Molecular liquids, bond orientational

Fig. 6.77. Calculations done using the statistical mechanical theory of electrolyte solutions. Probability density p(6,r) for molecular orientations of water molecules (tetrahedral symmetry) as a function of distance rfrom a neutral surface (distances are given in units of solvent diameter d = 0.28 nm) (a) 60H OH bond orientation and (b) dipolar orientation, (c) Ice-like arrangement found to dominate the liquid structure of water models at uncharged surfaces. The arrows point from oxygen to hydrogen of the same molecule. The peaks at 180 and 70° in p(0OH,r) for the contact layer correspond to the one hydrogen bond directed into the surface and the three directed to the adjacent solvent layer, respectively, in (c). (Reprinted from G. M. Tome and G. N. Patey, ElectrocNm. Acta 36 1677, copyright 1991, Figs. 1 and 2, with permission from Elsevier Science.

The orientation of bonds is strongly affected by local molecular motions, and orientation CF reflect local dynamics in a very sensitive way. However, the interpretation of multimolecular orientation CF requires the knowledge of dynamic and static correlations between particles. Even in simple liquids this problem is not completely elucidated. In the case of polymers, the situation is even more difficult since particules i and j, which are monomers or parts of monomers may belong to the same chain or to different Chains. Thus, we believe that the molecular interpretation of monomolecular orientation experiments in polymer melts is easier, at least in the present early stage of study. Experimentally, the OACF never appears as the complicated nonseparated function of time and orientation given in expression (3), but only as correlation functions of spherical harmonics... 

Figure 3. (a) Two-dimensional, bond orientational order parameter average values in the molecular fluid layers of LI ecu confined in a multi-walled carbon nanotube of diameter D=9norder parameter values for the contact, second, third and fourth layers, respectively. The dotted line represents the bulk solid-fluid transition temperature, (b) Positional and orientational pair correlation functions in the unwraiqred contact layer of U CCU confined in a multi-walled carbon nanotube of diameter D=9.1< (5 nm) showing liquid phase at 7=262 K and crystal phase at 7=252 K. 

In this chapter, the results of the molecular-dynamies simulation of Ni, Cu and Au in liquid and supercooled liquid states are displayed. The potentials of interatomie interaction within the framework of the embedded-atom method are used to generate realistic atomic configurations. The struetural analysis of the eluster stmeture has been conducted with the use of the bond orientational order parameter of the interatomic bonds. It is shown that the loeal ieosahedral order is present and it enhances at supercooling of the melts of the metals imder discussion. 

FIGURE 1.5. Local two-dimensional lattice for (a) a liquid and (b) for a hexatic phase with bond-orientational order. The molecular centers of mass are denoted by dots. 

Smectic liquid crystals are distinguished by having an intermediate degree of positional order in addition to molecular orientational and, in some cases, bond orientational order. 

We observed an extraordinary enhancement in bond orientational order, in ultrathin films deposited below their glass transition temperature, as revealed by extraordinary high values of the dielectric strength with respect to the bulk [1]. By varying the deposition conditions and the molecular size, we could tune the kinetic stability of the liquid phase enriched in bond orientational order (MROL) towards conversion into the ordinary liquid phase. 

Upon cooling, the molecules in proximity of the LFSs assume a correlated orientation, forming regions characterized by medium range order. These regions, called medium range crystalline ordered regions (MCRO), are surrounded by amorphous liquid. The extent of molecular orientation within the MRCOs is quantified by the bond orientational order parameter, for which the hexatic form described by Eq (1) is assumed. 

The range of systems that have been studied by force field methods is extremely varied. Some force fields liave been developed to study just one atomic or molecular sp>ecies under a wider range of conditions. For example, the chlorine model of Rodger, Stone and TUdesley [Rodger et al 1988] can be used to study the solid, liquid and gaseous phases. This is an anisotropic site model, in which the interaction between a pair of sites on two molecules dep>ends not only upon the separation between the sites (as in an isotropic model such as the Lennard-Jones model) but also upon the orientation of the site-site vector with resp>ect to the bond vectors of the two molecules. The model includes an electrostatic component which contciins dipwle-dipole, dipole-quadrupole and quadrupole-quadrupole terms, and the van der Waals contribution is modelled using a Buckingham-like function. 

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