Liquids molecular arrangement

In liquids, molecular arrangements change with time, and hence some functions can be devised to describe this dynamic behavior and to supplement the instantaneous or averaged information supplied by radial distribution functions. The essential idea in the constmction of these correlation functions is to consider the evolving distribution of some vectorial property as a function of time, with respect to a reference state the average value of the dot product between the actual distribution and the reference state must be close to unity for a perfect correlation, and decays to zero for complete loss of coherence. For example, consider an ensemble of N planar molecules and the unit vector, ut, perpendicular to the plane of the fc-th molecule. The correlation function for molecular orientation is given by ... 

One may now ask whether natural systems have the necessary structural evolution needed to incorporate high-performance properties. An attempt is made here to compare the structure of some of the advanced polymers with a few natural polymers. Figure 1 gives the cross-sectional microstructure of a liquid crystalline (LC) copolyester, an advanced polymer with high-performance applications [33]. A hierarchically ordered arrangement of fibrils can be seen. This is compared with the microstructure of a tendon [5] (Fig. 2). The complexity and higher order of molecular arrangement of natural materi-... 

In 1978, Bryan [11] reported on crystal structure precursors of liquid crystalline phases and their implications for the molecular arrangement in the mesophase. In this work he presented classical nematogenic precursors, where the molecules in the crystalline state form imbricated packing, and non-classical ones with cross-sheet structures. The crystalline-nematic phase transition was called displacive. The displacive type of transition involves comparatively limited displacements of the molecules from the positions which they occupy with respect to their nearest neighbours in the crystal. In most cases, smectic precursors form layered structures. The crystalline-smectic phase transition was called reconstitutive because the molecular arrangement in the crystalline state must alter in a more pronounced fashion in order to achieve the mesophase arrangement [12]. 

Novotny et al. [41] used p-polarized reflection and modulated polarization infrared spectroscopy to examine the conformation of 1 -1,000 nm thick liquid polyperfluoropropy-lene oxide (PPFPO) on various solid surfaces, such as gold, silver, and silica surfaces. They found that the peak frequencies and relative intensities in the vibration spectra from thin polymer films were different from those from the bulk, suggesting that the molecular arrangement in the polymer hlms deviated from the bulk conformation. A two-layer model has been proposed where the hlms are composed of interfacial and bulk layers. The interfacial layer, with a thickness of 1-2 monolayers, has the molecular chains preferentially extended along the surface while the second layer above exhibits a normal bulk polymer conformation. 

Fig. 5 Schematic representation of the molecular arrangement in A main-chain and B side-chain liquid crystal polymers...

Langmuir, in 1917, constructed the film balance for the measurement of the surface or spreading pressure. Thus, it became possible to experimentally observe that adsorbed films pass through several states of molecular arrangement. The various states resemble that of a two-dimensional gas, a low-density liquid, and finally a higher density or condensed-liquid state. In the latter case the spreading pressure can be described by the linear relationship,... 

For mixtures of lecithin plus Na cholate it appears possible to infer the molecular arrangement in the dispersed micelles from the most likely structure of the liquid crystalline phase suggested by x-ray analysis. However, there are cases where dispersion is not possible because neither component is sufficiently hydrophilic to be dispersed even when alone in water. This is shown by the association of cholesterol and lecithin in the presence of water. The ternary diagram of Figure 4 is relative to these systems. Here only the lamellar liquid crystalline phase is obtained (region 1< in Figure 4). This phase is already given by lecithin alone, which can absorb up to 55% water. Cholesterol can be incorporated within this lamellar phase up to the proportion of one molecule of choles-... 

Because the molecules in a liquid move freely, a simple stir with a spoon causes the molecules to mix and rearrange. But mixing doesn t rearrange the molecules to create a new substance. Mixing doesn t destroy the molecular arrangement of the liquid. The liquid remains a liquid. 

Some liquids can be supercooled when the temperature is decreased. It is easily supposed that a liquid-solid equilibrium might be achieved quickly when the molecular arrangement of a liquid is similar to that in the solid, which should be in equilibrium with the liquid at the... 

It shall be assumed in this chapter that molecular arrangement in the bulk of solid explosives, and all amorphous and liquid explosives, has no preferred orientation direction. The diffraction patterns in this case are isotropic around the primary X-ray beam, and the vector quantity, x, can be replaced by its scalar magnitude. It is customary to speak of diffraction profiles, rather than patterns, when isotropy obtains and the diffraction profiles are derived by integration of the (circularly-symmetric) diffraction pattern over the azimuthal component of the scattering angle. 

Figure 6.10 Liquid-crystalline SPM 133 and its molecular arrangement in the unit cell [106], Reprinted with permission from O. Y. Mindyuk, M. R. Stetzer, P. A. Heiney, J. C. Nelson, J. S.

A pure liquid (say water), once solute is added acts as the solvent and provided only relatively small amounts of solute are added we create a so called ideal solution (Frames 32 and 33). The entropy is increased (i.e. 5ideai solution > Spure solvent) by the addition of solute (more randomness in molecular arrangement is created compared with the pure solvent). The chemical potential is lowered ... 

Van der Waals forces usually contribute significantly to the cohesion energies and interfacial energies of solids and liquids. In those cases, in which determining interactions occur at interatomic spacings, there is no doubt of their mechanical importance. However, there is still doubt about the best way to formulate and compute forces while incorporating details of molecular arrangement. 

Liquids are neither characterised by the random chaotic motion of molecules, which one find in gases, nor by the perfect order of moleculars arrangement in solids. They occupy an intermediary position where molecules are more disorderly than those of a solid, but much less disorderly than those of gases. Because of this fact the enthalpy change when a crystal melts is always positive and the corresponding entropy change is also positive. This implies that there is less of order when a crystal melts. The liquid is thus intermediate between the complete order of the crystalline state and the complete disorder of the gaseous state. Because of this fact, the development of a molecular theory for liquids has posed formidable difficulties. 

These states of matter are summarized in Figure 8-3. It is not meant to be exclusive, in that there are other molecular arrangements, such as those found in the liquid crystalline state, that are both interesting and of great technological importance. Eut that is beyond the scope of what we want to cover here. (However, you may want to read about the discovery of Kevlar which involves liq-... 

M. Vacatello, G. Avitabile, P. Corradini, and A. Tuzi, ]. Chem. Phys., 73, 543 (1980). A Computer Model of Molecular Arrangement in a n-Paraffinic Liquid. 

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