Surface active compounds estimation

Before starting with the description of present theories of interfacial relaxations, the difference from adsorption kinetics studies has to be pointed out. The general difference lies in the composition of the adsorption layer. Adsorption kinetics processes, described in previous chapters usually start from an uncovered interface. The species with the highest concentration and surface activity adsorb first. The best measure to estimate the rate of adsorption at the beginning of the process is the ratio of surface concentration r over bulk concentration c. To compare the adsorption rate of two surface active compounds a simplification of Eq. (4.85) can be used, from which the time t needed by a surfactant to reach 95% of the equilibrium adsorption results. 

A sensitive method for the estimation of different surface active compounds (tetraalkylammonium salts, dextrans, crude oil components) in water is based on the measurements of the depression of the electrocapillary curve under conditions where the transport of the compound to the dropping Hg-electrode is accelerated by stirring of the solution being examined[12] The detection limit is in the range of 10-100 jjg 1 More details on "adsorptive polarographic analysis" are given in[13]. 

MW is often taken as the size descriptor of choice, while it is easy to calculate and is in the chemist s mind. However, other size and shape properties are equally simple to calculate, and may offer a better guide to estimate potential for permeability. Thus far no systematic work has been reported investigating this in detail. Cross-sectional area Ad obtained from surface activity measurements have been reported as a useful size descriptor to discriminate compounds which can access the brain (Ad<80A ) of those that are too large to cross the blood-brain barrier (BBB) [55]. Similar studies have been performed to define a cut-off for oral absorption [56]. 

The kinetic adsorption studies in different types of adsorbers were performed with two phenolic compounds of PNP and PCP on activated carbon. A technique of isotherm stepwise linearization has been proposed and applied to approximate nonlinear isotherms for GAC adsorption. The results showed that pore and surface diffusivity are estimated satisfoctorily using this stepwise linearization technique. This study also showed that the apparent diffusivity (De), which possesses concentration dependence, could be estimated on LCB by applying the technique in high-adsorption region. 

Differences in membrane damage caused by tricyclic compounds may be related to their surface activities and extent of adsorption on to cell membranes [190,192]. Although in experiments on enzyme leakage from isolated hepato-cytes the concentrations of tricyclic used was much higher than therapeutic plasma levels, this is perhaps reasonable as it has been found that the liver/plasma ratio of tricyclic antidepressants is of the order of 20-100 1 [192]. The order of toxicity of tricyclic compounds estimated by the extent of leakage of cytoplasmic and lysosomal enzymes from isolated hepatocytes, was in the order CPZ > amitryptiline > imipramine [191], which is the ranking of these compounds according to their surface activity and cell uptake. 

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