Molecular arrangement in liquids

E. MOLECULAR ARRANGEMENT IN LIQUIDS AND THE INVESTIGATION OF VERY MINUTE CRYSTALS... 

X-ray scattering experiments with water, for which we are indebted chiefly to Stewart and Meyer,and whose results are given in Table 70, have furnished the basis for an exact treatment of this problem. The subject has been pursued recently by Bernal and Fowler in particular, who endeavored to design from the structure of ice (see page 134), from the x-ray diagram of water mentioned above and from the known data for H2O molecule, a pattern of the molecular arrangement in liquid water the which would represent quantitatively as far as possible all known properties of this substance. 

Having described our present knowledge of molecular arrangement in liquids, we shall now discuss a few data on so-called liquid crystals or mesophases. The existence of such systems was discovered by 0. Leh-... 

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...

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-... 

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. 

Abstract The preparation of chiral functional materials with new, improved, and interesting properties is aided tremendously by control of the spatial arrangement of the functional units within them. The use of non-covalent interactions is absolutely critical in this regard, and the molecular-supramolecular balance has to be strictly controlled. The conducting, magnetic and optical properties of chiral materials whose function is profoundly influenced by supramolecular chemistry will be reviewed. Special emphasis is placed on the control of helical arrangements in liquid crystalline systems, in which both chiral induction and spontaneous resolution are important phenomena which can be controlled. 

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.

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. 

The study of molecular interactions in liquid mixtures is of considerable importance in the elucidation of the structural properties of molecules. Interactions between molecules influence the structural arrangement and shape of molecules. Dielectric relaxation of polar molecules in non-polar solvents using microwave absorption has been widely employed to study molecular structures and molecular interactions in liquid mixtures [81]. Ever since Lagemann and Dunbar developed a US velocity approach for the qualitative determination of the degree of association in liquids [82], a number of scientists have used ultrasonic waves of low amplitude to investigate the nature of molecular interactions and the physico-chemical behaviour of pure liquids and binary, ternary and quaternary liquid mixtures, and found complex formation to occur if the observed values of excess parameters (e.g. excess adiabatic compressibility, intermolecular free length or volume) are negative. These parameters can be calculated from those for ultrasonic velocity (c) and density (p). Thus,... 

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. 

Figure 10.2. Molecular arrangements in nematic and smectic-A liquid-crystalline phases. (From de Gennes and Frost, Copyright 1993, by Oxford University Press, Inc. Used by permission of Oxford...

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