Counteranion Effect on Micellization of Cationic Gemini Surfactants 14-2-14: Hofmeister and Other Counterions

The effect of counterions was investigated and analyzed to probe the principal ionic effects influencing the micellization behavior of dimeric cationic surfactant ethanediylbis(dimethyltetradecylammonium), referred to as gemini 14-2-14. The 30 counterions were classified to four different families depending on their nature: (1) small and inorganic counterions which are typically taken from the Hofmeister series were studied to focus on the effect of ion type; (2) n-alkyl carboxylate counterions were studied to focus on the effect of the hydrophobicity of counterions; (3) aromatic carboxylate counterions were included to focus on the effect of the position of substitutions; and (4) other counterions were included in order to shed light on other parameters. By investigating the critical micelle concentration (CMC), ionization degree of micelle (α), free energy of micellization (ΔGM), and aggregation numbers N of the gemini surfactant with these different types of anions, we demonstrated the effect of different ion properties independently. This approach allowed us to describe the effect of counterions on the micellization behavior of the gemini surfactant in terms of complex interplay between hydrophobicity of anions and other ion properties such as counterion hydration, interfacial packing of ions, and ionic morphology. Indeed, our results clearly demonstrate that a counterion effect on micellization properties cannot be described as a result of one single parameter of ions, as is too often assumed, but rather the balancing effects cooperatively affect the propensity of counterions to form ion pairs with surfactant headgroups and the entropy gain upon micellization. These results provide new insight in understanding the effect of ions on the delicate balance of forces controlling aggregate morphology and solution properties of charged amphiphilic molecules.