ANALYSIS OF THE BEHAVIOR OF A SERIES OF IMIDAZOLIUM-BASED IONIC LIQUIDS IN TWO DIFFERENT CHEMICAL PROCESSES. A CONTRIBUTION TO THE DESIGN OF NEW IONIC MATERIALS

C.G. ADAM, G. G. FORTUNATO, AND M. V. BRAVO

Conferencia; 13a Latin Conference on Physical Organic Chemistry.; 2015

Ionic Liquids (ILs) are called to become a new class of materials with a broad range of applications. At designing these materials, their physicochemical properties can be enhanced and/or modulated by changing the anion or cation nature and even the participation of a molecular solvent allows modulating a particular physicochemical property. In order to thoroughly understand the behavior of these ?ionic materials? a set of properties is required to enable correlating their chemical structure with the ?specific task? that they could develop.With this in mind, our aim was to get insight about the chemical structure of these materials analyzing their behavior during two different chemical processes in aqueous media.The selected ionic liquids are 1-alkyl-3-methylimidazolium bromide salts ([Cnmim] [Br]) with different alkyl chains (n = 4-12). The ionic liquid effect on the chemical acid-base equilibrium and on the hydrolysis of p-methoxybenzenesulfonyl chloride (CMBS) was studied. Moreover, in some cases, the obtained results were compared with the ones obtained employing [BF4] and [Cl] as counter ions.In both chemical processes the reactive systems exhibited a particular behavior as a function of the alkyl chain length of the ionic material, including the occurrence of self-aggregates like micelles for the systems comprising [C8-12mim] cations. While the ILs based on [C4mim] cation exhibited high pKa values and low dependence on the anion type, the ones with [C8-12mim] cations displayed a reduction in the pka values as the alkyl chain length is increased. With regard to the kinetic process, the kobs values decreased with respect to the value in pure water for all ILs. The system with [C4mim] [BF4] exhibited the lowest reaction rates comparing with the systems containing [Br] and [Cl]. For the systems based on [C8-12mim] cations, the kobs values diminished as the length of the alkyl chain increased. From the results achieved it was possible to infer the type of the cation-anion link and to evaluate how this association is enhanced or weakened by water molecules. Additionally, the occurrence of micelles greatly affects the acid-base equilibrium. The results accomplished in organized systems are related with the strong electrostatic interactions between the charged species of the employed microsensor and the cation in the IL. These interactions alter the acid-base equilibrium as a consequence of the electrostatic field that it is generated at the interface. The data obtained from the kinetic process allowed to confirm the self-aggregation process and the simultaneously determination of the CMC values. The pseudo-phase model was efficiently applied for the selected systems.