Kinetics partial melting

Melting and dissolution kinetics application to partial melting and dissolution of xenoliths. /. Geophys. Res. 91, 9395-9406. 

It would be experimentally very difficult to measure amounts of liquid present (including variations with a and temperature) in mixtures that have undergone partial melting. The kinetics of processes that demonstrably involve the participation of a liquid phase often resemble those for solid state decompositions, as illustrated by the following examples. 

Partial melting was reported [27] to take place during decomposition of this salt in air. Kinetic behaviour included a decrease in rate at a about 0.3. In vaeuum the reaction was accompanied by sublimation. Kinetic behaviour showed some irreproducibility and the decrease in rate occurred at a about 0.1. The residue contained metal and oxide, but, unlike the nickel salt, not the metal carbide. 

Kinetic Equations for Reactions Proceeding with Partial Melting... 

There is a triple poinf at which a liquid phase (in the case of Pb-Sn, a liquid solution of Pb and Sn), and two solid phases coexist. This is known as the eutectic point. It occurs at a specific composition, the eutectic composition, and melts/freezes completely at a specific temperature, the eutectic temperature. In the case of Pb-Sn, the eutectic composition is 61.9 weight % Sn, and the eutectic temperature is 181 °C. Notice that the eutectic solid is a two-phase mixture. In spite of this, both phases melt at the same temperature for the eutectic alloy. The maxima of solid solubility for Sn in solid Pb and Pb in solid Sn occur at the eutectic temperature and are, 19.2 weight % and 97.5 weight % Sn, respectively. A solid mixture having the eutectic composition on average is the only mixture that melts entirely at a single temperature. All other compositions (except pure elements) in a eutectic system partially melt leaving a solid with altered composition (if kinetics allow the solids to maintain equilibrium) in contact with a liquid of different composition. 

Reports of kinetic studies do not always include an explicit statement as to whether or not the reactant melted during reaction or, indeed, if this possibility was investigated or even considered (cf. p. 1). This aspect of behaviour is important in assessing the mechanistic implications of any data since reactions in a homogeneous melt, perhaps a eutectic, usually proceed more rapidly than in a crystalline solid. It is accepted that the detection of partial or localized melting can be experimentally difficult, but, in the absence of relevant information, it is frequently impossible to decide whether a reported reaction proceeds in the solid phase. 

The kinetic behaviour of metal salts of oxyacids may be influenced by water of crystallization. Where complete-dehydration precedes decomposition, the anhydrous material is the product of a previous rate process and may have undergone recrystallization. If water is not effectively removed, there may at higher temperature be the transient formation of a melt prior to decomposition. The usual problems attend the identification of partial or transient liquefaction of the reactant in the mechanistic interpretation of kinetic data. 

PBTP was depolymerised in excess methanol under high temperature (473-523 K) and high pressure (4-14 MPa) conditions. Depolymerisation was earried out at 483 K and 4-12 MPa, and at 513 K and 6-14 MPa. The temperature had a great effect on the depolymerisation rate, but the reaction pressure did not. Under the former eonditions, depolymerisation took over 80 min, but only about 20 min under the latter eonditions. The se results showed that the supereritieal state of methanol was not a key faetor in the depolymerisation reaction. A kinetic study of the reaetion at 473-523 K and 12 MPa showed that the deeomposition rate eonstant of PBTP increased dramatieally when the reaetion temperature was higher than the melting point of PBTP (500 K). This indicated that partial miseibility of the molten PBTP and methanol was an important faetor for the short-time depolymerisation. 9 refs. 

Lactam polymerizations (nonassisted as well as assisted) are usually complicated by heterogeneity, usually when polymerization is carried out below the melting point of the polymer [Fries et al., 1987 Karger-Kocsis and Kiss, 1979 Malkin et al., 1982 Roda et al., 1979]. (This is probably the main reason why there are so few reliable kinetic studies of lactam polymerizations.) An initially homogeneous reaction system quickly becomes heterogeneous at low conversion, for example, 10-20% conversion (attained at a reaction time of no more than 1 min) for 2-pyrrolidinone polymerization initiated by potassium t-butoxide and A-benzoyl-2-pyrrolidinone. The (partially) crystalline polymer starts precipitating from solution (which may be molten monomer), and subsequent polymerization occurs at a lower rate as a result of decreased mobility of /V-acyl lactam propagating species. 

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