Variation of CMC with Chemical Structure

2 Variation of CMC with Chemical Structure Hydrocarbon Chain Length 

For single alkyl chain surfactants, which are of principal interest in this treatment, the factor primarily determining the CMC is the size of the hydrophobic part. The dependence of the CMC on the number of carbon atoms (nc) in the alkyl chain can for many classes of amphiphiles be written2) 

Partial or total fluorination has striking effects on the CMC. Thus while perfluorinated compounds have much lower CMCs than the corresponding hydrocarbon surfactants (as an example the CMC is 0.030 M for sodium perfluorooctanoate and 0.38 M for sodium octanoate), partial fluorination increases the CMC. The strong deviation from an ideal behavior is illustrated in Fig. 2.425. A markedly nonideal behavior is also found in mixtures of hydrocarbon and fluorocarbon surfactants26.  

For nonionics, the CMCs may be markedly affected by the size and nature of the hydrophilic group. For ionic surfactants, the CMC is increased on introduction of additional ionic groups for example, the CMC of C10CH (COOK)2 is 0.13 M while it is 0.024 M for CnCOOK.1 

1 Cn denotes an n-alkyl chain with n carbon atoms. 

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