Aggregation Behavior of Aqueous Solutions of 1-Dodecyl-3-methylimidazolium Salts with Different Halide Anions.

J. Chem. Eng. Data, 2013, 58 (6), pp 1529–1534.

Mingqi Ao, Doseok Kim.

Department of Physics, Sogang University, Seoul 121-742, Korea.

 

Abstract

 

The interfacial and bulk properties of the aqueous solutions of imidazolium-based ionic liquids with different halide anions ([C12mim]Cl, [C12mim]Br, [C12mim]I) were investigated by surface tension and electrical conductivity measurements. The lowest surface tension ({Gamma}cmc) and maximum surface excess concentration ({GAMMA}max) values from the surface tension measurements showed that [C12mim]I had the highest surface activity. The thermodynamic pontentials of micellization ({DELTA}G0m, {DELTA}H0m, {DELTA}S0m) indicated that the micellization of [C12mim]Cl and [C12mim]Br was entropy-driven, while that of [C12mim]I was enthalpy-driven at 15 °C but entropy-driven above 20 °C. This distinct behavior for [C12mim]I was thought to be due to the higher binding affinity of I– to the micellar interface.

 

Copyright © 2013 American Chemical Society

 

 

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Additional Information

From your course in undergraduate electrostatics, charged particle like cation or anion in water cannot come up to the surface of the aqueous solutions as an image charge (which has the same sign as the original charge) formed above in the air side would repel it from coming close to the interface. This picture, however, was put to question as Jungwirth and Tobias using molecular dynamic simulations proposed that bulky anions like I can come readily to the air/water interface [1]. As this proposal has environmental implications with respect to the uptake of halide elements into the atmosphere, it drew attention from many researchers who wanted to test this proposition. Several experimental reports that followed were dubious, as some indeed observed surface excess of larger halide anions [2], while the others testified against such phenomenon [3].

 

Ao Mingqi and Doseok Kim at Sogang University in South Korea dissolved long-chain ionic liquid in water, which provided an environment in which relatively insoluble cations float at the water surface, while halide anions are dissolved in water [4]. By trying out with different halide anions (chloride, bromide, and iodide), they readily observed that there is a big difference in their tendencies to be attracted to the cationic headgroup at the interface; the bulkiest iodide anion was the most surface-active, followed by bromide and chloride anions. This trend shown from surface tension measurement, a classical method to investigate the liquid surface, was followed clearly as they monitored the bulk properties of these molecules forming aggregates. Although they used cationic surfactant to facilitate the adsorption of anions to the surface, it still demonstrates that ions in water cannot be treated as simple point charges, as water treats them differently in terms of their size [5].

 

[1] P. Jungwirth and D. J. Tobias, J. Phys. Chem. B 106, 6361 (2002).

[2] S. Ghosal et al., Science 307, 563 (2005).

[3] E. A. Raymond and G. L. Richmond, J. Phys. Chem. B 108, 5051 (2004).

[4] M. Ao and D. Kim, J. Chem. Eng. Data, 58, 1529 (2013).

[5 W. Wang et al., J. Phys. Chem. B, 117, 13884 (2013).

 

Aggregation Behavior of Aqueous Solutions of 1-Dodecyl-3-methylimidazolium Salts with Different Halide Anions

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