Perfluoroalkyl bile esters: A new class of efficient gelators of organic and aqueous-organic media

Article abstract of DOI:10.1039/c1jm11912e

A new class of fluorinated gelators derived from bile acids is reported. Perfluoroalkyl chains were attached to the bile acids through two different ester linkages and were synthesized following simple transformations. The gelation property of these deriv

Full text of DOI:10.1039/c1jm11912e

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PAPER
Perfluoroalkyl bile esters: a new class of efficient gelators of organic and
aqueous–organic media†
Supratim Banerjee, V. M. Vidya, A. J. Savyasachi and Uday Maitra*
Received 1st May 2011, Accepted 19th July 2011
DOI: 10.1039/c1jm11912e
A new class of fluorinated gelators derived from bile acids is reported. Perfluoroalkyl chains were
attached to the bile acids through two different ester linkages and were synthesized following simple
transformations. The gelation property of these derivatives is a function of the bile acid moiety, the
spacer and the fluoroalkyl chain length. By varying these parameters, gels were obtained in aromatic
2
hydrocarbons, DMSO and DMSO/DMF–H O mixtures of different proportions. Several derivatives
of deoxycholic and lithocholic acids were found to be efficient organogelators, while the reported bile-
acid based organogelators are mostly derived from the cholic acid moiety. The efficient gelators among
these compounds formed gels well below 1.0% (w/v) and hence they can be termed as supergelators. The
mechanical properties of these gels could be modulated by changing either the bile acid moiety or by
varying the length of the fluoroalkyl segment. The presence of CO -philic perfluoroalkyl groups is also
2
expected to enhance their solubility in supercritical CO
candidates for making aerogels.
2
and hence these compounds are promising
Introduction
amphiphilicity, which in turn drives their aggregation in aqueous
media. Bile salts have been known to form gels. For example, the
In the last two decades, supramolecular gels have drawn wide-
spread attention from scientists from different fields. They are
a class of soft materials where a liquid is entrapped by a 3D solid-
like fibrous network formed by the self-assembly of low mole-
cular mass organic compounds. The liquid component can either
sodium salts of deoxycholic acid and lithocholic acid are known
6
to form gels in water at neutral pH. Sodium cholate does not
form gels, but several divalent and trivalent metal salts of cholic
7
acid have been reported to form gels.
Other than the natural bile acids, a number of bile acid
8
derivatives have also been reported as potent hydrogelators.
1
2
be organic solvents (organogels) or water (hydrogels). The self-
assembly takes place through multiple weak interactions such as
H-bonding, p–p stacking, solvophobic and van der Waals
Fewer reports are available in the literature on bile acid derived
organogelators. One of the earlier reports documented the
9
formation of organogels by N-isopropyl cholamide. Willemen
3
interactions, metal co-ordinations, etc. The reversible nature of
these supramolecular interactions allows the tuning of the
functional properties of these gels in the macroscopic scale
through a molecular level modulation. Several reviews have
appeared in the literature highlighting the utility of supra-
molecular gels as hybrid materials and biomaterials, in the design
of sensors and in biomedicine (drug delivery, regenerative
et al. reported a series of N-cholyl amino acid alkyl esters which
were found to be efficient gelators of several aromatic hydro-
10
carbons. In an extension of this work, the same group investi-
gated amide, urea and ester derivatives of cholic acid in which the
amino acids were replaced by simple alkyl groups of variable
11
chain lengths. The amide and urea derivatives were found to be
4
medicine and tissue engineering).
efficient gelators of aromatic solvents but the ester derivatives
Bile salts are steroidal biodetergents which together with
lipids/fats/cholesterol form mixed micelles in the intestine to
(
octyl, decyl and dodecyl cholate) did not show gelation. In all
these compounds, the presence of the cholic acid moiety was
found to be essential for the gelation. Very recently, though,
Noponen et al. showed that the amino acid ester conjugates of
L-methionine methyl ester of not only cholic acid but also
5
enable fat digestion and absorption through the intestine wall.
They possess a rigid steroid backbone having polar hydroxyl
groups on the concave a-face and hydrophobic methyl groups on
the b-face. This arrangement creates
a
unique facial
12
lithocholic acid formed gels. These examples illustrate that the
majority of the organogelators derived from the bile acids con-
tained two key structural aspects (i) the cholic acid moiety and
Department of Organic Chemistry, Indian Institute of Science, Bangalore,
560 012, Karnataka, India. E-mail: maitra@orgchem.iisc.ernet.in
(
ii) an amide linkage on the side chain. Though the long chain
†
Electronic supplementary information (ESI) available: Synthesis and
alkyl esters of bile acids (octyl, decyl and dodecyl) did not show
gelation properties, a recent report from our group revealed that
characterization of all the new compounds and experimental details.
See DOI: 10.1039/c1jm11912e
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cholate esters with shorter alkyl chains such as ethyl, propyl, allyl
and propargyl formed gels in aromatic solvents like benzene,
aerogels which have many unique properties such as extremely
low densities (up to 95% of their volume is air), large pores and
high inner surface area, extremely low thermal conductivity, low
sound velocity and high optical transparency, etc. compared to
13
toluene, xylene and mesitylene efficiently.
The replacement of all the hydrogen atoms in a n-alkyl chain
by fluorine atoms transforms it into a perfluoroalkyl chain which
differs from the parent hydrocarbon chain in many respects.
These differences primarily arise from the very high electro-
negativity and low polarizability of the fluorine atoms. Thus,
fluorocarbons are known to have limited miscibility with
21
other solid materials. So, these perfluoroalkyl derivatives of bile
acids are also promising candidates for making aerogels directly
22
2
in sc-CO . In this paper, the aggregation behavior in organic as
well as in organic–aqueous mixtures of these unique molecules
will be discussed.
14
hydrocarbons. The higher hydrophobicity of the perfluoroalkyl
chains has been utilized for the design of fluorinated surfactants
which showed much lower critical micellar concentrations than
The synthesized fluoroalkyl derivatives of bile acids had the
following three variable structural motifs: (i) the bile acid moiety:
cholic (C), deoxycholic (DC) and lithocholic (LC), (ii) fluo-
roalkyl groups with different chain lengths and (iii) the spacer
15
their hydrogenated analogues. Other than their higher hydro-
phobicity, fluoroalkanes are also lipophobic in nature. The
unique combination of these two properties has also been
exploited to design different types of low molecular mass gela-
tors. One of the early examples of fluorinated gelators were
was either –(CO)–OCH – (P) or –O–(CO)– (Q). For the linker
2
P, nine ester derivatives were prepared having either a shorter
pentafluoropropyl (–CH
fluorooctyl [–CH (CF CF
CF ] groups connected to the cholic, deoxycholic and lithocholic
2
CF
2
CF
3
)
or longer pentadeca-
2
2
)
6
3
]/nonadecafluorodecyl [–CH
2
(CF )
2 8
16
semifluorinated n-alkanes. They were found to form gels in
various hydrocarbons. The gelation capability of these
compounds was dependent on the fluorocarbon to hydrocarbon
ratio. Nostro et al. subsequently reported the gelation of
mixtures of perfluorooctane and isooctane by a semifluorinated
3
acid moieties (Chart 1). A comparison of the aggregation
behavior of these derivatives with the already reported n-alkyl
analogues i.e., propyl, octyl and decyl esters of bile acids (vide
supra) would demonstrate the influence of the perfluoroalkyl
segment. On the other hand, compounds with the other ester
linkage Q were synthesized for five different perfluoroalkyl
groups (Chart 1). They were perfluorotridecyl (–C F ), per-
17
alkane. N-Alkyl perfluoroalkanamides were reported to be
18
efficient organogelators by George et al. The gelation ability of
these derivatives could be modulated by varying the chain length
of both the fragments and gel formation could be observed in
1
3
27
fluoroundecyl (ꢁC11
fluorononyl (–C
F
23), perfluorodecyl (–C10
F
21), per-
a variety of solvents like alkanes, alcohols, silicone oil, CCl
4
,
9
F
19
)
and perfluoroheptyl (–C F15). For
7
DMSO, etc. Interestingly, some of the derivatives formed gels in
n-perfluorooctane also. In order to investigate the role of the
linkage, the gelation of these amide derivatives was compared
with ester derivatives and they were found to be poorer gelators.
Yamanaka et al. reported organogelators derived from succinic
acid diesters containing perfluoroalkyl chains of variable
deoxycholic and lithocholic acids, all these derivatives were
synthesized whereas only two derivatives, perfluorotridecyl
9
(–C13F27) and perfluorononyl (–C F19), were prepared for cholic
acid. The alkyl analogues of bile acids with this linker have not
been reported in the literature. Hence, we have also synthesized
the derivatives of the three bile acids having the n-nonyl chain
(–C H ) connected through the Q linkage in order to compare
19
lengths. These compounds formed gels in a variety of solvents
including alcohols, ethyl acetate, acetonitrile, DMF, silicone oil,
etc. Interestingly, the coating of the xerogels of some of these
derivatives on glass surfaces generated superhydrophobic
9
19
the behavior of these derivatives with their perfluoroalkyl
analogues (Chart 2). Hereafter, the compounds will be desig-
nated by a set of notations for convenience (as shown in Chart 1
ꢀ
23
surfaces with contact angles greater than 150 .
and 2).
The design of the aforementioned gelators involved combining
two mutually incompatible segments (either directly or through
a linkage)—the alkyl and perfluoroalkyl chains. Herein, we
extend this principle to design a series of perfluoroalkyl
compounds where the alkyl part is replaced by the facially
amphiphilic bile acid derived backbone. The synthesis of these
compounds involved relatively simple steps by coupling per-
fluoroalkyl chains of varying lengths to the side chain of the bile
acids through two types of ester linkages. The presence of the
facially amphiphilic bile acids offers the possibility of tuning the
hydrophilicity of this segment by choosing cholic, deoxycholic or
lithocholic acid moieties (having 3, 2 and 1 hydroxyl group(s),
respectively). The perfluoroalkyl groups, besides being hydro-
phobic and lipophobic (vide supra), are also known to be
Experimental
Materials
All starting materials were of commercial grade and purchased
from Aldrich or Fluka. They were used without further
purification.
Characterization and instrumentation
1
13
H and C NMR spectra were recorded on a 300 MHz JEOL
Lambda-300 spectrometer and 400 MHz Bruker spectrometer in
DMSO-d and CDCl and referenced using the residual solvent
6
3
19
signal or TMS. F spectra were recorded on a 400 MHz (Bruker)
using trifluorotoluene as the reference. IR spectra were recorded
on JASCO-70 FT-IR. HRMS was recorded on Micromas Q-
TOF. Dynamic rheological measurements were done on the gels
on an AR 1000 (TA instruments) rheometer using a plate–plate
geometry. Transmission electron micrographs of the xerogels
were obtained using a TECNAI T-20 electron microscope
operating at 200 kV. Scanning electron micrographs were
2 2
CO -philic. This is of interest as the CO -philic nature of the
fluoroalkyl groups would be expected to enhance the solubility of
these derivatives in sc-CO . There are several reports available in
2
the literature where CO -philic fluoroalkyl groups have been
2
20
incorporated in polymers to improve their solubility in sc-CO2.
sc-CO , other than being a low cost, non-flammable, abundant
2
and easily recyclable solvent, is also used extensively to make
1
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Chart 1
recorded on a FEI-Quanta 200 and SIRION electron micro-
The synthesis of bile–fluoro esters having spacer Q was
accomplished by coupling the DBU salts of the perfluoro acids
scopes. Melting points were recorded in Raaga melting point
apparatus. The synthesis and characterization of all the new
compounds and the details of the experiments are given in the
ESI†.
26
with 23-iodo bile derivatives (Scheme 2).
Gelation properties
Synthesis
2
Spacer –(CO)–OCH – (P) (1a–i). A preliminary examination
of the gelation properties of the pentafluoropropyl esters (1a–c)
revealed that all of them were freely soluble (1–2% w/v) in
aliphatic and aromatic hydrocarbons, alcohols, etc. They did not
show gelation in polar aprotic solvents such as DMSO and DMF
also, but when water was added to these solutions in varying
proportions, gels were formed.
For the synthesis of compounds having the P linkage, the cor-
responding fluoroalcohols were first converted to tosylates by
24
reaction with tosyl chloride either in pyridine (for penta-
25
fluoropropyl alcohol) or in Et O in the presence of KOH (for
2
pentadecafluorooctyl and nonadecafluorodecyl alcohols). The
corresponding tosylates were then coupled with the DBU salts of
the bile acids in DMSO (Scheme 1).
A more detailed inspection revealed that depending on the
nature of the bile acid moiety, they formed gels in different
proportions of DMF–H
For example, the cholic derivative C24CH
: 1 and 2 : 1 DMSO–H O whereas the deoxycholic acid deriv-
ative DC24CH formed gels in 2 : 1 and 3 : 1 DMSO–H
mixtures. The litho derivative LC24CH was found to be the
2
O and DMSO–H
2
O mixtures (Table 1).
2 2 5
C F
formed gels in
1
2
2
C
2
F
5
2
O
2 2 5
C F
most versatile among these three. It formed gels in various ratios
of DMF–H O (2 : 1 to 6 : 1) and DMSO–H O (4 : 1 to 7 : 1)
2
2
mixtures. These observations suggested that the more hydro-
philic cholic derivative (three hydroxyl groups in the bile acid
backbone) could afford a larger volume of H O in its gels
2
compared to the less hydrophilic deoxy and litho derivatives. The
CGC (critical gelation concentration) and the thermal stability
(Tgel) of the gels of these derivatives were found to be dependent
Chart 2
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Scheme 1 Synthesis of perfluoroalkyl derivatives having spacer P.
ꢀ
on the ratio of the organic–aqueous mixtures. For instance,
exhibited a lower value of CGC in 1 : 1 DMSO–
O than the 2 : 1 mixture (0.1% and 0.4%, respectively). The
T
gel of a 1% gel of this derivative in 2 : 1 mixture was 40 C and
ꢀ
C
24CH
2
C
2
F
5
this was even lower than the Tgel (55 C) of a 0.5% gel formed in
H
2
a 1 : 1 mixture. A similar trend was also observed for the deoxy
Scheme 2 Synthesis of perfluoroalkyl derivatives having spacer Q.
1
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a
Table 1 Results of the gelation studies of the pentafluoropropyl esters
observed in organic solvents like aliphatic and aromatic hydro-
carbons, alcohols, DMF, etc. But, unlike the pentafluoropropyl
analogues, the deoxy and litho derivatives (DC CH C F and
C24CH
(1a)
2
C
2
F
5
DC24CH
(1b)
2
C
2
F
5
2 2 5
LC24CH C F
2
4
2 9 19
Solvent
(1c)
LC24CH
derivative, C24CH
different proportions of DMF–H
2
C
9
F
19) easily formed gels in DMSO. The cholate
19, on the other hand, formed gels in
O (2 : 1 to 5 : 1) mixtures
Table 2). As observed before for the gels of the three penta-
1
1
2
3
4
5
2
1
2
3
4
7
. DMSO/H
2
O (v/v)
2 9
C F
: 1
: 1
: 1
: 1
G (0.2)
G (0.4)
S (0.5)
N
I (0.5)
G (0.65)
G (0.3)
PG (1.0)
N
N
N
I
G (0.5)
G (0.5)
2
(
fluoropropyl esters in DMF–H
2
O and DMSO–H
2
O mixtures of
19 gels
were also found to be critically dependent on the ratio of DMF
: 1/6 : 1/7 : 1
. DMF/H O (v/v)
2
: 1
: 1
: 1
N
2 9
different proportions, the CGC and Tgel of C24CH C F
S (1.0)
WG (1.0)
N
N
I (0.5)
P (1.0)
N
N
and H O (see Table S1 in ESI†).
2
G (0.1)
G (0.2)
G (1.0)
The pentadecafluorooctyl esters (1d–f), the other longer chain
derivatives, on the other hand were found to have poorer gel
forming capabilities. Like the pentafluoropropyl and non-
adecafluorodecyl derivatives, they did not show gelation in
aromatic and aliphatic hydrocarbons (in most of the cases they
remained in solution and in some cases they precipitated out) and
: 1/5 : 1/6 : 1/
: 1
N
a
G: gel, S: solution, WG: weak gel, PG: partial gel, P: precipitate, I:
insoluble, N: not checked (the numbers in the parentheses are the
concentrations in % w/v in which the observations were made).
2 2
in aliphatic alcohols. In DMF–H O and DMSO–H O mixtures
of different proportions too, no gelation could be observed
except the formation of viscous solutions or weak/partial gels in
a few cases (for details see Table S2 in ESI†).
and the litho derivatives (see Table S1 in ESI†). The variation of
2 2 5
Tgel of LC24CH C F with the ratio of the organic and aqueous
components is shown in Fig. 1.
Spacer –O–(CO)– (Q) (2a–m). In this series, the per-
fluoroalkyl compounds which showed promising gelation
properties were the ones with deoxycholic and lithocholic acid
moieties having perfluoroalkyl groups of five different lengths
It is clear from these data that the presence of the penta-
fluoropropyl group significantly modulated the gelation proper-
ties of these bile acid derivatives compared to their alkyl
analogues. As mentioned previously, the propyl esters of the bile
acids were reported to show quite different gelation properties
and only propyl cholate was found to be an efficient gelator of
several aromatic hydrocarbons. Propyl deoxycholate and propyl
lithocholate were found to be non-gelators.
(
–C13F
27, –C11
F
23, –C10
F
21, –C
9
F
7
19, and –C F15). The two cholic
acid derivatives with –C13
F
27 and –C F
9
19 chains did not show gel
formation. Unlike the derivatives with spacer P, none of the
derivatives in this series formed gels in organic–aqueous media.
The gelation property of the three bile acid derivatives having
The gelation properties of the nonadecafluorodecyl esters
the longest fluoroalkyl chain, –C13F27 (2k–m) was investigated
(
1g–i), having a much longer fluoroalkyl chain, were investigated
first. There was a clear dependence of their gelation efficiency on
next. Similar to the pentafluoropropyl esters, no gelation was
the nature of the bile acid moiety. For example, DC C F
3 13 27
2
formed translucent gels in several aromatic hydrocarbons like
benzene, toluene, xylene, mesitylene, etc. LC C F also
23 13 27
formed gels in these aromatic hydrocarbons but the gels were
weaker at similar concentrations (ꢂ1.0%). The cholic derivative
23 13
C C F27 was found to be a non-gelator. Other than in aromatic
13
hydrocarbons, DC23C F27 formed gel in DMSO although it was
weak at ambient temperatures. The gelation studies are
summarized in Table 3.
A noticeable change in the gelation profile of the derivatives
was observed when the chain length of the fluoroalkyl segment
was shortened from –C F . The deoxycholic and lithocholic
1
3 27
derivatives with –C F chain were not efficient gelators of
11 23
aromatic hydrocarbons. Though DC C F formed weak gels
23 11 23
(
ꢂ1.0 to 2.0% w/v) in these solvents, no gel formation was
observed for the litho analogue. Rather, both of them formed
gels in DMSO (Table 3). For the derivatives having per-
fluoroalkyl chains shorter than –C11
and –C 15 (2a–j), gelation was observed only in DMSO (except
19 which was a non-gelator). Further investigations on the
9 19
F23, i.e., –C10F21, –C F
7
F
23 9
C C F
DMSO gels revealed that among the derivatives having the same
bile acid skeleton, the gelation capability was strongly dependent
on the fluoroalkyl chain length. For example, in the deoxy series,
the CGC increased on decreasing the chain length. DC C F
3 11 23
2
Fig. 1 Variation of Tgel of LC24CH
2
C
2
F
5
gels (1.0% w/v) (a) against the
O.
was found to be the most efficient gelator forming a gel even at
ratio of DMF–H O and (b) against the ratio of DMSO–H
2
2
0.07% concentration (ꢂ19 000 molecules of DMSO were
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,
a b
Table 2 Gelation properties of nonadecafluorodecyl esters
These compounds did not gel alcohols, aromatic hydrocarbons,
DMSO, DMF, etc. (see Table S3 in ESI†). In most of the cases
they remained in solution. These results highlighted the
importance of the fluoroalkyl segment in inducing gelation
properties in these derivatives.
2
C24CH C
9
F
19 DC24CH
2
C
9
F
2 9 19
19 LC24CH C F
Solvent
(1g)
(1h)
(1i)
Benzene
Toluene
DMSO
DMF
n-Propanol
n-Octanol
S
S
S
S
S
S
S
S
G
S
S
S
S
S
G
G (2.0)
Enthalpy of the gel–sol transition. For all the gelators, the Tgel
S
S
values increased with an increase in the concentration of the
gelators. A representative plot for LC24CH
O is shown in Fig. 4a.
An important parameter that can be derived from such a plot
2 2 5
C F in 2 : 1 DMF–
DMSO/H
: 1
DMF–H
2
O
H
2
1
S
N
S
N
P
2
O
2
5
: 1; 3 : 1; 4 : 1;
: 1
G
is the enthalpy associated with the gel–sol transition. This can be
obtained by plotting ln C (C ¼ concentration of the gelator)
a
b
G : gel, S: solution; P : precipitate, N: not checked. All the
observations were made at 1.0% w/v except otherwise mentioned.
versus 1/T and the molar enthalpy values (DH) can be obtained
gel
from the slope of the linear fit (Schr o€ der–van Laar equation—
17
eqn (1)).
ln C ¼ ꢁDH/RTgel + K
(1)
immobilized by one DC23
11
C F23 molecule!). CGC increased
slightly on going to DC C F and DC C F . Still they were
2
3
10 21
23 9 19
found to be very efficient gelators (CGCs were 0.15% and 0.25%
respectively) of DMSO. But there was a significant increase in the
Spacer –(CO)–OCH – (P). Using eqn (1) (a representative
2
plot in Fig. 4b), enthalpy values for the gel–sol transition were
CGC for the derivative DC23
C
7
F15. It could form gels only at
determined for six compounds (Table 4). DC CH C F showed
2
4
2 2 5
concentrations of 1.5% or higher in DMSO.
a higher value of enthalpy of gel melting for its gel in 3 : 1
In the lithocholic series also, there was a variation of the effi-
ciency of gel formation by the derivatives in DMSO depending
on the fluoroalkyl chain length albeit in a slightly different
manner than that observed for the deoxy analogues. The deri-
DMSO–H O than the other two pentafluoropropyl derivatives.
2
2
4
2 2 5
Interestingly, the gels of the two cholic derivatives, C CH C F
(in 1 : 1 DMSO–H O) and C CH C F (in 4 : 1 DMF–H O)
2
24
2
9
19
2
showed very similar values of enthalpy. Between the two gelators
vatives having –C11
F23 and –C
7
F15 chains could form gels in
of DMSO, DC CH C F and LC CH C F , the deoxy
2
4
2
9
19
24
2 9 19
DMSO only above 2.0% concentrations. On the other hand, the
derivative showed higher enthalpy value than the litho
derivative.
two intermediate derivatives, LC C F and LC C F were
23 9 19
2
3
10 21
efficient gelators of DMSO as demonstrated by their low CGCs
(
ꢂ0.2%). The DMSO gels obtained from both the deoxy and
Spacer –O–(CO)– (Q). As seen from Table 4, the enthalpy
values for the gel–sol transition did not vary significantly among
3 10 21
the deoxy and litho derivatives. In the deoxy series, DC C F
litho series were transparent. A few of them are shown in Fig. 2.
In terms of the thermal stability (Tgel), the DMSO gels of the
deoxy and litho derivatives were found to be quite similar.
2
2
3 9 19
and DC C F showed slightly higher enthalpy values than
DC23
C
11
F
23, DC23
C
10
ꢀ
F
21, LC23
C
10
F
21 and LC23
C
9
F
19 showed
23 11 23 23 10 21
DC C F . Between the two litho derivatives, LC C F
Tgel values of 63–66 C (for 1% gels, Fig. 3) except DC23
ꢀ
C
9
F
19
23 9 19
showed a slightly higher value of enthalpy than LC C F .
which showed a slightly lower Tgel value (57 C).
The alkyl analogues of these derivatives (having the Q
linkage) are not reported in the literature. Therefore, we
prepared decanoate esters of cholic, deoxycholic and lithocholic
acids [–O–(CO)C H ] and their gelation properties were
Rheology. The mechanical properties of the gels were investi-
gated by dynamic rheology. Gels are viscoelastic soft solid-like
8b,c
materials and hence they both store and dissipate energy. Two
kinds of rheology experiments were performed on these gels: (i)
frequency sweep at a fixed oscillatory stress of 1.0 Pa and (ii)
stress sweep at a fixed frequency of 1.0 Hz. From frequency
9
19
evaluated in solvents in which the perfluorinated analogues
formed gels and also in solvents in which gels did not form.
a
Table 3 Gelation studies of perfluoroalkyl derivatives (–C13
F27 and –C11
F
23
)
Solvent
C
23
C
13
F27 (2k)
DC23C
13
F
27 (2l)
LC23
C
13
F27 (2m)
DC23C
11
F
27 (2i)
11
LC23C F23 (2j)
Benzene
Toluene
Xylene
Mesitylene
DMSO
DMF
n-Hexane
n-Dodecane
EtOH
S
S
S
S
S
S
N
N
S
G
G
G
S
WG
S
N
S
S
WG
WG
WG
S
WG
WG
WG
N
G
S
S
S
S
S
S
S
N
WG
S
S
S
P
PG
PG
N
S
S
n-Octanol
S
S
N
S
S
a
S: solution; P: precipitate; G: gel; WG: weak gel; PG: partial gel; N: not checked (all the observations were made at 1.0% w/v).
1
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2 2 5
Fig. 2 Photographs of DMSO gels of LC24CH C F (0.5%, left),
DC23C
11
F
23 (0.25%, middle) and LC23
C
10
F
21 (0.5%, right).
0
sweep experiments the dynamic storage modulus (G ) and the loss
0
0
0
00
modulus (G ) are measured. For soft-solids like gels, G > G and
the ratio of these two characterizes the stiffness of the gels. On
the other hand, a stress sweep experiment provides the yield
stress, the limit beyond which the gel starts to flow.
As several derivatives with the spacer Q formed gels in DMSO,
a comparative study of these gels would demonstrate the effect of
the nature of the bile acid moiety and the fluoroalkyl chain length
on their mechanical properties. A similar study was not possible
in the P series as there was no common solvent where all of them
formed gels. Nonadecafluorodecyl deoxycholate and litho-
cholate (DC CH C F and LC CH C F ) formed gels in
Fig. 4 (a) Plot of Tgel vs. concentration for a 2 : 1 DMF–H
2
O gel of
2 2 5
LC24CH C F
and (b) Schr o€ der–van Laar plot for the same compound.
2
4
2
9
19
24
2 9 19
DMSO and hence their mechanical properties were compared
to be the highest for the DC23
shorter chain derivative DC23
C
11
F
23 gel (24 400 Pa). The next
with the derivatives from the other spacer series Q having the
F19. The concentration was kept at
C
10
F21 exhibited a lower value of
0
same fluoroalkyl chain –C
.0% (w/v) for all the gels.
9
1
4 000 Pa but on decreasing the chain length further, G again
1
9
increased to 19 700 Pa for the DC23C F19 derivative. A
0
comparative plot of G vs. frequency for these three derivatives is
Spacer –O–(CO)– (Q). In the deoxy series, the experiments
were performed for the DMSO gels of three derivatives—
DC C F , DC C F and DC C F whereas gels of
shown in Fig. 5A.
0
00
On the contrary, the stiffness (G /G ) of these gels showed
a clear dependence on the fluoroalkyl chain length. An increase
in the fluoroalkyl chain length resulted in a simultaneous increase
in the stiffness of the gels. Thus, the highest stiffness was
2
3
11 23
23 10 21
23 9 19
LC C F and LC C F were chosen for rheology from the
23 9 19
2
3
10 21
lithocholic series. The gels formed from the other derivatives
visually looked weaker and hence they were excluded from the
rheological investigations.
exhibited by the DC23
a linear fashion in going to the shorter chain derivatives
Fig. 6A). This decrease in stiffness suggested relatively more
11
C F23 gel and it decreased almost in
(
Frequency sweep. For the deoxy series, the variation in the
0
energy dissipation by viscous mechanisms in the shorter chain
8b
derivatives.
elasticity (G ) of the gels with the fluoroalkyl chain length did not
0
show any particular trend (Table 5). For example, G was found
0
0
00
The variation in elasticity (G ) and stiffness (G /G ) for the two
litho derivatives presented a different trend from that observed in
9
the deoxy series. LC23C F19 showed higher values for both of
Table 4 Molar enthalpy values (ꢃ10%) of the gel–sol transition of the
fluorogelators determined from Schr o€ der–van Laar equation
0
DH /kJ mol
ꢁ1
Gelator
Solvent
1
2
3
4
5
6
7
8
9
1
. C24CH
. DC24CH
. LC24CH
. C24CH
. DC24CH
. LC24CH
2
C
2
F
5
1 : 1 DMSO–H
3 : 1 DMSO–H
2
O
O
54
88
39
56
85
51
72
83
80
88
79
2
C
2
F
F
5
2
2
C
2
5
2 : 1 DMF–H
4 : 1 DMF–H
DMSO
2
O
O
2 9 19
C F
2
2
C
9
F
F
19
2
C
9
19
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
. DC23
. DC23
. DC23
C
C
C
11
F
F
F
23
21
10
9
19
Fig. 3 Bar diagram showing the Tgel of DMSO gels (1.0% w/v) of (2i)
0. LC23
C
10
F
21
DC23C
11
F
23; (2f) DC23
C
10
F
21; (2d) DC23
C
9
F
19; (2g) LC23
C
10
F
21 and (2e)
11. LC23C9F19
9 19
LC23C F .
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Table 5 Dynamic rheology of the gels of the perfluoroalkyl derivatives.
0
0
00
The G and (G /G ) values shown in the table were obtained from
frequency sweep experiments performed at a fixed stress of 1.0 Pa. The
values are at f ¼ 1.0 Hz. The s* values shown were obtained from stress
sweep experiments at a fixed frequency of 1.0 Hz (error ꢃ 10–15%)
0
0
00
System
Solvent
G /Pa
G /G
s*/Pa
DC23
DC23
DC23
C
C
C
11
F
F
23
21
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
4 : 1 DMF–H
24 400
14 000
19 700
160
24
17
11
5
480
215
160
5
15
320
50
65
85
380
1170
10
9
F
19
LC23
LC23
C
C
10
F
21
9
F
19
600
9
DC24CH
LC24CH
2
C
9
F
19
20 500
600
3600
4300
76 000
145 100
12
12
11
13
14
*
2
C
9
F
19
C
C
24CH
24CH
2
C
C
9
F
F
19
5
2
O
2
2
1 : 1 DMSO–H
3 : 1 DMSO–H
2 : 1 DMF–H O
2
2
O
O
DC24CH
LC24CH
2
C
2
F
5
2
2
C
2
F
5
10 9 19
these parameters than LC23C F21. For example, LC23C F
showed an elasticity of 600 Pa which was ꢂ4 times higher than
the value (160 Pa) obtained for LC C F . The stiffness was
2
3 10 21
also found to be almost 2 times higher (9 compared to 5). So, in
the litho series also, these two rheological parameters were
dependent on the fluoroalkyl chain length. However, in
comparison to the gels derived from the deoxycholic derivatives,
these gels exhibited lower elasticity (more than an order of
magnitude). The stiffness was also found to be higher in the
deoxycholic series.
0
0
Fig. 5 (A) G vs. frequency and (B) G vs. oscillatory stress for the
DMSO gels (1.0% w/v) of DC23 21 and DC23
C
11
F
23, DC23
C
10
F
9 19
C F .
Stress sweep experiments. In the deoxy series, the yield stress
values showed a strong dependence on the fluoroalkyl chain
length (shown in Fig. 6B). DC C F showed the highest value
C
24CH
2
C
2
F
5
, on the other hand showed a much lower value of
elasticity (4300 Pa) in 1 : 1 DMSO–H O though its stiffness was
2
2
3 11 23
comparable to the gels of the deoxycholic derivative (Table 5).
For the lithocholic derivative, there were large fluctuations in the
of yield stress (480 Pa) and then it decreased for DC C F
3 10 21
2
(
215 Pa) and DC C F (160 Pa, Fig. 5B). So, the yield stress
23 9 19
0
0
G
values in the experimental frequency range (0.1–10.0 Hz)
values decreased with the fluoroalkyl chain length (Fig. 6B). On
the other hand, in the lithocholic series, LC23 19 showed
higher yield stress than LC23
0
00
which led to a poor reproducibility of the stiffness (G /G ) values
from experiment to experiment and hence not discussed here.
Among the longer chain perfluoroalkyl derivatives, the non-
9
C F
10 21
C F .
In comparison to the deoxycholic derivatives, the yield stress
values were an order of magnitude lower for the litho derivatives.
In other words, the lithocholic derivatives were more susceptible
to flow under an ascending stress ramp than their deoxycholic
analogues.
adecafluorodecyl cholate C CH C F showed an elasticity of
24 2 9 19
3
600 Pa and stiffness of 11 for its gel in 4 : 1 DMF–H O. These
2
values, incidentally, were comparable to those obtained for the
gel of pentafluoropropyl cholate C24CH in 1 : 1 DMSO–
O (vide supra). The deoxycholic and lithocholic derivatives
having the nonadecafluorodecyl chain formed gels in DMSO and
comparison of their elasticity values showed that
DC24CH 19 exhibited more than an order of magnitude
ꢂ34 times) higher elasticity value than LC24CH 19. The
2 2 5
C F
H
2
In conclusion, the rheological studies on the DMSO gels of the
fluoroesters with the spacer Q demonstrated the influence of two
structural aspects on their mechanical properties: the nature of
the bile skeleton and the fluoroalkyl chain length. The results of
the frequency sweep and stress sweep experiments are listed in
Table 5.
a
2 9
C F
(
2 9
C F
Spacer –(CO)–OCH
showed gelation in mixtures of DMF–H
2
(P). As these perfluoroalkyl compounds
O and DMSO–H O of
2
2
different proportions and in DMSO, the rheology experiments
were performed for the derivatives in one particular solvent or
solvent mixture.
Frequency sweep experiments. Among the pentafluoropropyl
esters, DC24CH
2
C
2
F
5
and LC24CH
2
C
2
F
5
gels showed very high
O and
2
: 1 DMF–H O, respectively. The cholic derivative,
Fig. 6 Variation of stiffness (A) and yield stress (B) with fluoroalkyl
chain length of DMSO gels (1.0% w/v) of different deoxy derivatives
having spacer Q.
elasticity (76 000 Pa and 145 100 Pa) in 3 : 1 DMSO–H
2
2
1
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stiffness of these gels, though, was identical. A similar kind of
difference in the elasticity was also observed between the gels of
the deoxycholic and lithocholic derivatives in the Q series.
Stress sweep experiments. The gel of LC24CH
DMF–H O showed the highest value of yield stress. It started
flowing only at an oscillatory stress of 1170 Pa. DC24CH
also showed a reasonably high value of 380 Pa for its gel in 3 : 1
DMSO–H O. The two cholic derivatives, C24CH 19 and
showed quite similar values of yield stress for their
gels in 4 : 1 DMF–H O and 1 : 1 DMSO–H O (Table 5),
2 2 5
C F in 2 : 1
2
2 2 5
C F
2
2 9
C F
C
2 2 5
24CH C F
Fig. 7 Transmission electron microscopy images of xerogels obtained
2
2
from toluene gels of (a) DC23
13
C F27 (1%, scale bar 1.0 mm) and (b)
respectively. The DMSO gel of DC CH C F showed a higher
2
4
2 9 19
LC23 27 (1%, scale bar 0.5 mm).
13
C F
value of yield stress (more than 6 times) than a gel of
LC CH C F in the same solvent.
2 9 19
2
4
which were not observed for any of the deoxy derivatives. The
approximate diameters of these aggregates were ꢂ250 to 350 nm
Electron microscopy
and ꢂ240 to 300 nm for LC23
11 9
C F23 and LC23C F19, respec-
Transmission electron microscopy. The morphology of the
xerogels of the perfluoroalkyl derivatives was investigated by
TEM and SEM. Let us first consider the perfluoroalkyl
compounds with spacer Q. The TEM images of a xerogel of the
longest chain-length deoxy derivative DC C F (from toluene)
2 9 19
tively. The litho compound in the spacer series P, LC24CH C F
which formed gels in DMSO, showed similar features in the
TEM images of its xerogel with smaller fibers of ꢂ50 nm and
slightly larger fibers of 90–130 nm (Fig. 9c).
2
3
13 23
Four derivatives having spacer P formed gels in organic–
aqueous mixtures. We chose to investigate the morphology of the
xerogels of two of them (C CH C F and C CH C F ) by
showed entangled network of fibers (tape-like) of ꢂ300 nm
diameter along with some smaller ones of 180–200 nm. Some of
them were found to be twisted in a helical fashion (Fig. 7a). This
was not unexpected considering the fact that these derivatives
contained chiral bile acid backbone. The TEM images of the
2
4
2
2
5
24
2 9 19
TEM. C CH C F (in 1 : 1 DMSO–H O) showed a dense
2
2
4
2
2
5
network of fibers of diameters of 40–80 nm. The fibers were
found to be twisted in a helical fashion in many parts of the
sample (Fig. 10a). The nonadecafluorodecyl analogue of this
13
litho derivative LC23C F23 (xerogel obtained from a weak gel in
toluene), on the other hand, showed cylindrical fibers with
comparatively smaller diameters (90–135 nm, Fig. 7b). The fibers
were found to form bundles in some places and the average width
of these bundles was ꢂ200 nm.
2 9
derivative, C24CH C F19 showed tape-like, twisted fibers in its
xerogel obtained from 4 : 1 DMF–H
fibers were of 60–120 nm diameter.
2
O (Fig. 10b and c). The
The derivatives with fluoroalkyl chain lengths smaller than
Scanning electron microscopy. The SEM images of the xerogels
from DMSO of DC23 21 and DC24CH
13
–C F27 formed gels in DMSO (vide supra) and the TEM images
C
11
F23, DC23
C
10
F
2 9 19
C F
of their xerogels were compared in order to see whether there was
a variation in the morphology with the nature of the bile acid
skeleton and the fluoroalkyl chain length. The TEM images of
the deoxy derivatives demonstrated that though there was no
significant variation in the morphological features, a slight
increase was observed on going to shorter chain lengths (Fig 8a–
c). For example, DC23
C
11
F
23 showed fibers of diameter ꢂ50 to
7
0 nm with bundles having width of ꢂ100 nm. For DC C F ,
2
3 10 21
the fibers were generally of 90–120 nm diameter and they formed
bundles of ꢂ200 nm. Fibers of 130–150 nm were observed for
DC C F . In the junction zones, the fibers formed bundles of
2
3 9 19
ꢂ200 nm width. Interestingly, DC24CH
2 9
C F19, the deoxy deriv-
ative with the other spacer P, also showed fibers of similar
diameter (90–120 nm, Fig. 8d) in its xerogel obtained from
DMSO.
A similar trend in the morphological features was observed in
11
the litho series having the spacer Q. LC23C F23 showed a set of
smaller fibers ꢂ30 nm diameter along with some comparatively
larger fibers of 80–100 nm whereas LC C F showed two sets of
2
3 9 19
fibers of 40–50 nm and 90–120 nm diameter (Fig. 9a and b).
Hence, in the litho series also, there was a slight increase in the
diameter of the fibers while going to smaller chain lengths.
Interestingly, other than the entangled fibrous network, the TEM
images of both the litho derivatives showed the presence of
aggregates (spherical in shape) in different parts of the sample
Fig. 8 Transmission electron microscopy images of xerogels obtained
from DMSO gels of (a) 0.2% DC23 23 (scale bar 0.2 mm), (b) 0.5%
DC23C10F21 (scale bar 0.5 mm), (c) 1.0% DC23C9F19 (scale bar 1.0 mm) and
(d) 0.3% DC24CH 19 (scale bar 1.0 mm).
11
C F
2 9
C F
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showed similar entangled network of fibers (Fig. 11) as observed
from their TEM images (vide supra).
To investigate whether the concentration of the gelator had
any effect on the morphology of the gels, SEM images were
recorded for the xerogels of DC23
different concentrations. It was found that for both 1.0% and
.2% gels, the average diameters of the fibers were similar
Fig. 12). A similar observation was also noted for the SEM
9
C F19 (in DMSO) for two
0
(
10
images of the DMSO xerogels of DC23C F21 of different
concentrations (not shown).
It has been mentioned earlier that the TEM images of the
xerogels of the litho derivatives (from DMSO) showed the
presence of large aggregates. Interestingly, they were not
observed for the xerogels of LC C F obtained from toluene.
2
3 13 27
The SEM images of the xerogels of LC23
and LC23 19 (from DMSO) revealed similar kind of features
observed previously from their TEM images. LC23 27 did not
show the presence of aggregates whereas LC23 19 showed the
13
C F27 (from toluene)
9
C F
13
C F
9
C F
presence of aggregates along with the fibrous network (Fig. 13).
These aggregates probably originated from partial precipitation
of the gelator molecules inside the gel.
Fig. 10 Transmission electron microscopy images of xerogels of
C
C
24CH
24CH
2
C
C
2
F
F
5
(0.5%, 1 : 1 DMSO–H
2
O, scale bar 1 mm) (a) and of
2
9
19 (0.5%, 4 : 1 DMF–H O) (b) scale bar 1 mm and (c) scale bar
2
0
.2 mm.
Discussion
The present study involves the design of a new class of bile acid
based gelators. The side chains of the facially amphiphilic bile
the inter-gelator interactions between these fragments. These
variations would in turn influence the solubility of these deriva-
tives. This point is of interest as in general for effective gel
formation, the gelator should neither be too soluble nor too
insoluble in the gelling solvent (accompanied by its propensity to
preferentially aggregate in one-dimension). In other words,
gelation requires a fine balance between the solvent–gelator and
the gelator–gelator interactions. Thus, the gelator solubility and
27
acids were connected to perfluoroalkyl chains of different
lengths through two different ester linkages, –O–(CO)– and
–
2
(CO)–OCH –. Thus, the derivatives had three variable struc-
tural aspects—the steroidal backbone, the fluoroalkyl chain
length and the spacer. The choice of cholic, deoxycholic or
lithocholic acid would alter the polarity of the derivatives (also
a tuning of the potential H-bonding interactions) whereas the
variation in the fluoroalkyl chain length is expected to modulate
Fig. 11 Scanning electron microscopy images of xerogels (obtained
Fig. 9 Transmission electron microscopy images of xerogels (obtained
from DMSO gels) of (a) DC23
DC23 21 (0.5%, scale bar 2.0 mm) and (c) DC24CH
bar 2.0 mm).
C
11
F
23 (0.2%, scale bar 2.0 mm), (b)
from DMSO) of (a) LC23
C
11
F
23 (1.0%, scale bar 0.5 mm), (b) LC23
C
9
F
19
C
10
F
2 9
C F
19 (0.2%, scale
(
1.0%, scale bar 1.0 mm) and (c) LC24CH 19 (0.5%, scale bar 1.0 mm).
2 9
C F
1
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these gels on the ratio of the organic and aqueous components in
the mixtures. It is to be noted that the gelation properties of these
pentafluoropropyl esters are quite different from the already
reported n-propyl analogues (vide supra). Among the higher
derivatives, the cholate derivative with the nonadecafluorodecyl
group, C24CH
sponding deoxycholic and lithocholic derivatives formed gels in
DMSO (the only ones in –(CO)–OCH – series). The introduc-
2 9 2
C F19, formed gels in DMF–H O. The corre-
2
tion of much longer nonadecafluorodecyl group promoted
stronger inter-gelator perfluoroalkyl–perfluoroalkyl interactions.
For the cholic derivative, the higher polarity of the bile acid
Fig. 12 Scanning electron microscopy images of xerogels of DC23
9 19
C F
backbone could still govern its gel formation in DMF–H O
2
(from DMSO): (a) 1% (scale bar 5.0 mm) and (b) 0.2% (scale bar 2.0 mm).
mixtures whereas it was not possible for the less polar deoxy-
cholic and lithocholic derivatives.
the nature of the solvent strongly influence the gelation process
and there are several reports in the literature which highlight the
The deoxycholic and lithocholic derivatives with the spacer
–
O–(CO)– formed gels in aromatic hydrocarbons and also in
DMSO depending on the fluoroalkyl chain length. These deriv-
atives with the –C13 27 chain formed gels in many aromatic
28
importance of these parameters in the gelation process. In the
case of the perfluoroalkyl esters, we modulated the aforemen-
tioned parameters through a variation of the structural motifs
and we indeed found that the gelation abilities (nature of the
solvent gelated, the CGCs, the mechanical properties of the gels,
etc.) of these derivatives were dependent on them. With both
spacers, the deoxycholate and the lithocholate were found to be
more efficient in forming gels presumably because they possess
the optimum combination of polar/non-polar solubility. Two
cholates, C CH C F and C CH C F having the spacer
F
hydrocarbons like benzene, toluene, xylene, etc. A decrease in the
chain length significantly modulated the gelation properties of
the derivatives. The derivatives with –C11
7
F23 to –C F15 chains
formed gels only in DMSO (except DC C F which addi-
23 11 23
tionally formed weak gels in aromatic hydrocarbons). Even
among them, the gelation capability was dependent on the length
of the fluoroalkyl segment. Hence, in the deoxy series,
2
4
2
2
5
24
2 9 19
DC23
followed by DC23
a much less efficient gelator of DMSO with a CGC of 1.5%. In
the litho series, LC23 21 and LC23 19 were very efficient
gelators (CGC 0.2%) but both LC23 23 and LC23 15 were
C
11
F23 was the most efficient gelator (lowest CGC) closely
–
(CO)–OCH –, showed gelation in organic–aqueous media, but
2
C
10
F
21 and DC23 19. But DC23 15 was
C
9
F
7
C F
neither of the two cholates (C C F and C C F ) with the
2
3
13 27
23 9 19
other spacer –O–(CO)– showed gelation. This observation
points out the influence of the nature of the spacer on the gelation
properties. Interestingly, most of the reported bile acid based
organogelators have cholic acid as the bile acid backbone (vide
supra).
C
10
F
9
C F
C
11
F
7
C F
poorer gelators. These observations again highlight the mutual
interplay between the polarity of the bile acid backbone and side
chain perfluoroalkyl group. The derivatives with the –C F
3 27
1
2
The perfluorinated derivatives with the spacer –(CO)–OCH –
could form gels in non-polar aromatic hydrocarbons but when
the length of the fluoroalkyl segment was decreased, there was
also a simultaneous decrease in the inter-gelator interactions
showed gelation properties in organic–aqueous media and in
DMSO. The shorter chain pentafluoropropyl esters were found
to be efficient gelators of DMF–H O and DMSO–H O mixtures.
2
2
(
between the perfluoroalkyl groups) though the polarity of the
bile acid backbone remained unchanged. As a result the smaller
chain derivatives (–C11 23 to –C 19) could form gels efficiently
The cholic derivative C CH C F formed gels with higher
2
4
2 2 5
proportions of water than the deoxy and litho derivatives. In
other words, the aqueous–organic mixtures with higher contents
of water were more compatible with the more polar cholic
derivative whereas the least polar litho derivative formed gels in
mixtures with lower contents of water. This effect was further
manifested by the critical dependence of the CGC and Tgel of
F
9
F
in more polar DMSO where a suitable balance between the head
group polarity and the fluoroalkyl chain length was reached. This
balance was disturbed in decreasing the chain length further and
the derivatives with –C
DMSO.
7
F15 were found to be poorer gelators of
The mechanical properties of the gels formed in DMSO were
also found to be dependent on the bile acid moiety and the
fluoroalkyl chain length. In general, the deoxy derivatives
showed higher elasticity, stiffness and yield stress for their gels
than the litho analogues. Among the deoxy derivatives,
a decrease in the fluoroalkyl chain length resulted in a decrease of
the stiffness and yield stress values of the gels. On the other hand,
9 19
in the litho series, in going from the –C10F21 to the –C F
derivative, the elasticity, stiffness and yield stress increased. The
opposite trends in the aforementioned mechanical properties in
the deoxy and litho series probably indicate the difference in the
optimum conditions required for gel formation—more polar
deoxy derivatives need longer fluoroalkyl chains and smaller
fluoroalkyl chains are more compatible with comparatively non-
polar litho derivatives.
Fig. 13 Scanning electron microscopy of xerogels of (a) LC23
in toluene, scale bar 5.0 mm) and (b) LC23 19 (0.5% in DMSO, scale bar
.0 mm).
13
C F27 (1%
9
C F
2
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A closer inspection of the data on the gels derived from the
compounds having the same fluoroalkyl chain-length but
different spacers led to a few interesting correlations. Both
Bangalore, is thanked for financial support to AJS. We also
thank an anonymous referee for valuable comments.
9 2 9
DC23C F19 and DC24CH C F19 formed gels in DMSO and the
enthalpy of gel–sol transition (80 and 85 kJ mol), the storage
modulus (19 000 and 20 500 Pa) and the stiffness (11 and 12) of
their DMSO gels were very similar. The two corresponding litho
Notes and references
1
2
3
P. Terech and R. G. Weiss, Chem. Rev., 1997, 97, 3133.
L. Estroff and A. D. Hamilton, Chem. Rev., 2004, 104, 1201.
Molecular Gels: Materials with Self-assembled Fibrillar Networks, ed.
P. Terech and R. G. Weiss, Springer, Dordrecht, 2006.
derivatives (LC23
similarities in their CGCs (0.2 and 0.3%) and storage modulus
600 Pa for both). There were similarities between the shorter
chain (–C F ) derivatives, too. They are either non-gelators or
9 2 9
C F19 and LC24CH C F19) also showed some
4
(a) N. M. Sangeetha and U. Maitra, Chem. Soc. Rev., 2005, 34, 821;
(
(
b) A. R. Hirst, B. Escuder, J. F. Miravet and D. K. Smith, Angew.
7
15
Chem., Int. Ed., 2008, 47, 8002; (c) S. Banerjee, R. K. Das and
U. Maitra, J. Mater. Chem., 2009, 19, 6649; (d) M. Zelzer and
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poor gelators. For example, DC C F is not a very efficient
23 7 15
gelator (CGC 1.5%) of DMSO whereas LC C F is a poor
3 7 15
2
gelator capable of forming gels only above 2.0%. Similarly,
15, DC24CH 15 and LC24CH 15 were either
C24CH
2
C
7
F
2
C
7
F
2 7
C F
5
6
non-gelators or formed weak/partial gels in organic–aqueous
mixtures.
7
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Conclusions
We have reported the synthesis and gelation studies of a new
class of perfluorinated analogues of bile acids. They formed gels
2
in aromatic hydrocarbons, DMSO and in DMSO/DMF–H O
mixtures depending on the choice of the bile acid moiety, the
perfluoroalkyl segment and the spacer. The variation in the
gelation properties of the derivatives through a modulation of
the aforementioned structural aspects essentially points out the
fine balance that is required between the sovent–gelator and
gelator–gelator interactions for effective gel formation. These
gels were mostly transparent with reasonably high thermal
stability. The thermal stability, the mechanical properties and the
morphology of the gels were investigated in detail using appro-
priate techniques. Rheological investigations demonstrated that
these gels behaved as viscoelastic materials and the mechanical
properties of these gels could be modulated by changing either
the bile acid moiety or by varying the length of the fluoroalkyl
segment. Although there are a few reports in the literature on the
fluorinated gelators, to the best of our knowledge these are the
first examples of fluorinated gelators derived from the bile acids.
The presence of the perfluoroalkyl chains which are simulta-
neously hydrophobic and lipophobic introduced new properties
to these compounds. For example, the efficient organogelators
among these derivatives are the ones possessing deoxycholic and
lithocholic acid moieties whereas in the literature most of the
reported organogelators are based on the cholic acid skeleton.
8
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29
CO
2
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This work has been supported by the Indo-French Center for the
Promotion of Advance Research (IFCPAR), New Delhi. The
Institute Nanoscience Initiative (INI) is acknowledged for TEM
and SEM facility and Mr T.B.N. Satyanarayana for recording
the SEM images. The Department of Science & Technology is
thanked for a J.C. Bose Fellowship to UM. The Council of
Scientific and Industrial Research (CSIR), New Delhi is thanked
for the award of Research Fellowship to SB and JNCASR,
2
2
1
4704 | J. Mater. Chem., 2011, 21, 14693–14705
This journal is ª The Royal Society of Chemistry 2011

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2
3 The bile acid moiety will be indicated first by either using C (cholic), DC
(deoxycholic) or LC (lithocholic) with a subscript indicating the number
of carbons in the bile acid segment (as there are 23 and 24 carbons in the
bile acid segment for the spacers P and Q respectively). This will be
followed by indicating the perfluoroalkyl segment. For example,
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DC23
group –C11
linkage. Similarly, C24CH
derivative and the perfluoroalkyl chain –C
acid moiety through –(CO)–OCH – linkage.
C
11
F
23 means it is a deoxycholic derivative and the perfluoroalkyl
23 is joined to the bile acid moiety by the –O–(CO)–
19 suggests that it is a cholic acid
19 is connected to the bile
F
2 9
C F
9
F
2
24 K. Choi, R. Mruk, A. Moussa, A. M. Jonas and R. Zentel,
Macromolecules, 2005, 38, 9124.
2
2
5 H. Gilman and N. J. Beaber, J. Am. Chem. Soc., 1925, 47, 518.
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29 We have explored the aerogel formation ability of some of the longer
2
chain derivatives in supercritical CO in collaboration with Prof.
2
7 Interfacial properties of a few monofluorinated bile acids are
reported: J. F. Kauffman, R. Pellicciari and M. C. Carey, J. Lipid
Res., 2005, 46, 571.
Jean-Pierre Desvergne’s group in Bordeaux, France. The details of
aerogel formation and the investigations on their properties will be
published elsewhere.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 14693–14705 | 14705