Characteristics of gelation by amides based on trans-1,2-diaminocyclohexane: The importance of different substituents

Article abstract of DOI:10.1246/bcsj.20160360

Six diamides were prepared from trans-(1R,2R)-1,2-diaminocyclohexane and the corresponding racemate and were subsequently used as gelators. Three chiral compounds and their racemates were prepared. One of the chiral compounds and its racemate contained tw

Full text of DOI:10.1246/bcsj.20160360

Selected Paper
Characteristics of Gelation by Amides Based on trans-1,2-
Diaminocyclohexane: The Importance of Different Substituents
1
1
1
2
2,3
Haruka Nakagawa, Mamoru Fujiki, Takaaki Sato, Masahiro Suzuki, and Kenji Hanabusa*
1Faculty of Textile Science & Technology, Shinshu University, Ueda, Nagano 386-8567
2Interdisciplinary Graduate School of Science & Technology, Shinshu University, Ueda, Nagano 386-8567
3Division of Frontier Fibers, Institute for Fiber Engineering, ICCER, Shinshu University, Ueda, Nagano 386-8567
E-mail: hanaken@shinshu-u.ac.jp
Received: October 28, 2016; Accepted: December 12, 2016; Web Released: February 25, 2017
Kenji Hanabusa
In 1979, Kenji Hanabusa started his academic carrier at Shinshu University as an Assistant Professor. He
received Ph.D. from Osaka University in 1981 in Japan. In 1987, he was promoted to an Associate
Professor, and since 1999, he has been a Full Professor in the Interdisciplinary Graduate School of Science
and Technology at Shinshu University. His research interest is the development of low-molecular-weight
gelators and thickeners on the basis of supramolecular chemistry. He has published more than 300 articles
including original papers, reviews, and chapters of books.
Abstract
reason for researchers’ interest in gelators lies in their potential
industrial applications. Various compounds have been devel-
oped as gelators for use in cooking oils, spilled crude oil,
cosmetics, aromatic compounds, ink thickeners, and greases.
Six diamides were prepared from trans-(1R,2R)-1,2-diamino-
cyclohexane and the corresponding racemate and were sub-
sequently used as gelators. Three chiral compounds and their
racemates were prepared. One of the chiral compounds and its
racemate contained two n-dodecanoylamino groups as the same
substituents. The other two chiral compounds and their race-
mates contained different substituents: 10-undecenoylamino
and 2-heptyl-undecanoylamino groups, and 5-hydroxypenta-
noylamino and 2-heptylundecanoylamino groups. Their gela-
tion abilities were evaluated on the basis of the minimum gel
concentration using eight solvents. The thermal stability and
transparency of the gels were investigated by UV­vis spec-
troscopy using three-component mixed solvents of hexadecyl
2
3
For example, 12-hydroxystearic acid has been used practi-
cally as a gelator for cooking oil. As other examples, the
2
4
(1,3:2,4)-dibenzylidenesorbitol and the α,γ-bis-n-butylamide
of N-lauroyl-L-glutamic acid are used in antiperspirants, and
aromatic diureas are used in synthetic greases.
Physical gelation by low-molecular-weight compounds
results from noncovalent bonds, such as hydrogen bonding,
electrostatic interaction, van der Waals interaction, and π­π
interaction. During a gelation process, gelator molecules are
first self-assembled during a cooling process, producing fibrous
assemblies. These fibrous assemblies form a three-dimensional
network structure, and gelation occurs as solvent molecules are
trapped in the network.
Physical gelation by a gelator can usually be best understood
as resulting from competition between a gelator’s tendency to
dissolve via solvation and its tendency to self-assemble and
crystallize. That is, gels formed by gelators are reasonably
assumed to be metastable materials that are formed preceding
crystallization. Despite recent advances, designing a gelator is a
formidable challenge, if not impossible. An effective approach
to prevent the transformation from a metastable gel to a crys-
talline state has not been developed yet. Several other questions
associated with the design of gelators remain unanswered. For
instance, why do gelators in fluids form metastable gels instead
of thermally stable crystals and what are the necessary and
sufficient requirements, if any, for designing gelators? Notably,
most of the reported gelators have a chiral carbon center in their
2
-ethylhexanoate, liquid paraffin, and decamethyl cyclopenta-
siloxane (66 combinations). The gel-to-sol phase-transition
temperatures were also studied. The viscoelastic behavior of
the gels was studied by rheology measurements in the strain
sweep mode. Aggregates constructing three-dimensional net-
works were studied by transmission electron microscopy and
circular dichroism spectroscopy. The molecular packing of the
gels was evaluated by small angle X-ray scattering (SAXS).
1
. Introduction
Recently, low-molecular-weight gelators have attracted
special attention because large volumes of solvent can often
be immobilized by very small amounts of such compounds.
With advances in supramolecular chemistry, numerous reports
on gelators have been published in recent years.1 Another
­22
312 | Bull. Chem. Soc. Jpn. 2017, 90, 312–321 | doi:10.1246/bcsj.20160360
© 2017 The Chemical Society of Japan

structure, although exceptions are known.25­30 We previously
synthesized several types of gelators from chiral compounds
and observed that the corresponding racemates tend to crystal-
solvent evaporated. Recrystallization from 900 mL of ligroin
gave 67.87 g (70%) of trans-(1R,2R)-1-Boc-amino-2-amino-
cyclohexane. IR (KBr, cm ): 1695 (νC=O urethane), 1554
(δN-H amide II).
2.3.2 (R,R)-1: A solution of 34.07 g (0.159 mol) of trans-
(1R,2R)-1-Boc-amino-2-aminocyclohexane and 26.1 mL (0.238
mol) of N-methylmorpholine in 300 mL of dry THF was
cooled in an ice-water bath, and then 48.16 g (0.159 mol) of
2-heptylundecanoyl chloride was added drop-by-drop. The
mixture was stirred for 1 h in an ice water bath, followed by
¹
1
31­33
lize rather than form gels.
This behavior is explained by
3
4,35
Wallach’s rule;
specifically, racemic crystals tend to be
denser than their chiral counterparts and the former is more
stable than the latter. Despite racemic compounds being gen-
erally less expensive than chiral compounds, racemates are un-
fortunately unsuitable for use as gelators because of Wallach’s
rule. Resolving this apparent contradiction requires the devel-
opment of a method to prevent the denser packing of molecules
in racemates.
In the present paper, we focus on diamides derived from
trans-1,2-diaminocyclohexane and study the effect of different
substituents on the diamides’ gelation abilities. We attached
different substituents onto racemic trans-1,2-diaminocyclo-
hexane with the expectation that the different substituents
would prevent denser packing, resulting in physical gelation.
The incorporation of different substituents may lead to new
gelators if disordered molecular packing occurs.
3 h at room temperature. A filtrate without NEt /HCl salt was
3
evaporated and recrystallized from a mixture of 650 mL of
ethyl acetate and 250 mL of hexane. Yield; 71.78 g (94%). IR
¹1
(KBr, cm ): 1690 (νC=O urethane), 1644 (νC=O amide I),
1520 (δN-H amide II).
2.3.3 (R,R)-2: 130 mL of 25% HBr/AcOH was added to
34.07 g (0.159 mol) of (R,R)-1 in 100 mL of acetic acid and
stirred overnight. After completely removing HBr/AcOH, 400
mL of ether was added to the resulting oily product and cooled.
¹1
The oil crystallized. Yield; 66.25 g (96%). IR (KBr, cm ):
644 (νC=O amide I), 1525 (δN-H amide II).
.3.4 (R,R)-C12C18: A solution of 2.00 g (4.33 mmol) of
1
2
. Experimental
2
2
.1 Instrumentation. Elemental analysis was performed
(R,R)-2 and 0.88 g (8.67 mmol) of triethylamine in 60 mL of
dry THF was cooled in an ice water bath, and then 1.04 g (4.77
mmol) of n-dodecanoyl chloride was added drop-by-drop.
The mixture was stirred for 3 h at room temperature. 100 mL of
water was added to the reaction mixture and a precipitate was
filtered off and dried. Recrystallization from a mixture of 200
mL of ethyl acetate and 100 mL of hexane gave 2.18 g (89%) of
with a Perkin-Elmer 240B analyzer. Infrared spectra were
recorded on a Jasco FTIR-7300 spectrometer using KBr plate.
UV­vis and CD spectra were recorded on a Jasco V-570UV/
VIS/NIR and a Jasco J-600, respectively. Transmission electron
microscopy (TEM) and field emission SEM (FE-SEM) were
done with a JEOL JEM-SS and a Hitachi S-5000, respectively.
Rheology was measured with an Elquest Rheologia A300.
¹1
the product. IR (KBr, cm ): 3282 (νN-H), 1635 (νC=O amide
I), 1544 (δN-H amide II). Found: C 76.49 H 13.07, N 4.91%.
2
.2 Gelation Test. Gelation test was carried out by an
1
upside-down test tube method. A typical procedure is as
follows: A weighed sample and 1 mL of solvent in a septum-
capped test tube with internal diameter of 14 mm was heated
until the solid dissolved. The resulting solution was cooled at
Calcd for C H N O : C 76.81, H 12.53, N 4.98%. H NMR
3
6
70
2
2
(400 MHz, CDCl , TMS, 25 °C): δ = 5.95 (d, 1H, J = 6.52
3
Hz, NH-CO), 5.90 (d, 1H, J = 6.84 Hz, NH-CO), 3.59­3.70
(m, 2H, -NH-CH(CH )-CH(CH )-NH-), 1.72­2.12 (m, 7H,
2
2
2
5 °C for 2 h and then the gelation was checked visually. When
-CO-CHCH (CH )-, -CO-CH CH -, cyclohexane-CH -(CH ) -
2 2 2 2 2 2 2
no fluid ran down the wall of the test tube upon inversion of
tube, we judged it to be gel. The gelation ability was evaluat-
ed by the minimum gel concentration of a gelator necessary
for gelation at 25 °C. The unit is g L (gelator/solvent). The
solvents used for gelation test were ethyl acetate, isopropyl
myristate, γ-butyrolactone, toluene, liquid paraffin, silicone oil
CH -), 1.24­1.59 (br, 50H, alkyl, cyclohexane-CH -CH -),
2
2
2
0.85­0.89 (m, 9H, -CH CH ).
2
3
2.3.5 Racemic trans-1-Boc-amino-2-aminocyclohexane:
This compound was prepared from racemic trans-1,2-diamino-
cylohexane by a similar procedure to that described above.
¹
1
¹
1
Yield; 77%. IR (KBr, cm ): 1682 (νC=O urethane), 1517
(δN-H amide II). Found: C 61.68, H 10.73, N 13.16%. Calcd
for C H N O : C 61.65, H 10.35, N 13.07%.
(
2
KF-54), decamethyl cyclopentasiloxane (D5), and hexadecyl
-ethylhexanoate.
.3 Synthesis. trans-(1R,2R)-(¹)-1,2-Diaminocyclohexane
11
22
2
2
2
2.3.6 rac-1: This compound was prepared from racemic
trans-1-Boc-amino-2-aminocylohexane by a similar procedure
and racemic trans-1,2-diaminocyclohexane were purchased
from Tokyo Chemical Industry Co. Ltd.
¹
1
to that described in (R,R)-1. Yield; 93%. IR (KBr, cm ): 1696
(νC=O urethane), 1638 (νC=O amide I), 1528 (δN-H amide
II). Found: C 72.71, H 12.0, N 5.79%. Calcd for C H N O :
2
.3.1 trans-(1R,2R)-1-Boc-amino-2-aminocyclohexane:36
The 800 mL of methanol containing 16.53 g (0.453 mol) of
dry HCl was added dropwise to the solution of 51.78 g (0.453
mol) of trans-(1R,2R)-(¹)-1,2-diaminocyclohexane in 80 mL
of methanol for 2 h. After adding 50 mL of water, 98.97 g
2
9
56
2
3
C 72.45, H 11.74, N 5.83%.
2.3.7 rac-2: This compound was prepared from rac-1 by a
procedure similar to that described in (R,R)-2. Yield; 94%. IR
¹1
(0.453 mol) of di-t-butyl dicarbonate was added by portions for
(KBr, cm ): 1644 (νC=O amide I), 1525 (δN-H amide II).
Found: C 62.19 H 10.83, N 6.22%. Calcd for C H N OBr: C
1
h. The mixture was stirred for 2 h, and then methanol was
2
4
49
2
removed. The residue was rinsed with 900 mL of ether and the
insoluble matter was filtered off and dried. The crude product
was dissolved in 800 mL of water and then extracted with a
mixture of 500 mL of CH Cl and 340 mL of 2 M NaOH. The
62.45, H 10.70, N 6.07%.
2.3.8 rac-C12C18: This compound was prepared from rac-
2 by a similar procedure to that described in (R,R)-C12C18.
¹
1
Yield; 94%. IR (KBr, cm ): 3282 (νN-H) 1635 (νC=O amide
I), 1544 (δN-H amide II). Found: C 76.50 H 12.77, N 5.01%.
2
2
organic layer was retained and treated with MgSO and the
4
Bull. Chem. Soc. Jpn. 2017, 90, 312–321 | doi:10.1246/bcsj.20160360
© 2017 The Chemical Society of Japan | 313

1
¹1
Calcd for C H N O : C 76.81, H 12.53, N 4.98%. H NMR
and 28 nm . The preheated gel sample was poured into a
3
6
70
2
2
(
400 MHz, CDCl , TMS, 25 °C): δ = 6.03 (d, 1H, J = 6.56
cylindrical quartz cell having a diameter of 1 mm and the mea-
surement was started after standing for 1 h. A line-shaped mono-
chromatic primary beam (Cu Kα radiation, λ = 0.1542 nm) was
irradiated to the gel samples and the solvent. The background
contributions from capillary and solvent were corrected. Since
the scattering intensity from the gel sample rapidly converged
3
Hz, NH-CO), 5.96 (d, 1H, J = 6.88 Hz, NH-CO), 3.59­3.69
(
-
CH -), 1.24­1.59 (br, 50H, alkyl, cyclohexane-CH -CH -),
0
m, 2H, -NH-CH(CH )-CH(CH )-NH-), 1.72­2.12 (m, 7H,
2 2
CO-CHCH (CH )-, -CO-CH CH -, cyclohexane-CH -(CH ) -
2
2
2
2
2
2 2
2
2
2
.85­0.89 (m, 9H, -CH2CH3).
¹1
2
.3.9 (R,R)-C5H18: A solution of 2.90 g (6.23 mmol) of
to that of the solvent in the high-q range of q >10 nm , we
discuss the corrected scattering patterns of the gel samples ob-
(
R,R)-2 and 100 mL of dichloromethane was shaken with 100
¹
1
mL of 4 M NaOH in a separating funnel. The organic layer was
treated with MgSO and evaporated. Recrystallization from
2
2
1
served in 0.06 < q/nm < 10 in a practical manner. A model
independent collimation correction was made by relying on the
4
3
7
mL of ligroin gave 2.20 g (92%) of trans-(1R,2R)-1-amino-
-(2-heptylundecanoylamino)cyclohexane. To a solution of
.50 g (3.94 mmol) of trans-(1R,2R)-1-amino-2-(2-heptylunde-
Lake algorithm. The scattering intensity of the gel samples
were finally obtained on an absolute scale by referring to the
3
8
forward intensity of the secondary standard (water).
canoylamino)cyclohexane in 30 mL of dry THF, 0.423 mL
3.94 mmol) of δ-valerolactone was added and refluxed over-
3
. Results and Discussion
(
night. The precipitated matter after cooling the reaction mixture
was filtered off and washed with hexane. Recrystallization from
a mixture of 25 mL of ethyl acetate and 15 mL of hexane gave
3.1 Gelation Abilities. We prepared six diamides derived
from trans-1,2-diaminocyclohexane, as shown in Scheme 1.
The chiral species containing the different substituents®
specifically, trans-(1R,2R)-1-(dodecanoylamino)-2-(2-heptyl-
undecanoylamino)cyclohexane®is abbreviated as (R,R)-
C12C18. The corresponding racemate, which was prepared
from racemic trans-1,2-diaminocyclohexane, is abbreviated
as rac-C12C18. The chiral compound containing 5-hydroxy-
pentanoylamino and 2-heptylundecanoylamino groups is ab-
breviated as (R,R)-C5H18. The corresponding racemate is
abbreviated as rac-C5H18. The chiral and racemate containing
two n-dodecanoylamino groups as the same substituents are
denoted as (R,R)-C12C12 and rac-C12C12, respectively.
One objective of the present study is to develop gelators
based on racemic trans-1,2-diaminocyclohexane, which is
¹1
1
3
.63 g (86%) of (R,R)-C5H18. IR (KBr, cm ): 3407 (νO-H),
283 (νN-H), 1635 (νC=O amide I), 1543 (δN-H amide II).
Found: C 72.19, H 12.17, N 5.66%. Calcd for C H N O : C
29
56
2
3
7
2.45, H 11.74, N 5.83%.
.3.10 rac-C5H18: This compound was prepared from
2
racemic trans-1-amino-2-(2-heptylundecanoylamino)cyclo-
hexane via rac-2 by a similar procedure to that described
above. Recrystallization from a mixture of 80 mL of acetone
and 20 mL of methanol gave the product in a yield of 78%. IR
¹
1
(
KBr, cm ): 3407 (νO-H), 3283 (νN-H), 1636 (νC=O amide
I), 1543 (δN-H amide II). Found: C 72.69, H 12.29, N 5.83%.
Calcd for C29H56N2O3: C 72.45, H 11.74, N 5.83%.
2
.3.11 (R,R)-C12C12: A solution of 1.19 g (10.4 mmol)
of trans-(1R,2R)-(¹)-1,2-diaminocyclohexane and 2.11 g (20.9
mmol) of triethylamine in 300 mL of dry THF was cooled in an
ice water bath, and then 4.57 g (20.9 mmol) of n-dodecanoyl
chloride was added drop-by-drop. The mixture was stirred for
3
h at room temperature. 20 mL of acetone was added to the
reaction mixture and a precipitate was filtered off and washed
with water. Recrystallization from 100 mL of chloroform gave
¹
1
4
1
.20 g (92%) of the product. IR (KBr, cm ): 3283 (νN-H),
639 (νC=O amide I), 1546 (δN-H amide II). Found: C 75.28,
H 13.00, N 6.04%. Calcd for C H N O : C 75.26, H 12.21, N
3
2
62
2
2
1
5
2
.85%. H NMR (400 MHz, CDCl , TMS, 25 °C): δ = 5.88 (d,
3
H, J = 6.56 Hz, NH-CO), 3.62­3.67 (m, 2H, -NH-CH(CH )-
2
CH(CH )-NH-), 1.73­2.17 (m, 8H, -CO-CH -CH -, cyclo-
2
2
2
hexane-CH -), 1.55­1.58 (m, 4H, -CO-CH -CH -), 1.15­1.35
2
2
2
(
br, 36H, alkyl), 0.87 (t, 6H, J = 13.72, -CH2CH3).
.3.12 ra-C12C12: This compound was prepared from
racemic trans-1,2-diaminocyclohexane by a similar procedure
2
¹1
to that described above. Yield; 84%. IR (KBr, cm ): 3300
νN-H), 1636 (νC=O amide I), 1542 (δN-H amide II). Found:
C 75.22, H 12.98, N 6.08%. Calcd for C H N O : C 75.26, H
(
3
2
62
2
2
1
1
2.21, N 5.85%. H NMR (400 MHz, CDCl , TMS, 25 °C): δ =
3
5
.91 (d, 2H, J = 6.52 Hz, NH-CO), 3.59­3.69 (m, 2H, -NH-
CH(CH2)-CH(CH2)-NH-), 2.00­2.16 (m, 8H, -CO-CH2-CH2-,
cyclohexane-CH -), 1.55­1.58 (m, 4H, -CO-CH -CH -), 1.19­
2
2
2
1
.44 (br, 36H, alkyl), 0.87 (t, 6H, J = 13.64, -CH CH ).
2 3
2
.4 SAXS. Small angle X-ray scattering was recorded on an
Anton Paar SAXS. The accessible q-range was between 0.06
Scheme 1. Structures of gelators.
314 | Bull. Chem. Soc. Jpn. 2017, 90, 312–321 | doi:10.1246/bcsj.20160360
© 2017 The Chemical Society of Japan

Table 1. Results of gelation tests at 25 °C
Solvent
(R,R)-C12C18
rac-C12C18
(R,R)-C5H18
rac-C5H18
(R,R)-C12C12
rac-C12C12
Ethyl acetate
Isopropyl myristate
γ-Butyrolactone
Toluene
Liquid paraffin
Silicone oil (KF-54)
D5
GTL(8)
GTL(10)
P
GT(40)
GT(4)
GTL(8)
GTL(4)
GTL(8)
P
GTL(8)
GTL(10)
GTL(8)
GT(40)
GT(4)
GT(2)
GO(8)
GT(8)
P
GO(8)
GO(6)*
P
P
GT(20)*
GTL(4)
GT(4)
P
GTL(20)
GT(40)
GT(40)
GTL(10)
GT(8)
GTL(4)
GT(20)
GT(20)
GTL(8)
GT(2)
GTL(4)
GTL(8)
GTL(10)
GO(20)
GT(12)
GT(3)
GT(2)
GO(2)
GO(20)
GTL(20)
HDEH
GTL(4)
GTL(8)*
GT: Transparent gel. GTL: Translucent gel. GO: Opaque gel. P: Precipitation. I: Almost insoluble. PG: Partial gel. KF-54: Poly-
methylphenylsiloxane) of 400 cS. D5: Decamethyl cyclopentasiloxane. HDEH; Hexadecyl 2-ethylhexanoate. The values indicate the
minimum gel concentrations at 25 °C; the units are g/l (gelator/solvent). *Crystals precipitated from the formed gels after several hours.
(
much less expensive than chiral trans-1,2-diaminocyclohexane.
Because racemic compounds are unsuitable as gelators, as pre-
viously mentioned, the development of racemic gelators would
represent an important advancement. Gelation tests were per-
formed using an upside-down test-tube method. The macro-
scopic manifestation of successful gelation is the absence of
observable flow when a sample is inverted. The results of the
gelation tests with eight solvents are summarized in Table 1.
Regarding (R,R)-C12C12, we have already reported its promi-
nent gelation ability in a previous communication.30 Notably,
rac-C12C12 could gel some of the solvents, but the formed
gels were so unstable that crystallization occurred after several
hours. In general, crystallization of a racemic mixture gives a
racemic crystal comprising equivalent R- and S-configurations
but does not result in precipitation of each chiral crystal sepa-
rately. The facts that racemic crystals tend to be denser than
their chiral counterparts and that complimentarily packing of R-
and S-configurations gives more high-density crystals is known
(polaroil)
Hexadecyl 2-ethylhexanoate
0
1
0.8
䠖GTL
0.2
0
.6
0.4
0.4
0
.6
0.8
0.2
1
1
0
0
0.2
0.4
0.6
0.8
(non-polar oil)
Liquid Paraffin
(siliconeoil)
Decamethylcyclopentasiloxane
Figure 1. Gelation behavior of (R,R)-C12C18 in the mixed
solvent of HDEH, liquid paraffin, and D5 at concentration
¹1
of 8 mg mL
.
3
4,35
as Wallach’s rule.
This rule is the reason that racemic
of HDEH, liquid paraffin, and D5) is shown in Figure 1. We
observed that (R,R)-C12C18 formed translucent gels with all
these mixed solvent combinations. The same gelation behavior
described in Figure 1 was observed for rac-C12C18, (R,R)-
C5H18, and rac-C5H18.
mixtures are unsuitable as gelators.
New gelators (R,R)-C12C18 and (R,R)-C5H18 had high
gelation abilities comparable to that of (R,R)-C12C12. Surpris-
ingly, contrary to Wallach’s rule, rac-C12C18 and rac-C5H18
formed very stable gels in isopropyl myristate, γ-butyrolactone,
toluene, liquid paraffin, silicone oil (KF-54), decamethyl cyclo-
pentasiloxane (D5), and hexadecyl 2-ethylhexanoate (HDEH),
which were not transformed into crystals. The observation that
rac-C12C18 and rac-C5H18 formed more stable gels than
rac-C12C12 can be explained on the basis of the disorder of
molecular packing. The different substituents in rac-C12C18
and rac-C5H18 prevent the complimentarily denser packing;
consequently, rac-C12C18 and rac-C5H18 are not precipitated
as racemic crystals. The order of gelation ability to form sta-
ble gels is (R,R)-C12C12 μ (R,R)-C12C18 μ (R,R)-C5H18 >
rac-C12C18 μ rac-C5H18 º rac-C12C12.
The phase-transition temperatures of the prepared gels were
investigated; the results are shown in Figure 2. As shown in
Figure 2A, the greatest thermal stability was observed for gels
formed by (R,R)-C5H18; their gel-to-sol phase-transition tem-
peratures were within 95 to 115 °C. The second-most thermally
stable gels were formed by rac-C5H18, as shown in Figure 2B.
In the case of (R,R)-C12C18, when the content of HDED
(polar oil) in the mixed solvents was increased, the phase-
transition temperature which decreased from 65 °C to 74 °C is
shown in Figure 2C. Figure 2D shows that the lowest phase-
transition temperatures were observed in the gels formed by
rac-C12C18. The high phase-transition temperatures of (R,R)-
C5H18 and rac-C5H18 compared to those of (R,R)-C12C18
and rac-C12C18 is explained by the hydrogen bonding among
hydroxyl groups. Although the reason of the low phase-
transition temperatures of rac-C12C18 compared to those of
(R,R)-C12C18 remains unclear, the enantiomer may prevent
aggregate growth.
The gelation abilities of (R,R)-C12C18, (R,R)-C5H18,
rac-C12C18, and rac-C5H18 were investigated using three-
component mixed solvents frequently used in cosmetics:
HDEA as a polar oil, liquid paraffin as a non-polar oil, and
D5 as a silicone oil. The concentrations of (R,R)-C12C18,
rac-C12C18, (R,R)-C5H18, and rac-C5H18 were fixed at 8,
¹
1
1
0, 8, and 8 mg mL (gelator/solvent), respectively. The gela-
Transparent gels are widely desirable from a practical appli-
cation perspective. We used UV­vis spectroscopy to evaluate
the transparency of the prepared gels. As shown in Figure S1
tion behavior of (R,R)-C12C18 in the mixed solvent (66
combinations, with weight ratios ranging from 10:0:0 to 0:0:10
Bull. Chem. Soc. Jpn. 2017, 90, 312–321 | doi:10.1246/bcsj.20160360
© 2017 The Chemical Society of Japan | 315

polar oil
Hexadecyl 2-ethylhexanoate
100
A
90
80
70
0
1
0
.2
0
.8
0
.4
(R,R)-C12C18
rac-C12C18
0
.6
95䡚115Υ
60
50
0
.6
0
.4
(
R,R)-C5C18
rac-C5C18
R,R)-C12C12
4
0
0
0
.8
0
.2
1
3
(
1
0
20
rac-C12C12
0
0.2
0.4
0.6
0.8
non-polaroil
siliconeoil
10
Liquid Paraffin
Decamethylcyclopentasiloxane
0
polar oil
Hexadecyl 2-ethylhexanoate
300
350
400
450
500
550
600
650
700
B
Wavelength / nm
0
1
7
5䡚94 Υ
0
.2
Figure 3. Transmittance change of gels at 10 mg mL¹1 in
0
.8
0
.4
the mixture of polar oil:non-polar oil:silicone oil = 4:3:3,
as measured using a UV­vis spectrophotometer.
0
.6
95䡚115Υ
0
.6
0
.4
0
1
.8
Table 2. GAve¤, GAve¤¤, and tan δAve under a strain of 0.02 at
25 °C
0
.2
1
5
0䡚74Υ
0
0
0.2
0.4
0.6
0.8
non-polaroil
siliconeoil
Liquid Paraffin
Decamethylcyclopentasiloxane
GAve¤ (Pa)
GAve¤¤ (Pa)
tan δAve
(
R,R)-C12C18
1455
488
303
83
0.20
0.18
0.33
0.36
0.45
0.38
polar oil
Hexadecyl 2-ethylhexanoate
rac-C12C18
(R,R)-C5C18
rac-C5C18
(R,R)-C12C12
rac-C12C12
0
1
C
6
2507
1467
1150
857
681
442
456
291
0
.2
5䡚74 Υ
0
.8
75䡚94 Υ
0
.4
0
.6
0
.6
0
.4
0
.8
0
.2
1
9
5䡚100Υ
the aggregates becomes equivalent to the wavelength of visible
light, the formed gels will be opaque because they will scatter
visible light. Because the disordered arrangements of structur-
ally different substituents in (R,R)-C12C18 and (R,R)-C5H18
form aggregates with random molecular packing, the aggre-
gates cannot grow to a size comparable to the wavelength of
visible light. The different substituents in these compounds are
thought to disturb the molecular packing. The introduction of
structurally different substituents will be useful for developing
gelators to form transparent gels.
1
0
0
0.2
0.4
0.6
0.8
non-polaroil
siliconeoil
Decamethylcyclopentasiloxane
Liquid Paraffin
polar oil
Hexadecyl2-ethylhexanoate
0
1
3
0䡚35 Υ
D
0.2
0.8
0.4
0.6
0.6
0.4
0
.8
3.2 Rheology. As shown in Figures S2­S4, the viscoelastic
behavior of the gels was studied by rheology measurements in
strain sweep mode and frequency sweep mode at 25 °C. The
solvent used for the measurements was a mixture of HDEH,
liquid paraffin, and D5 (vol. ratio 4:3:3) and the concentrations
0
.2
1
0
0
0.2
0.4
0.6
0.8
1
non-polar oil
silicone oil
Liquid Paraffin
Decamethylcyclopentasiloxane
Figure 2. Gel-to-sol phase-transition temperatures in the
mixed solvent of HDEH, liquid paraffin, and D5 at concen-
tration of 8 mg mL : (A) (R,R)-C5C18, (B) rac-C5C18,
C) (R,R)-C12C18, and (D) rac-C12C18.
¹
1
were 10 mg mL . In strain sweep mode at 0.03 Hz, the storage
elastic moduli (G¤) of almost all of the investigated gels were
larger than the loss elastic moduli (G¤¤) up to a strain of 0.1 and
the G¤¤ of the gels exceeded the G¤ at a strain of 0.5, which
demonstrates the collapse of gels. In frequency sweep mode at
a strain of 0.2, plateau regions of G¤ and G¤¤ of all the samples
were observed up to 1 Hz, where the G¤ exceeded the G¤¤. In
particular, the G¤ of (R,R)-C12C18, (R,R)-C5H18, and rac-
C5H18 exceeded the G¤¤ up to 5 Hz. Average G¤ and G¤¤ in the
plateau regions of 0.002 to 1.0 Hz, referred to as GAve¤ and
GAve¤¤, and loss angle tan δAve are summarized in Table 2,
where the tangent of the phase angle (tan δAve)­the ratio of
GAve¤¤ to GAve¤ is a useful quantifier of the presence and extent
¹1
(
the six gelators investigated in this work formed transparent or
translucent gels. The transmittance spectra of gels composed of
a mixture of HDEH, liquid paraffin, and D5 (vol. ratio 4:3:3)
are shown in Figure 3. The transmittances at 400 nm of gels of
R,R)-C12C18, (R,R)-C12C12, (R,R)-C5H18, rac-C12C18,
rac-C12C12, and rac-C5C18 at a concentration of 10 mg mL
were 94, 79, 95, 58, 79, and 82%, respectively. The order of
transparency of gels over the total investigated wavelength
band is (R,R)-C12C18 μ (R,R)-C5H18 > (R,R)-C12C12 μ
rac-C5H18 > rac-C12C18 μ rac-C12C12. The transmittance
of the rac-C12C12 gel rapidly decreased because the gel
crystallized. The transparency of gels is thought to depend on
the size of the aggregates constructing the three-dimensional
networks responsible for physical gelation. When the width of
(
¹1
3
9­41
of elasticity in a gel system.
The tan δAve values of less than
unity indicate elastic-dominant behavior and values greater
than unity indicate viscous-dominant behavior. Regarding
chiral compounds, the order of GAve¤ is (R,R)-C12C12 <
(R,R)-C12C18 < (R,R)-C5C18 and that of tan δAve is (R,R)-
316 | Bull. Chem. Soc. Jpn. 2017, 90, 312–321 | doi:10.1246/bcsj.20160360
© 2017 The Chemical Society of Japan

C12C12 > (R,R)-C5C18 > (R,R)-C12C18. Namely, (R,R)-
C5C18 and (R,R)-C12H18 with different substituents could
form the harder gels. The hardness of gels seems to depend on
size and density of aggregate; namely, the fibers consisting of
the racemates are thicker than those of the enantiomers. For
example, (R,R)-C12C18 formed fine thread-like aggregates
with nearly homogeneous diameters of ³26 nm, whereas the
widths of fibers in rac-C12C18 ranged from 42 to 50 nm.
Results in SAXS stated later also suggests an importance of
substituents; (R,R)-C12C12 and rac-C12C12 with the same
substituent formed denser packed aggregates than (R,R)-
C12C18 and rac-C12C18 with the different substituent.
Because (R,R)-C12C18 and (R,R)-C5H18 with different sub-
stituents form fine aggregates, consequently, 3D networks in
gels are crowded, and finally the hard gels are formed. The
large GAve¤ of (R,R)-C5H18, compared to those of (R,R)-
C12H18, will be attributed to the participation of hydrogen
bonding among hydroxyl groups.
B
A
C
E
D
F
Next, we compared the behavior of the racemates; rac-
C12C12, rac-C12C18, and rac-C5H18. Their behavior of
GAve¤ is almost similar to that of chiral, i.e., the order of GAve¤ is
rac-C5C18 > rac-C12C12 > rac-C12C18. GAve¤ and GAve¤¤,
and tan δAve of rac-C5C18 were 1467 Pa, 442 Pa, and 0.36
respectively. Meanwhile, those of (R,R)-C5H18 were 2507 Pa,
Figure 4. TEM images of gels prepared from (A) (R,R)-
C12C18 (0.5 mg mL¹1), (B) rac-C12C18 (0.5 mg mL¹1),
6
81 Pa, and 0.33 respectively. These results indicate that the
gel strength of rac-C5C18 was comparable to that of (R,R)-
C5C18. The large GAve¤ of (R,R)-C5H18 and rac-C5H18
suggests that the hydrogen bonding among hydroxyl groups
plays an important role in gelation.
¹1
(
C) (R,R)-C5C18 (0.5 mg mL ), (D) rac-C5C18 (0.5
¹1
¹1
mg mL ), (E) (R,R)-C12C12 (0.8 mg mL ), and (F) rac-
C12C12 (0.8 mg mL ) in a mixture of polar oil:non-polar
¹1
oil:silicone oil = 4:3:3.
3
.3 TEM.
The transparency and rheology behavior of
the formed gels depended on the substituents and chirality of
the gelators. Aggregates that formed three-dimensional net-
works consisting of gelator molecules were observed by TEM.
Figure 4 shows TEM images of loose gels formed by (R,R)-
C12C18, rac-C12C18, (R,R)-C5C18, rac-C5C18, (R,R)-
C12C12, and rac-C12C12. The solvent was a mixture of
HDEH, liquid paraffin, and D5 (vol. ratio 4:3:3). The concen-
by TEM. Notably, as shown in Figure 4F, helical aggregates
were observed in gels formed by rac-C12C12, although the
helicity was unclear. We reasonably assume that the (R,R)-
C12C12 and trans-(1S,2S)-1,2-bis(dodecanoylamino)cyclohex-
ane of the enantiomer included in rac-C12C12 form helical
fibers individually in the gel. However, this gel was unstable
and crystallized via reconstitution of the enantiomers because of
Wallach’s rule.
¹1
trations of gelators for preparing samples were 0.5 mg mL for
R,R)-C12C18, rac-C12C18, (R,R)-C5C18, and rac-C5C18
(
¹
1
and 0.8 mg mL for (R,R)-C12C12 and rac-C12C12. Because
the concentrations of gelators used to prepare TEM samples
were considerably lower than the minimum gel concentrations,
the images in Figure 4 show fibers consisting of loose gels
before actual gelation. The image of (R,R)-C12C18 shows fine
thread-like aggregates with nearly homogeneous diameters
of ³26 nm. By contrast, the widths of fibers in rac-C12C18
ranged from 42 to 50 nm, which is larger than the widths of the
fibers in (R,R)-C12C18. The transparent gels formed by (R,R)-
C12C18 may be a consequence of the fine fibers. The widths
of fibers of (R,R)-C5C18 and rac-C5C18 were ³20 nm and
To confirm the helical aggregates of (R,R)-C12C18, which
we failed to detect in TEM and FE-SEM images, we recorded
the gel’s CD spectra. The solvent for CD spectroscopy was a
mixture of HDEH, liquid paraffin, and D5 (vol. ratio; 4:3:3),
and the concentration of (R,R)-C12C18 in the prepared sam-
¹
1
ples was 8 mg mL . The CD spectrum of (R,R)-C12C18 at
25 °C exhibited markedly strong peak for the amide unit:
[θ]210 = ³50 mdeg. The intensity of this peak decreased with
increasing temperature and disappeared at 50 °C because the
loose gel was transformed into an isotropic solution. The disap-
pearance of the CD signal in the isotropic solution led us to
conclude that the strong CD band originates from helical aggre-
gates of (R,R)-C12C18 and not from the chirality of (R,R)-
C12C18 itself. The strong CD band supports the existence of
helical aggregates in the gels of (R,R)-C12C18.
3
0­58 nm, respectively. The fibers formed by the racemates
are thicker than those formed by the enantiomers. The helicity
of fibers of (R,R)-C12C12 in acetonitrile was left-handed, as
reported previously.42 As shown in Figure S5, FE-SEM images
also confirmed the left-handed helicity in the gels of toluene
and isopropyl myristate. For reasons not clear to us, Figure 4A
showed helical aggregates were not observed in gels formed by
3.4 SAXS. The TEM observations suggested that the size
and shape of aggregates in the gels depended on the type of
substituents and the chirality of the gelators. To study the static
structure of the aggregates in detail, we employed SAXS analy-
ses. Figure 5 shows the SAXS intensities, I(q), of dilute gel
(
R,R)-C12C18. The fibers of (R,R)-C12C18 are so thin that
they did not grow into helical aggregates that could be observed
Bull. Chem. Soc. Jpn. 2017, 90, 312–321 | doi:10.1246/bcsj.20160360
© 2017 The Chemical Society of Japan | 317

tration. The greater forward intensities and the steeper slope of
rac-C12C12 compared to those of other systems demonstrated
that this system forms larger aggregates than the other com-
pounds. Because rac-C12C12 has identical substituents in its
molecules, it can easily achieve denser packing compared to
other compounds with different substituents. In fact, rac-
C12C12, comprising both the R- and S-configurations, tended
¹
4
to form high-density crystals due to Wallach’s rule. The q
behavior is generally called the Porod scattering and is ascribed
to interface formation. At low concentrations, such as 10 and
¹
1
2
0 mg mL , the slope of the I(q) of (R,R)-C12C12 in the
¹
4
forward direction was still gentler than q , indicating that,
¹
1
even at 40 mg mL , the aggregation tendency of the gelator
was not strong enough to form a well-defined interface between
the solvent and the fiber bundle-like higher-order aggregates.
The slope of the I(q) of (R,R)-C12C18 and rac-C12C18
in the region 0.1 < q/nm < 1 was virtually independent of
the concentration. Generally, the fractal dimension of the scat-
Figure 5. SAXS intensities, I(q), of the gels prepared from
rac-C12C18, (R,R)-C12C18, rac-C12C12, and (R,R)-
¹
1
¹1
C12C12 at 10 mg mL at 25 °C on an absolute scale.
HDEH was used as the solvent.
tering object, d , is manifested in the asymptotic exponent
f
¹
df 43
¹1
as I(q) £ q
.
The asymptotic behavior of I(q) £ q and
¹
2
I(q) £ q in the forward direction can be assigned to rod
and planar structures, respectively. As for (R,R)-C12C18,
¹
1.4
¹1
¹1
we observed a slope of q
for q < 0.3 nm instead of q ,
which implied the formation of aggregating rods. A steeper
¹
1.9
slope of q
was observed for rac-C12C18, which was still
¹
2
slightly gentler than but close to q , which may be assigned to
a planar-like structure or alternatively an aggregating rod struc-
ture. If the gelling systems at low gelator concentrations can be
regarded as a dilute dispersion of colloidal particles, I(q) may
be interpreted as the Fourier transformation of the pair-distance
distribution function, p(r), as given by
Z
1
sin qr
4
4;45
IðqÞ ¼ 4³
pðrÞ
dr
ð1Þ
qr
0
To obtain p(r) corresponding to the structure function in real
space, we performed Fourier inversion analyses of the SAXS
intensities I(q) of (R,R)-C12C18 and rac-C12C18 at 40
Figure 6. Concentration-dependence of the SAXS inten-
sities I(q) of gels at 25 °C in HDEH on an absolute scale.
A; (R,R)-C12C18, B; rac-C12C18, C; (R,R)-C12C12, D;
rac-C12C18.
¹1
mg mL . First, we used an indirect Fourier transformation
46,47
(
IFT) technique,
assuming an isotropic dilute dispersion.
The IFT fits to the experimental I(q) and the resulting p(r)
values are displayed in Figure 7. As shown in Figure 7B, the
deduced p(r) of (R,R)-C12C18 exhibits typical features of a
rod-like structure, namely a local maximum in the low-r regime
and an extended tail in the high-r regime. The maximum
length, exceeding 50 nm, and the cross-sectional diameter,
about 6 nm, can be determined from the maximum distance,
where p(r) approaches zero and the infraction point of p(r) is
seen on the slightly higher-r side of the local maximum. The
latter is highlighted by an arrow and a dotted line in Figure 7B.
Importantly, we simultaneously observed the emergence of a
second additional local maximum at r μ 13 nm, as indicated by
a dotted arrow. Note that this behavior is quite similar to that
observed for the p(r) of the aggregating rod-like normal and
samples prepared from (R,R)-C12C18, rac-C12C18, (R,R)-
C12C12, and rac-C12C12 at 25 °C on an absolute scale, where
the solvent was HDEH and the concentration of the gelator was
¹
1
1
0 mg mL . The forward intensities of (R,R)-C12C12 and rac-
C12C12 were significantly greater than those of rac-C12C18
and (R,R)-C12C18, demonstrating that (R,R)-C12C12 and rac-
C12C12 formed larger aggregates. We infer that (R,R)-C12C12
and rac-C12C12 form more densely packed aggregates than
(
R,R)-C12C18 and rac-C12C18. The denser packing of (R,R)-
C12C12 and rac-C12C12 appears to be a consequence of their
ordered compact structures, because these gelators have the
same substituents in their molecules, i.e., two n-dodecanoyl
groups.
4
8
49
reverse micelles in aqueous as well as lipophilic solvent
conditions. The rac-C12C18 system showed a more pro-
nounced second local maximum, which is overwhelming and
We next conducted a concentration series of SAXS experi-
ments, and the results are demonstrated in Figure 6. Compar-
ison of the I(q) values of rac-C12C12 and (R,R)-C12C12 in
appears to hinder the first local maximum. Strongly linked
¹
1
¹1.9
the region 0.1 < q/nm < 1 reveals that the I(q) of rac-
with the q
slope of the forward intensity, the p(r) of rac-
¹
3
C12C12 is nearly proportional to q , regardless of the concen-
C12C18, at a glance, resembles that of a planar structure, rather
318 | Bull. Chem. Soc. Jpn. 2017, 90, 312–321 | doi:10.1246/bcsj.20160360
© 2017 The Chemical Society of Japan

Figure 8. Schematic diagram of two structures discussed
about rac-C12C18. Planer structure (left) and planer-like
aggregates by aggregated-rods (right).
scale of the local concentration fluctuations, n is the particle
number density, and » is the isothermal osmotic compressi-
T
bility. We treated ² and the amplitude, nk T» , as the varia-
B
T
ble parameters, which grow when approaching the critical
5
2,53
point.
In general, the OZ structure factor model is applied
to the description of a temperature-induced critical fluctua-
4
3
tion. In this study, we used the same model for the gelling
systems in an extended manner. As shown in Figure 7, the
GIFT results demonstrated that the experimental I(q) can be
excellently reproduced by the product of P(q), having the
overall shape of the rod-like particles and SOZ(q) with the
optimized parameters, yielding typical shapes of p(r) for the
long rod structure, having a cross-sectional diameter of about
Figure 7. Fourier inversion analyses of SAXS intensities
of (R,R)-C12C18 (A and B) and rac-C12C18 (C and D)
gels (40 mg mL ) at 25 °C using IFT/GIFT technique.
The IFT and GIFT fit results respectively without and with
the Ornstein­Zernike structure factor model (A and C)
and the resulting pair-distance distribution functions, p(r)
6
nm for both (R,R)-C12C18 and rac-C12C18. We obtained
¹1
² = 8 nm and nk T» = 2 for (R,R)-C12C18 and ² = 25 nm
B
T
and nk T» = 5 for rac-C12C18, indicating a larger concen-
B
T
tration fluctuation for rac-C12C18. The difference in struc-
tural morphogenesis between (R,R)-C12C18 and rac-C12C18
can be explained by the existence of equimolar (S,S)-C12C18
in rac-C12C18. The complimentarily packing of R- and S-
configurations in rac-C12C18 gives more high-density crys-
tals, according to Wallach’s rule. The IFT/GIFT results for
these two gel systems led us to conclude that the presence of a
long rod-like or strand structure is most likely and that these
were aggregated to some extent in an aligned manner, but did
not form a crystalline-like structure.
(B and D).
than that of a rod structure. However, we point out that the
aggregating rods, closely positioned with a small mutual tilt
angle, say less than 30°, can mimic the p(r) of a planar-like
structure; see Figure 8 or the literature.43
We tried to further confirm the above-mentioned IFT results
and verify the aggregating rod-like structures in a more quanti-
tative manner. An alternative interpretation of the enhanced
As we already discussed, neither a change in the position of
the local maximum, nor a change in the slope of I(q) was
observed for (R,R)-C12C18 and rac-C12C18 with increasing
concentration, as shown in Figures 7A and 7B. By contrast,
with increasing concentration of (R,R)-C12C12 and rac-
¹
1.4
forward scattering, i.e., I(q) £ q
I(q) £ q
for (R,R)-C12C18 and
¹
1.9
for rac-C12C18, could be a critical-like concen-
tration fluctuation due to the attractive interaction between the
rod-like aggregates and the resulting upturn increase of the
structure factor in the low-q region (S(q) > 1). We used a gen-
¹
1
C12C12, an interference peak (q ) at about 6 nm appeared,
h
corresponding to a d-spacing of ³1 nm if Bragg’s law was
5
0,51
¹1
eralized indirect Fourier transformation (GIFT) technique,
applied to q . Judging from the fact that the q μ 6 nm condi-
h
h
taking the local concentration fluctuations into account. For the
gel systems, we assumed a relation describing a concentrated
colloidal dispersion, I(q) = nP(q)SOZ(q), where P(q) is the form
factor of the primary rod-like aggregates and SOZ(q) is a modi-
fied Ornstein­Zernike (OZ) structure factor, in which SOZ(q) is
approximated by a Lorentzian function:
tion did not change with concentration, the peak appears to be
attributed to the short distance between gelator molecules within
the aggregates. Broad peaks of (R,R)-C12C12 and rac-C12C12
¹
1
in the region 1 < q/nm < 5 are derived from the internal
structure of the primary aggregates, which are likely to have a
rod-like structure, thus do not exhibit inter-aggregate interfer-
ence scattering. Similar broad peaks of (R,R)-C12C18 and rac-
nkBT»T
¹
1
SOZðqÞ ¼ 1 þ
ð2Þ
C12C18 observed in the region 1 < q/nm < 4 are also as-
cribed to the intra-aggregate interference contributions. How-
ever, when the concentration of (R,R)-C12C12 was increased to
2
2
1
þ q ²
where kB is the Boltzmann constant, T is the absolute
temperature, ² is the correlation length describing the length
¹
1
40 mg mL , sharp interference peaks suddenly appeared in the
Bull. Chem. Soc. Jpn. 2017, 90, 312–321 | doi:10.1246/bcsj.20160360
© 2017 The Chemical Society of Japan | 319

Table 3. SAXS diffraction peak dataa)
Compound
R,R)-C12C12
rac-C12C12
q1* (nm¹1)
q2* (nm¹1
)
q3* (nm¹1
)
qh (nm¹1
)
q1*:q2*:q3*
d1 (nm)
dh (nm)
(
1.21
®
2.20
®
2.41
®
6.17
6.00
1:√3:√4
®
5.19
®
1.02
1.05
a) The scattering vectors corresponding to the position of the characteristic reflections qn* (n = 1, 2, *) and the
interlayer spacing d are determined from the SAXS intensities. The interlayer spacing d is calculated using Bragg’s
law, dn = 2π/qn*.
4
. Conclusion
We prepared three gelators, each of chiral and racemic types,
based on trans-1,2-diaminocyclohexane. One is chiral and
racemate containing n-dodecanoylamino groups as the same
substituents. The others are chiral and racemates containing
structurally different substituents: 10-undecenoylamino and 2-
heptylundecanoylamino groups as the substituents in one case,
and 5-hydroxypentanoylamino and 2-heptylundecanoylamino
as the substituents in the other case. Results of gelation tests
with eight solvents demonstrated that the order of gelation
ability to form stable gels is (R,R)-C12C12 μ (R,R)-C12C18 μ
(
R,R)-C5H18 > rac-C12C18 μ rac-C5H18 º rac-C12C12.
Although the gels formed by rac-C12C12 were so unstable
that they crystallized after several hours, both rac-C12C18
and rac-C5H18, which included different substituents, formed
very stable gels. The gelation abilities were investigated using
three-component mixed solvents of HDEH, liquid paraffin,
and D5 (66 combinations). (R,R)-C12C18, (R,R)-C5H18, rac-
C12C18, and rac-C5H18 all formed translucent gels with
all of the investigated mixed solvent combinations. The ther-
mal stabilities of the gels were evaluated on the basis of
their phase-transition temperatures. The highest thermal stabil-
ity was observed for gels formed by (R,R)-C5H18, and the
second-most thermally stable gels were formed by rac-C5H18.
The transmittance of gels composed of a mixture of HDEH,
liquid paraffin, and D5 was studied. The order of transpar-
ency of gels over the total investigated wavelength band
was (R,R)-C12C18 μ (R,R)-C5H18 > (R,R)-C12C12 μ rac-
C5H18 > rac-C12C18 μ rac-C12C12. The viscoelastic be-
havior of gels of a three-component mixed solvent was studied
via rheology measurements. The (R,R)-C5C18 and (R,R)-
C12H18 with different substituents formed the harder gels
compared with (R,R)-C12C12. Electron microscopy images
of (R,R)-C12C18 showed fine thread-like aggregates with
nearly homogeneous ³26 nm diameters. The widths of fibers of
rac-C12C18 ranged from 42 to 50 nm. The fibers formed by
racemates were thicker than those formed by enantiomers. The
left-handed helicity was confirmed in the gels formed by toluene
and isopropyl myristate in (R,R)-C12C12. Helical aggregates
in gels formed by (R,R)-C12C18 were not observed by TEM;
however, the appearance of a strong CD band supported the
existence of helical aggregates in the gels. The IFT/GIFT
results for (R,R)-C12C18 and rac-C12C18 indicate that the
presence of a long rod-like or strand structure is most likely and
that these were aggregated to some extent in an aligned manner,
but not a crystalline-like structure. (R,R)-C12C12 formed rod-
like aggregates at low concentrations, which then transformed
Figure 9. Organogel structure of (R,R)-C12C12 with high-
er-order aggregates formed in hexadecyl 2-ethylhexanoate.
region 0.9 < q/nm 1 < 4, as shown in Figure 7C, which are
likely to be interference peaks arising from the crystalline-like
positional correlations of the aggregates. The interlayer spacing,
d, values for (R,R)-C12C12 and rac-C12C12, calculated from
the interference peak positions, are summarized in Table 3. As
previously mentioned, only in the case of (R,R)-C12C12, the
forward intensity approached Porod scattering, and sharp peaks
¹
¹
1
suddenly appeared at 40 mg mL , which we call the scattering
vectors corresponding to the peak positions q * and q * from the
1
2
lower q-side. As shown in Figure 7C, the third reflection (q *)
3
could also be vaguely identified. Since the positional ratio
q1*:q2*:q3* was nearly 1:√3:√4, the packing of the aggregates of
¹
1
(
R,R)-C12C12 at 40 mg mL can be assigned to a hexagonal
5
4
structure. As shown in Figure 9, we can conclude that (R,R)-
C12C12 formed rod-like aggregates at low concentrations of
¹
1
1
0 and 20 mg mL , which then transformed into hexagonal
¹
1
aggregates at 40 mg mL .
From the results showing that the forward scattering inten-
sities of (R,R)-C12C18 and rac-C12C18 are smaller than those
of (R,R)-C12C12 and rac-C12C12, and moreover, the absence
of interference peaks in (R,R)-C12C18 and rac-C12C18, we
can conclude that the different substituents in (R,R)-C12C18
and rac-C12C18 moderate the denser packing. Consequently,
excessive aggregation is hindered in small aggregates.
¹1
into hexagonal aggregates at 40 mg mL . The forward scatter-
ing intensities of (R,R)-C12C18 and rac-C12C18 demonstrated
320 | Bull. Chem. Soc. Jpn. 2017, 90, 312–321 | doi:10.1246/bcsj.20160360
© 2017 The Chemical Society of Japan

that the different substituents in (R,R)-C12C18 and rac-
C12C18 moderate the denser packing. The restrain of excessive
aggregation results in small aggregates. Finally, we clarified
the importance of different substituents in diamides based on
racemic trans-1,2-diaminocyclohexane on physical gelation.
A concept of beingness of structurally different substituents
will be helpful in development and molecular design of new
gelators.
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Supporting Information
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Figure S1, Figure S2, Figure S3, Figure S4, and Figure S5.
This material is available on http://dx.doi.org/10.1246/bcsj.
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