determined to be 13.3 kJ mol¹1, while that of the gel was
determined to be 15.3 kJ mol¹1. On the other hand, the relation-
ship of diffusibility and viscosity of solvent molecules in gel
follows as the Stokes-Einstein equation below:19
D ¼ kT=c³©rS
ð4Þ
where k is the Boltzmann constant, T is the temperature, c is the
constant (6 or 4), ³ is the circular constant, © is the viscosity,
and rS is the Stokes radius. As can be seen in Figure 6b, the D
values in neat PC and its gel were determined to be 4.34 ©
¹10
¹1
10
and 4.30 © 10¹10 m2 s at 20 °C, respectively. Herein,
assuming that the rS is equal to the van der Waals radius (rv =
0.276 nm)20 and c = 4, the © of PC in neat and gel were
determined to be both 2.7 mPa s at 20 °C. Their values were
similar to the literature data which is 2.8 mPa s at 20 °C.21
These results indicate that the micro-viscosity is similar to neat
PC, although this gel has non-fluidity, that is, the macro-
viscosity is very high by gelation.
Figure 6. (a) ¦-dependence of D in neat PC and its gel at
30 °C and (b) T-dependence of D in neat PC and its gel
with 3 wt % 1-12-6.
kJ mol¹1). These melting temperatures of xerogel and powder
were both 94 °C. These results indicate that the stability of
xerogel and powder were similar. In fact, PC gel with 1-12-6
has been stable at 20 °C for >12 months.
Conclusion
Aprotic low-molecular-mass organic compounds 1-4 based
on a gemini 4-[2-(perfluoroalkyl)ethylsulfanyl]phenoxy deriv-
ative unit formed physical gels in several alcoholic and polar
solvents at low concentration. It was found MGC depends
on alkylene chain length (m) and oxidation state of sulfur
through screening for the molecular structures. In particular, the
organic gels with 3 wt % 1-12-n have been stable at 20 °C
for >12 months and syneresis is not observed in sealed tubes,
thus far. Tgel-sol depends on perfluoroalkyl chain length (n).
Probably, MGC and Tgel-sol significantly depends on steric and
electrostatic balance. From the results of microscope observa-
tion and NMR spectroscopy, 1-12-6 aggregates into the tape
shaped molecular nanofibers, and entangle to form the 3D
networks in PC. On the other hand, D values of PC molecules
in gel are similar to those in neat PC, although PC molecules
have no fluidity by gelation.
To the best of our knowledge, there are few reports per-
taining to the self-diffusion coefficient (D) of solvent molecules
in gels. The values were determined with pulsed field gradient
(PFG)-NMR spectroscopy.15 Figure 6a shows diffusion time
(¦)-dependence of D of PC in neat PC and its gel with 3 wt %
1-12-6 at 30 °C. The D values were independent between 50
and 100 ms, in addition these D values of its gel were similar
to those of the neat, and these D values of neat PC and its gel
¹10
¹1
were determined to be 5.8 © 10
and 5.4 © 10¹10 m2 s
,
respectively. The 3D diffusion distance of particular based on
Brownian motion follows as the equation below:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
R ¼ 6D ꢀ ꢀ
ð2Þ
where R is the diffusion distance (square displacement). The
Brownian motion of PC molecules in the gel was not affected
at least radius 18 ¯m by gel networks. Thus, the mesh size of
the nanofiber in the gel (Figure 5) is probably micron scale,
thereby it is natural and predictable that solvents molecules in
the gel behaved as the neat-like solvent.
Experimental
Materials. 2-(Perfluoroalkyl)ethyl iodide were purchased
from Daikin Industries, Ltd. reagent chemicals; 4-hydroxy-
benzenethiol was purchased from Sankyo Kasei Co., Ltd.
reagent chemicals; 1,ω-dibromoalkane were purchased from
Tokyo Chemical Industry Co., Ltd. reagent chemicals. All
reagents and solvents were obtained from generally commercial
sources.
Instrumentation. Melting point (mp) was obtained with a
Yanaco MP-J3 micro melting point apparatus. High-perform-
ance liquid chromatograms were recorded on a Shimadzu
Prominence HPLC System. Infrared (IR) spectra were recorded
on a Shimadzu IR Prestige-21 spectrometer using KBr disc or
KRS-5 film. 1H NMR spectra were recorded with a JEOL JMN-
LA500 and JNM-ECA500 spectrometer (500 MHz), where
tetramethylsilane (TMS) was used as an internal standard.
High-resolution mass spectra (HRMS) were recorded with a
Waters LCT PremierTM XE. Phase transition latent heats were
recorded on a Seiko Instruments SSC/5200, where α-alumina
was used as a caloric standard material. Gel strength was
measured with Rheometer CR-500X (Sun Scientific Co., Ltd.).
Scanning probe microscopy (SPM) and scanning electron
Figure 6b shows temperature (T)-dependence of D in neat
PC and its gel between 0 and 80 °C. Herein, above room
temperature, a convection of neat PC molecules in an NMR
tube resulted from the deviation of D values,16 therefore such
D values corrected with Arrhenius plot at low temperature, as
shown in Figure S10. In contrast, the D values in gels (includ-
ing sol phase) were not affected by convection. It is thought
that gel networks inhibit the convection of PC molecules with
increasing temperature. Same phenomenon has been reported
by Takekawa et al.17 As can be seen in Figure 6b, the D values
of its gel were similar to those of the neat between 0 and 80 °C.
In addition, the relationship of T-dependence and activation
energy is shown in Figure 6b, with the Arrhenius equation
below:18
D ¼ D0 expðꢁEA=RTÞ
where D0 is the constant (frequency factor), EA is the activation
energy, R is universal gas constant, and T is the temperature.
The EA for self-diffusion of PC molecules in neat PC was
ð3Þ
© 2015 The Chemical Society of Japan
Yoshida, Tomohiro
