Abstract

The effect of imidazolium-based ionic liquids, ([C12mim][Cl] and [C8mim][Cl]), on the acid-base equilibria of two sulfonated indicators has been studied. The presence of ILs leads to decreased pKa values because of the stronger electrostatic interaction of cationic ILs with the basic forms of the indicators with more negative charge. The longer alkyl side chain of [C12mim][Cl] compared to [C8mim][Cl] results in stronger hydrophobic interaction of this IL with the basic forms of the dyes leading to a more effective decrease in the pKa values. Also, the transition points and transition intervals of the acid-base titration curves of the indicators were affected by the presence of ILs. It was found that the IL interaction with acid-base indicators also results in sharpening the acid-base titration curves of the indicators. From these observations, it is concluded that the presence of ILs can tune the pKa values of indicators. All the experiments were performed spectrophotometrically and the results were obtained using curve fitting methods.

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