Phase liquid melt

Ordered dialkoxy PPV derivative has been prepared by Yoshino et al. [491. oly(2 -nonoyloxy-1,4-phenylene vinylene) 27a forms a nematic liquid-crystalline phase upon melting. The material retains its order upon cooling to room temperature, and its band gap (2.08 eV) is measurably smaller than in an unoricnted sample. Oriented electroluminescence may be achieved by rubbing a thin fdin of the material to induce molecular orientation [50],... 

Eutectic It is a mixture of two or more substances that solidifies as a whole when cooled from the liquid state, without changing composition. It is the composition within any system of two or more crystalline phases that melts completely at the minimum temperature. 

On heating from a crystalline phase, DOBAMBC melts to form a SmC phase, which exists as the thermodynamic minimum structure between 76 and 95°C. At 95°C a thermotropic transition to the SmA phase occurs. Finally, the system clears to the isotropic liquid phase at 117°C. On cooling, the SmC phase supercools into the temperature range where the crystalline solid is more stable (a common occurrence). In fact, at 63°C a new smectic phase (the SmF) appears. This phase is metastable with respect to the crystalline solid such phases are termed monotropic, while thermodynamically stable phases are termed enantiotropic. The kinetic stability of monotropic LC phases is dependent upon purity of the sample and other conditions such as the cooling rate. However, the appearance of monotropic phases is typically reproducible and is often reported in the phase sequence on cooling. It is assumed that phases appearing on heating a sample are enantiotropic. 

An example of a binary eutectic system AB is shown in Figure 15.3a where the eutectic is the mixture of components that has the lowest crystallisation temperature in the system. When a melt at X is cooled along XZ, crystals, theoretically of pure B, will start to be deposited at point Y. On further cooling, more crystals of pure component B will be deposited until, at the eutectic point E, the system solidifies completely. At Z, the crystals C are of pure B and the liquid L is a mixture of A and B where the mass proportion of solid phase (crystal) to liquid phase (residual melt) is given by ratio of the lengths LZ to CZ a relationship known as the lever arm rule. Mixtures represented by points above AE perform in a similar way, although here the crystals are of pure A. A liquid of the eutectic composition, cooled to the eutectic temperature, crystallises with unchanged composition and continues to deposit crystals until the whole system solidifies. Whilst a eutectic has a fixed composition, it is not a chemical compound, but is simply a physical mixture of the individual components, as may often be visible under a low-power microscope. 

Application of the ignition stimulus (such as a spark or flame) initiates a complex sequence of events in the composition. The solid components may undergo crystalline phase transitions, melting, boiling, and decomposition. Liquid and vapor phases may be formed, and a chemical reaction will eventually occur at the surface... 

The diagram shows the homogeneous liquid (melt) region (with/= 2) as well as the three distinct two-phase regions (each with/= 1). The junction of these regions is the eutectic point (triangle), a triple point (with/= 0). 

The successful conversion of graphite to diamond involves crystallizing the diamond from a liquid melt. The solvent most often used is nickel metal, or alloys of nickel with other ferrous metals. The reason for this success can be seen by referring to Figure 15.7, the binary (solid + liquid) phase diagram for (nickel + carbon).u8 We note from the figure that (Ni + C) forms a simple... 

About 14 g of choline chloride are stirred with a solution of about 20 g of phosgene in 100 g of chloroform for about two hours at room temperature. The mixture becomes a two-phase liquid mixture. Hydrochloric acid and excess phosgene are removed by distillation in vacuo. Chloroform is added to the syrup, and the mixture is then added to a solution of excess ammonia in chloroform which was cooled with solid carbon dioxide-acetone. The mixture is filtered, and the solid is extracted with hot absolute alcohol. The solid in the alcoholic solution is precipitated with ether, and filtered. It is recrystallized from a methyl alcohol-ether mixture the carbaminoyl-choline chloride obtained has a melting point of about 208°-210°C. 

BASF s original acid scavenger, triethylamine, created viscosity and work-up problems because of the need to maintain anhydrous conditions. BASF s use of 1-methylimidazole instead of triethylamine, at their reaction temperature of 80°C, led to the formation of two liquid phases, an upper diethoxyphenylphosphine phase, and a lower methylimidazolium chloride phase (fortuitously, this ionic liquid melts at 75°C ). Moreover, methylimidazolium chloride proved to be a nucleophilic catalyst. The processing revolution generated by these discoveries enabled creation of a high-productivity continuous process for diethoxyphenylphosphine manufacture and created a whole new business in acid scavenging technology. 

Fig. XVI-2.—Gibbs free energy as function of temperature, for simplified model of solid and liquid, illustrating change of phase on melting.

The solid white form really is only in a state of false equilibrium, being unstable with respect to the polymerised forms at all realisable temperatures. There are also the other forms—red, scarlet and black phosphorus—the behaviour of which under definite conditions of pressure and temperature cannot be stated with any certainty. Further, the melting-point even of the well-crystallised white phosphorus can be made fcto vary under certain conditions (see p. 15). In fact, all the condensed phases, liquid and solid, behave as mixtures rather than as single pure substances. 

Molecules are finite figures with at least one singular point in their symmetry description. Thus, point groups are applicable to them. There is no inherent limitation on the available symmetries for molecules. Molecules in the gas phase are considered to be free. They are so far apart that they are unperturbed by interactions from other molecules and thus can be considered isolated from each other. On the other hand, intermolecular interactions may occur between the molecules in condensed phases, i.e., in liquids, melts, amorphous solids, or crystals. In the present discussion all molecules will be assumed unperturbed by their environment, regardless of the phase or state of matter in which they exist. 

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