Ionization of a Nonpolar Liquid with an Alcohol

Abstract

Nonpolar liquids whose dielectric permittivities are close to 2 have very low conductivities, usually below 10 × 10(-10) S/m. Their ionization is suppressed by the lack of solvation resulting from the negligible dipole moment of such liquids' molecules. Ionization could be enhanced by the addition of other substances that could serve as solvating agents, creating inverse micelles around ions and preventing them from reassociating into ion pairs and neutral molecules. Surfactants are normally used for this purpose, but we show here that alcohols could perform a similar function. However, the mechanism of ionization by alcohols turns out to be quite different compared to the mechanism of ionization by surfactant. For instance, the conductivity of poly-α-olefin oil (PAO) depends on the concentration of added octanol (alcohol) as an exponential function above 10% of the octanol content. At concentrations below approximately 10%, octanol does not affect the conductivity at all. This phenomenon has never been observed for surfactant solutions. Apparently, octanol is completely dissolved at concentrations below 10% and forms micelles only above this concentration, which is the cmc for octanol-PAO mixtures. Below the cmc, octanol molecules do not dissociate, despite being able to dissociate in pure octanol, which has a conductivity of about 10 × 10(-7) S/m. This again stresses the importance of the solvating factor in the ionization of liquids. Above 10% concentration, octanol molecules form micelles, which become charged by the disproportionation mechanism when they collide. To explain the exponential dependence of conductivity on octanol content, we assume that charged micelles grow in volume with increasing octanol content faster than neutral ones. Ion-dipole interactions are responsible for the preferential adsorption of octanol molecules onto charged micelles. Additional ionization occurs in such larger micelles, which then break down into smaller ones carrying individual electric charges.

Application of Classical Thermodynamics to Conductivity in Nonpolar Media: Experimental Confirmation

“…The conductivity increases sharply with the mass concentration of alcohol. This sharp increase was …rst shown by Dukhin et al in a recent series of articles [9][10] [11]. <Fig 1.…”

“…Twenty years later, they adequately gave the results as a linear plot of the logarithm of the equilibrium constant as a function of the inverse of the permittivity [4]. Recently, there has been a growing interest for the conductivity of surfactants in non-polar medium [6] [7] [8] and for the conductivity of mixtures of pure solvents [9][10] [11]. In some cases, the chemical potential of the ionogenic species is not a¤ected by the creation of ions or of ion-pairs.…”

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

“…Recently, experimental work has been published on mixture of alcohol and hydrocarbons by Dukhin and co-workers. 10,11 On these mixtures, only one other publication can be found, on the conductivity of mixtures of ethanol and gasoline. 32 In the first article, 10 written by Dukhin and Bombard, the conductivity of a mixture of n-octanol and an alcane (PAO) of similar viscosity was measured.…”

“…10,11 On these mixtures, only one other publication can be found, on the conductivity of mixtures of ethanol and gasoline. 32 In the first article, 10 written by Dukhin and Bombard, the conductivity of a mixture of n-octanol and an alcane (PAO) of similar viscosity was measured. The authors found a very non-linear dependence (almost an exponential dependence) of the conductivity on the fraction of alcohol.…”

Electrical Conductivity and Viscosity in Binary Organic Liquid Mixtures: Participation of Molecular Interactions and Nanodomains

“…A recent study by Bombard and Dukhin [32] showed that the electrical conductivity of poly(α-olefin) (PAO)/1-octanol mixtures increased by five orders of magnitude between 100% PAO and 100% octanol. This prompted the authors to propose an ionization mechanism, involving alcohol autoprotolysis above a "critical micelle concentration" (≈20 wt% octanol), as being responsible.…”