boundaries were indistinct. Heating of this gelled emulsion to
> 50 °C resulted in the ‘melting’ of the mass which separated
into discrete aqueous and organic layers. When this was allowed
to cool undisturbed, as before only the organic layer was gelated
selectively. Similarly, a given volume of commercially availa-
ble petrol could be gelated from a two-phase mixture.
Importantly the gelation was unaffected even in the presence of
(a) NaCl (at various concentrations) or (b) chelation inducing
metal salts (e.g. CuSO4), or (c) oxidizing agents (e.g. KMnO4)
or (d) other impurities present in water from natural sources (not
shown).
Notes and references
‡ All new compounds, 1–4, were synthesized from readily available
precursors and were characterized by FT-IR, 1H-NMR, LR-MS and
elemental analysis. Selected data: N-n-dodecanoyl-(S)-alanine (1) was
synthesized by the hydrolysis of 2 in MeOH with 1.0 eq. of 1 M NaOH for
2 h at ~ 5 °C followed by careful acidification in cold conditions. Isolated
as a solid, mp: 84 °C (97% yield). [a]D25 (c = 2 in CHCl3) = +16.6°. IR
(cm21): 3348, 1704, 1646, 1520. 1H-NMR (300 MHz, CDCl3) d: 0.88 (t, J
= 6.5 Hz, 3H), 1.25 (br m, 16H), 1.45 (d, J = 7 Hz, 3H), 1.61 (m, 2H), 2.22
(t, J = 8 Hz, 2H), 4.54 (m, 1H), 6.4 (d, J = 9 Hz, 1H). LR-MS: 271 (M+,
2%). Anal. calcd. for C15H29NO3: C, 66.38; H, 10.77; N, 5.16. Found: C,
66.62; H, 10.94; N, 4.92%. Methyl N-n-dodecanoyl-(S)-alaninate (2) was
Next, the roles of various functional groups involved in the
self-assembly process were investigated by blocking one by one
the hydrogen bonding sites in 1 via chemical modification. First
the carboxylic acid (-CO2H) in 1 was converted to its methyl
ester, 2. Then the amide moiety (-NH-C(O)) in 1 was subjected
to N-methylation to give 3. Interestingly, neither 2 nor 3 induced
any gelation. Thus the presence of both the -CO2H and the
secondary amide (-NH-C(NO)) appear to be essential for self-
association of the monomer into fibers (Fig. 1) a necessary
prerequisite for gelation.
synthesized by reaction of L-alanine methyl ester hydrochloride (Fluka)
with dodecanoyl chloride (1.1 eq.) in dry CHCl3 and Et3N (2.2 eq.). Isolated
as a solid, mp: 65 °C, (97% yield). [a]D 25 (c = 2 in CHCl3) = +14.0°. IR
(cm21): 3300, 1732, 1650, 1537. 1H-NMR (300 MHz, CDCl3): d: 0.88 (t, J
= 7 Hz, 3H), 1.26 (br m, 16H), 1.41 (d, J = 6.5 Hz, 3H), 1.62 (m, 2H), 2.18
(t, J = 8 Hz, 3H), 3.73 (s, 3H), 4.56 (m, 1H), 5.93 (d, J = 9 Hz, 1H). LR-
MS: 285 (M+, 2%). Anal calcd. for C16H31NO3: C, 67.33; H, 10.95; N, 4.91.
Found: C, 67.52; H, 11.03; N, 5.05%. N-n-Dodecanoyl-N-methyl-(S)-
alanine (3) was synthesized by the reaction of N-methyl-L-alanine (Fluka) in
dry DMF with dodecanoyl chloride (Fluka) and Et3N (1.1 eq.). Isolated as
a solid, mp: 79 °C, (64% yield). [a]D25 (c = 2 in CHCl3) = +10.1°. IR
(KBr) (cm21): 1701, 1645, 1541. 1H-NMR (300 MHz, CDCl3) d: 0.87 (t, J
= 6.5 Hz, 3H), 1.26 (br m, 16H), 1.44 (d, J = 7 Hz, 3H), 1.62 (m, 2H), 2.21
(t, J = 8 Hz, 3H), 2.91 (s, 3H), 4.53 (m, 1H). LR-MS: 285 (M+, 2%). Anal.
calcd. for C16H31NO3: C, 67.33; H, 10.95; N, 4.91. Found: C, 67.22; H,
10.58; N, 4.74%. N-Dodecyl-(S)-alaninamide (4) was prepared by catalytic
hydrogenation (10% Pd/C) in MeOH of N-benzyloxycarbonyl-NA-hex-
adecyl-(S)-alaninamide, a compound that was prepared by DCC coupling of
(S)-N-Benzyloxycarbonylalanine (Fluka) and n-hexadecylamine (Fluka) in
dry THF. Isolated as a hygroscopic solid, mp: 54 °C (87% yield). [a]D25 (c
In order to understand the precise roles of the -CO2H and
-NH-C(NO) residues in 1 in the process of gelation, detailed FT-
IR studies were carried out.∑ First, FT-IR spectra of (a) the solid
(KBr pellet) from a dried benzene gel of 1, (b) solutions of 1 (c
= 60 mg mL21) in benzene and (c) in a non-gelatable solvent
such as CHCl3 were compared (not shown). The amide and the
–CO2H moieties in 1 are as strongly hydrogen bonded in the gel
state as they are in the solid state. Benzene does not interfere
with the intermolecular association. The amide I band in the gel
state was almost as strongly hydrogen bonded as in the solid
1
= 2 in CHCl3) = +12.8°. IR (KBr) (cm21): 3320, 1630, 1560. H-NMR
(300 MHz, CDCl3) d: 0.86 (t, J = 7 Hz, 3H), 1.2 (br m, 18H), 1.26 (m, 2H),
1.43 (d, J = 7 Hz, 3H), 3.2 (m, 2H), 4.09 (m, 1H), 7.5 (br s, 1H). LR-MS:
256 (M+, 40%). Anal. calcd. for C15H32N2O·0.25 H2O: C, 69.04; H, 12.56;
N, 10.74. Found: C, 69.36; H, 12.83; N, 10.48%.
§ The ability of 1 to gelate a given solvent was tested by solubilizing 1 (1
mmol) in the desired solvent (7.5 mL) by gentle heating and allowing the
solution to spontaneously cool to rt. The gel was allowed to stand for ca. 15
min at rt. MGC was calculated as described in the literature.3a
¶ Scanning electron micrograph (SEM) was recorded using a Cambridge
stereoscan S-360 SEM. A glass plate bearing a droplet of 1 dissolved in
benzene or n-heptane was attached to the sample stage after completion of
gelation and sputtered with gold to 100–150 Å.
(1646 cm21). However, in CHCl3 the amide I band (1669 cm21
)
evidenced a weakly hydrogen bonded species probably between
the oxygen of the amide carbonyl and the acidic H of CHCl3.
This is confirmed by efficient gelation of CCl4 by 1. Thus
solvents capable of hydrogen bonding suppress gelation in these
systems by disallowing the self-assembly process.
The -CO2H sites of 1 in both solvents form dimers of
comparable strength (1732 cm21). Thus the factor controlling
gelation of 1 seems to be hydrogen bonding at the amide sites
(-N–H…(O)NC–N–H…(O)NC-), which is significantly affected
by the polarity and protic nature of the solvent. However, the
strength of the amide I band is considerably weaker in CHCl3
(1669 cm21) than in benzene (1648 cm21). It is this difference
in strength that most probably determines whether the solvent
promotes or inhibits perpetuation of the superstructure. In
addition to the amide site, the availability of the free -CO2H is
mandatory for the dimer formation leading to gelation. This was
confirmed with 4 where the free amine cannot form analogous
dimers and despite the presence of amides, the gelation did not
occur.
∑ 1 was dissolved in the desired concentrations in benzene or CHCl3 and
loaded into a solution cell of a JASCO 410 FT–IR spectrometer. Spectra
were corrected for solvent absorption.
1 S. Miyazaki, F. Suisha, N. Kawasaki, M. Shirakawa, K. Yamatoya and D.
Attwood, J. Controlled Release, 1998, 56, 75; Y. Ito, N. Sugimura, K. Oh
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2 See for a recent highlight: J. H. van. Esch and B. L. Feringa, Angew.
Chem., Int. Ed., 2000, 39, 2263.
In summary, in order to exhibit gelation, the molecule must
have the capacity to self-assemble in three-dimensions to form
fibrous networks. Self-assembly of 1 is evident from SEM. IR
studies show that this process involves at least two interactions
(-CO2H and (O)NC–N–H) where each residue promotes the
formation of a supramolecular array. In water, due to the
presence of a lipophilic alkyl chain, 1 exerts a hydrophobic
effect4 and excludes water. Additional stabilization of such
aggregates most likely originates from Van der Waals contacts
of the polymethylene chains. This promotes the self-organiza-
tion of 1 and in the process it is able to gelate hydrocarbon-
based fuels or solvents even in the presence of water. While the
present system is interesting, the necessity to heat and cool the
samples in order to achieve phase separation significantly limits
its use for the containment of oil-spills. Clearly issues such as
requirement of heating to achieve gelation have to be addressed
before a real-life application is possible. Nevertheless the
present system demonstrates its unique ability to confer phase-
selective gelation of toxic solvents from complex mixtures.
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