The below is a summary of the literature search I performed in order to explain the shoulder in [C4mim][TF] around .8 to 1.1 inverse angstroms and why it might possibly vanish for the doped [C4mim][TF] samples. After the individual summaries from each reference, I will make my own concluding remarks.
Some of the resource materials are too large to upload but I have them saved to a folder on my flashdrive.
J. Phys. Chem. B 114, 16838 (2010)
The compounds explored in this reference are as follows: [C6mim][Cl], [C8mim][PF6], [C10mim][PF6].
The authors make a claim that for the above compounds that the structure factors for the liquid below 2 inverse angstroms are often similar of the structure factor in a powdered crystal simulation of the same sample. They continue by saying this is from a similar structuring of the liquid as was in the solid crystal, although the length scale is on a longer range in the liquid. The specific contributions of the peak at .9 inverse angstroms in liquid arise from the positive cation-cation and anion-anion terms summed with the negative cross terms. They make the definite claim the peak/shoulder arises from “short interionic distance between polar groups of the same charge.” The authors make an equivalent interpretation later on that the feature can be, “Also interpreted as a periodicity in the absence of ions of opposite charge at that particular distance”.
J. Chem. Eng. Data. 59, 3120 (2014)
See Table 1 of reference for specific cation to anion pairings (there are 14 different pairings).
The authors in this work call the feature which is generally around 8.8 inverse nanometers an intermediate peak which shows “the periodicity of the polar network for each IL (alternation between ions of opposite and similar charge.” This statement is in agreement with the reference from the last section.
The authors explore structure factors of [C6mim][C4SO3] and [C6mim][Ntf2], noting the intermediate peak at 8.8 inverse nanometers in [C6mim][Ntf2] shifts to 9.3 inverse nanometers when [Ntf2]– is replaced with [C4SO3]–. The authors state that a smaller anion polar head allows for a more compact polar network. The intermediate peak becomes a shoulder in this case because “the combination of lower intensities with shifts to higher q-values (and the corresponding approach to the contact peak [strong peak at 14 inverse nanometers]) justifies the transformation of the intermediate peak into a shoulder at 9.3 inverse nanometers.”
The authors make the intermediate peak in [C4NH3][Ntf2] (at 9.1 inverse nanometers) vanish by replacing the anion with Carboxylates [CnCOO]–. The authors state that, “the polar network is composed by small ionic heads (the –NH3 and –COO groups).” The authors then go on to describe the resulting structure which forms in [C4NH3][C1COO]:
We have a necklace-like polar network that is extremely thin and flexible in the midst of nonpolar regions that even with only C4 chains occupy a relatively large proportion of the available volume. Such a fact will hinder the emergence of any intermediate-range ordering/periodicity.
In other words, the bonding of the cation and anion heads eliminates the intermediate peak which means the polar network no longer has intermediate ordering.
A comparison of four anions is made, keeping the cation as [C4mim]+. The anions are [Ntf2]–, [PF6]–, [S1SO3]–, [C1COO]–. The authors go on and list the anions which have the strongest intermediate peak (around 8.5 inverse nanometers) to anions having more subdued peaks (around 9.5 inverse nanometers). The order is [Ntf2]–, [PF6]–, [C1SO3]–, and [C1COO]–. Acetate ([C1COO]–) is the smallest anion and it also has the most subdued intermediate peak. A summarizing claim is made by the authors which states that the diminishing or vanishing of the intermediate peak occurs when, “the protic Ils revert to the corresponding neutral species.” Protic simply means that the species is able to donate a Hydrogen atom, resulting in Hydrogen bonding.
J. Chem. Phys. 134, 104509 (2011)
In this article the structure of [Cnmim][Br] with n = 2,4,6 is explored. The authors note a shoulder at 1.1 inverse angstroms which is most evident for n=2. They contribute the shoulder to the positive peaks in the ring center to ring center and anion to anion correlations with the strong negative peak in the cross terms and state that this is exhibited in molten salts through charge ordering. A reference (J. Phys. Chem. B 114, 12623 (2010)), which only looked at the n=2 case, reached these conclusions and initially explained the phenomenon. The cation ring and the anion have 34 and 36 electrons respectively which is how the authors explain the shoulder/peak’s small amplitude.
After reading the above articles, along with many others, we now have a better understanding of the intermediate shoulder/peak feature present in some ionic liquids. The feature’s cause becomes apparent in the molecular dynamic simulations. The partial structure factors of the cation to cation and anion to anion terms contribute positively to the total structure factor around the q values for this feature whereas the cross terms contribute negatively. These structure factors give insight to the intermediate peak resulting from an intermediate ordering of the polar species, giving rise to what the above authors refer to as a polar network. Experiments have explored different combinations of cations to anions and have provided fruitful results. In general, the larger cation and anion polar heads have a more distinct intermediate peak. Smaller polar heads lead to an intermediate peak which is hidden by the main “contact peak” around 1.5 inverse angstroms. The intermediate peak will also go away if the cation and anion form a neutral species in the ionic liquid, as is the case for protic ionic liquids, which effectively disrupts the intermediate ordering of the polar network.
The above conclusion leads me to two possibilities happening in the doped [C4mim][TF] samples. The first involves the doping element forming a neutral species (ion pair) which would mean the intermediate ordering of the polar network would no longer be present (or would be dampened) in our experiment as was the case for the protic ionic liquids. The other possibility is that the smaller doping element makes the ordering in our liquid more compact and therefore shifts the intermediate feature into the larger scattering peak around 1.5 inverse angstroms, effectively hiding it in the much larger peak.