Supercooled, phases

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. 

The mesophase is locked-in indefinitely by simple cooling from the isotropic phase. Supercooling at Txn is from V-IO to 20 C. 

Bartell and co-workers have made significant progress by combining electron diffraction studies from beams of molecular clusters with molecular dynamics simulations [14, 51, 52]. Due to their small volumes, deep supercoolings can be attained in cluster beams however, the temperature is not easily controlled. The rapid nucleation that ensues can produce new phases not observed in the bulk [14]. Despite the concern about the appropriateness of the classic model for small clusters, its application appears to be valid in several cases [51]. 

The action of this and other anti-bumping devices e.g., minute carborundum chips) is dependent upon the fact that the transformation of a superheated liquid into the vapour will take place immediately if a vapour phase e.g., any inert gas) is introduced. The effect may be compared with that produced by the introduction of a small quantity of a solid phaM into a supercooled liquid, e.g., of ice into supercooled water. 

The separation of the solid phase does not occur readily with some liquid mixtures and supercooling is observed. Instead of an arrest in the cooling curve at /, the cooling continues along a continuation of c/ and then rises suddenly to meet the line f g which it subsequently follows (Fig. 1,13, 1, iii). The correct freezing point may be obtained by extrapolation of the two parts of the curve (as shown by the dotted line). To avoid supercooling, a few small crystals of the substance which should separate may be added (the process is called seeding ) these act as nuclei for crystallisation. 

The positional order of the molecules within the smectic layers disappears when the smectic B phase is heated to the smectic A phase. Likewise, the one-dimensional positional order of the smectic M phase is lost in the transition to the nematic phase. AH of the transitions given in this example are reversible upon heating and cooling they are therefore enantiotropic. When a given Hquid crystal phase can only be obtained by changing the temperature in one direction (ie, the mesophase occurs below the soHd to isotropic Hquid transition due to supercooling), then it is monotropic. An example of this is the smectic A phase of cholesteryl nonanoate [1182-66-7] (4), which occurs only if the chiral nematic phase is cooled (21). The transitions are aH reversible as long as crystals of the soHd phase do not form. 

Although 16 different crystalline modifications have been identified (24,25), the a-pentahydrate is the stable form below 48°C. Solutions of sodium thiosulfate in the absence of seed crystals can be easily supercooled below their normal crystallisation temperatures. The dotted line extension of the dihydrate phase in Figure 1 is an indication that, if supercooling takes place below this line, solutions normally giving the pentahydrate may form the dihydrate [36989-90-9] s1ste2id. 

Figure 9.2. Constitutional supercooling in alloy solidification (a) phase diagram (b) solute-enriched layer ahead of the solid/liquid interface (c) condition for a stable interface (d) condition...

States such as superheated liquid and supercooled vapour are known as metastable, they are not of themselves unstable, but become so on introduction of a small amount of the stable phase. 

Although Carnelley once thought he had been able to superheat ice ( hot ice ), it is almost certain that no solid can be maintained alone at a temperature higher than its melting-point. Tammann (Zeitschr. physik. Chem., 68, 257, 1910) finds, however, that a crystalline solid may, under certain circumstances, be superheated in the presence of its melt. This occurs when the supply of -heat to the crystal is sufficiently great in comparison with the linear velocity of crystallisation of the supercooled liquid (cf. Findlay Phase Hide). 

Figure 4.8 Heat capacity of glycerol as a function of temperature. The solid line indicates Cp,m for the liquid and glassy phase. The dashed line represents Cp m for the solid. The dotted line at the melting temperature of 291.05 K. indicates the change in heat capacity upon melting. A glass transition occurs in the supercooled liquid at approximately 185 K. The heat capacities of the solid and the glass approach one another as the temperature is lowered they are almost identical below 140 K.

Fe(CN)6]3-(aq) + 6 H20(1). substrate The chemical species on which an enzyme acts, superconductor An electronic conductor that conducts electricity with zero resistance. See also high-temperature superconductor. supercooled Refers to a liquid cooled to below its freezing point but not yet frozen, supercritical fluid A fluid phase of a substance above its critical temperature and critical pressure. supercritical Having a mass greater than the critical mass. 

Similar methods to the above can be used to determine the 7-weight of a system composed of N molecules of vapor and N molecules of supercooled liquid. In this case, however, an exceedingly important difference arises when we consider the number of ways of arranging the molecules which make up the liquid phase. 

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