Effects of interfaces on structure and dynamics of water droplets on a graphene surface: A molecular dynamics study

Abstract

The structure and dynamics of water droplets on a bilayer graphene surface are investigated using molecular dynamics simulations. The effects of solid/water and air/water interfaces on the local structure of water droplets are analyzed in terms of the hydrogen bond distribution and tetrahedral order parameter. It is found that the local structure in the core region of a water droplet is similar to that in liquid water. On the other hand, the local structure of water molecules at the solid/water and air/water interfaces, referred to as the interface and surface regions, respectively, consists mainly of three-coordinated molecules that are greatly distorted from a tetrahedral structure. This study reveals that the dynamics in different regions of the water droplets affects the intermolecular vibrational density of states: It is found that in the surface and interface regions, the intensity of vibrational density of states at ∼50 cm−1 is enhanced, whereas those at ∼200 and ∼500 cm−1 are weakened and redshifted. These changes are attributed to the increase in the number of molecules having fewer hydrogen bonds in the interface and surface regions. Both single-molecule and collective orientation relaxations are also examined. Single-molecule orientation relaxation is found to be marginally slower than that in liquid water. On the other hand, the collective orientation relaxation of water droplets is found to be significantly faster than that of liquid water because of the destructive correlation of dipole moments in the droplets. The negative correlation between distinct dipole moments also yields a blueshifted libration peak in the absorption spectrum. It is also found that the water–graphene interaction affects the structure and dynamics of the water droplets, such as the local water structure, collective orientation relaxation, and the correlation between dipole moments. This study reveals that the water/solid and water/air interfaces strongly affect the structure and intermolecular dynamics of water droplets and suggests that the intermolecular dynamics, such as energy relaxation dynamics, in other systems with interfaces are different from those in liquid water.

Nanoconfinement effects on water in narrow graphene-based slit pores as revealed by THz spectroscopy

“…90 Whereas for water at hydrophobic surfaces the librational band is expected to blue-shift as a consequence of steric hindrance, 91 here both librational modes (L1 and L2) are found to be red-shifted with respect to those found in bulk water, as also observed experimentally for water confined in micelles 92 and for water droplets at the interface with a graphene surface. 93 Moreover, a population increase in the hard-librations and a loss in population of soft-librations at d int = 0.8 nm is highlighted by the steeper slope of the librational band 80 between 350 and 650 cm À1 with respect to the systems at d int = 1.0 and 1.4 nm, as observed in Fig. 2.…”

Anisotropic and Finite Effects on Intermolecular Vibration and Relaxation Dynamics: Low-Frequency Raman Spectroscopy of Water Film and Droplet on Graphene by Molecular Dynamics Simulations

“…More importantly, the cross term in the droplet is found to be dominant in the TCF and has a slow relaxation with τ = 22.1 ps (Table 1). We note that this behavior of the collective polarizability relaxation in the droplet is qualitatively different from that of the collective dipole relaxation, 38 which shows the negative correlation between distinct dipole moments and the resultant much faster decay than that of bulk water. Thus, the present result suggests that, unlike in the film and bulk, the cross correlation between distinct molecules plays an essential role in the collective polarizability relaxation in the droplet.…”

“…However, these spectral differences between the droplet and bulk are smaller than those between the film and bulk because the molecules in the core region, which are more than half of the molecules in the droplet, have similar structural and dynamical properties to those in the bulk. 38 In addition, the decomposition analyses for the TCF of the droplet (green lines in Figure 3 and Figure 4) show that the self term in eq 10 and the molecular and the molecular-induced cross terms in eq 11 yield the difference in spectral shape between the droplet and bulk, as seen in the results obtained in the film. Thus, we find that these spectral features observed in the film and droplet systems are caused by the changes in the orientation motion and the associated polarizability fluctuation of the interfacial water molecules whose anisotropic local environments differ from those in the bulk.…”

“…Figure 8b shows that the increased libration peak at 600−800 cm −1 is attributed to the perpendicular component. The further analyses indicate that the increased libration peak is due to the self term (Figure 9) of the orientation-ordered water molecules 26,38,59,60 on the graphene surface (Figure S4) that have a large polarizability fluctuation in the perpendicular component (Figure S5). Note that although a libration peak shift to a higher frequency has been observed in the theoretical far IR absorption spectra, the shift is due to the negative correlation between distinct dipole moments, 38 which is different from the mechanism of the spectral change found in the present Raman spectra.…”

“…The two implications are consistent with the vibrational density of states (vDOS) that has a redshifted libration band and an enhanced intermolecular HB stretching peak (Figure S1) and with the previous result. 38 On the other hand, the increased libration peak at 600−800 cm −1 is not found in the vDOS, implying that the spectral change is attributed to an increased polarizability fluctuation, not to the change in vibration motions due to the anisotropic interfacial environment. The polarizability fluctuation will be explained in section 3.3 as being related to the orientationordered water molecules on the graphene surface.…”