Critical concentration of ion‐pairs formation in nonpolar media

Abstract

It is known that nonpolar liquids can be ionized by adding surfactants, either ionic or nonionic. Surfactant molecules serve as solvating agents, building inverse micelles around ions, and preventing their association back into neutral molecules. According to the Bjerrum-Onsager-Fuoss theory, these inverse micelle ions should form "ion pairs." This, in turn, leads to nonlinear dependence of the conductivity on the concentration. Surprisingly, ionic surfactants exhibit linear conductivity dependence, which implies that these inverse micelle ions do not form ion pairs. Theory predicts the existence of two ionic strength ranges, which are separated by a certain critical ion concentration. Ionic strength above the critical one is proportional to the square root of the ion concentration, whereas it becomes linear below the critical concentration. Critical ion concentration lies within the range of 10(-11) -10(-7) mol/L when ion size ranges from 1 to 3 nm. Critical ion concentration is related, but not equal, to a certain surfactant concentration (critical concentration of ion-pairs formation (CIPC)) because only a fraction of the surfactant molecules is incorporated into the micelles ions. The linear conductivity dependence for ionic surfactants indicates that the corresponding CIPC is above the range of studied concentrations, perhaps, due to rather large ion size. The same linearity is a sign that charged inverse micelles structure and fraction are concentration independent due to strong charge-dipole interaction in the charge micelle core. This also proves that CIPC is independent of critical concentration of micelle formation. Nonionic surfactants, on the other hand, exhibit nonlinear conductivity dependence apparently due to smaller ion sizes.

Structure Characterization and Properties of Metal–Surfactant Complexes Dispersed in Organic Solvents

“…This is characteristic of a strong interaction between the ion pairs. 55 This corroborates the SAS analysis and the MD simulations, which indicate that there is no ion dissociation when the complex is dispersed in alcohols. Figure 8 shows the electrochemical behavior of the metallic complexes in toluene.…”

Molecular Dynamics Simulation of Aggregates in the Dodecane/span80 System and Their Behaviour in an Electric Field

“…Impurity ions can reside inside the water core, electrostatically shielded by the surfactant molecules around, thereby preventing recombination with counter ions. Reverse micelles with electric charges in the core serve as ions in nonpolar liquids [1][2][3][4][5].…”

Application of classical thermodynamics to the conductivity in non-polar media

“…Dukhin did not find a convincing explanation for the absence of ionpairing in the available literature and hypothesized that the phenomena might be linked to the size of micelles. 9 He concludes by saying that more experiments are needed to confirm this hypothesis. There is however a simple explanation.…”

“…However, the difference is due to an error in the Taylor expansion of a square root. 9 From Eqs. (1) and (6) we can also deduce that…”

“…An extended review on ion, ion-pair, and inverse micelles in non-polar media has been done by Dukhin and Parlia. 8 In a following article, Dukhin et al 9 state that the formation of ion-pair decreases the conductivity in the case of surfactant in non-polar solvent, by analogy with the work of Onsager, Fuoss, and Kraus. However, this analogy is questionable as we will discuss in the present article.…”