Isotropic compounds

A real human shin-bone has a highly complex geometry. It consists of several types of substances, for instance, bone tissue and marrow, with very different mechanical properties. The substances are also inhomogeneous, anisotropic, and vast biological variations exist between different individuals. The inhomogeneities of the various substances cannot directly be eliminated in the research object. But when it comes to theoretical and mathematical considerations it is necessary to increase the level of idealization and construct a model object which fits into the theoretical framework. The inhomogeneities must in some way or other be reduced before it is possible to fit the shin-bone into a theoretical framework. In the example under consideration the research object was represented as a so-called Timoschenko beam. Only some of the constitutive properties of the shin-bone were then represented in the model object. They comprised, in the final definition, dynamic and structural properties of a rectilinear, twisted, non uniform Timoschenko beam which was made up of two linearly elastic and transversally isotropic compounds and one perfectly flexible compound (Thomsen 1990). 

Sohd rocket propellants represent a very special case of a particulate composite ia which inorganic propellant particles, about 75% by volume, are bound ia an organic matrix such as polyurethane. An essential requirement is that the composite be uniform to promote a steady burning reaction (1). Further examples of particulate composites are those with metal matrices and iaclude cermets, which consist of ceramic particles ia a metal matrix, and dispersion hardened alloys, ia which the particles may be metal oxides or intermetallic compounds with smaller diameters and lower volume fractions than those ia cermets (1). The general nature of particulate reinforcement is such that the resulting composite material is macroscopicaHy isotropic. 

In Raman spectroscopy the intensity of scattered radiation depends not only on the polarizability and concentration of the analyte molecules, but also on the optical properties of the sample and the adjustment of the instrument. Absolute Raman intensities are not, therefore, inherently a very accurate measure of concentration. These intensities are, of course, useful for quantification under well-defined experimental conditions and for well characterized samples otherwise relative intensities should be used instead. Raman bands of the major component, the solvent, or another component of known concentration can be used as internal standards. For isotropic phases, intensity ratios of Raman bands of the analyte and the reference compound depend linearly on the concentration ratio over a wide concentration range and are, therefore, very well-suited for quantification. Changes of temperature and the refractive index of the sample can, however, influence Raman intensities, and the band positions can be shifted by different solvation at higher concentrations or... 

Phase composition changes as a function of temperature are similar to these that were observed for the lithium-containing system (see Fig. 21), except that individual stable ammonium-cobalt-oxyfluoroniobate occurs prior to the formation of CoNbOF5 (Fig.21, curve 4). It is assumed that the composition of this intermediate phase, formed at 330-350°C, is (NH CoNbOFy [129]. Complete removal of ammonium occurs at about 400°C (Fig. 21, curve 5) and leads to the formation of CoNbOFs. The compound is defined as practically isotropic rose-colored ciystals with a refractive index of N = 1.500. The... 

Currently, theories are not yet able to predict the transition temperatures based on molecular structure of the constituent molecules. However, for several compounds there is considerable empirical data relating the transition temperature between isotropic and nematic phases (Tni) to molecular structure. Higher implies greater nematic stability. For example, it is... 

The first report on the liquid crystalline properties of these compounds was published by Gray and Mosley [44] in 1976. The series of 4 -n-alkyl-4-cyanobiphenyls (CBn) have been widely studied by different methods due to their readily accessible nematic ranges around room temperature. The compounds have the phase sequences crystal-nematic-isotropic for CBS, CBIO, and monotropic nematic for CBS, CB4 crystal-smectic A-nematic-isotropic for CB9 crystal-smectic A-isotropic for CBll. The lower homologous CB2 is nonmesogenic. The general chemical structure of the compounds CBn is presented in Fig. 1. 

In this section, we will describe the crystal structures of the mesogenic A -n-alkoxy-4-cyano-biphenyls (Fig. 1) [53-57]. The compounds have the phase sequences crystal-nematic-isotropic for CB05—CB07 and monotropic nematic for CB01-CB04 crystal-smectic A-nematic-isotropic for CB08. The crystal and molecular data of the investigated compounds CBOn are summarised in Table 2. 

Kurogoshi and Hori [ 104] determined the crystal structures of the mesogenic ethyl and butyl 4-[4-(4-n-octyloxybenzoyloxy)benzylidene]aminobenzoates. The compounds have different phase sequences crystal-smectic A-nematic-isotropic and crystal-smectic C-smectic A-nematic-isotropic for the ethyl and butyl compounds, respectively. Both compounds have layer structures in the solid phase. The butyl compound contains two crystallographically independent molecules. Within the layers, adjacent molecules are arranged alternately so as to cancel their longitudinal dipole moments with each other. In the ethyl compound the core moieties are almost perpendicular to the layer plane, while in the butyl compound these moieties are tilted in the layer. 

Both compound I and a mixture of the two derivatives M70 (70% 1 30% II) display the usual monolayer smectic Aj phase on cooling down from the isotropic liquid. At lower temperatures the particular smectic phases (X and X ) showing polar behaviour have been observed [78]. The smectic X phase in... 

The conventional liquid state is described as the isotropic phase. The temperature at which the compound passes from the solid phase into a mesophase is described as the melting point and the transition temperature between a mesophase and an isotropic liquid is described as the clearing point. 

The introduction of a second chiral atom in the system leads to a reduction in the mesogenic properties and only a monotropic chiral nematic transition is observed for compound 23. However, when this compound is cooled down from the isotropic liquid state at a cooling rate of 0.5 °Cmin , very unusual blue phases BP-III, BL-II and BP-I are observed in the range 103-88 °C. Blue phases usually require pitch values below 500 nm. Hence the pitch value of the cholesteric phase for 23 must be very short, suggesting that the packing of two chiral carbons forces a faster helical shift for successive molecules packed along the perpendicular to the director. 

Besides this, the remarkable properties of gold(I) compounds, which often give rise to aurophilic interactions and/or to luminescence, are of interest when these properties are transported into the liquid crystal field. Although there is much still to be studied, it is already clear that luminescence can survive in the condensed but mobile state of a mesophase, and even in the isotropic liquid state of a molten gold compound. It also seems that aurophilicity can contribute in some cases to the formation of mesophases. 

Conversely, in a membrane model, acetylcholine showed mean log P values very similar to those exhibited in water. This was due to the compound remaining in the vicinity of the polar phospholipid heads, but the disappearance of extended forms decreased the average log P value somewhat. This suggests that an anisotropic environment can heavily modify the conformational profile of a solute, thus selecting the conformational clusters more suitable for optimal interactions. In other words, isotropic media select the conformers, whereas anisotropic media select the conformational clusters. The difference in conformational behavior in isotropic versus anisotropic environments can be explained considering that the physicochemical effects induced by an isotropic medium are homogeneously uniform around the solute so that all conformers are equally influenced by them. In contrast, the physicochemical effects induced by an anisotropic medium are not homogeneously distributed and only some conformational clusters can adapt to them. 

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