Structure and Dynamics of the LiquidâLiquid Interface of an Aqueous

Article pubs.acs.org/JPCC

Structure and Dynamics of the Liquid−Liquid Interface of an Aqueous NaCl Solution with Liquid Carbon Tetrachloride from FirstPrinciples Simulations Jyoti Roy Choudhuri and Amalendu Chandra* Department of Chemistry, Indian Institute of Technology, Kanpur, India 208016 ABSTRACT: The present work deals with a first-principles theoretical study of the liquid−liquid interfacial system of an aqueous NaCl solution with the hydrophobic liquid of carbon tetrachloride (CCl4). Various structural and electronic properties of the system such as the inhomogeneous density profiles of ions and water molecules, hydrogen bond distributions, orientational profiles, vibrational frequency distributions, and also dipole moments of water at the bulk and interface are investigated. It is found that the chloride ions have a higher propensity for the interface and the sodium ions have a tendency to reside in the inner side of the aqueous medium. The dynamical aspects of the interfaces are analyzed in terms of the diffusion, orientational relaxation, hydrogen bond dynamics, and vibrational spectral diffusion and are compared with those of the bulk regions. It is found that the interfacial molecules have a faster diffusion and orientation relaxation with respect to the bulk molecules. The interfacial water molecules are also found to have a shorter hydrogen bond lifetime than those of bulk water molecules. We have also investigated the dynamics of vibrational frequency fluctuations of water molecules in the bulk and interfacial regions.

1. INTRODUCTION The adsorption and distribution of ions at the interface is considered to be one of the most important processes in a variety of biological and chemical systems.1−3 The phenomenon of the solvation of ions at aqueous surfaces and interfaces is of particular importance. The process of ion transport across an interface also has its relevance in many environmental problems.4 Because of this importance, there have been a number of experimental studies on the behavior of water and ions at interfaces by using various surface specific techniques.5−7 The use of X-ray refractivity measurements has provided the ion density profiles at the liquid−liquid interface.7 Vibrational sum-frequency studies (VSFS) of interfacial water molecules have revealed the behavior of common ionic solutes at the interface between different aqueous solutions and the organic liquid of CCl4.8 The study also showed how the water hydrogen bonding gets affected by specific ions at the interfacial region.8 Studies concerning the ions at the liquid−liquid interfaces are rather limited. The mechanism for transport of a chloride ion has been investigated using molecular dynamics simulations.9,10 These studies have provided useful information regarding free energies and solvent structures as the ion moves across the interface. Recently, Richmond et al.11 have given a more detailed molecular-level picture of the distribution of ions in the interfacial region and their effects on water molecules at the interface. Their study also revealed details about the location of ions and water and hydrogen bonding environment across the interface.11 Computational studies of the neat CCl4− water interface have shown a high degree of orientational ordering of the water molecules in the interfacial region.12−15 It © 2014 American Chemical Society

was also suggested that CCl4 molecules exhibit orientational layering near the interface which can be extended up to several molecular layers.16 The orientational ordering in the aqueous and organic phases generates an interfacial field across the boundary region which influences the absorption of charge species in the interfacial region from the aqueous phase.12 The above simulation studies on liquid−liquid interfaces of aqueous ionic solutions and organic liquids were conducted using empirical potential models. The simulation study presented here is based on the method of ab initio molecular dynamics involving a quantum mechanical many-body treatment of the energies and forces without employing any model potential. This method has been successfully used in studies of liquid−vapor interfaces of pure water17−19 and water−ammonia mixtures20 and also of liquid−liquid interfaces of pure water and carbon tetrachloride without any ions.21 In this method, the forces on the nuclei are directly determined from quantum electronic structure calculations performed “on the fly” via an adiabatic dynamics principle.22,23 The main aim of our study is to gain a molecular-level understanding of structural and dynamical aspects of liquid−liquid interfaces of aqueous NaCl solution and the organic liquid of carbon tetrachloride. We have also made a detailed analysis of the structural and dynamical aspects of interfacial hydrogen bonds and their relations to vibrational frequency fluctuations of interfacial molecules. In recent years, the correlations between hydrogen bond fluctuations and vibrational frequency fluctuations of aqueous Received: June 22, 2014 Revised: September 12, 2014 Published: September 25, 2014 23083

dx.doi.org/10.1021/jp506193n | J. Phys. Chem. C 2014, 118, 23083−23091

The Journal of Physical Chemistry C

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surface normal and the center of the simulation box is located at z = 0. In our present calculations, we kept the thickness of each slab to be 0.05 Å. For our present system, the liquid− liquid interfacial region can be defined where the densities of both the liquids start decreasing from their bulk values. From Figure 1a, it is clearly observed that two density profiles cross at

systems have drawn significant attention both experimentally24−29 and theoretically.19,20,30−37 The time-dependent frequencies of OD modes are calculated by employing the method of wavelet analysis.36−40 The current study also provides detailed information about the density and orientational profiles, distributions of hydrogen bonds, diffusion, and orientational relaxation of interfacial molecules.

2. DETAILS OF SIMULATIONS We have performed ab initio molecular dynamics simulations of the liquid−liquid interfaces of an aqueous ionic solution and organic liquid at room temperature. The aqueous ionic system is a concentrated solution (3.9 M NaCl) with 15 Na+, 15 Cl−, and 191 water molecules. The other liquid phase contains 39 CCl4 molecules. The box lengths are determined from the number of molecules so as to be consistent with experimental densities of the ionic solution41 and the hydrophobic phase at standard condition (298 K, 1 atm). Then, classical molecular dynamics simulations were performed using classical models of water, ions, and carbon tetrachloride42−46 to generate the initial configurations for ab initio simulations. The simulation systems are periodically replicated in all three dimensions. After proper equilibration of the bulk liquids, the two simulation boxes are joined along the z-direction, and then the rectangular box is considered as the new simulation box for the next phase of the simulation run. Thus, dimensions of the new box are 18.41 Å along x and y directions and 36.82 Å along the z direction. The system was then re-equilibrated again by imposing periodic boundary conditions along all three dimensions. The final configuration of the classical run was used as the initial configuration for the ab initio molecular dynamics simulations. The ab initio molecular dynamics simulations were performed by employing the Car−Parrinello22,23 method and the CPMD code.47 The electronic structure of the extended system was represented by the Kohn−Sham (KS) formulation48 of density functional theory (DFT) within a plane wave basis. The Vanderbilt ultrasoft49 pseudopotentials were used to treat the core electrons. We note that the ultrasoft pseudopotentials allow a lower value of the energy cutoff in the basis set expansion. The plane wave expansion of the KS orbitals was truncated at a kinetic energy of 25 Ry as used in earlier studies.19,20 Like earlier ab initio simulation studies50 of aqueous interfacial systems, we have employed the BLYP51,52 functional for electronic structure calculations of the interfacial system. A fictitious mass of μ = 800 au was assigned to the electronic orbitals, and the coupled equations of motion describing the system dynamics were numerically solved by using a time step of 5 au The hydrogen atoms were assigned the mass of deuterium which ensured electronic adiabaticity and energy conservation during the simulations. We equilibrated the system through ab initio molecular dynamics for 10 ps in a canonical ensemble at 298 K by using the Nosé−Hoover chain method.53 Then, we continued the run in the microcanonical ensemble for another 25 ps for calculations of various equilibrium and dynamical quantities.

Figure 1. (a) Density (g cm−3) profiles of water (black), CCl4 (green), Cl− (red), and Na+ in the bulk and liquid−liquid interfacial regions. (b) Density (g cm−3) profiles of water (black), CCl4 (green), Cl− (red), and Na+ in the bulk and liquid−liquid interfacial regions from classical simulation.

around z = 0, which is considered as the center of the interfacial zone. It is notable that toward the center of the interfacial zone there occurs lowering in density for both the liquids, but it is not as sharp as found earlier for liquid−vapor interfaces.17−19 The interfacial region is defined by the change in total number density from 90% to 10% of the bulk liquid density as followed in previous work.17,50,54−58 The width of the interface for water is found to be 4.8 Å. The average bulk density of water is 0.95 g cm−3, and that of CCl4 is 1.5 g cm−3. The most interesting fact is that the chloride ions have higher propensity of staying in the interfacial region than sodium ions, which is in agreement with the earlier classical simulation results using polarizable models.11 This phenomenon can be explained with respect to polarizabilities of the ions. Sodium ions being “hard” with no valence electron stay out of the interface where as the “soft” polarizable chloride anions show a higher tendency of being at the interface. In Figure 1b the density profiles of water, CCl4, and the ions are shown from classical simulation. The classical simulation involved nonpolarizable force fields which have produced similar density profiles for the positive and negative ions at the interface. We next discuss the variation of hydrogen bond profiles in the bulk and interfacial regions. The present system exhibits two types of hydrogen bonds: water−water and water−chloride ion hydrogen bonds. A geometric criterion has been set to define these hydrogen bonds. A water pair is considered to be hydrogen bonded if their respective hydrogen−oxygen distance is less than 2.5 Å, and for chloride ion−water, the hydrogen and chloride ion distance should be less than 2.9 Å. In Figure 2, the hydrogen bond profiles of the system are shown. It is found

3. STRUCTURAL PROPERTIES 3.1. Density and Hydrogen Bond Profiles. An important property of an interfacial system is its inhomogeneous density. We have calculated the inhomogeneous density profile by analyzing the average number of molecules of a given species in the slabs of thickness Δz where the z coordinate is along the 23084



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3.3. Average Frequencies and Hydrogen Bonds. We have used the wavelet method38,39 of time series analysis to calculate time-dependent frequencies of OD stretch modes from the ab initio molecular dynamics trajectories. This method provides information regarding the frequency content of a fluctuating function over a time window. The details of this method for OD stretch frequency calculations have been described in previous work.36,37 We have calculated the average frequencies of the bulk OD groups and also of free OD groups at the interface. The hydrogen bonded water molecules that reside in the bulk water are found to be red-shifted with respect to the frequencies of free OD modes at the interface. The frequency shifts of the molecules are largely influenced by the surrounding hydrogen bonding states. The stronger the hydrogen bonding interaction, the higher is the red-shift in OD stretching frequency. The average stretch frequency of the OD modes in the bulk phase is 2382 cm−1, which is very close to the corresponding result for pure bulk water (2380 cm−1).36,37 The average frequency of the dangling OD modes at the interface is found to be 2560 cm−1. It may be noted that the water molecules at the interface can be in three different types of donor hydrogen bonding states: (i) water without any donor hydrogen bonds (ND), (ii) water with a single donor hydrogen bond and a free OD bond (SD), and (iii) water with double donor hydrogen bonds (DD). From Figure 4, it is clear

Figure 2. Number of different hydrogen bonds per molecule in different regions. The red and blue dashed lines indicate chloride ion− water hydrogen bonds per chloride ion and water−water hydrogen bonds per water molecule.

that at the interface reduction in number density results in the decrease of the number of hydrogen bonds. Thus, the number of hydrogen bonds gradually decreases from bulk to the interface region. The average number of hydrogen bonds per water molecule is found to be 3.0 and 2.4 in the bulk and interfacial regions, respectively. 3.2. Molecular Orientation. The orientation of water molecules is expressed in terms of the orientational distribution of the angle (θ) that the molecular dipole aligns with the surface normal along the z-axis. In Figure 3, we have

Figure 3. Probability distributions of the orientation of water dipole (blue solid) and OD vectors (red dashed) in the interfacial region.

represented the orientational distributions of water dipoles at the interface. A uniform distribution is observed for bulk water, meaning no preferred orientation in the bulk phase as expected. However, a maximum is found at around cos θ = −0.48 for the interfacial molecules which means a preferred orientational order. The molecules at the interface have a tendency to orient their dipoles by an angle of about 115° with the surface normal. We have also determined the angular distribution of OD vectors with the surface normal which reveals two maxima at around θ = 60° and 180°. So, from these results, we can conclude that the first type of OD modes point toward the CCl4 side which are mostly dangling modes and the second type points toward the bulk side to participate in the formation of hydrogen bonds. We also note that the angular distribution curves have rather broad peaks which mean the OD modes can have a rather wide range of orientations apart from the most probable ones.

Figure 4. Population distributions of water molecules in different hydrogen bonding states in the (a) bulk and (b) at the interface.

that single donor (SD) water molecules occupy the maximum population (56%) at the interface. Besides SD population, 39% of the molecules are found in the double donor (DD) hydrogen bonding state, and very few water molecules (5%) also exist with both OD modes dangling. The probabilities for SD, DD, and ND in the bulk phase are 0.33, 0.66, and 0.01, respectively. The difference in hydrogen bonding environment of water 23085



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interface.15 The variation of dipole moment of CCl4 in the interfacial and bulk regions is shown in Figure 7.

clearly reveals a very different nature of the interfacial and bulk regions.

4. ELECTRONIC PROPERTIES We have calculated the dipole moments of water and CCl4 molecules from locations of the nuclear charges and distributions of the electronic charges. We have used the method of maximally localized Wannier functions59,60 to calculate the molecular charge distributions. The average molecular dipole moment of a water molecule is shown as a function of interfacial depth in Figure 5. The average molecular

Figure 7. Variation of the average dipole moment of CCl4 molecules with position along the z-direction.

5. DYNAMICAL PROPERTIES 5.1. Diffusion. The translational diffusion of the constituent molecules is one of the important dynamical properties to be looked at for an inhomogeneous interfacial system. We have calculated the diffusion coefficient from the mean-square displacement which increases linearly with time at long times. Since the molecules at the interface do not remain continuously at the interface, we define Pc(t) as the survival probability62 for a molecule to remain in the interfacial region continuously from time t = 0 to t. The quantity ⟨Δx(t)2⟩i is defined as the meansquare displacement (MSD) along the x-direction which is averaged over the particles which remain in the interfacial region from t = 0 to t. The MSD of the interfacial molecules is related to the diffusion coefficient in the long time limit by the following equation:62

Figure 5. Variation of the average dipole moment of water molecules with position along the z-direction.

dipole moment of water is found to be around 2.85 D in the bulk region, and it reduces to a value around 2.1 D in the interfacial region. A similar qualitative trend was observed in the study of liquid−vapor interface using first-principles simulations.50 The current study reveals that the presence of ions does not affect the dipole moment of water profoundly in the bulk. In Figure 6 the variation of dipole moment of Cl− ion

⟨Δx(t )2 ⟩i = 2Pc(t )Dx (I )t

(1)

Therefore, the parallel diffusion coefficient of the interfacial molecules along the x and y directions is given by Dx , y(I ) = lim

t →∞

⟨Δx(t )2 ⟩i + ⟨Δy(t )2 ⟩i 4tPc(t )

(2)

Figure 8 shows the MSD of water molecules in the bulk and interfacial regions. The diffusion coefficients are calculated from the slopes of the curves in the long time region for the bulk and interfacial regions, and the respective values are found to be 0.42 × 10−5 and 0.96 × 10−5 cm2 s−1 for water molecules. The diffusion coefficient of chloride ion is 0.2 × 10−5 and 0.35 × 10−5 cm2 s−1 at the bulk and interface. The diffusion coefficient of sodium ion at the bulk is 0.12 × 10−5 cm2 s−1. It is clear that the interfacial molecules are more diffusive than the bulk molecules. This is due to the fact that at the interface the molecules possess fewer number of hydrogen bonds and reduced collisional effects than with the bulk water molecules. 5.2. Orientational Relaxation. The influence of hydrogen bond environment on the single particle rotational motion of water is investigated by calculating the orientational time correlation function. The orientational time correlation function Cl(t) is given by

Figure 6. Variation of the average dipole moment of chloride ions with position along the z-direction.

is shown. The average dipole moment in the bulk is 0.7 D, and it increases to a value 1.2 D at the interface. This can be explained by the unsymmetrical solvation of chloride ion at the interface. Thus, the net water dipole surrounding the chloride ion polarizes the ion and causes a slight increase in the dipole moment at the interface with respect to homogeneously solvated chloride ion in bulk. The average bulk dipole moment of chloride ion is in agreement with earlier result.61 The presence of ions and water at the interface affects the dipole moment of CCl4. The local electric field induced by the D2O molecules and ions gives rise to a significant dipole moment of CCl4 at the interface. This was also observed in an earlier classical polarizable model based study of the liquid−liquid

Cl(t ) = 23086

⟨Pl(e (0). e (̂ t ))⟩ ̂ ⟨Pl(e (0) ⟩ ̂ ·e (0)) ̂

(3)



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interfacial water molecules are calculated by integrating Cμ2 (t) (COD 2 (t)), and the values are found to be 4.0 (4.35) and 3.15 (3.6) ps, respectively. It is found that at the interface water molecules can rotate at a faster rate compared to that of bulk molecules. This is due to fewer number of hydrogen bonds per molecule at the interface compared to that in the bulk, which makes the rotational motion of a water molecule at the interface less hindered compared to bulk. 5.3. Hydrogen Bond Dynamics. The relaxation of hydrogen bonds in the bulk and interfacial region are analyzed by using the so-called population correlation function approach.63−73 There are two types of hydrogen bonds in our system: water−water and chloride ion−water. The geometric criteria for these hydrogen bonds have been described in earlier section. Here, we have calculated the relaxation of these two types of hydrogen bonds. Following earlier work,63−73 we define a population variable h(t) which is unity when a water−water or ion−water pair is hydrogen bonded at time t and zero otherwise. The variable H(t) = 1 if a hydrogen bond exists continuously from time t = 0 to t, or it is zero otherwise. The continuous hydrogen bond time correlation function SHB(t) is defined as63−73

Figure 8. Time dependence of the mean-square displacement of bulk and interfacial water molecules.

where Pl is the Legendre polynomial of rank l and ê(t) denotes the unit vector along the dipole or the OD vector of a water molecule. The second-rank rotational function C2(t) directly reflects the experimentally measured rotational anisotropy. We have calculated the reorientational relaxation of the molecular dipole (Cμ2 (t)) and OD vector (COD 2 ) of water molecules. The orientational correlation function of the dipole and OD vectors of the bulk and interfacial water molecules are shown in Figure 9. The relaxation times for dipole (OD) vectors of the bulk and

SHB(t ) =

⟨h(0)H(t )⟩ ⟨h(0)2 ⟩

(4)

where ⟨...⟩ denotes an average over all the pairs of a given type. The correlation function SHB(t) gives the probability that a hydrogen bonded pair remains bonded at all times from time t = 0 up to t. The corresponding integrated relaxation time τHB measures the average lifetime of a hydrogen bond of that particular pair type. In Figure 10, the time dependence of SHB(t) of different types of hydrogen bonds is shown for the bulk and interfacial regions. In the bulk phase, the calculated lifetime of chloride ion−water hydrogen bond is 4.27 ps and that at the interface is 3.52 ps. The lifetime of a hydrogen bond between two water molecules in the two regions are found to be 2.71 and 2.25 ps, respectively. It is found that the hydrogen bonds at the interfaces have a shorter lifetime than those in the bulk phase. We have also studied the dynamics of dangling OD bonds by calculating the dangling OD correlation function SDH(t) which gives the probability that an initially non-hydrogen-bonded OD group remains dangling at all times up to t. The function provides information on the hydrogen bond re-formation dynamics. The results of SDH(t) are shown in Figure 10. The dangling lifetime of OD modes in bulk phase is found to be 0.22 ps, but it increases to 1.92 ps at the interface. This may be attributed to the lower density of water at the interface which makes an initially dangling OD mode to remain dangling for a longer time. The strong cooperative effects in the bulk region clearly influence the hydrogen bonds to undergo faster breaking and re-formation. As a result, the lifetime of dangling OD modes is very short in the bulk water phase. The values of all the relaxation times are included in Table 1. 5.4. Vibrational Spectral Diffusion. In an aqueous solution, the solvation environment of water molecules changes with time due to thermal motion of surrounding molecules. As a result, the vibrational frequencies of water molecules also fluctuate, and the influence of the surrounding environmental fluctuations gets captured in the vibrational frequency fluctuations of water. The dynamics of vibrational frequency

Figure 9. Temporal relaxation of the water dipole and OD vector orientational time correlations for the bulk and interfacial molecules. 23087



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vibrational spectral diffusion essentially captures the dynamics of hydrogen bond fluctuations that spontaneously take place in the system due to thermal effects. In our present work, we have calculated the frequency correlation for the OD stretch modes of water, and the results are shown in Figure 11 for both bulk

Figure 11. Relaxation of the time correlation function of OD fluctuating frequencies of water molecules in the bulk (blue dashed) and interfacial regions (red dashed). The smooth gray lines represent the fits by a triexponential function. The resulting plots are averaged over the OD modes which are present in a given region.

and interfacial regions. We have fitted the decay of the frequency correlation functions to a triexponential function and obtained three relaxation times. The relaxation times and the weights are included in Table 2. The oscillation in the short Table 2. Relaxation Times and Weights of the Frequency Correlations of OD Modes of Water in the Bulk and Interfacial Regionsa Figure 10. Relaxation of the continuous hydrogen bond correlation function SHB(t) between (a) chloride ion and water molecules and (b) between two water molecules. The relaxation of the continuous dangling correlation function SDH(t) of OD bonds in the bulk and interfacial regions are shown in (c).

bulk

interface

water−water chloride−water

2.71 4.27

2.25 3.52

⟨δω(t )δω(0)⟩ δω(0)2

τ2 (ps)

τ3 (ps)

a1

a2

a3

0.13 0.09

3.15 2.50

5.25 4.20

0.55 0.61

0.21 0.23

0.24 0.11

The results are averaged over the OD modes which are present in the respective regions.

time basically comes from the underdamped motion of intact hydrogen bonded pairs. The existing long time decay captures the hydrogen bond dynamics, as the time scales of the long time decay agree well with our calculated lifetimes of water− water and chloride ion−water hydrogen bonds. The present calculations reveal that the average hydrogen bond lifetimes at the interface are slightly shorter than that of the bulk aqueous region. This result is different from earlier simulation studies of liquid−vapor interfaces using classical models.15,62,74−78 It was found that, due to lower number density and reduced cooperative effects between the molecules, hydrogen bonds have a slower dynamics at the interface.73,79 In order to understand the faster relaxation of hydrogen bonds at the interface, we have looked at the distribution of D···Cl distance between a water hydrogen and its nearest chlorine of CCl4 at the interface, and the results are shown in Figure 12. This figure clearly reveals a significant probability of finding a D···Cl pairs within a distance of about 3.0 Å, which is the typical cutoff distance for the formation of a hydrogen bond between a hydrogen atom of water and a charged chlorine atom.72 So, the partial charges on the chlorine atoms which arise due to higher electronegativity and higher polarizability of

fluctuations are calculated through the vibrational frequency time correlation function defined by Cω(t ) =

τ1 (ps)

a

Table 1. Hydrogen Bond Lifetimes (τHB) in ps type of hydrogen bond

region bulk interface

(5)

where δω(t) is the fluctuation from the average frequency at time t. The average of this equation is over those modes which are present in the respective regions. The time-dependent infrared spectroscopic studies look at the fluctuations of vibration frequencies with time which are caused by fluctuations in local solvation environment in bulk liquids.19,20,24−37 In an aqueous ionic solution, hydrogen bonds exist between the anions and water molecules and also between the water molecules themselves. Thus, the changes in solvation environment in an aqueous solution particularly involve the changes in hydrogen bonds. Hence, the dynamics of 23088



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shorter lifetime of interfacial hydrogen bonds compared to that of the bulk which differs from the results of earlier studies of similar interfaces using classical potential models.79 The slight acceleration of hydrogen bond dynamics is attributed to faster rotational and translational motion of interfacial molecules and also to the possibility of weak H···Cl hydrogen bonding interactions between water and CCl4 at the interface. The existence of such weak hydrogen bonding interactions, in addition to other hydrogen bonding interactions, adds to the cooperativity among interfacial molecules and facilitates hydrogen bond breaking and re-formation events.

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Figure 12. Probability distribution of the distance (RD···Cl) between D (of water) and its nearest Cl (of CCl4) at the interface.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.C.). the chlorine atoms promote a weak interaction between water and CCl4. The binding energy of H2O−CCl4 dimer is also calculated, and its value is 0.7 kcal/mol and that of water−water is 4.5 kcal/mol. So, it is clear that interaction between water and CCl4 is much weaker than the water−water hydrogen bonding interaction. The existence of weak interaction is also supported by the orientational profile of water that one of the OD bonds is pointing toward CCl4 which facilitates the formation of weak hydrogen bonds.

Notes

6. SUMMARY AND CONCLUSIONS We have presented a theoretical study of the structural and dynamical properties of liquid−liquid interface of a 3.9 M aqueous solution of NaCl and carbon tetrachloride from firstprinciples simulations without employing any empirical potential models. The present study reveals many interesting results regarding the difference in behavior of the bulk and interfacial molecules. The density profiles show a decrease of water and CCl4 density at the interface. The more polarizable chloride ions show a higher tendency of residing at the interface in comparison to the sodium ions. The high propensity can be attributed to asymmetric solvation which is favorable for the more polarizable ions at the interface. The lower density and reduced collisional effects allow the molecules to rotate more freely at the interface. The reduced density also leads to decrease in the number of hydrogen bonds at the interface. At the interface, the water dipoles prefer to align in directions perpendicular to the surface normal. The orientational distributions also reveal that one OD mode points toward the CCl4 side and the other OD mode points toward the inner side to form hydrogen bonds with other molecules on the bulk side. The analysis of electronic properties shows that the presence of water molecules and ions at the interface gives rise to a finite dipole moment of CCl4 at the interface. The dynamical aspects of the interfacial system have been investigated by studying the diffusion, orientational relaxation, dynamics of hydrogen bonds, and vibrational spectral diffusion. Because of diminished density and fewer number of hydrogen bonds at the interface, the molecules at the interface show faster diffusion and faster rotational relaxation in comparison to the bulk molecules. We have also studied hydrogen bond dynamics by using the population correlation function approach. The vibrational frequency fluctuations of water molecules are investigated through calculations of frequency time correlations. The long-time decay of the frequency fluctuations is found to have time scales that are close to the lifetimes of water−water and chloride ion−water hydrogen bonds in the bulk and interfacial regions. The present study reveals a slightly

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Department of Science and Technology (DST) and Council of Scientific and Industrial Research (CSIR), Government of India, for financial support. The authors are also thankful to the High Performance Computing Facility at the Computer Centre, IIT Kanpur, where part of the calculations was done. REFERENCES

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