Initial solid-forming condition determination

Figure I. Flow diagram showing computation procedure used for determining initial solid forming condition...

The kinetic principles operating during the initiation and advance of interface-controlled reactions are identical with the behaviour discussed for the decomposition of a single solid (Chaps. 3 and 4). The condition that overall rate control is determined by an interface process is that a chemical step within this zone is slow compared with the rate of arrival of the second reactant. This condition is not usually satisfied during reaction between solids where the product is formed at the contact of a barrier layer with a reactant. Particular systems that satisfy the specialized requirements can, however, be envisaged for example, rate processes in which all products are volatilized or a solid additive catalyzes the decomposition of a solid yielding no solid residue. Even here, however, the kinetic characteristics are likely to be influenced by changing effectiveness of contact as reaction proceeds, or the deactivation of the catalyst surface. 

It is of interest to examine quantitatively such potential-dependent redox equilibria as determined by SERS in comparison with that obtained by conventional electrochemistry. Figure 1 shows such data determined for Ru(NH3 )6 3" 2+at chloride-coated silver. The solid curves denote the surface concentrations of the Ru(III) and Ru(II) forms as a function of electrode potential, normalized to values at -100 and -500 mV vs SCE. These are determined by integrating cyclic voltammograms for this system obtained under conditions [very dilute (50 yM) Ru(NH3)63 +, rapid (50 V sec-1) sweep rate] so that the faradaic current arises entirely from initially adsorbed, rather than from diffusing, reactant (cf. ref. 6b). The dashed curves denote the corresponding potential-dependent normalized Ru(III) and Ru(II) surface concentrations, obtained from the integrated intensities of the 500 cm 1 and 460 cm-1 SERS bands associated with the symmetric Ru(III)-NH3 and Ru(II)-NH3 vibrational modes.(5a)... 

The urea-formaldehyde polymer is formed by a multi-step reaction process between urea and formaldehyde. The initial phase is a methylolation of the urea under slightly alkaline conditions with a formaldehyde-urea (F/U) molar ratio of 2.0 1 to 2.4 1. Condensation of the methylolureas from the methylolat ion reaction is at atmospheric reflux with a pH of 4 to 6. This condensation polymerization continues to a pre-determined viscosity, at which time the pH is adjusted with a suitable base to 7-3 to 8.0. The adhesive is then concentrated to a total solids content of 50 to 60 percent by vacuum distillation. Additional urea is then normally added to produce a final F/U molar ratio of 1.6 1 to 1.8 1. 

Rapid crystallization in the lower temperature range tends to yield Form I of sulindac. The use of chlorinated solvents (methylene chloride, chloroform, 1,1-dichloroethane, etc.), either under carefully controlled conditions or more chaotic circumstances, results in the isolation of Form I of sulindac after drying off of the solvent. The initially isolated solid in these cases was determined to be a sulindac solvate of the corresponding chlorinated solvent. Once the solvates are broken by drying. Form I of sulindac results, indicating a strong structural relationship between Form I and the solvates. 

A well-defined monodisperse penta(L-alanine)- -butylamide H-[Ala]5-NHBu was synthesized by an activated ester method " and other natural abundant polypeptides, [Ala]n-5, [Leu]n-1 and [Leu]n-2, were synthesized by the N-carboxy a-amino-acid anhydride (NCA) method.Fully N-labelled homopolypeptides, [Ala ]n (99 at.% of N purity MASSTRACE, Inc.) and [Leu ]n (99 at.% of N purity MASSTRACE, Inc.), which show characteristic differences in conformation such as the a-helix and /3-sheet forms, were prepared by the heterogeneous polymerization of the corresponding NCAs in acetonitrile with -butylamine as an initiator. Conformational characterization of these samples was made on the basis of the conformation-dependent C and chemical shifts determined from the CP-MAS NMR method and from the characteristic bands in the IR and far-IR spectra. Figs. 38 and 39 show the 75.5 MHz C and 30.4 MHz N CP-MAS NMR spectra respectively of these fully N-labelled (99 at.% purity of N) homopolypeptides adopting the a-helical and /3-sheet forms (A) [Ala ]n-2 (a-helix), (B) [Ala ]n-1 (/3-sheet), (C) [Leu ]n-2 (a-helix), (D) [Leu ]n-1 (/3-sheet) in the solid state. Synthetic conditions and conformational characteristics of these samples are summarized... 

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