| Collective and single-molecule relaxation dynamics |
We have investigated collective and single-molecule relaxation dynamics.
Effects of interfaces on structure and dynamics of water droplets on a graphene surface: A molecular dynamics study
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.
Maurya, Metya, Singh, & Saito, J.Chem.Phys., 154, 164704 (2021).
Molecular mechanism of acceleration and retardation of collective orientation relaxation of water molecules in aqueous solutions
The collective orientation relaxation (COR) of water molecules in aqueous solutions is faster or slower with an increase in the concentration of the solutions than that in pure water; for example, acceleration (deceleration) of the COR is observed in a solution of sodium chloride (tetramethylammonium chloride) with increasing concentration. However, the molecular mechanism of the solution and concentration dependence of the relaxation time of the COR has not yet been clarified. We theoretically investigate the concentration dependence of the COR of water molecules in solutions of tetramethylammonium chloride (TMACl), guanidinium chloride (GdmCl), and sodium chloride (NaCl). Based on the Mori-Zwanzig equation, we identify two opposing factors that determine the COR of water molecules in any aqueous solution: the correlation of dipole moments and the single-molecule orientation relaxation. We reveal the molecular mechanism of the concentration dependence of the relaxation time of the COR in the TMACl, GdmCl, and NaCl solutions in terms of these two factors.
Moritsugu, Nara, Koda, Tominaga, & Saito, J.Phys.Chem.B, 124, 11730 (2020).
Dynamics and relaxation of an intermediate size water cluster (H2O)108
The potential surface, melting, surface structure, and hydrogen bond network of an intermediate size water cluster (H2O)108 are investigated. The orientation relaxations of single molecule and of collective molecules are analyzed and compared with those of liquid water. The collective orientation relaxation (COR) (i.e., dielectric relaxation) of the water cluster is found to be much faster than that of liquid water due to different boundary conditions. In both liquid and cluster, the cross correlation between individual molecular dipoles plays an important role in static and dynamic quantities. COR of the cluster yields a so-called 1/f fluctuation in contrast to the well-known Debye relaxation in liquid water. In order to understand these differences of COR between the water cluster and liquid water, the wave vector dependence of the transverse and longitudinal components of COR is examined. A surface effect on hydrogen bond network and the correlation between structural change and coordination number are analyzed.
Saito & Ohmine, J.Chem.Phys, 101, 6063 (1994).