32
August/September 2011 260.00 240.00 220.00
For the purposes of this experiment, the relative response is equal to the peak area of the solute in ionic liquid divided by the peak area of the solute in DMAC.
200.00 180.00
(Note: ΔδX = (δX(solute) /δX(DMAC)) - ((δX(solute) /δX[BMIM][PF6])
160.00 Solute 140.00 120.00 Pentane Cyclohexane 100.00 80.00
Table 1: Difference in Hansen solubility parameters from DMAC to [BMIM][PF6]
60.00 40.00 20.00 0.00 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 Minutes 4.50 5.00 5.50 6.00 6.50 7.00 7.50 8.00
The first functional group to be considered are the alkanes. In order of relative response: cyclohexane > pentane. The dispersion interaction difference between DMAC and
Figure 5: Chromatogram of [BMIM][PF6]; showing residual solvent impurities. In all cases the residual solvents response in
[BMIM][PF6] was superior to [BMIM][BF4]. The response of cyclohexane has been increased almost 33 fold when compared to its DMAC response.
As can be seen in the chromatogram below,
no peak relating to [BMIM][PF6] is observed during the run (Figure 4). However, the chromatogram of blank showed two large peaks corresponding to the retention time of dichloromethane, acetone and two smaller peaks corresponding to low levels of toluene and n-butyl acetate (Figure 5) were also present. Therefore this solvent would not suitable for residual solvent analysis of the aforementioned solvents since significant levels of these volatile organic compounds are already present in the ionic liquid.
[BMIM][BF4] has an interfering peak (Figure 4) making quantification of acetone and acetonitrile difficult. The difference in behaviour between ionic solvents may be explained by their difference
in hydrogen bond basicity. [BMIM][PF6] has a lower hydrogen bond bascity (1.579 at 70°C) than [BMIM][BF4] (1.967°C)10
. Due to
the unique functionality of the ionic liquids their intermolecular interactions are numerous and complex (Figure 1). The main solute solvent interactions involve: hydrogen bonding, dispersion and polar interactions. Several linear free energy relationships have been proposed to account for the nature of
Propan-2-ol Ethanol
Methanol n-Propanol t-Butanol
ΔδD 0.19
0.19 0.18 0.19 0.19
ΔδP 0.17
0.25 0.35 0.20 0.17
• δd - The energy from dispersion bonds between molecules
• δp - The energy from dipolar intermolecular force between molecules
• δh - The energy from hydrogen bonds between molecules.
(Note: ΔδX = (δX(solute) /δX(DMAC)) - ((δX(solute) /δX[BMIM][PF6]) Solute
.
the intermolecular interactions, the most recent being the Abraham solvent parameter modelx
The decrease in partition coefficient of the residual solvents in the ionic liquids may account for the observed increase in response. The aim of the sample matrix is to lower the partition coefficient of the analyte (i.e. increase the analyte activity coefficient so that a greater concentration can be sampled in the headspace). Thus ionic liquids have lowered the residual solvent’s partition coefficient and hence greater sensitivity has been achieved.
The relative responses of the analytes in
[BMIM][PF6] may additionally be understood by the Hansen solubility parameters when classified according to functional group (Table 1). Hansen solubility parameters were developed by Charles Hansen as a way of predicting if one material will dissolve in another. They are based on the idea that like dissolves like, with three descriptors being used to define the energy of a molecule;
ionic liquid is greater for cyclohexane (ΔδD = 0.2) than pentane (Table 1). Thus dispersion forces may be the dominant interaction for the alkanes. However, the response of only 2 alkanes cannot be used as reliable indication of a trend for the entire homologous series and thus further work should be undertaken to correlate response with dispersion interaction difference.
However, there is no clear trend for alcohols. The strength of all three interaction parameters need to be taken into account to describe for the difference in relative response (Table 2). The solute which displays the largest change in relative response is propan-2-ol (13), however there is no
difference in δH (ΔδH = 0.1) compared to the t-butanol which has a smallest change in relative response (2.3). It is worth noting that the solvent boiling points may have an effect on the observed response. The equilibration temperature is 85°C and thus under these conditions we can assume that the analytes are fully vapourised. However, t-butanol and n-propanol have higher boiling points, 99°C and 97°C respectively and therefore may only be partially vapourised in the headspace, and consequently t-butanol and n-propanol may have low relative responses because only low concentrations of solvents are present in the headspace, thus the difference between
DMAC and [BMIM][PF6] is too small for an accurate comparison to be made.
Δ δH 0.10
0.12 0.14 0.11 0.10
Boiling Point/o 82
78.3 64.7 97 99
Table 2: to illustrate difference in Hansen solubility Parameters from DMAC to [BMIM][PF6]
C rel. response 13 11
7.1 4.6 2.3
Reproducibility The peak area %RSD peak area for each volatile organic impurity needed to be less than 10% and the retention time %RSD for each volatile organic impurity needed to be less than 1% as per the regulatory requirement.
The reproducibility criteria for %RSD peak
0.17 0.2
0 0
0 0
ΔδD ΔδP Δ δH rel.
response 1.5 33
mV
n-pentane - 2.621 ethanol - 2.781
acetone - 3.231
DCM - 3.748
toluene - 6.603 n-butyl acetate - 6.856
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