Nucleation-controlled mechanisms

Aspartame is relatively unstable in solution, undergoing cyclisation by intramolecular self-aminolysis at pH values in excess of 2.0 [91]. This follows nucleophilic attack of the free base N-terminal amino group on the phenylalanine carboxyl group resulting in the formation of 3-methylenecarboxyl-6-benzyl-2, 5-diketopiperazine (DKP). The DKP further hydrolyses to L-aspartyl-L-phenyl-alanine and to L-phenylalanine-L-aspartate [92]. Grant and co-workers [93] have extensively investigated the solid-state stability of aspartame. At elevated temperatures, dehydration followed by loss of methanol and the resultant cyclisation to DKP were observed. The solid-state reaction mechanism was described as Prout-Tompkins kinetics (via nucleation control mechanism). 

When underpotential deposition adsorption/desorption takes place randomly at any substrate site M, the following random adsorptioncontrolling treatment is to be employed, and when the process is controlled by a two-dimensional nucleation-growth mechanism, the process analysis should be carried out according to Section ni.l.(b). 

The values of n for LiBr and Ii2S04 he between 1 and 2, implying a two-dimensional diffusion-controlled mechanism with deceleratory nucleation. However, the liNOs process has n = 0.5, indicating that this process is completely nucleation controlled. liOH has n = 2.2, consistent with a phase boundary controlled process in two dimensions, with deceleratory nucleation again. 

The objective is to reduce volatiles to below 50-100-ppm levels. In most devolatilization equipment, the solution is exposed to a vacuum, the level of which sets the thermodynamic upper limit of separation. The vacuum is generally high enough to superheat the solution and foam it. Foaming is essentially a boiling mechanism. In this case, the mechanism involves a series of steps creation of a vapor phase by nucleation, bubble growth, bubble coalescence and breakup, and bubble rupture. At a very low concentration of volatiles, foaming may not take place, and removal of volatiles would proceed via a diffusion-controlled mechanism to a liquid-vapor macroscopic interface enhanced by laminar flow-induced repeated surface renewals, which can also cause entrapment of vapor bubbles. 

Inasmuch as surface tension is an important factor for nucleation, one may conclude that nucleation is not rate-controlling during film boiling. The controlling mechanism must be the transfer of heat across the vapor film. 

Figure 6.17 Schematic of the size distribution control mechanism of the hot injection and heat-up methods. In the left boxes, the monomer supply modes are shown as the plots of supersaturation vs. time. In the right boxes, the resulting time evolutions of the nucleation rate, the mean size, and the relative standard deviation are shown. The injection time and the start of the heat procedure are set as t = 0 in the hot injection and the heat-up processes, respectively.

In both cases, homogeneous flow or localized banded flow, the fundamental mechanism involves the nucleation-controlled formation of STs. Moreover, in the case of intense shear in narrow bands the material is in the flow state, with a hquid-like material content having pe 0.5. There the scale of the spatially percolating STs will be much smaller than in the homogeneous-flow case, as for their form in the range above Tg in the sub-cooled hquid (Johnson et al. 2007). We hasten to add that this happens without a significant temperature rise inside the bands (Zhou et al. 2001), as discussed in Section 7.8.3 below. 

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