Sigma Profiles and Potentials of Ionic Liquids

The sigma profile and potential of cations and anions are obtained using Equations 3.55 and 3.56, respectively (Figures 3.5 and 3.6). For such an exercise, we have chosen a common cation, that is, 1-alkyl-3-methylimidazolium

FIGURE 3.5

Sigma profiles of individual cations (EMIM: 1-ethyl-3-methylimidazolium; BMIM: 1-butyl- 3-methylimidazolium; OMIM: 1-Octyl-3-methylimidazolium).

FIGURE 3.6

Sigma potential of individual cations.

FIGURE 3.7

Sigma profile of individual anions.

([RMIM]; R=O, H, B etc.; O:Octyl; H:Hexyl; B:Butyl) (Figure 3.5) and then paired it up with three common anions, as given in Figure 3.7.

The sigma profiles and potentials for cations are shown in Figures 3.5 and 3.6, respectively. In all the normalized profiles, a small part of the sigma profile lies to the left of the hydrogen bonding cutoff radius (i.e. о < -0.0082 e/A2), indicating a hydrogen bonding donor capacity. The sigma profiles for [BMIM] and [OMIM] are of the same nature. Most of the prominent peaks lie in the negative side of the sigma profile. The negative screening charges are due to the positive charge residing inside the imidazolium ring. [EMIM], [BMIM], [OMIM], all show peaks at the outmost position in the negative direction. They also show up in the sigma potential (Figure 3.6), that is, negative values of chemical potentials are encountered on the positive side of the screening charge densities (SCDs). The negative values are encountered since extra free energy is gained by forming hydrogen bonds. It should be noted that extra energy is gained with the formation of hydrogen bonds; this energy is negative in value when compared with the free energy required (which is positive in nature) for removing the SCDs.

The sigma profiles and potentials for anions are shown in Figures 3.7 and 3.8, respectively. For the anions, it is evident that a negative contribution to the sigma potential is due to positive screening charges, as obtained from their sigma profiles. It should be noted that the contribution of chemical or segment potential is a sum of two values, that is, the misfit term and the hydrogen bond term. For the anions, a negative value on the profile implies that it is the hydrogen-acceptor bonding that dominates the left-hand side of

FIGURE 3.8

Sigma potentials of individual anions.

Figure 3.7. However, this is the opposite in the donor region, where the peak rises sharply owing to the difference in the shape and size of the segments or the misfit contribution is known to dominate the total interaction.

For all the anions, peaks are lying on the right of the cutoff zone for hydrogen bonding, that is, <5hb = +0.0082 e/A2. All the prominent peaks for the anions, that is, 0.0085 e/A2 for [PF6], 0.014 e/A2 for [CF3SO3], 0.015 e/A2 for [EtSO4] and 0.011 e/A2 for [BF4], lie to the right of the cutoff SCD, that is, ahb = +0.0082. This is due to the inherent negative charges of the anions. The sigma potential is a similar one as observed for cations, except that negative values of chemical potentials are encountered on the negative side of the SCDs.

 
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