High-pressure crystallization kinetics

Most of the high-pressure crystallization kinetics studies have shown that ECC grows one dimensionally at high pressures [113-116]. 

Hoehn HH, Ferguson RC, Hebert RR. Effect of molecular-weight on high-pressure crystallization of linear polyethylene. 1. Kinetics and gross morphology. Polym Eng Sci 1978 18(6) 457-65. 

The authors further tested the Pt(l 11) and Pd(l 10) surfaces [71, 72] using in situ STM and SXRD. All these single crystals show a similar kinetic behavior in CO oxidation. The gradual roughening of the surface corresponds to the formation of surface oxides and a higher CO oxidation rate. The structure insensitivity observed at high pressure is in contrast with the results obtained in UHV, where the reactivity shows a strong orientational dependence. 

High-pressure liquid chromatographic (HPLC) analysis performed with a chiral mobile phase (57,58) confirmed in all the conglomerate systems that the S inhibitors are selectively occluded only in the bulk of the S substrate crystals, typically in amounts of 0.5-1% (and, by symmetry, occlusion of R occurs only in R crystals). The selective adsorption causes, furthermore, a drastic decrease in the growth (and possibly nucleation) rate of the affected enantiomer, leading to efficient kinetic resolution various conglomerate systems have been resolved by this method (54). 

Recently, the steady-state reaction kinetics of CO oxidation at high pressure over Ru , Rh " , Pt, Pd, and Ir single crystals have been studied in our laboratory. These studies have convincingly demonstrated the applicability and advantages of model single crystal studies, which combine UHV surface analysis techniques with high pressure kinetic measurements, in the elucidation of reaction mechanisms over supported catalysts. 

When very high pressures (> 1 GPa) are applied to liquid phases, glasses, or molecular crystals, mobility is reduced and steric effects become more important both in equilibrium and in kinetic aspects. Equations (9) and (14) are still valid, but equilibria and kinetics of chemical reactions must take into account the energetic, structural, and dynamic properties of the environment as well. 

As has already been noted, polymerization is a common output of high-pressure reactions. The kinetics of solid-state pressure-induced polymerizations have been treated within the nuclei growth [see eq. (17)] model. These reactions, as we will discuss in Section IV, are a typical example of how the crystal structure plays a fundamental role in sohd-state chemistry. Kinetic data of polymerizations are usually analyzed according to Eq. (17) by inserting an additional parameter fo accounting for the nucleation step ... 

The use of solid electrolytes in batteries and fuel cells is another important application. Examples are zirconia based fuel cells and sulphur batteries with Na-/ -A1203 as electrolyte. Many other interesting and practical aspects of solid electrolytes are worth mentioning, for example, the possibility to detect stresses, to build up high pressures, or to monitor mass accelerations. Also, solid electrolytes have recently been used to investigate the interface kinetics in crystals (Section 10.4.2). 

These studies are beginning to produce important kinetic information only in recent years. We shall summarize the experimental information available from studies of these reactions on the stepped [6(111) x (100)] and low Miller index (111) crystal surfaces and compare the low- and high-pressure rates when these data are available. 

Main factors which affect a hydrothermal reaction are the initial eomposition, reaction temperature and time. In mild hydrothermal synthesis, reaction temperatures lower than 240 °C are respected for both safety of high pressure in normal autoclaves and protection of softness of Teflon line. In our specific synthesis system, high temperature favorites the reaction and the most important factor was the base concentration in the initial reaction mixtures. The reaction time associated with reaction temperature affected the reaction. Crystallization kinetic experiment for a typical reaction showed that a reaction time more than lOh gave well-crystallized product and the further crystal growth needed additional time. Table 1 lists the starting reaction compositions and phase identification of products obtained at 240 °C for lOh. 

An important use of high mechanical pressiues is to force solids to assume crystal structures and coordination numbers in which they are normally unstable. Thus, four-coordinate Si in Si02, typified by quartz or try dimite, can be forced to assume the six-coordinate rutile stracture under very high pressures. Once formed, these structures are kinetically stable. The high-pressure polymorph of silica, called stishovite is found terrestrially as the result of meteor impact. 

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