Nucleation properties

Fig. 1.2.5. The helix nucleating properties of AcHel depend on the two conformational states of the N-terminal imide bond.

In-situ growth monitoring, in particular by reflective high energy electron diffraction (RHEED), has provided some fundamental information on the surface and nucleation properties of nitrides. Early RHEED studies by Hughes et al [34] and Hacke et al [49] were completed by Smith et al [50], The observed surface reconstructions for (0001) and (0001) GaN surfaces have allowed the modelling of the quasi-equilibrium surface, which has been calculated to be preferentially Ga-terminated [51], Feuillet et al [52] have followed the evolution of surface lattice constants in RHEED for the nucleation of GaN on AIN or InN on GaN (and vice versa) and extracted a wide range of information on the character of nucleation and misfit relaxation. 

In an example from the pharmaceutical industry Sudo et a/. (1991) studied the relative nucleation properties of forms A and B of cimetidine, which is reported to have four polymorphic non-solvated forms and three polymorphic monohydrates. Modification A is preferred for pharmaceutical formulations. The waiting time method was used to study the primary nucleation process (Harano and Oota 1978), mainly for competitive crystallization of the A and B modifications. A is a thermodynamically metastable form and is more soluble than B in any solvent. At high supersaturation... 

In almost all industrial crystallizations, both nucleation and growth contribute to the final result (purity, morphology, size distribution) in a major way. All growth processes, emphasized in this book as being desirable for maximum control and robustness in many or most situations, still require an understanding of the nucleation properties of the system being studied in order to minimize the contribution of nucleation to the final result. 

Mixed-phase cloud processes A variety of mixed-phase and ice cloud models exist, describing the homogeneous and heterogeneous formation of water droplets and ice crystals. The implications of aerosol particles on mixed-phase clouds may be evaluated if their ice nucleating properties are known. 

An understanding of GPC (both nucleation and growth) requires knowledge of which of the possible species participate in nucleation, their concentrations, and their thermodynamic and nucleation properties. For the atmosphere, one needs the concentrations not only of precursor gases but also of the oxidants, ozone,... 

Lammel, G., and Novakov, T. (1995) Water nucleation properties of carbon black and diesel soot particles, Atmos. Environ. 29, 817-823. 

Polycarbonate was foimd to have nucleating properties towards poly(lactide). It increases its crystalhzation rate and improves mechanical properties. ... 

Carbon black is an example of a nucleating reagent which can adjust nucleating properties of PEKK, which depend on its composition. ... 

A static instability occurs when a small perturbation from original steady-state flow leads to a new stable operating condition which is not in the vicinity of the original state. The mechanism and the threshold conditions are predicted using steady-state characteristics of the system. Pressure drop characteristics of a flow channel, nucleation properties, and... 

Importance of the position which the guest residue (G) occupies in the host peptide. From these investigations asymmetric conformation nucleation properties of the guest amino acid might possibly be derived. 

Crystallization behavior is affected by the temperature of crystallization, the nucleation properties of additives (such as dyes), the relative molecular mass distribution and co-existing monomers, etc. It is important to investigate the crystallization behavior of the polymers, since the properties of polymeric products, such as appearance, dimensional stability and thermal stability, are influenced by crystallization processes. 

In addition to the numerous elemental characterisations of ambient particles, we have recently seen more applications in fundamental physico-chemistry. This includes laboratory studies on gas-particle reactions, e.g. sea salt or soil dust with nitric acid. Environmental SEM (ESEM) or environmental TEM (ETEM) does offer excellent future prospects in this field. In ESEM and ETEM, it is possible to obtain high quality images and to do some chemical analysis while the gaseous environment around the sample is controlled i.e. vacuum in the neighbourhood of the sample is not necessary, the relative humidity can be varied and a temperature around — 30 °C can be maintained). Ice nucleation properties of individual atmospheric particles have also been studied recently. Both ESEM and ETEM are relatively new techniques, of which the potential has not fully been explored. 

Table 5 Chemical and physical properties of aerosols, which affect their nucleating properties... 

Their decomposition at elevated temperatures causes two problems the lost of the nucleating property, and the formation of aldehydes as decomposition products. Although the odor of the aldehydes is pleasant it is undesirable in most applications. 

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