# Thermodynamics of the Micellization of a Simple Zwitterionic Surfactant in the Presence of Imidazolium Salts

## TL;DR

This study explores how different imidazolium salts affect the formation of zwitterionic micelles, revealing how anions and cations influence micellization thermodynamics.

## Contribution

The paper introduces a new method to separate the effects of cations and anions on micellization using a modified thermodynamic model.

## Key findings

- Imidazolium salts with different anions modulate the critical micelle concentration (CMC) of the zwitterionic surfactant SB3–14.
- C4mimI and C4mimClO4 initially decrease the CMC of SB3–14 before increasing it at higher salt concentrations.
- A modified thermodynamic model successfully separates cation and anion contributions to micellization.

## Abstract

Although formally
neutral, in aqueous media, zwitterionic micelles
selectively concentrate the anions of added inorganic salts at the
micelle-water interface. The resultant negative electrostatic potential
of the micelles can be additionally modulated by the metal cations
of the added salt. In the present work, isothermal titration calorimetry
(ITC) has been employed to determine the critical micelle concentrations
(CMC) and standard thermodynamic parameters of micellization (ΔG
mic
0, ΔH
mic
0, and ΔS
mic
0) of the zwitterionic
surfactant N-tetradecyl-dimethylammoniumpropanesulfonate
(SB3–14) in aqueous solution at 298.15 K as a function of the
concentration (100–1000 mmol L–1) of six
salts with organic cations: the chloride salts of 1-methylimidazolium
(ComimCl) and 1-ethyl-3-methylimidazolium (C2mimCl) and the chloride, bromide, iodide, and perchlorate salts of
1-butyl-3-methylimidazolium (C4mimCl, C4mimBr,
C4mimI, and C4mimClO4, respectively).
ComimCl, C2mimCl, C4mimCl, and C4mimBr increase the CMC of SB3–14 in this concentration
range, while C4mimI and C4mimClO4 initially decrease the CMC, followed by an increase in the CMC at
higher salt concentrations. Although the ΔH
mic
0 values
of SB3–14 are consistently negative and the ΔS
mic
0 values decrease relative to those in the absence of added salt,
the overall behavior is distinct from that in the presence of the
corresponding sodium salts, especially for C4mimI and C4mimClO4 with the most strongly interacting anions.
We demonstrate that, by explicitly incorporating specific cation-dependent
interactions into the formalism developed earlier for inorganic salts
[

Patiño-AgudeloÁ. J.,

Colloids Surf., A
2025, 725, 137728], it is possible
to separate the contributions of the organic cation to the micellization
of SB3–14 from those of the anion, providing a coherent picture
of the complex interplay between anion, cation, and the zwitterionic
interface.