Factors affecting the CMC and micellar size

5 Factors affecting the CMC and micellar size 3.5.1 Nature of the hydrophobic group 

The large majority of amphiphiles, whether ionic or non-ionic have hydrophobic regions composed of hydrocarbon chains. For ionic amphiphiles, increase in the number of carbon atoms in unbranched hydrocarbon chains leads to a decrease in the CMC. Fig. 3.6 shows a linear relationship between log CMC and the number 

As might be expected, the increased hydrophobicity conferred by an increase of chain length also causes an increase in micellar size. In many cases a linear relationship has been established between log (micellar weight) and the hydrocarbon chain length. Arnarson and Elworthy [65] have demonstrated a linear relationship between the aggregation numbers of polyoxyethylene non-ionic surfactants and the number of carbon atoms in the hydrocarbon chain (see Fig. 3.7) for surfactants with similar ratios of (number of ethylene oxide units)/(number of carbon atoms in hydrocarbon chain). 

The effect on micellar properties of the introduction of a fluorine atom into the hydrocarbon chain has been investigated. Muller et al [24-27] reported that substitution of the CF3 group for the terminal CH3 group of the surfactants 

Some surfactants possessing two hydrocarbon chains attached to a single head group, e.g. the dialkyldimethylammonium chlorides, form lamellar structures when dispersed in aqueous solution. When such turbid solutions are subjected to ultrasonic irradiation, optically clear solutions are formed in which the surfactant is dispersed in the form of closed vesicles [104] similar in structure to the liposomes formed by phospholipids. The use of these totally synthetic bilayer vesicles as model membranes and as potential drug delivery systems is currently under investigation. 

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