Hydrogen bonded structure and dynamics of liquid-vapor interface of water-ammonia mixture: An ab initio molecular dynamics study

Abstract

We have carried out ab initio molecular dynamics simulations of a liquid-vapor interfacial system consisting of a mixture of water and ammonia molecules. We have made a detailed analysis of the structural and dynamical properties of the bulk and interfacial regions of the mixture. Among structural properties, we have looked at the inhomogeneous density profiles of water and ammonia molecules, hydrogen bond distributions, orientational profiles, and also vibrational frequency distributions of bulk and interfacial molecules. It is found that the interfacial molecules show preference for specific orientations so as to form water-ammonia hydrogen bonds at the interface with ammonia as the acceptor. The structure of the system is also investigated in terms of inter-atomic voids present in the system. Among the dynamical properties, we have calculated the diffusion, orientational relaxation, hydrogen bond dynamics, and vibrational spectral diffusion in bulk and interfacial regions. It is found that the diffusion and orientation relaxation of the interfacial molecules are faster than those of the bulk. However, the hydrogen bond lifetimes are longer at the interface which can be correlated with the time scales found from the decay of frequency time correlations.

A first principles simulation study of vibrational spectral diffusion in aqueous NaBr solutions: Dynamics of water in ion hydration shells

“…[32], the inverse of the scale factor a is proportional to the frequency and thus the wavelet transform at each b gives the frequency content of f ðtÞ over a time window about b. Following our recent work on other aqueous systems [26][27][28][29]40], the time dependent function f ðtÞ for a given OD bond is constructed to be a complex function with its real and imaginary parts corresponding to the instantaneous fluctuations in OD distance and the corresponding momentum along the OD bond at time t. The stretch frequency of this bond at a given time t ¼ b is then determined from the scale a that maximizes the modulus of the corresponding wavelet transform at b and the process is then repeated for all the OD bonds that are present in the current simulation systems.…”

“…Since a bi-or triexponential fit does not reproduce the small oscillation or bump that is found at very short time, we used the following function including a damped oscillatory function to fit the calculated results of spectral diffusion [22,23,[26][27][28][29] f ðtÞ ¼ a 0 cosðx S tÞe…”

“…The temporal evolution of such frequency fluctuations, known as vibrational spectral diffusion, has been studied in many time dependent vibrational spectroscopic experiments to unearth the dynamics of hydrogen bond fluctuations in water and aqueous solutions [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. There have also been a number of theoretical studies on vibrational spectral diffusion in aqueous media [20][21][22][23][24][25][26][27][28][29] and all these studies have provided much detailed information on the dynamical fluctuations in these systems at molecular level.…”

“…On the theoretical side, the method of classical molecular dynamics simulation was employed to calculate the frequency fluctuations in aqueous NaI and NaBr solutions [23,25]. Very recently, theoretical studies were also conducted that went beyond the use of classical models and looked at the dynamics of spectral diffusion in water, supercritical water and aqueous solutions from first principles without using any empirical models [26][27][28][29]. In the latter studies, the method of ab initio molecular dynamics [30,31] was employed for the generation of dynamical trajectories and the fluctuating frequencies were calculated directly from the dynamical trajectories through a time series analysis [32].…”

Pressure and temperature dependence on the hydrogen bonding and dynamics of ammonium ion in liquid water: A molecular dynamics simulations study

“…To calculate the hydrogen bond properties and dynamics of NH 4 + -water and water-water hydrogen bonds, we use a set of geometric criteria [33][34][35][36][37][38][39][40][41][42][43][44][45] where it is assumed that a hydrogen bond between two species exists, if the following distance and angular criteria are satisfied, i.e.,…”