Carboxyl groups Chain length, dependence

Usmani and Wahab ° determined the adsorption of lower aliphatic and some organic unsaturated fatty acids from their aqueous solutions on activated carbons prepared from wood, rice husk, and lignite coal. The adsorption depended on the porosity and the chemical stfucture of the carbon surface. It was also found that for a given acid the adsorption depended on the polarity of the carboxylic group, the length of the side chain, and the nature of the type of bonding. The adsorption followed both the Langmuir and the Freundlich adsorption equations. 

One of the most sensitive tests of the dependence of chemical reactivity on the size of the reacting molecules is the comparison of the rates of reaction for compounds which are members of a homologous series with different chain lengths. Studies by Flory and others on the rates of esterification and saponification of esters were the first investigations conducted to clarify the dependence of reactivity on molecular size. The rate constants for these reactions are observed to converge quite rapidly to a constant value which is independent of molecular size, after an initial dependence on molecular size for small molecules. The effect is reminiscent of the discussion on the uniqueness of end groups in connection with Example 1.1. In the esterification of carboxylic acids, for example, the rate constants are different for acetic, propionic, and butyric acids, but constant for carboxyUc acids with 4-18 carbon atoms. This observation on nonpolymeric compounds has been generalized to apply to polymerization reactions as well. The latter are subject to several complications which are not involved in the study of simple model compounds, but when these complications are properly considered, the independence of reactivity on molecular size has been repeatedly verified. 

The reactions that occur when dicarboxylic acids are heated depend critically upon the chain length separating the carboxyl groups. Cyclization usually is... 

In another paper Fendler et al.59) investigated again the CMC dependence of alkylammonium propionates on the number of carbon atoms in the alkyl group of the quaternary ammonium cation (Fig. 12). The same feature as mentioned with octylammonium carboxylates is observed, i.e., the constancy of the CMC values for these alkylammonium propionates in carbon tetrachloride, while in benzene a linear plot is obtained. The latter is taken to indicate the decreased solubility of the monomers with increasing alkyl chain length in benzene. Table 471 exhibits in addition... 

It is interesting to note that the enhancement of skin permeation of drugs by fatty acid is dependent upon the alkyl chain length (Figure 6). In addition, the esterification of the carboxylic acid group reduces the effectiveness of fatty acids in skin permeation enhancement. The type of enhancer used and its concentration in the adhesive coating also play an important role in the extent of skin permeation (Figure 7). 

Fatty acid salts and many polar derivatives of fatty acids are amphiphilic, possessing both hydrophobic and hydrophilic areas within the one molecule. These are surface-active compounds that form monolayers at water/air and water/surface interfaces and micelles in solution. Their surface-active properties are highly dependent on the nature of the polar head group and, to a lesser extent, on the length of the alkyl chain. Most oleochemical processes are modihcations of the carboxyl group to produce specihc surfactants. 

As mentioned above, the hydrophilic head group may be unionised [e.g., alcohols or poly(ethylene oxide) (PEO) alkane or alkyl phenol compounds], weakly ionised (e.g., carboxylic acids), or strongly ionised (e.g., sulphates, sulphonates, and quaternary ammonium salts). The adsorption of these different surfactants at the A/W and O/W interfaces depends on the nature of the head group. With nonionic surfactants, repulsion between the head groups is small and these surfactants are usually strongly adsorbed at the surface of water from very dilute solutions. Nonionic surfactants have much lower cmc values when compared to ionic surfactants with the same alkyl chain length typically, the cmc is in the region of... 

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