydrogen bonds to DMSO [11,12]. The influence of

DMSO on the structure of liquid water has been

investigated by calculating the pair correlation func-

tions between different atoms of water–water, water–

DMSO and DMSO–DMSO molecules by neutron

diffraction and computer simulation studies [12,13–15]

and the results suggest that the water structure is

*Corresponding author. Email: [email protected]

References

[1] L. Wasungu, M. Scarzello, G. van Dam, G. Molema,

A. Wagenaar, J.B.F.N. Engberts and D. Hoekstra,J. Mol. Med. 84, 774 (2006).

[2] A.J. Kirby, P. Camilleri, J.B.F.N. Engberts,

M.C. Feiters, R.J.M. Nolte, O. Soderman,M. Bergsma, P.C. Bell, M.L. Fielden, C.L. GarcıaRodrıguez, P. Guedat, A. Kremer, C. McGregor,

C. Perrin, G. Ronsin and M.C.P. van Eijk, Angew.Chem. Int. Ed. 42, 1448 (2003).

[3] B.K. Kim, K.O. Doh, Y.U. Bae and Y.B. Seu,J. Microbiol. Biotechnol. 21, 93 (2011).

[4] P. Yang, J. Singh, S. Wettig, M. Foldvari, R.E. Verralland I. Badea, Eur. J. Pharm. Biopharm. 75, 311 (2010).

[5] A.M.S. Cardoso, H. Faneca, J.A.S. Almeida,

A.A.C.C. Pais, E.F. Marques, M.C.P. Lima andA.S. Jurado, Biochim. Biophys. Acta 1808, 341 (2011).

[6] S.D. Wettig, R.E. Verrall and M. Foldvari, Curr. Gene

Ther. 8, 9 (2008).[7] C. Bombelli, L. Giansanti, P. Luciani and G. Mancini,

Curr. Med. Chem. 16, 171 (2009).

[8] A. Colomer, A. Pinazo, M.A. Manresa, M.P. Vinardell,M. Mitjans, M.R. Infante and L. Perez, J. Med. Chem.54, 989 (2011).

[9] N.A. Negm, M.F. Zaki and M.A.I. Salem, J. Dispersion

Sci. Technol. 31, 1390 (2010).[10] M.A. Hegazy, M. Abdallah and H. Ahmed, Corros. Sci.

52, 2897 (2010).

[11] F.A. Ansari and M.A. Quraishi, Arab. J. Sci. Eng. 36,11 (2011).

[12] Z. Huang, H. Lu, T. Zhang, R. Wang and D. Qing,

Pet. Sci. Technol. 28, 1621 (2010).[13] M.N. Maithufi, D.J. Joubert and B. Klumperman,

Energy Fuels 25, 162 (2011).[14] B. Weng, R. Shepherd, J. Chen and G.G. Wallace,

J. Mater. Chem. 21, 1918 (2011).[15] M. In and R. Zana, J. Dispersion Sci. Technol. 28, 143

(2007).

[16] M. Johnsson and J.B.F.N. Engberts, J. Phys. Org.Chem. 17, 934 (2004).

[17] S.K. Hait and S.P. Moulik, Curr. Sci. 82, 1101

(2002).[18] F.M. Menger and J.S. Keiper, Angew. Chem. Int. Ed.

39, 1906 (2000).

[19] S. Zhu, L. Liu and F. Cheng, J. Surfactants Deterg. 14,221 (2011).

[20] X.P. Liu, J. Feng, L. Zhang, Q.T. Gong, S. Zhao andJ.Y. Yu, Colloids Surf., A 362, 39 (2010).

[21] M. Cai, M. Zhang and P. Ma, J. Dispersion Sci.Technol. 31, 1633 (2010).

[22] C.F.J. Kuo, L.H. Lin, M.Y. Dong, W.S. Chang andK.M. Chen, J. Surfactants Deterg. 14, 195 (2011).

[23] T.R. Prytkova, I. Eryazici, B. Stepp, S. Nguyen and

G.C. Schatz, J. Phys. Chem. B 114, 2627 (2010).[24] J.H. Allen, E.T. Schoch and J.M. Stubbs, J. Phys.

Chem. B 115, 1720 (2011).

[25] M. Muller, K. Katsov and M. Schick, Phys. Rep. 434,113 (2006).

[26] S.V. Bennun, M.I. Hoppes, C. Xing and R. Faller,Chem. Phys. Lipids 159, 59 (2009).

[27] K. Patterson, M. Lisal and C.M. Colina, Fluid PhaseEquilib. 61, 48 (2011).

[28] J. Zhang, W. Li, J. Wang, M. Qin, L. Wu, Z. Yan,

W. Xu, G. Zuo and W. Wang, IUBMB Life 61, 627(2009).

[29] G. Bellesia, A.I. Jewett and J.E. Shea, Protein Sci. 20,

818 (2011).[30] F. Schmid, Macromol. Rapid Commun. 30, 741 (2009).[31] M.J. Stevens, J. Chem. Phys. 121, 11942 (2004).

[32] J.A.S. Almeida, S.R. Pinto, Y. Wang, E.F. Marques andA.A.C.C. Pais, Phys. Chem. Chem. Phys. 13, 13772(2011).

[33] J.A.S. Almeida, H. Faneca, R.A. Carvalho,

E.F. Marques and A.A.C.C. Pais, PLoS ONE 6 (1),e26965 (2011).

[34] J.A.S. Almeida, E.F. Marques, A.S. Jurado and

A.A.C.C. Pais, Phys. Chem. Chem. Phys. 12, 14462(2010).

[35] Y. Marcus, J. Chem. Soc. Faraday Trans. 87, 2995

(1991).[36] P. Pullmannova, M. Bastos, G. Bai, S.S. Funari,

I. Lacko, F. Devınsky, J. Teixeira and D. Uhrıkova,

Biophys. Chem. 160, 35 (2012).[37] R. Ionov, A. El-Abed and M. Goldmann, Eur. Biophys.

J. 38, 229 (2009).[38] P. Linse, MOLSIM, version 4.0.8 (2004). Available from:

http://130.235.71.204/home.php?Program¼Molsim.[39] M.P. Allen and D.J. Tildesley, Computer Simulation of

Liquids (Clarendon, Oxford, 1987).

Dow

nloa

ded

by [

Um

eå U

nive

rsity

Lib

rary

] at

05:

34 0

8 Se

ptem

ber

2014

136 S. Chowdhuri and S.K. Pattanayak

weakly affected by the presence of DMSO molecules,but the percentage of hydrogen-bonded water mole-cules is substantially reduced compared with purewater. The single-particle dynamics of water andDMSO molecules in the mixtures have been calculatedusing several experimental techniques, such as NMR[16–20], Raman [20], quasi-elastic neutron scattering(QNS) [12,21,22] and depolarized Rayleigh scattering[23], and also by computer simulation [7,8,24–26]. Thestudies show that the translational [7,16–20,25] androtational dynamics [23] of both water and DMSOmolecules slow significantly in the mixture comparedwith the pure liquids, which indicates the formation ofinter-species complexes in the mixtures, but with alifetime shorter than the characteristic rotationalrelaxation times (�10 ps), which means that DMSOmolecules do not carry the water molecules with themas reorientation takes place.

Recently, Ladanyi and coworkers [26] have shownthat the water motion is less hindered in water-rich andwater-poor mixtures than it is at near-equimolecularcomposition, which suggests that the coupling betweenthe rotational and translational motion increasesas the equimolecular composition of the mixture isapproached. Dielectric studies [27,28] reveal that therelaxation times of the mixtures show a maximum atan intermediate molar fraction of DMSO, and this isattributed to the spatial constraints of DMSO mole-cules due to the hydrogen- bonded network rather thanthe hydrophobic hydration of the methyl groups.Recent studies of Bagchi and coworkers [29] haveshown that there is a sharp increase in pair hydropho-bicity even when a small amount of DMSO is addedto water up to composition XDMSO¼ 0.15, and theenhancement weakens at higher DMSO concentra-tions. The study was further extended to determine theorigin of both the thermodynamic and dynamicanomalies in aqueous binary mixtures of DMSO [30].Avalos et al. computed the tetrahedral order param-eter of water molecules in different concentrations ofaqueous DMSO solution [31]. This parameter indicatesa reduction in the local tetrahedral order of water whenthe solute concentration is increased, followed by aclear minimum at the equimolecular concentrationnear the locus of the maximum density of the mixture,probably due to the formation of water–DMSOcomplexes. In the context of the present study, wemust discuss the work of Baker and Jonas [20], wherethe pressure dependence of the self-diffusion andorientational relaxation of water in many water–DMSO solutions were measured under ambient con-ditions. It was found that the pressure anomaly inthe self-diffusion and orientational relaxation of waterexists in mixtures with low concentrations of DMSO.

However, no such anomalies were observed at higherconcentrations of DMSO. The self-diffusion and ori-entational relaxation of DMSO molecules were notinvestigated in these studies. On the theoretical side,we are not aware of any such study on water–DMSOsystems at high pressure.

In this work, we carried out a series of moleculardynamics simulations of water–DMSO mixtures at fivedifferent concentrations at varying temperatures andpressures. We simulated the systems at two differenttemperatures, T¼ 298 and 258K, and seven differentpressures, ranging from 0.1 to 500MPa. The primarygoal was to investigate the effects of pressure on thesingle-particle translation and rotational dynamicsof both water and DMSO molecules in their binarymixtures and also the hydrogen-bond structural relax-ation properties of water–water and water–DMSOhydrogen bonds in the five different solutions. Thetranslational and rotational dynamics of solvent mol-ecules were investigated by calculating the values of theself-diffusion coefficients and orientational relaxationtimes of solvent molecules. We also calculated theaverage number and energies of intra- and inter-specieshydrogen bonds present in these binary mixtures,as these quantities are intimately related and can helpto explain the dynamical behavior of water and DMSOmolecules in the solutions. We also calculated thewater–water and water–DMSO hydrogen-bond struc-tural relaxation dynamics. In total, we simulated70 different systems.

2. Models and simulation details

In this study, water and DMSO molecules are charac-terized using multisite interaction models. In thesemodels, the interaction between atomic sites isexpressed as

uðri, rj Þ ¼ 4"ij�ijrij

� �12

� �ijrij

� �6" #

þ qiqjrij

, ð1Þ

where qi is the charge of the ith atom or ion. TheLennard–Jones parameters �ij and "ij are obtainedusing the combination rules �ij¼ (�iþ �j)/2 and"ij¼

ffiffiffiffiffiffiffi"i"j

p, where �i and "i are the Lennard–Jones

diameter and well-depth parameter for the ith atomor ion. For water, we employed the extended simplepoint charge (SPC/E) potential [32] where each watermolecule consists of a Lennard–Jones interactionsite located on oxygen and three charge interactionsites located on oxygen and two hydrogen atoms.The DMSO molecules are modeled by the four-siteP2 model of Luzar and Chandler [10], which comprisestwo methyl, one sulfur and one oxygen site, with all

S. Chowdhuri and S.K. Pattanayak

Dow

nloa

ded

by [

Um

eå U

nive

rsity

Lib

rary

] at

05:

34 0

8 Se

ptem

ber

2014

Molecular Physics 137

sites acting both as Lennard–Jones and charge inter-

action sites. Each methyl group is considered to be a

single interaction site and thus the hydrogen atoms

of DMSO are not considered explicitly. The values of

the Lennard–Jones and electrostatic interaction poten-

tial parameters of the different atomic site of water and

DMSO are given in Ref. [25].The molecular dynamics simulations were carried

out in a cubic box of 256 particles of both water and

DMSO depending on their composition in the binary

mixtures. The composition of each solvent mixture is

characterized by the mole fraction of DMSO (XDMSO)

and in the present study we considered five different

compositions with XDMSO¼ 0.012, 0.09, 0.20, 0.35 and

0.50. The simulations of various solutions were per-formed at two different temperatures (298 and 258K)

and at seven different pressures (0.1, 50, 110, 175, 250,

375 and 500MPa). We employed the minimum image

convention for calculation of the short-range Lennard–

Jones interactions. The long-range electrostatic inter-

actions were treated using the Ewald method [33] and,

for the integration over time, we adapted the leap-frog algorithm with a time step of 10–15 s (1 fs). In the

starting configuration, the water and DMSO molecules

were located on a face-centered cubic lattice with

random orientations. In order to determine the appro-

priate box size for a desired temperature at a given

pressure, we first carried out MD runs of 400–600 ps at

a constant pressure by employing the weak couplingscheme of Berendsen et al. [34]. During this initial

phase of the simulations, the volume of the simulation

box was allowed to fluctuate, and the average vol-

ume was determined at the end of the simulation.

Subsequently, we carried out simulations in the

microcanonical ensemble keeping the box size fixed

at the average value obtained previously for a givensystem at a given temperature and pressure. While

carrying out the simulations in the microcanonical

ensemble, each system was equilibrated for 0.8–1 ns

and the simulations were run for another 2.5–5 ns for

the calculation of the structural and dynamical quan-

tities. The average values of the pressure and temper-

ature of a system during the production phase of eachsimulation were found to be very close to the desired

pressure and temperature.

3. Dynamical properties: Self-diffusion coefficientsand orientational relaxation times

The translational self-diffusion coefficient Di of species

i (either water or DMSO) is related to the time

integral of the velocity–velocity autocorrelation

function (VAF) as

Di ¼kBT

mi

Z 1

o

CvðtÞdt, ð2Þ

where kB is Boltzmann’s constant, mi is the mass

of species i and CvðtÞ is the velocity–velocity time

correlation function, CvðtÞ, defined as

CvðtÞ ¼hviðtÞ � við0Þihvið0Þ � við0Þi

, ð3Þ

where viðtÞ is the velocity of species i at time t [35]

and the average is carried out over all the species

in the system and over the initial time. The transla-

tional self-diffusion coefficient can also be calculated

from the long-time limit of the mean-square displace-

ment (MSD),

Di ¼ limt!1

jrðtÞ � rð0Þj2� �

6t, ð4Þ

where r(t) is the position of species i at time t, by a

least-square fit of the long-time region of MSD as

obtained from simulations. The diffusion coefficients

calculated using these two different routes have been

found to be quite close to each other and we have

taken the average of the values obtained from these

two routes for a given type of solvent.The single particle orientational motion of solvent

molecules is analysed by calculating the orientational

time correlation function, C�l ðtÞ, defined as

C�l ðtÞ ¼Pl ½e�ðtÞ � e�ð0Þ��

Pl ½e�ð0Þ � e�ð0Þ�� , ð5Þ

where Pl is the Legendre polynomial of rank l and e� is

the unit vector which points along the �-axis in the

molecular frame. In this work, we calculated the time

dependence of C�l ðtÞ for l¼ 2, and for the three

different e�, the molecular dipole vector �, the H–H

vector and the O–H vector of water. At short times, the

decay of C�l ðtÞ is generally non-exponential because

of inertial and non-Markovian effects. At long times,

when these effects are not important, the relaxation is

diffusional and C�l ðtÞ decays exponentially. The orien-

tational correlation time ��l , defined as the time integral

of the orientational correlation function

��l ¼Z 1

0

dtC �l ðtÞ ð6Þ

was obtained by explicit integration of the data for

C �l ðtÞ obtained from simulations up to 50 ps with a

time interval of 0.01 ps. Under ambient conditions,

depending on their concentrations, we calculated

the data up to 40 ps for water and 30 ps for DMSO,

weakly affected by the presence of DMSO molecules,but the percentage of hydrogen-bonded water mole-cules is substantially reduced compared with purewater. The single-particle dynamics of water andDMSO molecules in the mixtures have been calculatedusing several experimental techniques, such as NMR[16–20], Raman [20], quasi-elastic neutron scattering(QNS) [12,21,22] and depolarized Rayleigh scattering[23], and also by computer simulation [7,8,24–26]. Thestudies show that the translational [7,16–20,25] androtational dynamics [23] of both water and DMSOmolecules slow significantly in the mixture comparedwith the pure liquids, which indicates the formation ofinter-species complexes in the mixtures, but with alifetime shorter than the characteristic rotationalrelaxation times (�10 ps), which means that DMSOmolecules do not carry the water molecules with themas reorientation takes place.

Recently, Ladanyi and coworkers [26] have shownthat the water motion is less hindered in water-rich andwater-poor mixtures than it is at near-equimolecularcomposition, which suggests that the coupling betweenthe rotational and translational motion increasesas the equimolecular composition of the mixture isapproached. Dielectric studies [27,28] reveal that therelaxation times of the mixtures show a maximum atan intermediate molar fraction of DMSO, and this isattributed to the spatial constraints of DMSO mole-cules due to the hydrogen- bonded network rather thanthe hydrophobic hydration of the methyl groups.Recent studies of Bagchi and coworkers [29] haveshown that there is a sharp increase in pair hydropho-bicity even when a small amount of DMSO is addedto water up to composition XDMSO¼ 0.15, and theenhancement weakens at higher DMSO concentra-tions. The study was further extended to determine theorigin of both the thermodynamic and dynamicanomalies in aqueous binary mixtures of DMSO [30].Avalos et al. computed the tetrahedral order param-eter of water molecules in different concentrations ofaqueous DMSO solution [31]. This parameter indicatesa reduction in the local tetrahedral order of water whenthe solute concentration is increased, followed by aclear minimum at the equimolecular concentrationnear the locus of the maximum density of the mixture,probably due to the formation of water–DMSOcomplexes. In the context of the present study, wemust discuss the work of Baker and Jonas [20], wherethe pressure dependence of the self-diffusion andorientational relaxation of water in many water–DMSO solutions were measured under ambient con-ditions. It was found that the pressure anomaly inthe self-diffusion and orientational relaxation of waterexists in mixtures with low concentrations of DMSO.

However, no such anomalies were observed at higherconcentrations of DMSO. The self-diffusion and ori-entational relaxation of DMSO molecules were notinvestigated in these studies. On the theoretical side,we are not aware of any such study on water–DMSOsystems at high pressure.

In this work, we carried out a series of moleculardynamics simulations of water–DMSO mixtures at fivedifferent concentrations at varying temperatures andpressures. We simulated the systems at two differenttemperatures, T¼ 298 and 258K, and seven differentpressures, ranging from 0.1 to 500MPa. The primarygoal was to investigate the effects of pressure on thesingle-particle translation and rotational dynamicsof both water and DMSO molecules in their binarymixtures and also the hydrogen-bond structural relax-ation properties of water–water and water–DMSOhydrogen bonds in the five different solutions. Thetranslational and rotational dynamics of solvent mol-ecules were investigated by calculating the values of theself-diffusion coefficients and orientational relaxationtimes of solvent molecules. We also calculated theaverage number and energies of intra- and inter-specieshydrogen bonds present in these binary mixtures,as these quantities are intimately related and can helpto explain the dynamical behavior of water and DMSOmolecules in the solutions. We also calculated thewater–water and water–DMSO hydrogen-bond struc-tural relaxation dynamics. In total, we simulated70 different systems.

2. Models and simulation details

In this study, water and DMSO molecules are charac-terized using multisite interaction models. In thesemodels, the interaction between atomic sites isexpressed as

uðri, rj Þ ¼ 4"ij�ijrij

� �12

� �ijrij

� �6" #

þ qiqjrij

, ð1Þ

where qi is the charge of the ith atom or ion. TheLennard–Jones parameters �ij and "ij are obtainedusing the combination rules �ij¼ (�iþ �j)/2 and"ij¼

ffiffiffiffiffiffiffi"i"j

p, where �i and "i are the Lennard–Jones

diameter and well-depth parameter for the ith atomor ion. For water, we employed the extended simplepoint charge (SPC/E) potential [32] where each watermolecule consists of a Lennard–Jones interactionsite located on oxygen and three charge interactionsites located on oxygen and two hydrogen atoms.The DMSO molecules are modeled by the four-siteP2 model of Luzar and Chandler [10], which comprisestwo methyl, one sulfur and one oxygen site, with all

Dow

nloa

ded

by [

Um

eå U

nive

rsity

Lib

rary

] at

05:

34 0

8 Se

ptem

ber

2014


whereas in the case of lower temperatures the valueswere calculated up to 50 ps to obtain the relativechanges of the orientational relaxation of these sol-vents in their binary mixtures. The re-orientationalrelaxation process of DMSO and water molecules intheir binary mixtures has been measured experimen-tally using depolarized Rayleigh scattering [23], NMRspectroscopy [16,20,36], IR pump–probe and opticalheterodyne detected optical Kerr effect (OHD-OKE)spectroscopy [37]. Such studies have mainly beencarried out to explore the temperature and composi-tion effects on the rotational diffusion of DMSO andit has been observed that the orientational relaxation ismaximum at XDMSO¼ 0.65 [23,37] and with respectto temperature the relaxation times of DMSO followsArrhenius behavior for the pure liquid and also foraqueous solution [23,36]. The pressure dependence ofthe intra- and inter-molecular proton relaxation ratesin water–DMSO mixtures has also been calculatedexperimentally [20], and it was found that the inter-molecular rate increases, whereas the intra-molecularrate decreases up to 5% DMSO in binary mixtures.

The anomalous behavior of the pressure dependence[38–41] of pure water is maintained at low concentra-tions of DMSO in solution. Experimentally, theorientational relaxation of the H–H and O–H vectorof water can be measured by 1H–1H and 17O–1Hdipolar relaxation NMR experiments. However, weare not aware of any such experimental measurementof aqueous DMSO systems under similar conditions.

Figure 1 shows the pressure dependence of the self-diffusion coefficients of DMSO and water moleculesfor different water–DMSO mixtures at both 298 and258K. Figure 2 shows the variation of the second rankdipole orientational relaxation times of water andDMSO molecules with pressure, and the pressuredependence of the orientational relaxation times of theO–H vector and H–H vector of water are shown inFigure 3. The actual (un-normalized) values of thediffusion coefficients and the orientational relaxationtimes at P¼ 0.1MPa are given in Tables 1–3. It isfound that, at 258K, for low DMSO concentrationsolutions, the diffusion coefficients of water moleculesinitially increase and then decrease with pressure, thus

0.2

0.4

0.6

0.8

1

1.2

D (

P)

/ D (

P0)

(a)

(b)

(c)

(d)

Water T=298K

XDMSO=0.012XDMSO=0.09XDMSO=0.20XDMSO=0.35XDMSO=0.50

Water T=258K

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500

D (

P)

/ D (

P0)

P (MPa)

DMSO T=298K

0 100 200 300 400 500P (MPa)

DMSO T=258K

Figure 1. Pressure dependence of the self-diffusion coefficients of water and DMSO molecules in water–DMSO mixtures ofvarious compositions at 298 and 258K.

Dow

nloa

ded

by [

Um

eå U

nive

rsity

Lib

rary

] at

05:

34 0

8 Se

ptem

ber

2014


passing through a maximum. Correspondingly, theorientational relaxation times of water show an initialdecrease with pressure mainly for lower DMSO con-centrations and the result is quite prominent at 258K.However, at higher DMSO concentrations, no suchanomalous behavior of the dynamical properties isfound on the application of pressure. The standarddeviation of the data related to the translational androtational dynamics of water and DMSO moleculespresented here is about 2–3% of the average values.The behavior of our calculated results with theapplication of pressure at 298K is very similar inpattern to the experimental observations reported byBaker and Jonas [20]. The pressure anomaly of thedynamical properties of water can mainly be explainedon the basis of the increased number of interstitialwater molecules at the expense of hydrogen-bondstability. On the other hand, the DMSO moleculeshows an average pressure dependency of the dynam-ical properties at lower DMSO concentrations and thiscan perhaps be explained in terms of the fewer number

of hydrogen bonds and their greater stability. Sincehydrogen-bond breaking plays an important role indetermining the rate of the translational and rotationaldiffusion of solvent molecules, the formation of thesestrong inter-species hydrogen bonds also means aslower dynamics of water and DMSO in the mixturescompared with those of the pure solvent. The resultsobserved here are quite expected since it is well knownthat water, a tetrahedral associating liquid, will breakits local hydrogen-bonding order and will concomi-tantly exhibit faster translational and rotationaldynamics upon the application of external pressure.It will then become more of an ‘ordinary’ liquid and itsdynamics will slow with a further increase of pressure.Hence, the pressure dependence of the diffusioncoefficients will pass through a local maximum. Onthe other hand, DMSO, a polar non-associating liquid,will not exhibit this dynamical anomaly and DMSO’sdynamics will slow with increasing pressure. This effectis mild because of DMSO’s incompressibility, as forany other non-hydrogen-bonding liquid. A mixture of

0.6

0.8

1

1.2

1.4

1.6

1.8

2

τ 2µ

(P)

/ τ2µ

(P0)

Water T=298KXDMSO=0.012XDMSO=0.09XDMSO=0.20XDMSO=0.35XDMSO=0.50

Water T=258K

0.8

1.2

1.6

2

2.4

2.8

0 100 200 300 400 500

τ 2µ

(P)

/ τ2µ

(P0)

P (MPa)

DMSO T=298K

0 100 200 300 400 500

P (MPa)

DMSO T=258K

(a)

(b)

(c)

(d)

Figure 2. Pressure dependence of the second rank dipole orientational relaxation times of water and DMSO molecules in water–DMSO mixtures of various compositions at 298 and 258K.

whereas in the case of lower temperatures the valueswere calculated up to 50 ps to obtain the relativechanges of the orientational relaxation of these sol-vents in their binary mixtures. The re-orientationalrelaxation process of DMSO and water molecules intheir binary mixtures has been measured experimen-tally using depolarized Rayleigh scattering [23], NMRspectroscopy [16,20,36], IR pump–probe and opticalheterodyne detected optical Kerr effect (OHD-OKE)spectroscopy [37]. Such studies have mainly beencarried out to explore the temperature and composi-tion effects on the rotational diffusion of DMSO andit has been observed that the orientational relaxation ismaximum at XDMSO¼ 0.65 [23,37] and with respectto temperature the relaxation times of DMSO followsArrhenius behavior for the pure liquid and also foraqueous solution [23,36]. The pressure dependence ofthe intra- and inter-molecular proton relaxation ratesin water–DMSO mixtures has also been calculatedexperimentally [20], and it was found that the inter-molecular rate increases, whereas the intra-molecularrate decreases up to 5% DMSO in binary mixtures.

The anomalous behavior of the pressure dependence[38–41] of pure water is maintained at low concentra-tions of DMSO in solution. Experimentally, theorientational relaxation of the H–H and O–H vectorof water can be measured by 1H–1H and 17O–1Hdipolar relaxation NMR experiments. However, weare not aware of any such experimental measurementof aqueous DMSO systems under similar conditions.

Figure 1 shows the pressure dependence of the self-diffusion coefficients of DMSO and water moleculesfor different water–DMSO mixtures at both 298 and258K. Figure 2 shows the variation of the second rankdipole orientational relaxation times of water andDMSO molecules with pressure, and the pressuredependence of the orientational relaxation times of theO–H vector and H–H vector of water are shown inFigure 3. The actual (un-normalized) values of thediffusion coefficients and the orientational relaxationtimes at P¼ 0.1MPa are given in Tables 1–3. It isfound that, at 258K, for low DMSO concentrationsolutions, the diffusion coefficients of water moleculesinitially increase and then decrease with pressure, thus

0.2

0.4

0.6

0.8

1

1.2

D (

P)

/ D (

P0)

(a)

(b)

(c)

(d)

Water T=298K


Water T=258K

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500

D (

P)

/ D (

P0)

P (MPa)

DMSO T=298K

0 100 200 300 400 500P (MPa)

DMSO T=258K

Figure 1. Pressure dependence of the self-diffusion coefficients of water and DMSO molecules in water–DMSO mixtures ofvarious compositions at 298 and 258K.

Dow

nloa

ded

by [

Um

eå U

nive

rsity

Lib

rary

] at

05:

34 0

8 Se

ptem

ber

2014


water and DMSO, therefore, is expected to go fromwater-like (anomalous) to DMSO-like (ordinary)behavior upon the application of pressure as theDMSO mole fraction increases.

4. Hydrogen-bond properties and structuralrelaxation times

To calculate the hydrogen-bond properties and struc-tural relaxation times of water–water and DMSO–water hydrogen bonds, we use a set of geometric

criteria [41–49] where it is assumed that a hydrogenbond between two molecules exist if the followingdistance and angular criteria are satisfied, i.e. R(OX)5RðOXÞ

c , R(OH)5RðOHÞc , and �5 �c. For the water–water

hydrogen bond, R(OX) and R(OH) denote the oxygen(water)–oxygen (water) and oxygen (water)–hydrogen(water) distances and the corresponding quantitieswith subscript ‘c’ denote the cut-off values. Theangle � ð¼ �ðOwOwHÞÞ is the oxygen (water)–oxygen(water)–hydrogen angle and �c is the upper limit that

0.6

0.8

1

1.2

1.4

1.6

1.8

2

τ 2O

H (

P)

/ τ2O

H (

P0)


Water T=258K

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 100 200 300 400 500

τ 2H

H (

P)

/ τ2H

H (

P0)

P (MPa)

Water T=298K

0 100 200 300 400 500P (MPa)

Water T=258K

(a)

(b)

(c)

(d)

Figure 3. Pressure dependence of the second rank O–H vector and H–H vector orientational relaxation times of water moleculesin water–DMSO mixtures of various compositions at 298 and 258K.

Table 2. The second rank dipole orientational relaxationtimes, ��2 (in ps), of water and DMSO molecules in water–DMSO mixtures at P¼ 0.1MPa.

Solution

��2;DMSO

(298K)

��2;H2O

(298K)

��2;DMSO

(258K)

��2;H2O

(258K)

XDMSO¼ 0.012 3.86 1.57 11.6 4.80XDMSO¼ 0.09 7.50 2.90 23.5 10.2XDMSO¼ 0.20 13.1 7.37 41.0 27.2XDMSO¼ 0.35 19.7 19.2 57.1 54.8XDMSO¼ 0.50 17.3 25.5 51.9 62.0

Table 1. The self-diffusion coefficients (in 10–5 cm2 s–1) ofwater and DMSO molecules in water–DMSO mixtures atP¼ 0.1MPa.

Solution Dð298KÞDMSO Dð298KÞ

H2ODð258KÞ

DMSO Dð258KÞH2O


Dow

nloa

ded

by [

Um

eå U

nive

rsity

Lib

rary

] at

05:

34 0

8 Se

ptem

ber

2014


is allowed for a hydrogen bond to exist between theoxygen of water and the hydrogen of another watermolecule. In the case of the DMSO–water hydrogenbond, R(OX) and R(OH) denote the oxygen (DMSO)–oxygen (water) and oxygen (DMSO)–hydrogen(water)distances and the angle � ð¼ �ðOwODHÞÞ is the oxygen(water)–oxygen (DMSO)–hydrogen angle and �c is theupper limit that is allowed for a hydrogen bond to existbetween the oxygen of DMSO and the hydrogen of thewater molecule. The cut-off values for the Ow–Ow, Ow–Hw, Ow–OD and OD–Hw distances are determined fromthe positions of the first minimum of the correspondingradial distribution functions. Figure 4 shows the O–Oradial distribution functions (RDF) between watermolecules for different compositions of water–DMSOmixtures at varying temperatures and pressures. It isfound that the first and second peaks of gOO(r) aresharp with well-defined minima at r¼ 3.45 and 5.60 A,respectively. The effect of pressure on these water–DMSO mixtures is significant at lower DMSO con-centrations, where it is believed that some of the watermolecules occupy the interstitial position due to theapplication of pressure. The greater peak height of theradial distribution function at 258K indicates thatliquid water is more structured at lower temperature.It is observed from the oxygen (DMSO)–oxygen(water) RDF that the nearest-neighbor distanceof ODMSO–Owater is shorter by �0.2–0.3 A than theOwater–Owater distance, and from the oxygen (DMSO)–hydrogen (water) RDF, it is clear that the oxygen ofthe DMSO molecule is hydrogen bonded to the watermolecules and the effects of pressure on these radialdistribution functions are not significant and hence arenot shown here. For the calculation of the statisticsof the water–water hydrogen bond, we chose theOw–Ow and Ow–Hw distance values 3.50 and 2.45 A,whereas for the DMSO–water hydrogen bond, theOw–OD and OD–Hw distances were taken as 3.20 and2.45 A, respectively. For the geometric criteria, gener-ally a cut-off angle of �c¼ 30� is used, but to providemore flexibility due to thermal motion, sometimes the

less strict definition with cut-off angle �c¼ 45� can alsobe used [42]. In the present study, we used a cut-offangle of �c¼ 30� for the existence of a hydrogen bondtogether with the above distance criteria. We alsochecked variations of the cut-off angle (�c) up to 75�

due to concern about the greater angular orientation ofDMSO–water hydrogen bonds [12], and observed anegligible change in the calculation of the averagenumber of hydrogen bonds and their probabilitydistribution. Our calculated numbers of water–waterhydrogen bonds between two water molecules andDMSO–water hydrogen bonds per DMSO moleculewith probability distribution at P¼ 0.1MPa are givenin Figure 5. It is observed that, as the number ofDMSO molecules increases, the peak of the distribu-tion of water–water hydrogen bonds shifts towardsfewer bonds. This indicates the decreasing availabilityof water in the solution and also the competition toform inter-species hydrogen bonds. At lower DMSOconcentrations (up to XDMSO¼ 0.20), about 70–80%of the inter-species hydrogen bonds represents oneDMSO–two water molecules, whereas a significantnumber of hydrogen bonds with one DMSO–threewater molecules (�4–18%) are also present in thesesolutions. The formation of one DMSO–threewater hydrogen-bonded aggregates has been supportedexperimentally [9] and also by computer simulation[6,8,10]. The average number of hydrogen bondsbetween water–water and DMSO–water decreasessignificantly with increasing DMSO concentrationand consequently the effects of pressure on thesequantities are insignificant and are not shown here.The pressure anomaly exhibited by low DMSO con-centration solutions can be explained in terms of theincreasing number of water molecules with fifthneighbors, which destabilizes the hydrogen-bondedtetrahedral structure of water, and is believed tospeed up the dynamics of water molecules with theapplication of pressure. At higher DMSO concentra-tions, the water–water hydrogen-bond network struc-ture is disrupted so strongly by the DMSO moleculesthat the presence of interstitial water molecules in thesecases is not significant. The formation of these stablewater–DMSO complexes and also the disruption ofthe water–water hydrogen-bond network are believedto be the reasons for the absence of any pressure-induced anomalous behavior of these concentratedwater–DMSO solutions.

The pair dynamics of two hydrogen-bonded solventmolecules (either water–water or water–DMSO) in thesolutions were investigated by calculating the followinghydrogen-bond correlation function [31,42,44–49] :

CHBðtÞ ¼ hhHBð0ÞhHBðtÞi=hhHBi, ð7Þ

Table 3. The second rank O–H vector and H–H vectororientational relaxation times (�OH

2 and �HH2 in ps) of water

molecules in water–DMSO mixtures at P¼ 0.1MPa for 298and 258K.

Solution

�OH2;H2O

(298K)

�OH2;H2O

(258K)

�HH2;H2O

(298K)

�HH2;H2O

(258K)


water and DMSO, therefore, is expected to go fromwater-like (anomalous) to DMSO-like (ordinary)behavior upon the application of pressure as theDMSO mole fraction increases.

4. Hydrogen-bond properties and structuralrelaxation times

To calculate the hydrogen-bond properties and struc-tural relaxation times of water–water and DMSO–water hydrogen bonds, we use a set of geometric

criteria [41–49] where it is assumed that a hydrogenbond between two molecules exist if the followingdistance and angular criteria are satisfied, i.e. R(OX)5RðOXÞ

c , R(OH)5RðOHÞc , and �5 �c. For the water–water

hydrogen bond, R(OX) and R(OH) denote the oxygen(water)–oxygen (water) and oxygen (water)–hydrogen(water) distances and the corresponding quantitieswith subscript ‘c’ denote the cut-off values. Theangle � ð¼ �ðOwOwHÞÞ is the oxygen (water)–oxygen(water)–hydrogen angle and �c is the upper limit that

0.6

0.8

1

1.2

1.4

1.6

1.8

2

τ 2O

H (

P)

/ τ2O

H (

P0)


Water T=258K

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 100 200 300 400 500

τ 2H

H (

P)

/ τ2H

H (

P0)

P (MPa)

Water T=298K

0 100 200 300 400 500P (MPa)

Water T=258K

(a)

(b)

(c)

(d)

Figure 3. Pressure dependence of the second rank O–H vector and H–H vector orientational relaxation times of water moleculesin water–DMSO mixtures of various compositions at 298 and 258K.

Table 2. The second rank dipole orientational relaxationtimes, ��2 (in ps), of water and DMSO molecules in water–DMSO mixtures at P¼ 0.1MPa.

Solution

��2;DMSO

(298K)

��2;H2O

(298K)

��2;DMSO

(258K)

��2;H2O

(258K)


Table 1. The self-diffusion coefficients (in 10–5 cm2 s–1) ofwater and DMSO molecules in water–DMSO mixtures atP¼ 0.1MPa.

Solution Dð298KÞDMSO Dð298KÞ

H2ODð258KÞ

DMSO Dð258KÞH2O


Dow

nloa

ded

by [

Um

eå U

nive

rsity

Lib

rary

] at

05:

34 0

8 Se

ptem

ber

2014


where hHBðtÞ ¼ 1 if a pair of solvent molecules (eitherwater–water or water–DMSO) is hydrogen bonded attime t and zero otherwise. The distance and angularcriteria for water–water and water–DMSO hydrogen

bonds are discussed above. The correlation functionCHBðtÞ describes the probability that a hydrogen bondis intact at time t, given that it was intact at time zero.Thus, the dynamics of CHBðtÞ describes the structural

0

1

2

3

4

g Ow

−O

w(r

)g O

w−

Ow

(r)

g Ow

−O

w(r

)

T=298K

(a)

XDMSO=0.012

P0.1P110P250P500

T=258K

(d)

XDMSO=0.012

P0.1P110P250P500

0

2

4

6

T=298K

(b)

XDMSO=0.20

P0.1P110P250P500

T=258K

(e)

XDMSO=0.20

P0.1P110P250P500

0

3

6

9

1 3 5 7 9r(Å)

T=298K

(c)

XDMSO=0.50

P0.1P110P250P500

1 3 5 7 9r(Å)

T=258K

(f)

XDMSO=0.50

P0.1P110P250P500

Figure 4. The oxygen (water)–oxygen (water) radial distribution function of water–DMSO mixtures of various compositions at298 and 258K with varying pressure.

Dow

nloa

ded

by [

Um

eå U

nive

rsity

Lib

rary

] at

05:

34 0

8 Se

ptem

ber

2014


relaxation of hydrogen bonds, and the associatedrelaxation times �OW...HW

and �OD��HWcan be interpreted

as the time scales of reorganization of water–waterand DMSO–water hydrogen bonds. In both cases, thedecay of the time correlation function is calculated upto 100 ps, depending on the concentration, and thecorresponding results of the structural relaxation timesof water–water (�OW...HW

) and water–DMSO (�OD...HW)

hydrogen bonds are shown in Figure 6. The actual(un-normalized) values of the structural relaxationtimes at P¼ 0.1MPa are given in Table 4 and thecalculated values are quite close to the previouslyreported MD simulation results for different aqueousDMSO solutions [31]. It is found that the water–waterhydrogen-bond structural relaxation time decaysat a comparatively faster rate with the application ofpressure at 258K. On the other hand, the averagepressure dependency of the DMSO–water hydrogen-bond structural relaxation time is observed at 258K,which can be explained in terms of the greaterstability and fewer inter-species hydrogen bonds in

these solutions. The decay of the time correlationfunctions occurs at a slower rate with increasingDMSO concentration. Hence, a larger value of thestructural relaxation time indicates stable water–DMSO complexes in the solution.

The strength of the DMSO–water hydrogen-bondinteraction is reflected in the long lifetimes of thesebonds compared with water–water hydrogen bonds inwater–DMSO mixtures. On the other hand, the verylong hydrogen-bond lifetimes in the case of water–water hydrogen bonds in these binary mixtures com-pared with the hydrogen-bond lifetime in pure watercan be explained in relation to the distortion of theenvironment of the water molecules and hence increas-ing local rigidity. The existence of chains of watermolecules with greater stability is responsible for thevery long lifetime of these hydrogen bonds and this isexplained very precisely in Ref. [31]. The standarddeviation of the hydrogen-bond structural relaxationtimes presented here it is about 4% of the averagevalues.

0

0.2

0.4

0.6

0.8

1

Pro

babi

lity

H2O...H2O


H2O...H2O

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5

Pro

babi

lity

nHB

0 1 2 3 4 5nHB

DMSO...H2O

T = 298K (a) T = 258K (c)

T = 298K (b) DMSO...H2O T = 258K (d)

Figure 5. The probability of finding water–water and DMSO–water hydrogen bonds (nHB) in water–DMSO mixtures of variouscompositions at 298 and 258K.

where hHBðtÞ ¼ 1 if a pair of solvent molecules (eitherwater–water or water–DMSO) is hydrogen bonded attime t and zero otherwise. The distance and angularcriteria for water–water and water–DMSO hydrogen

bonds are discussed above. The correlation functionCHBðtÞ describes the probability that a hydrogen bondis intact at time t, given that it was intact at time zero.Thus, the dynamics of CHBðtÞ describes the structural

0

1

2

3

4

g Ow

−O

w(r

)g O

w−

Ow

(r)

g Ow

−O

w(r

)

T=298K

(a)

XDMSO=0.012

P0.1P110P250P500

T=258K

(d)

XDMSO=0.012

P0.1P110P250P500

0

2

4

6

T=298K

(b)

XDMSO=0.20

P0.1P110P250P500

T=258K

(e)

XDMSO=0.20

P0.1P110P250P500

0

3

6

9

1 3 5 7 9r(Å)

T=298K

(c)

XDMSO=0.50

P0.1P110P250P500

1 3 5 7 9r(Å)

T=258K

(f)

XDMSO=0.50

P0.1P110P250P500

Figure 4. The oxygen (water)–oxygen (water) radial distribution function of water–DMSO mixtures of various compositions at298 and 258K with varying pressure.

Dow

nloa

ded

by [

Um

eå U

nive

rsity

Lib

rary

] at

05:

34 0

8 Se

ptem

ber

2014


5. Summary and conclusions

In this paper, we have presented a series of molecular

dynamics simulation results of the pressure dependencebehavior of water–DMSO mixtures under ambient

and cold conditions. We have considered five different

concentrations of aqueous DMSO solutions with

pressure variation ranging from 0.1 to 500MPa.

The primary goal was to investigate the effects ofpressure on the single-particle dynamics and hydrogen-bond properties of both water and DMSO moleculesin their binary mixtures under ambient and coldconditions. It was found that, at 258K, the self-diffusion coefficients of water molecules initiallyincrease with pressure and then decrease, thus passingthrough a maximum for low DMSO concentrationsolutions. On the other hand, the correspondingorientational relaxation times are found to decreasewith increasing pressure. However, at higher concen-trations of DMSO, no such anomalous behavior of thedynamical properties is found on the application ofpressure. The absence of such anomalous behaviorat higher DMSO concentrations is attributed todisruption of the hydrogen-bond network caused bythe presence of DMSO molecules and also to theformation of strongly hydrogen-bonded water–DMSOcomplexes.

In this context, we also calculated the statisticsof intra- and inter-species hydrogen bonds in these

0.8

1

1.2

1.4

1.6

1.8

2

2.2

τ OW

...H

W (

P)

/ τO

W...

HW

(P

0)τ O

D...

HW

(P

) / τ

OD

...H

W (P

0)

(a) (c)

(d)

H2O...H2O T=298K

XDMSO =0.012XDMSO =0.09XDMSO =0.20XDMSO =0.35XDMSO =0.50

H2O...H2O T=258K

0.8

1

1.2

1.4

1.6

1.8

2

0 100 200 300 400 500P (MPa)

0 100 200 300 400 500P (MPa)

(b)DMSO...H2O T=298K DMSO...H2O T=258K

Figure 6. Pressure dependence of the hydrogen-bond structural relaxation times between water–water and DMSO–water forwater–DMSO mixtures of various compositions at 298 and 258K.

Table 4. The hydrogen-bond structural relaxation times ofvarious compositions of water–water and DMSO–water(�OW...HW

and �OD ...HWin ps) mixtures at P¼ 0.1MPa for

298 and 258K.

Solution

�OW...HW

(298K)

�OW...HW

(258K)

�OD ...HW

(298K)

�OD ...HW

(258K)


Dow

nloa

ded

by [

Um

eå U

nive

rsity

Lib

rary

] at

05:

34 0

8 Se

ptem

ber

2014


aqueous DMSO solutions, and it was observed that

the tetrahedral hydrogen-bonded network of waterbreaks down at higher DMSO concentrations due tothe absence of any hydrogen-bond-donating site of the

DMSO molecule. We also found that the applicationof pressure increases the average number of hydrogenbonds per water molecule, whereas these values are not

significant except at lower mole fractions of DMSO.This can be attributed to the fact that, at higherpressure, some of the water molecules are forced to

occupy interstitial positions, which leads to an increasein the number of molecules with five neighbors at theexpense of the more stable four-hydrogen-bonded

water structure.In addition, we also calculated the hydrogen-bond

structural relaxation times between water–water andwater–DMSO molecules. It was observed that thehydrogen-bond structural relaxation time of both

water–water and water–DMSO increases with increas-ing DMSO concentration in the solutions, whereas inall cases the hydrogen-bond structural relaxation time

increases with the application of pressure. Therefore,the addition of even a small amount of DMSO restrictsboth intra- and inter-species hydrogen-bond reorgani-

zation and prevents the natural increase with increas-ing number of DMSO molecules in the solution, whichis further amplified by the application of pressure.

Water–DMSO hydrogen bonds have been shown to bestronger than water–water hydrogen bonds and theformation of these strong inter-species hydrogenbonds is believed to slow the dynamics of water and

DMSO in the mixture compared with those of puresolvent.

In this work, we have considered the structure,dynamics and hydrogen-bond properties of water–

DMSO mixtures under different thermodynamic con-ditions. It would also be interesting to study thestructure and dynamics of aqueous peptide bond

environments in the presence of DMSO, which mayreveal how hydrogen-bond structures and their energycan be affected by increasing DMSO concentrations

in an aqueous peptide mixture. We hope to addressthese studies in the future.

Acknowledgements

One of the authors (S. Chowdhuri) would like to thankProf. Amalendu Chandra for his kind support and manyhelpful communications. The authors are grateful tothe CSIR for financial support of this work through grantNo. 01 (2352)/09 EMR-II and also to the Indian Instituteof Technology, Bhubaneswar, for the initial help in executingthis project.

References

[1] S.W. Jacob and R. Herschler, Biological Actions of

Dimethyl Sulfoxide (New York Academy of Science,

New York, 1975), p. 508.[2] Z.-W. Yu and P.J. Quinn, Biosci. Rep. 14, 259 (1994).

[3] G. Christoph, W. Dana, C. Frederic, N. Gerhard and

U.E. Stefan, J. Chem. Phys. 136, 015102 (2012).[4] A. Luzar, Faraday Discuss. 103, 29 (1996).[5] D.F. Mierke and H. Kessler, J. Am. Chem. Soc. 113,

9466 (1991).[6] I.I. Vaisman and M.L. Berkowitz, J. Am. Chem. Soc.

114, 7889 (1992).[7] I.A. Borin and M.S. Skaf, Chem. Phys. Lett. 296, 125

(1998).

[8] I.A. Borin and M.S. Skaf, J. Chem. Phys. 110, 6412

(1999).

[9] D.H. Rasmussen and A.P. Mackenzie, Nature (London)

220, 1315 (1968).[10] A. Luzar and D. Chandler, J. Chem. Phys. 98, 8160

(1993).[11] P.P Wiewior, H. Shirota and E.W. Castner, J. Chem.

Phys. 116, 4643 (2002).[12] J.T. Cabral, A. Luzar, J. Teixeira and M.-C. Bellissent-

Funel, J. Chem. Phys. 113, 8736 (2000).[13] A.K. Soper and A. Luzar, J. Chem. Phys. 97, 1320

(1992).

[14] A.K. Soper and A. Luzar, J. Phys. Chem. 100, 1357

(1996).

[15] A. Luzar, A.K. Soper and D. Chandler, J. Chem. Phys.

99, 6836 (1993).[16] K.J. Packer and D.J. Tomlinson, Faraday Trans. Chem.

Soc. 67, 1302 (1971).[17] B.C. Gordalla and M.D. Zeidler, Mol. Phys. 59, 817

(1986).[18] B.C. Gordalla and M.D. Zeidler, Mol. Phys. 74, 975

(1991).[19] J. Kowalewski and H. Kovacs, Z. Phys. Chem 149, 49

(1986).[20] E.S. Baker and J. Jonas, J. Phys. Chem. 89, 1730

(1985).

[21] J.T. Cabral, A. Luzar, J. Teixeira and

M.-C. Bellissent-Funel, Physica B 276, 508 (2000).

[22] H.N. Bordallo, K.W. Herwig, B.M. Luther and

N.E. Levinger, J. Chem. Phys. 121, 12457 (2004).[23] Y. Higashigaki, D.H. Christensen and C.H. Wang,

J. Phys. Chem. 85, 2531 (1981).[24] M.S. Skaf, J. Phys. Chem. 103, 10719 (1999).[25] S. Chowdhuri and A. Chandra, J. Chem. Phys. 119,

4360 (2003).[26] M.R. Harpham, N.E. Levinger and B.M. Ladanyi,

J. Phys. Chem. B 112, 283 (2008).

[27] Z. Lu Manias, D.D. Macdonald and M. Lanagan,

J. Phys. Chem. A 113, 12207 (2009).

[28] L.J. Yang, X.Q. Yang, K.M. Huang, G.Z. Jia and

H. Shang, Int. J. Mol. Sci. 10, 1261 (2009).[29] S. Banerjee, S. Roy and B. Bagchi, J. Phys. Chem. B

114, 12875 (2010).

5. Summary and conclusions

In this paper, we have presented a series of molecular

dynamics simulation results of the pressure dependencebehavior of water–DMSO mixtures under ambient

and cold conditions. We have considered five different

concentrations of aqueous DMSO solutions with

pressure variation ranging from 0.1 to 500MPa.

The primary goal was to investigate the effects ofpressure on the single-particle dynamics and hydrogen-bond properties of both water and DMSO moleculesin their binary mixtures under ambient and coldconditions. It was found that, at 258K, the self-diffusion coefficients of water molecules initiallyincrease with pressure and then decrease, thus passingthrough a maximum for low DMSO concentrationsolutions. On the other hand, the correspondingorientational relaxation times are found to decreasewith increasing pressure. However, at higher concen-trations of DMSO, no such anomalous behavior of thedynamical properties is found on the application ofpressure. The absence of such anomalous behaviorat higher DMSO concentrations is attributed todisruption of the hydrogen-bond network caused bythe presence of DMSO molecules and also to theformation of strongly hydrogen-bonded water–DMSOcomplexes.

In this context, we also calculated the statisticsof intra- and inter-species hydrogen bonds in these

0.8

1

1.2

1.4

1.6

1.8

2

2.2

τ OW

...H

W (

P)

/ τO

W...

HW

(P

0)τ O

D...

HW

(P

) / τ

OD

...H

W (P

0)

(a) (c)

(d)

H2O...H2O T=298K

XDMSO =0.012XDMSO =0.09XDMSO =0.20XDMSO =0.35XDMSO =0.50

H2O...H2O T=258K

0.8

1

1.2

1.4

1.6

1.8

2

0 100 200 300 400 500P (MPa)

0 100 200 300 400 500P (MPa)

(b)DMSO...H2O T=298K DMSO...H2O T=258K

Figure 6. Pressure dependence of the hydrogen-bond structural relaxation times between water–water and DMSO–water forwater–DMSO mixtures of various compositions at 298 and 258K.

Table 4. The hydrogen-bond structural relaxation times ofvarious compositions of water–water and DMSO–water(�OW...HW

and �OD ...HWin ps) mixtures at P¼ 0.1MPa for

298 and 258K.

Solution

�OW...HW

(298K)

�OW...HW

(258K)

�OD ...HW

(298K)

�OD ...HW

(258K)


Dow

nloa

ded

by [

Um

eå U

nive

rsity

Lib

rary

] at

05:

34 0

8 Se

ptem

ber

2014


[30] S. Roy, S. Banerjee, N. Biyani, B. Jana and B. Bagchi,J. Phys. Chem. B 115, 685 (2011).

[31] C. Nieto-Draghi, J.B. Avalos and B. Rousseau,J. Chem. Phys. 119, 4782 (2003).

[32] H.J.C. Berendsen, J.R. Grigera and T.P. Straatsma,J. Phys. Chem. 91, 6269 (1987).

[33] M.P. Allen and D.J. Tildesley, ComputerSimulations of Liquids (Oxford University Press, NewYork, 1987).

[34] H.J.C. Berendsen, J.P.M. Postma, W.F. Gunsteren,A. DiNola and R. Haak, J. Chem. Phys. 81, 3684(1984).

[35] J.P. Hansen and I.R. McDonald, Theory of SimpleLiquids (Academic Press, London, 1986).

[36] T. Tokuhiro, L. Menafra and H.H. Szmant, J. Chem.Phys. 61, 2275 (1974).

[37] D.B. Wong, K.P. Sokolowsky, M.I. El-Barghouthi,E.E. Fenn, C.H. Giammanco, A.L. Sturlaugson andM.D. Fayer, J. Phys. Chem. B 116, 5479 (2012).

[38] T. DeFries and J. Jonas, J. Chem. Phys. 66, 896(1977).

[39] J. Jonas, T. DeFries and D.J. Wilbur, J. Chem. Phys. 65,582 (1976).

[40] F.X. Prielmeir, E.W. Lang, R.J. Speedy andH.-D. Ludemann, Phys. Rev. Lett. 59, 1128 (1987).

[41] A. Chandra and S. Chowdhuri, J. Phys. Chem. B 106,6779 (2002).

[42] S. Chowdhuri and A. Chandra, J. Phys. Chem. B 110,9674 (2006).

[43] S. Chowdhuri and A. Chandra, Chem. Phys. Lett. 313,

79 (2003).[44] A. Luzar and D. Chandler, Phys. Rev. Lett. 76, 928

(1996).

[45] A. Luzar and D. Chandler, Nature (London) 379, 53(1996).

[46] A. Chandra, Phys. Rev. Lett. 85, 768 (2000).[47] B.S. Mallik, A. Semparithi and A. Chandra, J. Chem.

Phys. 129, 194512 (2008).[48] B.S. Mallik, A. Semparithi and A. Chandra, J. Phys.

Chem. A 112, 5104 (2008).

[49] H. Xu and B.J. Berne, J. Phys. Chem. B 105, 11929(2001).

Dow

nloa

ded

by [

Um

eå U

nive

rsity

Lib

rary

] at

05:

34 0

8 Se

ptem

ber

2014

pressure dependence on the single-particle dynamics and hydrogen-bond structural relaxation of...

Documents