Complexation, chemical interaction

As with povidone, crospovidone also forms chemical complexes or associates with a large number of drugs and other substances. For a typical example of comparison see Section 3.4.3.1. Here, too, the formation of the complexes is reversible and they do not form in alkaline medium. Whether crospovidone forms a complex with a drug depends on its chemical structure. 

Systematic investigations of aromatic compounds have shown that phenol and carboxyl groups have a strong influence on complexation. This is shown in Fig. 69. 

For most drugs, the degree of complexation lies within the range that provides an acceleration in dissolution rate. 

Normally, the degree of complexation is so low that, at best, an acceleration in the dissolution of the drug can be observed (see Section 3.4.3). The complexation constants of a number of active substances in 0.1 N hydrochloric acid can be seen in Table 120. They were also determined by adsorption on crospovidone in 0.01 N hydrochloric acid and, in some cases, in synthetic gastric juice according to USP [158]. 

The shaded area in Fig. 70 shows the usual systemic concentration range of crospovidone after medication. Thus for a complexation constant of less than 201 mol-i,the bound portion of the drug is always considerably less than 10%. Tannin and hexylresorcinol are exceptions with higher complexation constants [158]. Catechin also has a complexation constant of more than 1000 1 mol-i. Certain halogen compounds may have complexation constants in excess of 201 mol 1. 

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Fig. 6.3 The more complex chemical interactions at the membrane cathode.

The high concentrations of numerous chemicals in urban media contribute to complex chemical interactions and transformations within urban chemical mixtures. One example of this complexity is chemical speciation and distribution in stormwater, which affects bioavailability, toxicity and fate. High particle and colloidal concentrations suggest that many compounds in stormwater are not bioavailable, although the extent to which bioavailability is reduced depends on location. 

Perhaps most importantly, these results show that the potential health effects of complex earth materials cannot be assessed based solely on the toxicity of their together individual components— the integrated effects of the whole material must be considered, along with potentially complex chemical interactions between individual components and the body s fluids. 

Local flora and fauna analysis to permit determination as to whether the contaminants have entered the food chain and to assess the tendency of various species to concentrate or eliminate individual contaminants. In some cases, it is necessary to supplement the field investigations with controlled bench- or pilot-scale studies. These studies may be performed to simulate a mobilization or dispersion mechanism, or the complex chemical interactions between the waste form, surrounding matrix, or soil pathways, and/or the effectiveness of certain technologies in preventing migration or providing the required level of isolation. These pilot studies are often defined as feedback and obtained from the assessment of remedial alternatives. 

In summary, the commercial AgCuTi and Pd-base braze alloys exhibit complex chemical interactions with the ZrBz-SiC composites which involve dissolution, diffusion, and rxrmpositional... 

Humic substances deriving from aquatic and terrestrial environments are not identical in their composition and structure but are very similar, which is mentioned by Suffet and MacCarthy [20] as well as by Chen and Inbar [66]. Such complex chemical interactions with ecological consequences may therefore appear in all ecosystems. 

Characterizes molecular stmcture also by their propensity to engage chemical reactivity, with specific frontier measure appropriately modeled by the Fukui function (the density to number of electrons derivatives) as well as the chemical hardness iteratively evolution among adducts in a complex chemical interaction or through various chemical interactions channels (so explaining by quantum chemical reactivity principles the Le Chatelier-Braim principle of reactive equilibrium and bonding, or the electronic delocalization by novel sharing index, just to name few preeminent applications) ... 

The R2Se3-Cu2Se systems (R = Sm, Gd, Tb, Yb) exhibit complex chemical interaction resulting in the formation of several intermediate phases or solid solutions based on these phases. 

All these experiments have exhibited, at the beginning of the interaction, an incubation period characterized by a rapid dissolution of the oxides. This initial period is followed by the establishment of the diffusion process. Since the av able experiments were carried out with initial 0 tygen-fi ee Zr, some corrective factors have been introduced in the code modelling to tiy to extrapolate the experimental results to the case of Zr initially pre-loaded with otygen. A more detailed description of this complex chemical interaction is provided in [5]. 

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