Thermodynamics of Aggregation of Ammonium/Tetraalkylammonium Perfluorooctanoates: Effect of Counterions

Ammonium/tetraalkylammonium perfluorooctanoates [C7F15COON(CnH2n+1)4, n = 0, 1, 2, 3, 4; abbreviated as APFO, TMAPFO, TEAPFO, TPAPFO, and TBAPFO, respectively] were investigated by electrical conductivity measurements at different temperatures T. The critical micelle concentration (cmc) as a function of T showed a typical U-shaped relationship for APFO, TMAPFO, TEAPFO, and TPAPFO, similar to that for ordinary ionic surfactants. However, TBAPFO presented behavior more like that of nonionic surfactants. APFO exhibited a complicated dependence of the degree of counterion association of aggregates (β) on T, probably because of its hydrolysis in solution. Both the cmc and standard Gibbs energy of aggregation (ΔaggrG°) decreased with increasing counterion size, while the degree of counterion association (β) was found to increase with the counterion size. There was a linear relationship between ln(cmc) and the chain length of the counterions (n), suggesting that the hydrophobic chains of counterions made contributions to aggregation. APFO, TMAPFO, TEAPFO, and TPAPFO all exhibited the enthalpy−entropy compensation phenomenon. TBA+ showed its own specialty in comparison with the other counterions and induced significant deviation from normal thermodynamic cases of surfactants. The β value for TBAPFO was very large and increased with increasing T, whereas β for the other surfactants decreased with increasing T.