Carboxylate salts micelle formation

F Salts of Carboxylic Acids as Soaps. Micelle Formation... 

The correlation between fluorescence intensity and salt concentration can be explained as follows the salt concentration in the organic phase increases as more hydrated trioctylammonium carboxylate is formed [Eq. (13)] and consequently micelle formation increases. The chromophore is distributed between the micellar phase and the solution, and with the rise in micelle concentration the number of fluorescent chromophores also rises. 

Emulsifiers such as fatty alcohol sulfates, alkane sulfonates, alkali salts of fatty acids, and others are slightly to marginally effective (77). Fluormated surfactants, particularly perfiuorinated carboxylic acids containing seven or eight fluorine atoms, are specially effective in maintaining a high rate of polymerization after about 40% conversion. Fluorinated surfactants are characterized by low values of critical concentration of micelle formation (74). They are thermally and chemically stable, and their incorporation does not impair the PVF properties. 

The cmc of bile salts is strongly influenced by its structure the trihydroxy cholanic acids have a higher cmc than the less hydrophilic dihydroxy derivatives. As expected, the pH of solutions of these carboxylic acid salts has an influence on micelle formation. At sufficiently low pH, bile acids which are sparingly soluble will be precipitated from solution, initially being incorporated or solubilized in the existing micelles. The pH at which precipitation occurs, on saturation of the micellar system, is generally about one pH unit higher than the pK of the bile acid. 

Recently Bloom and Reinsborough have measured the surface tension of pyrdinium chloride as well as tra nsport properties and used it to interpret micelle formation of this salt with cetyl and myristyl cationic soaps. Earlier Coleman and Prideaux studied mixtures of carboxylic acids with diethylamine and piperidine, but these are complicated systems which are too far afield of this discussion. 

A study of the solubilization of decanol in solutions of sodium octanoate showed that at low surfactant concentrations the solubilization of the additive increased rapidly after the cmc was exceeded, and continued to do so for some time as the concentration of sodium chloride was increased. At higher surfactant concentrations, however, it was found that there was an initial increase in decanol incorporation, which reached a maximum and then began to decrease as the salt level continued to increase. When the octanoate concentration well exceeded the cmc, the addition of salt resulted in an immediate decrease in the ability of the system to incorporate the additive. Such complex interactions have been attributed to alterations in the thermodynamics of mixed micelle formation for the decanol and carboxylate salt. Similar results may be seen in systems where the increased electrolyte content produces a change in the character of the micellar system a sphere-to-rod micellar transformation or the development of a mesophase, for example. 

The formation of micelles and their properties are responsible for the cleansing action of soaps Water that contains sodium stearate removes grease by enclosing it m the hydrocarbon like interior of the micelles The grease is washed away with the water not because it dissolves m the water but because it dissolves m the micelles that are dis persed m the water Sodium stearate is an example of a soap sodium and potassium salts of other C12-C1S unbranched carboxylic acids possess similar properties... 

In recent studies, Friberg and co-workers (J, 2) showed that the 21 carbon dicarboxylic acid 5(6)-carboxyl-4-hexyl-2-cyclohexene-1-yl octanoic acid (C21-DA, see Figure 1) exhibited hydrotropic or solubilizing properties in the multicomponent system(s) sodium octanoate (decanoate)/n-octanol/C2i-DA aqueous disodium salt solutions. Hydrotropic action was observed in dilute solutions even at concentrations below the critical micelle concentration (CMC) of the alkanoate. Such action was also observed in concentrates containing pure nonionic and anionic surfactants and C21-DA salt. The function of the hydrotrope was to retard formation of a more ordered structure or mesophase (liquid crystalline phase). 

On the basis of surface and bulk interaction with water. Small [85] classified bile acids as insoluble amphiphiles and bile salts as soluble amphiphiles. On account of the undissociated carboxylic acid group, the aqueous solubility of bile acids is limited [35] in contrast, many bile salts have high aqueous solubilities as monomers [33] and, in addition, their aqueous solubilities are greatly enhanced by the formation of micelles [5,6]. Because many bile salts are weak electrolytes, their ionization and solubility properties are more complicated than those of simple inorganic or organic electrolytes [5,35]. For example, the p/Tj, values of bile acids in water vary markedly as functions of bile salt concentration and, because micelles formed by the A (anionic) species can solubilize the HA (acid) species [5,35], the equilibrium precipitation pH values of bile acids also vary as functions of bile salt concentration. Finally, certain bile salts are characterized by insolubility at ambient temperatures [2,5,6,86,87], only becoming soluble as micelles at elevated temperatures (the critical micellar temperature) [6]. 

The conductance of the water/ammonium salt of perfluoropolyether carboxylic acid/perfluoropolyether system as a function of water content at different oil/sur-factant mass ratios has shown a well-shaped maximum in the curve [551. The initial rise in conductance has been considered to be due to the increased dissociation of counterions from the head groups by their solvation with water. The formation of water droplets (reverse micelles) begins at the conductance maximum, which has been supported by scattering measurements. A working model and quantitative analysis for this distinct system have been attempted. 

---