Micelle formation of tetradecyltrimethylammonium X-benzoate surfactants: Benzoate counterion substituent effects

Tetradecyltrimethylammonium (TTA+) benzoate surfactants have a range of CMC values dependent on the substituent effects of the benzoate counterions. Twenty-three TTA+/X-benzoate CMC values were determined by conductivity and surface tension. The 12 TTA+/para- and meta-benzoate and the unsubstituted benzoate CMC values yield a correlation to the hydrophobicity parameter (log P) of the benzoic acid, as the counterion. The logarithm of conductivity CMCs correlated to the log P value of the benzoic acid yields a slope of—0.4399. All 10 of the TTA+/ortho-benzoate CMC values do not yield a correlation with hydrophobicity yet the TTA+/ortho-halogenated benzoates give a strong correlation to hydrophobicity (log P). For this subset, the correlation was nearly identical to one of the TTA+/para- and meta-benzoates. The slope of the logarithm of TTA+/ortho-halogenated benzoate CMC values to the log P of benzoic acid is—0.413. Another subset is the TTA+/ortho-hydrogen bond donating benzoates which clearly show micelle formation is favored in addition to the hydrophobic effect. The surface tension data CMCs yield a similar correlation to the conductivity data. This combined data yields further insight on the structural features of the counterion contribution to micelle formation. Specifically, the benzoates show both hydrophobicity effects and when ortho-hydrogen bond donating substituents are present, micelle formation is strongly enhanced in addition to the hydrophobic effect of the anion.