then dried at 60 1C, under reduced pressure, over P4O10, until
constant weight. Aliquots of the probe solution in acetone
were pipetted into 2 mL volumetric tubes, followed by
evaporation of acetone at room temperature, under reduced
pressure, in the presence of P4O10. IL, W, or IL-W mixtures
were added so that the probe final concentration was
2 ꢂ 10ꢀ5 mol Lꢀ1. A Shimadzu UV 2550 UV-vis spectro-
photometer was used. The temperature inside the thermo-
statted cell-holder was controlled to within ꢃ0.05 1C with a
digital thermometer (model 4000A, Yellow Springs Instruments,
Yellow Springs). Each spectrum was recorded twice at a rate
of 140 nm minꢀ1; the values of lmax were determined from the
first derivative of the absorption spectra. The uncertainty in
ET(MePMBr2) is r0.15 kcal molꢀ1. Thermo-solvatochromism
was studied in the temperature range from 10 to 60 1C.
Densities of the liquids were determined at 25, 35, 40 and
45 1C with a DMA 40 resonating-tube densimeter (Anton
Paar, Graz).
non-ideal; this may be attributed to several factors and/or
solute–solvent interaction mechanisms. Enrichment of the
probe solvation shell in the solvent of higher er leads to
‘‘dielectric enrichment’’, and consequently to non-ideal
behavior.14 However, er (W) is 4 er (IL),15 i.e., if dielectric
enrichment were operative, all curves of Fig. 2 should lie
above-, not below the straight line that connects the polarities
of the two pure liquids; this is not the case.
Non-ideal behavior can originate from preferential solvation
of the probe by a component of the mixture, due to solute–
solvent specific interactions, e.g., hydrogen-bonding and
dipole–dipole interactions. As discussed elsewhere,8a,b most
binary mixtures of solvents are micro-heterogeneous; there
exists the possibility of preferential solvation of the probe
by the less polar micro-domains, leading to below-the-line
deviation, as shown in Fig. 2. In summary, non-ideal solvation
behavior is not unexpected.
An important objective of the present work is to evaluate
the application to aqueous ILs of the solvation model that we
have applied to analyze solvatochromism in binary mixtures of
water with protic or aprotic solvents. The basis of this model is
that the binary mixture is made of the two pure solvents, plus a
‘‘complex’’ one; the latter is constituted of hydrogen-bonded
species. Therefore, the medium employed is composed of IL,
W, and a 1 : 1 IL-W hydrogen-bonded complex, as given by
eqn (2). These three solvents compete for the solvation of the
probe, as described by eqn (3)–(5):8a,b,9
Results and discussion
Note: Details of all calculations performed are given in the
Calculations section.
The probe and the ILs employed
We have employed MePMBr2 because its empirical polarity
scale, ET(MePMBr2) is linearly correlated with ET(30) (34 solvents,
correlation coefficient, r = 0.9685), the most extensively
investigated polarity scale. Although the susceptibility of
MePMBr2 and RB to solvent ‘‘acidity’’ and dipolarity/
polarizability are practically the same, the former probe is
sensitive to solvent lipophilicity, as given by the empirical
scale log P.11 The latter is extensively employed as a measure
of lipophilicity or hydrophobic character; it refers to the
partition coefficient of a substance between 1-octanol and
IL + W " IL-W
(2)
(3)
Probe(IL)m + m (W) " Probe(W)m + m IL
Probe(IL)m + m (IL-W) " Probe(IL-W)m + m IL (4)
Probe(W)m + m (IL-W) " Probe(IL-W)m + m W (5)
where (m) represents the average number of solvent molecules
whose exchange in the probe solvation shell affects ET(probe);
usually m r 2; (m) should not be confused with the total
number of solvent molecules that solvate the probe. An
important consequence of this model is that the mole fractions
that we have employed in our calculations (except those of
Table 1) are ‘‘effective’’ not analytical ones. In order to keep
these calculations tractable, working assumptions are included,
namely, a single value is attributed to (m); the stoichiometry of
IL-W species is taken as 1 : 1. The former has proved to be
valid for the solvation of many probes, including the one
employed here.8a,b The formation of hydrogen-bonded
complexes between ILs, e.g., BuMeImBF4 or AlBuImCl and
water has been demonstrated by IR,16a,b NIR,17 1H NMR,9
and predicted by theoretical calculations.18 A plausible
structure for this complex is water-mediated hydrogen bonding
between the anion and the cation, in particular the relatively
acidic H2 of the imidazolium ring. Formation of complexes
composed of the IL with one or more water molecules can be
envisaged;19 the species with stoichiometry other than 1 : 1
may be treated, to a good approximation, as mixtures of the
1 : 1 structure plus excess solvent (IL or W). The appropriate-
ness of the 1 : 1 stoichiometry, and the model itself can be
further tested by the goodness of fit of the regression analyses
of ET(MePMBr2) versus wW, vide infra, and agreement of the
water, both mutually saturated: log P = log ([substance]1-octanol
/
[substance]water).13 The values of log P were found to
be ꢀ1.467, ꢀ1.080, and ꢀ0.858, for AlMeImCl, AlBuImCl,
and AlHxImCl, respectively, i.e., all ILs are more soluble in
water than in 1-octanol. As expected, their lipophilicity in-
creases as a function of increasing the number of carbon
atoms, NC, of the (variable) alkyl group R in AlRImCl,
according to the equation: Log P = 1.583 + 0.122 NC;
r = 0.9968, see Fig. S2.w In summary, we have employed a
probe that is sensitive to the hydrophobic character of the
medium; the ILs employed differ in their solubility in 1-octanol
(hence lipophilicty) by a factor of 4.1. Note that log P of the
reference molecular solvent, PrOH, is 0.25, i.e., its lipophilicity
is 52.1, 21.4, and 12.8 times that of AlMeImCl, AlBuImCl, and
AlHxImCl, respectively. As given in the Experimental section,
the m.p. of AlMeImCl is 52–53 1C; once melted, this IL takes
several hours to solidify at room temperature. Therefore,
we were able to determine its empirical polarity in the
temperature range studied, except at 10 1C.
Thermo-solvatochromism in mixtures of ILs and water
Fig. 2 shows the solvatochromic responses of MePMBr2 as a
function of the mole fraction of water in the binary mixture,
wW., at 25 1C. All plots shown are non-linear, i.e., solvation is
ꢁc
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1766 | Phys. Chem. Chem. Phys., 2010, 12, 1764–1771