Chemical Physics 368 (2010) 7–13

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Chemical Physics

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Liquid–vapour interfaces of aqueous trimethylamine-N-oxide solutions:A molecular dynamics simulation study

Sandip Paul *

Department of Chemistry, Indian Institute of Technology, Guwahati, Assam 781039, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 18 May 2009In final form 27 November 2009Available online 6 January 2010

0301-0104/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.chemphys.2009.11.017

* Tel.: +91 3612582321; fax: +91 3612582349.E-mail address: sandipp@iitg.ernet.in

The liquid–vapour interfaces of aqueous trimethylamine-N-oxide (TMAO) solutions of varying composi-tion are investigated by means of molecular dynamics simulations. The inhomogeneous density, aniso-tropic orientational profiles, surface tension and the pattern of hydrogen bonding are calculated inorder to characterize the location, microscopic structure and the thermodynamic aspects of the interfacesand to explore their effects on the interfacial dynamical properties of water and TMAO molecules. Thedynamical aspects of the interfaces are investigated in terms of the single-particle dynamical propertiessuch as the relaxation of velocity autocorrelation and the translational diffusion coefficients along theperpendicular and parallel directions of the interfacial water and TMAO molecules at 298 K. The resultsof the interfacial dynamics are compared with those of the corresponding bulk phases.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

The microscopic structure and dynamics of aqueous solutions atliquid–vapour interfaces have been subject of great interest inrecent years. Studies of these interfacial systems are importantnot only in chemistry and biology but also in the areas of environ-mental and atmospheric sciences. The molecular properties of li-quid–vapour interfaces of molecular liquids is important in theunderstanding of equilibrium and dynamical aspects of variouschemical processes that occur at such interfaces. Earlier experi-mental studies on liquid–vapour interfaces primarily focused onthermodynamic quantities such as surface tension and surface po-tential measurements which could only provide a macroscopicdescription of these interfaces. More recent experimental tech-niques such as surface second harmonic [1] and sum frequencygeneration [2–4] now provide more detailed microscopic informa-tion of the structure and dynamics of liquid–vapour interfaces.These methods have been used to investigate molecular orienta-tion at a wide variety of interfaces involving pure water or aqueoussolutions [5–19]. Computer simulation studies have also been usedto study liquid–vapour interfaces of pure water and aqueous solu-tions [20–31]. Note that, the existing theoretical and experimentalstudies at liquid–vapour interfaces mainly deal with the structuraland thermodynamical properties. Only very few molecular dynam-ics simulation study investigated the dynamical properties at theliquid–vapour interfaces [21,31–33].

The present paper deals with the equilibrium and dynamicalbehavior of liquid–vapour interfaces of water–trimethylamine-N-

ll rights reserved.

oxide (TMAO) mixtures of varying composition. TMAO (see Fig. 1),a polar nonelectrolyte, tends to stabilize folded protein states andcan counteract the effects of urea on protein structure [34–40]and it is believed that hydrogen bonding plays an important role[37]. Although, there have been numerous study of water–TMAOmixtures in the liquid state [39–44], the properties at the liquid–va-pour interfaces of water–TMAO mixtures are yet to explore. It isknown that the hydrogen bonds between the water and TMAO mol-ecules is very strong [44]. So, for water–TMAO mixtures, the dynam-ics of the interfaces is expected to be intimately related to thestructure and energetics of hydrogen bonds that are present at suchinterfaces. Thus, it would be worthwhile to make a detailed molec-ular-level investigation of the hydrogen bonding and dynamicalproperties of liquid–vapour interfaces of water–TMAO mixtures.The present work makes a contribution toward this end.

In this work, the molecular dynamics simulations of liquid–vapour interfaces of mixture of water–TMAO at varying composi-tion have been carried out. The focus has been to calculate thehydrogen bond properties such as the average number of hydrogenbonds per molecule in different regions, the single-particle dynam-ical properties such as the relaxation of the perpendicular and par-allel components of velocity autocorrelation function and thetranslational diffusion coefficients of the interfacial water andTMAO molecules. The dynamical properties of the interfaces arecompared with those of the bulk phases. Furthermore, the inhomo-geneous density and orientational profiles, surface tension and alsothe distribution of hydrogen bonds have been calculated as thedynamical properties of the interfaces are intimately related tothese equilibrium quantities.

The outline of the present paper is as follows. The simulationdetails including the construction of the interfaces and their

N

O

CH CHCH3

33

Fig. 1. TMAO.

Table 2The number of water ðNwaterÞ; TMAO molecules ðNtmaoÞ and the length of the simulationbox along x ðLxÞ, y ðLyÞ and z-directions ðLzÞ for each of the systems studied in thiswork.

xtmao Nwater Ntmao Lx (Å) Ly (Å) Lz (Å)

0.0 500 0 24.64 24.64 73.920.10 450 50 26.44 26.44 79.320.25 375 125 27.70 27.70 83.100.50 250 250 32.00 32.00 96.000.75 125 375 34.66 34.66 103.980.90 50 450 36.38 36.38 109.141.0 0 500 37.00 37.00 111.00

8 S. Paul / Chemical Physics 368 (2010) 7–13

characterization in terms of density profiles are presented in Sec-tion 2. Calculations of interfacial structures, orientation of waterand TMAO molecules both at the bulk and at the interfacial regionsand surface tensions are described in Section 3. Section 4 dealswith the hydrogen bond patterns in the bulk liquid and at theinterfaces. In Section 5, the simulation results of the single-particledynamics at the interfaces have been discussed and the conclu-sions are summarized in Section 6.

2. Details of models, simulation method and construction of theinterfaces

The molecular dynamics simulations of the liquid–vapour inter-face of water–TMAO mixtures have been carried out at 298 K. Theall atom model for TMAO molecules have been used where themethyl hydrogen atoms are explicitly taken account [39]. SPC/Ewater model has been used for water molecules [45]. In these mod-els, the interaction between atomic sites of two different moleculesis expressed as

uabðra; rbÞ ¼ 4�abrab

rab

� �12

� rab

rab

� �6" #

þqaqb

rab; ð1Þ

where rab is the distance between the atomic sites a and b and qa isthe charge of the ath atom. The Lennard-Jones parameters rab and�ab are obtained by using the combination rules rab ¼ ðra þ rbÞ=2and �ab ¼

ffiffiffiffiffiffiffiffiffiffi�a�bp

. The values of the potential parameters qa, ra and�a for TMAO and water are summarized in Table 1.

For each system, the bulk simulation is carried out in a cubicbox of 500 molecules (water and TMAO), periodically replicatedin all three dimensions. The box length L was adjusted in such away that the pressure would be close to the atmospheric pressureat 298 K. After this bulk solution was properly equilibrated, twoempty cubic boxes of equal size were added on either side of theoriginal simulation box along the z-direction and this larger rectan-gular box (of dimension L� L� 3L) was taken as the simulationbox in the next phase of the simulation run and the system wasreequilibrated by imposing periodic boundary conditions in allthree directions. This resulted in a liquid slab of approximate widthL separated by vacuum layers of approximate width 2L. Some ofthe molecules were found to vapourize to the empty space to forma liquid–vapour interface on both sides of the liquid slab. Table 2shows the number of water and TMAO molecules and also the

Table 1The Lennard-Jones parameters and charges used in the models considered. e is theelementary charge.

Atom r ðÅÞ � ðkJ=molÞ Charge ðeÞ

Water (SPC/E model)O 3.166 0.646 �0.8476H – – +0.4238

TMAOC 3.041 0.281 �0.26N 2.926 0.8314 +0.44O 3.266 0.6344 �0.65H 1.775 0.0769 +0.11

length of the simulation box along x, y and z-dimensions for eachsystem. In the simulations, the long range electrostatic interactionswere treated by using the three dimensional Ewald method [46]. Aspherical cut-off at a distance L=2 was used for the real spaceEwald and for the Lennard-Jones interactions. The quaternion for-mulation of the equations of rotational motion is employed and, forthe integration over time, the leap-frog algorithm with a time stepof 10�15 s (1 fs) is adapted. MD runs of 500 ps were used to equil-ibrate each system in the bulk phase and then the liquid–vapourinterfacial systems in rectangular boxes were equilibrated for1 ns. During the equilibration, the temperature of the simulationsystem was kept at 298 K through rescaling of the velocities. Thesimulations of the interfacial systems were then continued for an-other 1 ns production run using microcanonical ensemble. For allthe systems, the total energy was found to be well conserved dur-ing the production phase of the simulations. The average temper-ature during the production run of the simulations were found tobe 300 K and no systematic drift in the temperature of the simula-tion systems was observed. Since the average temperature is veryclose to the desired temperature of 298 K, the later value will becontinued in subsequent discussion. The interfaces were found tobe stable over the simulation time. The density, orientational pro-files, surface tensions, hydrogen bonds and the dynamical proper-ties of the interfaces were calculated during the last productionphase of the simulations.

The surface tension is calculated by using the following virialexpression which is obtained from the well-known Kirkwood–Bufftheory [47]

c ¼ 12A

Xi<j

Xa;b

@uab

@rab

1rabðrij � rab � 3zijzabÞ

" #* +; ð2Þ

where uab is the interaction energy between sites a and b of mole-cules i and j, rij and zij are the centers of mass distance and the dis-tance along z direction between molecules i and j and rab and zab arethe corresponding distances between sites a and b. A is the totalsurface area which is equal to 2L2. The quantity within the thirdbrackets in the above expression is calculated at each MD stepand finally the averaging was done over the total number of MDsteps that were run during the production phase of the simulations.

3. Interfacial structure and surface tension

The number density profiles (of water and TMAO) of water–TMAO mixtures have been calculated for various concentration ofTMAO as a function of z by computing the average number of mol-ecules in slabs of thickness Dz ¼ 0:05 Å lying on either side of thecentral plane at z = 0 and the results are shown in Fig. 2. Followingprevious work [20], the thickness ðdlvÞ of a liquid–vapour interfaceis defined as the distance over which the total number density de-creases from 90% to 10% of the bulk liquid density. From these den-sity profiles, it is clear that TMAO molecules do not prefer to stay atthe surface and hardly any TMAO molecules can be seen at low bulk

ρ(z

)o

−3

(A

)ρ

(z)

o−

3(A

)

ρ(z

)o

−3

(A

)

ρ(z

)o

−3

(A

)

oz (A)

oz (A)

(a)

(b)

(c)

(d)

(e)

(f)

(g)

x tmao= 0.0

x tmao= 0.10

x tmao=

x tmao=

x tmao=

x tmao=x tmao=

0.25

0.50

0.75

0.90

1.0

−20

0.04

0.03

0.02

0.01

0

0.04

0.03

0.02

0.01

0

0.04

0.03

0.02

0.01

0

0.04

0.03

0.02

0.01

0−40 −20 20 40 0 −40 0 20 40

Fig. 2. Number density profiles of liquid–vapour interfaces of water–TMAO mixtures of varying composition. The solid curves are for water and the dashed curves are forTMAO. xtmao represents the mole fraction of TMAO.

S. Paul / Chemical Physics 368 (2010) 7–13 9

TMAO concentrations. The TMAO molecules start accumulating atthe interfaces above xtmao ¼ 0:25. A precise analysis of the enrich-ment of the interfaces by water or TMAO molecules can be madeby plotting the mole fractions of water and TMAO molecules atthe interfaces against that of the bulk liquid phases. Such plotsare shown in Fig. 3. It is clearly seen that the interfacial regionsare slightly enriched by TMAO molecules up to xtmao ¼ 0:25. Forlow TMAO mole fraction, the value of xL

tmao is found to be slightlysmaller than xtmao which is the TMAO mole fraction for the entiresimulation system.

Figs. 4 and 5 show the orientational profiles of bulk and interfa-cial water molecules at various TMAO concentrations. The orienta-tion of a water molecule is described in terms of the angle h thatthe molecular dipole vector makes with the surface normal alongz-axis. In Fig. 4, the results of the normalized probability functionPðcos hÞ as a function of cos h is shown for water molecules. Inthe bulk phase, the probability function, as expected, is found tobe uniform, i.e., there is no preferred orientation of the water mol-ecules in the bulk phase of the liquid slabs as one would expect. Inthe interfacial region, however, the probability function is nonuni-form which shows an orientational structure of the interfacial mol-

ecules. For pure water ðxtmao ¼ 0:0Þ at liquid–vapour interface,Pðcos hÞ is maximum at around cos h ¼ 0 which means that watermolecules at the interface prefer to orient with their dipoles paral-lel to the surface. One hydrogen atom projects into the liquid sidesof the interface and other projects toward the vapour side of theinterface. This molecular orientation is confirmed by a calculationof the H–H vector of interfacial water molecules with respect to thesurface normal (not shown). For example, for xtmao ¼ 0:10 thewater H–H vector at the interfacial region, makes an angle of about0� with the surface normal which clearly indicates that one hydro-gen atom of interfacial water molecules is pointed inward (towardsliquid region) and other is pointed outward. As expected, there isno preferred orientation of water H–H vector in the bulk liquid re-gion. With increasing TMAO mole fraction, up to xtmao ¼ 0:25, theangle h decreases very slightly which indicates that the dipole vec-tor is tilted slightly toward the vapour phase. For example, for purewater ðxtmao ¼ 0:0Þ liquid–vapour interface, the maximum ofPðcos hÞ appears at around cos h ¼ 0:0 (i.e., h ¼ 90�) and forxtmao ¼ 0:25 the maximum of Pðcos hÞ appears at around cos h ¼0:20 (i.e., h ¼ 78�). This is because, with increasing TMAO concen-tration the number of water molecules at the interfacial region de-

x Ltmao

xtm

aoI

xI w

ater

(b)

(a)

0

0.25

0.5

0.75

1

0

0.25

0.5

0.75

1

0 0.25 0.5 0.75 1

Fig. 3. The variation of mole fraction of (a) water and (b) TMAO at interfaces withthe mole fraction of TMAO in the bulk liquid phases. The dashed lines are only toconnect the simulation results (squares). The solid lines correspond to the equalmole fractions of the interfacial and bulk liquid phases.

10 S. Paul / Chemical Physics 368 (2010) 7–13

creases and since TMAO molecules do not prefer to stay at the sur-face (see Fig. 2), there are some ‘free non H-bonded’ water mole-cules present at the interfaces. Above xtmao ¼ 0:25, there is a shiftin the angle the water dipole vector makes with the surface nor-

cos θ

(a)

(b)

(c)

x tmao= 0.0

x tmao=

x tmao=

0.10

0.25

θθ

θP

(cos

)

P (c

os

)P

(cos

)

0.033

0.022

0.011

0

0.033

0.022

0

0.011

0.033

0.022

0.011

0−1 −0.5 0 0.5 1

Fig. 4. Probability of the orientation of water dipole vector in the bulk (solid) and in thethe surface normal.

mal. In this concentration region, water molecule at the interfacialregion prefers to orient in such a way so that its both hydrogenatoms project towards liquid region (see also Fig. 2d). This is dueto the fact that above xtmao ¼ 0:25, there are more TMAO moleculespresent at the interfacial region and the dipole vector of interfacialwater molecules orient itself to maximize number of hydrogenbonds. Note that, water–TMAO hydrogen bond is much strongerthan the water–water hydrogen bond [44].

Fig. 5 shows the probability function of the orientation of the di-pole vector of a TMAO molecule in the interfacial and bulk regions;the orientation of a TMAO molecule is described by the angle h thatthe above vector makes with the surface normal. The orientationaldistribution of pure TMAO ðxtmao ¼ 1:0Þ liquid–vapour interface isbroad and the maximum of Pðcos hÞ appears at around cos h ¼ 0:0which indicates that the angle between the dipole vector and sur-face normal ðhÞ is about 90�. This means that the dipole vector liesparallel to the surface plane. With decreasing TMAO concentration,the dipole vector of surface TMAO molecules is projected towardsthe vapour region.

Fig. 6 shows the calculated surface tension values, c, for variousmixtures at 298K. The standard deviations of the surface tensiondata, which were calculated by using block averages over 100 ps,are about 4.4% of the average values reported. It is found that thesurface tension decreases with increase of TMAO concentrationand the decrease is more rapid after xtmao ¼ 0:50. This is because,up to xtmao ¼ 0:25, there is weak accumulation of TMAO moleculesat the liquid–vapour interfacial region and TMAO molecules startaccumulating at the interfacial region at xtmao ¼ 0:50 (confirmedby the number density plot, Fig. 2 and the surface enrichment plot,Fig. 3). The experimental surface tension values of water–TMAOmixtures also decrease with increasing TMAO concentration, for

cos θ

(d)

(e)

(f)

x tmao=

x tmao=

x tmao=

0.50

0.75

0.90

−1 −0.5 0 0.5 1

interfaces (dashed). h is the angle between the dipole vector of water molecule and

cos θ cos θ

(a)

(b)

(c)

(d)

(e)

(f)

x tmao=

x tmao=

x tmao=

x tmao=

x tmao=

x tmao=

0.10

0.25

0.50

0.75

0.90

1.0

θθ

θP

(cos

)

P (c

os

)P

(cos

)

0.033

0.022

0.011

0

0.033

0.022

0.011

0

0.033

0.022

0−1

0.011

−0.5 0 0.5 1 −1 −0.5 0.5 10

Fig. 5. Probability of the orientation of the dipole vector of TMAO molecule in the bulk (solid) and in the interfaces (dashed). h is the angle between the dipole vector of TMAOmolecule and the surface normal.

γ(d

ynes

/cm

)

x tmao

80

60

40

200 0.25 0.5 0.75 1

Fig. 6. Dependence of surface tension on the mole fraction of TMAO. Open squaresrepresent the simulated surface tension values and open triangle represents thesimulated surface tension value of pure water liquid–vapour interface with tailcorrection (Ref. [21]).

S. Paul / Chemical Physics 368 (2010) 7–13 11

low TMAO concentration (the TMAO concentration used in theexperiment is much lower than that of this study) [51].

4. Hydrogen bond distribution at interfaces

The hydrogen bond properties of water and TMAO molecules inthe bulk and in the interfacial regions are calculated by computingthe average number of hydrogen bonds that each molecule has in agiven region and also the probability of finding a molecule with agiven number of hydrogen bonds. Following earlier studies on

hydrogen bonds in water and aqueous solutions [48–50] a set ofgeometric criteria have been used to define the presence of ahydrogen bond between two molecules. Two water moleculesare taken to be hydrogen bonded if their interoxygen distance isless than 3.45 Å and simultaneously hydrogen–oxygen distance isless than 2.37 Å and the oxygen–oxygen–hydrogen angle is lessthan a cut-off value of 45�. A hydrogen bond between a water mol-ecule and a TMAO molecule exists if their interoxygen distance isless than 3.45 Å and simultaneously hydrogen of water–oxygenof TMAO distance is less than 2.64 Å. The cut-off value of the oxy-gen–oxygen–hydrogen angle is again 45�. The oxygen–oxygen andhydrogen–oxygen distances are determined from the positions ofthe first minimum of the corresponding radial distribution func-tions of the liquid mixtures. Following previous nomenclature inthe context of water–water hydrogen bonds [49], a ‘less strict’ def-inition of the hydrogen bonds is used in the present study. Thequantities of interest are the percentages, fn, of water or TMAO mol-ecules that engage in n hydrogen bonds and the average number ofhydrogen bonds per molecule nHB. The above quantities are calcu-lated for two types of hydrogen bonds: water–water (ww) andwater–TMAO (wt).

Fig. 7 shows the variation of the number of hydrogen bonds permolecule with change of TMAO concentration in bulk liquid phasesand also at interfaces. For pure water (xtmao ¼ 0:0), in the bulk li-quid phase, majority of water molecules participate in four hydro-gen bonds whereas, in the liquid–vapour interfacial region, most ofthe molecules are found to have either three or two hydrogenbonds. The average number of hydrogen bonds per water moleculeis also significantly smaller than that in the bulk liquid phase. Thissmaller number of hydrogen bonds at the vapour–water interfacesis likely due to the lower density and the presence of vapour

n HB

n HB

x tmao

(a)

(b)

ww

ww

wt/t

wt/t

wt/w

wt/w

0

1

2

3

4

0

1

2

3

4

0 0.25 0.5 0.75 1

Fig. 7. Variation of number of hydrogen bonds per molecule with TMAO concen-tration. (a) is for bulk and (b) is for the interfacial regions.

fn

fn

fn

(a)

(b)

(c)

ww

wt/t

wt/w

n

0.6

0.4

0.2

0

0.6

0.4

0.2

0

0.6

0.4

0.2

00 1 2 3 4

Fig. 8. Fraction of molecules with n number of hydrogen bonds formed in bulk(open squares) and at the interfacial regions (crosses) for 50:50 mixtures of water–TMAO.

12 S. Paul / Chemical Physics 368 (2010) 7–13

(essentially vacuum) on one side of the liquid. With the increase ofTMAO concentration, the numbers of ww (water–water) and wt/t(water–TMAO per TMAO) hydrogen bonds decrease and that ofwt/w (water–TMAO per water) increases. Fig. 8 shows the fractionof molecules with n number of hydrogen bonds in bulk liquid andat the interfacial region for 50:50 mixture of water and TMAO.

5. Diffusion of interfacial molecules

In this section, the single molecule relaxation of water andTMAO molecules at the liquid–vapour interfaces are reported. Animportant issue concerning the single-particle dynamics of aninterface is: How different the single-particle dynamical propertiesat the interface are compared to that of the corresponding bulkphases? The dynamical behavior of interfacial molecules is ex-pected to be different from bulk molecules translationally. Also,the translational motion of molecules at the interface can be highlyanisotropic in contrast to the bulk molecules which move in an iso-tropic environment. In view of this anisotropic aspect, the perpen-dicular and parallel components of the translational diffusion arecalculated separately.

The ath component of velocity of a molecule is denoted byvaðtÞ ða ¼ x; y; zÞ and its normalized autocorrelation functionCv ;aðtÞ is defined by

Cv;aðtÞ ¼hvaðtÞvað0Þihv2

ai; ð3Þ

where h� � �i denotes an equilibrium ensemble average. In these cal-culations, the average of Eq. (3) is carried out over those moleculeswhich are found in the interfacial region at time 0 and also at time t.

The diffusion coefficient Da ða ¼ x; y; zÞ is calculated from thevelocity–velocity autocorrelation function by using the followingrelation

Da ¼kBTm

Z 1

0Cv;aðtÞdt; ð4Þ

where m is the mass of a molecule and kB is Boltzmann constant.The anisotropic nature of the translational motion is clearly illus-trated in Fig. 9 where the diffusion coefficients values in the parallel(x) and perpendicular (z) direction of the interface are shown andthe values are compared with their corresponding bulk liquid val-ues at different mole fractions of TMAO. The experimental diffusioncoefficient values for water–TMAO mixtures are yet to be known. Upto xtmao ¼ 0:50 the diffusion coefficients of both water and TMAO inthe bulk liquid region decreases significantly indicating there isstrong interaction between the water and TMAO molecules whichis consistent with the recent finding [44]. Above xtmao ¼ 0:50, bothDbulkðH2OÞ and DbulkðTMAOÞ begin to increase. At low TMAO concen-trations, the TMAO is strongly solvated by water molecules andthereby retarding the diffusion of water molecules. At high concen-tration of TMAO, there are insufficient number of water moleculespresent to interact with all the TMAO molecules. Thus, the relativeimportance of the strength of water–TMAO interactions decreasesand, as a result, an increase of the diffusion coefficients is observed.The diffusion coefficients of both water and TMAO molecules alongx-direction at the interface (parallel diffusion) follow the sametrend as that of the corresponding bulk liquid values with the min-imum of the diffusion coefficient values appearing at xtmao ¼ 0:50.For the perpendicular diffusion at interfaces, however, the mini-mum occurs at xtmao ¼ 0:25 for both water and TMAO molecules.Note that, for pure water ðxtmao ¼ 0:0Þ, the relative changes of thediffusion coefficients between the bulk and interfacial regions arequalitatively similar to the results obtained by Liu et al. [33] whoused the method of survival probabilities to calculate the diffusioncoefficients.

D (

10

cm

/s

)−

52

D (

10

cm

/s

)−

52

x tmao

H O2(a)

(b)TMAO

0

1.5

3

4.5

6

0

1.5

3

4.5

6

0 0.25 0.5 0.75 1

Fig. 9. Variation of diffusion coefficients with the concentration of TMAO. Opensquares are for bulk liquids, crosses are for interfacial diffusion coefficients alongparallel (x, y) direction and the open triangles are for diffusion of interfacialmolecules along perpendicular (z) direction.

S. Paul / Chemical Physics 368 (2010) 7–13 13

6. Summary

Molecular dynamics simulations have been performed to inves-tigate the various equilibrium and dynamical properties of liquid–vapour interfaces of water–TMAO mixtures of varying composition.The goal was to investigate the changes of interfacial properties ofwater and TMAO molecules with the change in TMAO concentra-tion. Simulations are carried out at room temperature and variousinterfacial properties that are calculated include density and orien-tational profiles, surface tension, structure of hydrogen bonds andmolecular diffusion of both water and TMAO molecules.

It is found that the orientational distribution of the dipole vec-tor of interfacial water molecules changes with change of TMAOconcentration and there is a shift in the orientational angle ofwater dipole vector above xtmao ¼ 0:25. The dipole vector of pureTMAO ðxtmao ¼ 1:0Þ molecules lies parallel to the surface and forwater–TMAO mixtures this vector orients itself to maximize num-ber of hydrogen bonds. The surface tension is found to increasewith decrease of TMAO mole fraction. The distribution of differenttypes of hydrogen bonds (ww, wt/w and wt/w) and the total num-ber of hydrogen bonds per molecule are also calculated in bothbulk liquid and interfacial regions.

The translational motion of water and TMAO molecules is foundto be highly anisotropic in the interfacial regions. The minimum of

translational diffusion coefficients for both water and TMAO mole-cules appear at xtmao ¼ 0:50 (in both bulk and interfacial regions).

Acknowledgment

This research was enabled by the use of Orang computing re-sources of Indian Institute of Technology, Guwahati, Assam, India.

References

[1] K.B. Eisenthal, Chem. Rev. 96 (1996) 1343, and references therein.[2] Q. Du, E. Freysz, Y.R. Shen, Science 264 (1994) 826.[3] Q. Du, R. Superfine, E. Freysz, Y.R. Shen, Phys. Rev. Lett. 70 (1993) 2313.[4] S. Gopalakrishnan, P. Jungwirth, D.J. Tobias, H.C. Allen, J. Phys. Chem. B 109

(2005) 8861.[5] K. Wolfrum, H. Graener, A. Laubereau, Chem. Phys. Lett. 213 (1993) 41.[6] J.Y. Huang, M.H. Wu, Phys. Rev. E 50 (1994) 3737.[7] C.D. Stanners, Q. Du, R. Chin, P. Cremer, G.A. Somorjai, Y.R. Shen, Chem. Phys.

Lett. 232 (1995) 407.[8] M.C. Goh, J.M. Hicks, K. Kemnitz, G.R. Pinto, K. Bhattacharyya, K.B. Eisenthal,

T.F. Heinz, J. Phys. Chem. 92 (1988) 5074.[9] E.V. Sitzmann, K.B. Eisenthal, J. Phys. Chem. 92 (1988) 4579.

[10] K. Kemnitz, K. Bhattacharyya, J.M. Hicks, G.R. Pinto, K.B. Eisenthal, T.F. Heinz,Chem. Phys. Lett. 131 (1986) 285.

[11] K. Bhattacharyya, E.V. Sitzmann, K.B. Eisenthal, J. Chem. Phys. 87 (1987) 1442.[12] T. Petralli-Mallow, T.M. Wong, J.D. Byers, H.I. Yee, J.M. Hicks, J. Phys. Chem. 97

(1993) 1383.[13] R. Superfine, J.Y. Huang, Y.R. Shen, Opt. Lett. 15 (1990) 1276.[14] J.C. Conboy, J.L. Daschbach, G.L. Richmond, J. Phys. Chem. 98 (1994) 9688.[15] T. Raising, T. Stehlin, Y.R. Shen, M.W. Kim, P. Valint Jr., J. Chem. Phys. 89 (1988)

3386.[16] X. Shi, E. Borguet, A.N. Tarnovski, K.B. Eisenthal, Chem. Phys. 205 (1996) 167.[17] K. Wolfrum, J. Lobau, A. Laubereau, Appl. Phys. A 59 (1994) 605.[18] D. Zimdars, K.B. Eisenthal, J. Phys. Chem. B 105 (2001) 3993.[19] D. Zhang, J.H. Gutow, K.B. Eisenthal, T.F. Heinz, J. Chem. Phys. 98 (1993) 5099.[20] R.S. Taylor, L.X. Dang, B.C. Garrett, J. Phys. Chem. 100 (1996) 11720.[21] S. Paul, A. Chandra, Chem. Phys. Lett. 373 (2003) 87.[22] J. Alejandre, D.J. Tildesley, G.A. Chapela, J. Chem. Phys. 102 (1995) 4574.[23] P. Jungwirth, D.J. Tobias, J. Phys. Chem. B 104 (2000) 7702.[24] P. Jungwirth, D.J. Tobias, J. Phys. Chem. B 105 (2001) 10468.[25] P. Jungwirth, D.J. Tobias, J. Phys. Chem. B 106 (2002) 6361.[26] M. Matsumoto, Y. Takaoka, Y. Kataoka, J. Chem. Phys. 98 (1993) 1464.[27] I. Benjamin, J. Chem. Phys. 110 (1999) 8070.[28] I. Benjamin, Phys. Rev. Lett. 73 (1994) 2083.[29] I. Benjamin, Annu. Rev. Phys. Chem. 48 (1997) 407.[30] I. Benjamin, Chem. Phys. Lett. 469 (2009) 229.[31] S. Paul, A. Chandra, J. Phys. Chem. B 107 (2003) 12705.[32] S. Paul, A. Chandra, J. Chem. Phys. 123 (2005) 174712.[33] P. Liu, E. Harder, B.J. Berne, J. Phys. Chem. B 108 (2004) 6595.[34] P.H. Yancey, M.E. Clark, S.C. Hand, R.D. Bowlus, G.N. Somero, Science 217

(1982) 1214.[35] P.H. Yancey, W.R. Blake, J. Conley, Comp. Biochem. Physiol. A 133 (2002) 667.[36] Q. Zou, B.J. Bennion, V. Daggett, P. Murphy, J. Am. Chem. Soc. 124 (2002) 1192.[37] B.J. Bennion, V. Daggett, Proc. Natl. Acad. Sci. USA 101 (2004) 6433.[38] B.J. Bennion, M.L. DeMarco, V. Daggett, Biochemistry 43 (2004) 12955.[39] K.M. Kast, J. Brickmann, S.M. Kast, R.S. Berry, J. Phys. Chem. A 107 (2003) 5342.[40] M.V. Athawale, J.S. Dordick, S. Garde, Biophys. J. 89 (2005) 858.[41] R. Noto, V. Martorana, A. Emanuele, S.L. Fornili, J. Chem. Soc. Faraday Trans. 91

(1995) 3803.[42] A. Fornili, M. Civera, M. Sironi, S.L. Fornili, Phys. Chem. Chem. Phys. 5 (2003)

4905.[43] S. Paul, G.N. Patey, J. Phys. Chem. B 110 (2006) 10514.[44] S. Paul, G.N. Patey, J. Am. Chem. Soc. 129 (2007) 4476.[45] H.J.C. Berendsen, J.R. Grigera, T.P. Straatsma, J. Phys. Chem. 91 (1987) 6269.[46] M.P. Allen, D.J. Tildesley, Computer Simulation of Liquids, Oxford, 1987.[47] J.G. Kirkwood, F.P. Buff, J. Chem. Phys. 17 (1949) 338.[48] A. Luzar, D. Chandler, Nature (London) 379 (1996) 53;

A. Luzar, D. Chandler, Phys. Rev. Lett. 76 (1996) 928.[49] A. Luzar, J. Chem. Phys. 113 (2000) 10663.[50] A. Chandra, Phys. Rev. Lett. 85 (2000) 768;

A. Chandra, J. Phys. Chem. B 107 (2003) 3899.[51] Y. Kita, T. Arakawa, T.-Y. Lin, S.N. Timasheff, Biochemistry 33 (1994) 15178.