Discontinuous melting

Figure 5.31 Concentration dependence of CD spectra at 205 nm (top) and specific heat (bottom) of DCg.gPC tubules prepared in ethanol-water (70 30) at lipid concentrations of (a) 0.2, (b) 0.5, (c) 1.0, (d) 2.0, and (e) 5.0 mg/ml. These results show crossover from continuous to discontinuous melting as lipid concentration is increased. Reprinted with permission from Ref. 137. Copyright 1997 by the American Chemical Society.

With a discontinuous melt, the glass strand is stopped by a suitable fixture after completion of the casting process. 

In the discontinuous melting process, the individual manufacturing steps previously described do not take place at different locations, but are carried out one after the other. The batch is placed into the empty melting tank with a shovel and melted by arch heating as well as by electrical bot-... 

Fig. 4.21. Side view of the manufacturing facility for the 8-m mirror substrates Discontinuous Melting...

The nature of melting for argon films has also been studied and discussed. Evidence for continuous melting has been obtained from neutron diffraction studies [120-121], from heat capacity measurements [117], and from x-ray diffraction techniques [64,97]. Adsorption isotherms data [62] and LEED results [125,126] indicate a rather sharp transition. Some computer simulations have been interpreted in terms of continuous melting [131], whereas others [132,133] have found a first-order behavior (discontinuous melting). Finally, heat capacity measurement indicates that the melting transition is weakly first order, occurring at a triple line temperature [63]. 

Although there have been few studies about the order or phase transitions, indications of a discontinuous melting in monolayer films have been obtained from heat capacity measurements [12]. 

The heat capacity can be computed by examining the vibrational motion of the atoms and rotational degrees of freedom. There is a discontinuous change in heat capacity upon melting. Thus, different algorithms are used for solid-and liquid-phase heat capacities. These algorithms assume different amounts of freedom of motion. 

To 9 g of n-butyllithium in 200 ml of dry ether 20 g of 3-bromopyridine Is added as quickly as possible at -40°C without raising the temperature. When the addition Is finished the mixture is stirred for another 30 minutes. Thereafter 32.5 g of 0>dimethylamino-4 -bromopro-piophenone is added in such a way that the temperature does not exceed -40°C. The cooling is discontinued and the mixture is stirred during the night whereupon the reaction mixture is poured onto ice and diluted HCI, which is washed with ether and is extracted with 20 ml of methylene dichloride. The methylene dichloride is dried and evaporated. The crystals are dissolved in water, which then is made alkaline with a solution of Na2C03, is extracted with ether, dried, and evaporated and recrystallized from isopropyl ether, petroleum ether 1 1. Yield 4 g of 1 -(4 -bromophenyl)-3-(N,N-dimethylamino)-1 -(3"-pyridyl)-propanol. Melting point 67°C. 

Cyclic conjugation is continuous in o-benzoquinone and discontinuous in p-benzoquinone (Scheme 15, cf. Scheme 4). The donors (the C=C bonds) are on one side of the cyclic chain and the acceptors (the C=0 bonds) are on the other side in o-benzoquinone. In p-benzoquinone the donors and the acceptors are alternatively disposed along the chain. The thermodynamic stability of o-benzo-quinone is under control of the orbital phase property. The continuity conditions are not satisfied. o-Benzoquinone is antiaromatic. The thermodynamic stability of p-benzoquinone is free of the orbital phase (neither aromatic nor antiaromatic) and comes from the delocalization between the four pairs of the neighboring donors and acceptors. In fact, p-benzoquinone, which melts at 116 °C, is more stable than o-benzoquinone, which decomposes at 60-70 °C. 

The ionic conductivities of most solid crystalline salts and oxides are extremely low (an exception are the solid electrolytes, which are discussed in Section 8.4). The ions are rigidly held in the crystal lattices of these compounds and cannot move under the effect of applied electric fields. When melting, the ionic crystals break down, forming free ions the conductivities rise drastically and discontinuously, in some cases up to values of over 100 S/m (i.e., values higher than those of the most highly conducting electrolyte solutions). 

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