Communications
200L119 and 300L33. The viscometers were immersed in a water
bath with an HCTB-3030 immersion cooler from Omega Instruments
that was capable of maintaining temperatures within 0.058C. The
measured viscosities were reproducible to within 0.1 cSt.
Like unsaturated fats, ILs 2, 4–7, 9, and 11 discolor over
time if left in contact with air. This observation indirectly
suggests a high solubility of gases (e.g., O2) in these new
species, as in unsaturated fats such as olive oil.[17] Although
the degree of oxidation is sufficiently low as to be undetect-
able by NMR spectroscopy after a month, discoloration is
apparent sooner, and these ILs gradually acquire a faint odor
of decomposition products associated with side-chain oxida-
tion. Although likely to render the cations biodegradable,[18]
this reactivity will be deleterious for many other applications.
Oxidation can be suppressed by storing the ILs under N2 or by
adding to them small quantities (ca. 1 mol%) of butylated
hydroxytoluene (BHT), an antioxidant used to prevent
rancidification in processed foods.
Crystal data for 11a: C38H45BN2, Mr = 540.57, 0.17 ꢀ 0.02 ꢀ
0.02 mm3, monoclinic, P21/c, a = 18.1597(8), b = 35.5151(11), c =
21.3555(7) ꢁ, b = 111.634(5)8, V= 12802.9(8) ꢁ3, Z = 16, 1calcd
=
1.122 gcmÀ3, m = 0.479 mmÀ1, CuKa radiation (l = 1.54178 ꢁ), T=
100(2) K, 2tmax = 59.068, 38370 measured and 17595 independent
reflections, Rint = 0.1026, R = 0.0579, wR = 0.1084, Oxford Diffraction
Ltd., Xcalibur CCD system, solution with direct methods and
refinement with 1490 parameters, difference map between + 0.193
and À0.209 eꢁÀ3. CCDC 752914 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
Not surprisingly, the new ILs are good solvents for
cholesterol, a major membrane component in many organ-
isms. Whereas little cholesterol dissolves in “typical” ILs, such
as [BMIM]Tf2N (BMIM = 1-butyl-3-methylimidazolium), we
have observed the dissolution of up to 30 mass percent in 4 at
room temperature. This result prompts us to suggest that
these new unsaturated ILs and similar ILs may be useful as
solvents for biomolecules that are membrane-localized or
drug molecules that act on membrane components or
passively diffuse through the phospholipid bilayer to enter
the cell. One category of species that are expressly interesting
in this context is the lipases, which have been investigated for
their catalytic behavior in more typical ILs.[19] Although the
factors governing enzyme activity and stability in ILs are
complex,[19] it seems reasonable to propose that IL solvents
more akin to those environments in which such enzymes
naturally function may offer advantages over other IL
solvents.
In conclusion, the present data show that ILs with long
alkyl appendages respond to side-chain modification—specif-
ically unsaturation—in a fashion reminiscent of homeoviscous
adaptation by living organisms. It is a phenomenon that is
eminently exploitable from a utilitarian standpoint and that we
believe creates new opportunities for manipulating the proper-
ties of ILs and other synthetic soft materials. Studies aimed at
assessing an array of such possibilities are in progress.
Received: November 2, 2009
Revised: January 28, 2010
Published online: March 8, 2010
Keywords: fluid-mosaic model · homeoviscous adaptation ·
.
ionic liquids · lipids · phase transitions
[3] A. A. H. Padua, J. N. A. C. Lopes, ACS Symp. Ser. 2007, 975, 86 –
101.
[4] I. Lꢂpez-Martin, E. Burello, P. N. Davey, K. R. Seddon, G.
Rothenberg, ChemPhysChem 2007, 8, 690 – 695.
[5] A. E. Bradley, C. Hardacre, J. D. Holbrey, S. Johnston, S. E. J.
[7] C. M. Gordon, J. D. Holbrey, A. R. Kennedy, K. R. Seddon, J.
October 31, 2009), National Institute of Standards and Technol-
ogy, Gaithersburg MD, 20899, 2009.
[12] C. M. Gordon, M. J. Muldoon in Ionic Liquids in Synthesis,
Vol. 1 (Eds.: P. Wasserscheid, T. Welton), Wiley-VCH, Wein-
heim, 2007, pp. 9 – 20.
[14] M. C. Costa, M. P. Rolemberg, L. A. D. Boros, M. A. Kraehen-
[15] X. Wang, F. W. Heinemann, M. Yang, B. U. Melcher, M. Fekete,
[16] We think it both important and noncoincidental that this length
(about seven carbon atoms) also corresponds to the length at
which the organic domain of alkyl imidazolium ILs appears to
become effectively continuous in character (see references [3]
and [4]).
[17] F. D. Gunstone, The Chemistry of Oils and Fats: Sources,
Composition, Properties and Uses, CRC, Boca Raton, 2004,
pp. 141 – 143.
Experimental Section
High-purity (> 99%) fatty-alcohol mesylates were obtained from Nu-
Chek Prep, Inc. and used as received for the preparation (under
Finkelstein conditions)[20] of the corresponding iodides. The ILs were
prepared and characterized by using standard techniques,[7] with the
exception that all reactions were performed under N2. Melting points
were determined by using a Seiko Instruments differential scanning
calorimeter, model SSC/5200-DSC 220U, with a heating rate of
58minÀ1. The reported data are the average of at least three trials, and
were reproducible to within 18C. The phase transition identified as
the melting point is the one for which a liquid is the product; the
former state may have been either crystalline or amorphous. Several
of the new ILs were difficult to crystallize even by cooling to À1708C.
However, we found that preheating of the samples at 758C for 10 min
followed by slow cooling to À1708C resulted in an exothermic event
on subsequent heating that was consistent with a glass transition or
crystallization from a gel. Subsequent warming then reproducibly
yielded a reliable Tm value. Viscosities were measured by using the
traditional flow technique in Cannon Ubbelohde Viscotesters
2758
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 2755 –2758