Ionic Liquids: Past, Present and Future

Scientific conference (2011, August 24)

Hammett Acidity Scale in Ionic Liquids : An Indication of Their Weak Dissociating Character

Poster (2010, August)

Ionic liquids are salts with the particularity to exhibit melting points near room temperature (below 100°C, by convention) with no vapour tension. For this last reason, ionic liquids are sometimes called ... [more ▼]

Ionic liquids are salts with the particularity to exhibit melting points near room temperature (below 100°C, by convention) with no vapour tension. For this last reason, ionic liquids are sometimes called “green solvents”. In addition, their exclusive materials and solvent properties has led to an amazing increase of interest from both academic and industrial community, confirmed by the explosion of the number of published papers in the last decade. The many combinations of organic and inorganic cations and anions allow an infinity of new ionic solvents then permitting the selection of the desired properties for a given application. Nevertheless, it is impossible to investigate all these combinations and the unusual complexity of these new solvents gives rise to many controversies. Consequently, the development of the general rules for understanding the chemistry in ionic liquids is crucial. A fundamental property of solvent is its solvating power, for instance towards the proton. Therefore, we are interested to investigate the acid-base properties in ionic liquids in order to ultimately find a correlation with the acidic catalysis activity. We then have proposed a colorimetric method to determine the acidity levels accessible in these new media: the Hammett acidity function H0. This spectroscopic method is based on the protonation equilibrium for a family of coloured indicator with pKa’s assumed as solvent independent (following the Hammett proposition). This presentation will summarize our Hammett acidity measurements in several ionic liquids. - At first, we will show that it is possible to evaluate the Hammett acidity function with two different coloured indicators, in the same ionic liquid. Since the Hammett acidity of a given mixture was found to depend on the choosen coloured indicator, this suggests the formation of ions associations in ionic liquids. As a result, the ionic liquids are clearly not as dissociating as initially thought and the Hammett acidity function is in fact an apparent function, underestimating the real acidity level. - The apparent acidity functions have then been compared for several ionic liquids to which an acid has been added ([BMIm][NTf2], [BMIm][BF4], [BMIm][OTf], [BHIm][NTf2], [BMIm][PF6], [HNEt3][NTf2]). The conclusions are as follows: 1) the accessible acidity level is not influenced by the nature of the cation; 2) on the contrary, the nature of anion is very critical and the solvating power towards the proton follows the order: OTf- > NTf2- > BF4- > PF6-. The more the proton is solvated, the less it is acidic. - Finally, the difference of acidity of two acids, HOTf and HNTf2 (both strong acids in water), has been investigated in [BMIm][BF4], [BMIm][NTf2] and [BMIm][OTf]. In [BMIm][OTf], these two acids show the same acidity (they behave as strong acids) due to the solvent levelling effect; on the other hand, in [BMIm][NTf2] and [BMIm][BF4] allowing higher acidity levels, HNTf2 is stronger than HOTf . The observed difference is also another indication of the lower proton solvation in [BMIm][BF4] or [BMIm][NTf2] versus that in [BMIm][OTf]. [less ▲]

The Hydrogen Electrode in Ionic Liquids: Acidity Measurements and Titrations

Poster (2010, March)

The acidity level in ILs containing acid was first determined using the Hammett acidity function (H0)1-2 in our laboratory. It was demonstrated that this attainable acidity, extending from -3 to -8, is ... [more ▼]

The acidity level in ILs containing acid was first determined using the Hammett acidity function (H0)1-2 in our laboratory. It was demonstrated that this attainable acidity, extending from -3 to -8, is exclusively depending of the nature of anion and follow the order: PF6 > BF4 > NTf2 > OTf. Nevertheless, the Hammett acidity function is an apparent function in this media and must then be corrected for. Consequently, in a second step, we tried to evaluate directly the proton activity from the determination of a potentiometric acidity function (R0) based on the extrathermodynamic Strehlow assumption.3 Therefore, the equilibrium potential of the H+/H2 couple was measured with an hydrogen electrode versus the ferricinium/ferrocene couple for which the potential is considered as independent of the solvent. [less ▲]

Tentative determination of the acidity level in room temperature ionic liquids by electrochemical methods

in ECS Transactions (2009), 16(49), 3

In our attempt to evaluate the acidity levels reached by acidified ionic liquids (BMImBF4, BMImNTf2 and BMImOTf + HOTf or HNTf2), the uncertainty on the pKas of the indicators needed for the Hammett ... [more ▼]

In our attempt to evaluate the acidity levels reached by acidified ionic liquids (BMImBF4, BMImNTf2 and BMImOTf + HOTf or HNTf2), the uncertainty on the pKas of the indicators needed for the Hammett spectrophotometric procedure was pointed out. As consequence another method is proposed, based on the H+/H2 couple potential measurement. In this purpose, if dynamic methods failed mainly for lack of sufficient reversibility, potentiometry with a hydrogen electrode gave meaningful results. The R0(H+) Strehlow function, could be calculated, using the Fc+-Fc couple as reference assumed as solvent independent. The obtained results show that i) the acidities are much higher than those in water; ii) the acidities measured by the hydrogen electrode are higher than those measured by the Hammett method; iii) the sequence of acidities for solutions of similar content of added acid is still BF4 > NTf2 > OTf as previously measured with the Hammett method. [less ▲]

About the Acidity Level in Room Temperature Ionic Liquids

in ECS Transactions (2007), 3(35), 71

The Brønsted acidity level was evaluated for ionic liquids to which a strong acid has been added. The evaluation method is based on the determination of the Hammett acidity functions H0, using UV-Visible ... [more ▼]

The Brønsted acidity level was evaluated for ionic liquids to which a strong acid has been added. The evaluation method is based on the determination of the Hammett acidity functions H0, using UV-Visible spectroscopy. The acidity of protons is mainly determined by their solvation state and consequently, the properties of protons depend on both the nature of the solvent and the nature and concentration of the acid. In practice, it was found that, for the investigated ionic liquids, the cation as well as the added acid nature does not play a dominant role, whereas changing the anion nature may lead to very different acidities. Indeed, for a similar content of added acid, the measured acidity levels are in the order: PF6- > BF4- > NTf2- > OTf-. The problems of the influence of impurities on the final acidity and of the weakly dissociating character of the ionic liquid are addressed. [less ▲]