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melting temperature of the gelator in an oil bath until the solid
dissolved. Then, the solutions/sols were shaken to ensure homoge-
neity. For the fast-cooled samples, the glass tubes were then
placed directly into an ice-water bath for 10 min and were warmed
to room temperature for 1 h. For the slow-cooled samples, the hot
mixtures were kept in the oil bath while it was returned slowly to
room temperature by removing the heating unit. Then, each tube
was opened, a steel ball (4 mm diameter; 250 mg) was carefully
placed on the surface of the mixture, and the tube was flame-
sealed again. Gelation was considered to have occurred qualitative-
ly if the samples supported the ball at room temperature. The ap-
pearances of the gels were classified as opaque (OG) or clear (CG).
Gelators that did not dissolve after heating were deemed “insolu-
ble” (I). If the gelators dissolved after heating, but precipitated
when cooled, the samples were classified as precipitates (P). If
a gel-like mixture was formed after cooling, but flowed slowly
when the tube was reversed, the sample was considered a gelati-
nous precipitate (GP). Systems in which only a solution/sol re-
mained after heating and cooling are referred to as solutions (S).
Finally, if a potential gelator dissolved on heating and formed an
inhomogeneous particle suspension on cooling, the sample was
denoted as a suspension (Sus). CGCs were determined from
a series of fast-cooled samples with different gelator concentra-
tions by the falling-ball method. The lowest concentration at
which the ball did not fall was considered the CGC. Tg ranges were
determined from fast-cooled gels by the falling-ball method in
flame-sealed glass tubes that were heated in an oil bath at
28CminÀ1. The temperature ranges reported are between when
the ball began to fall and when it arrived at the bottom of the
tube under the influence of gravity.
sealed after the samples had cooled to their gel phases. The dif-
fraction data were collected for 10 h. The diffraction peaks of the
gels were identified by subtracting empirically the amorphous scat-
tering of liquids from the total gel diffractograms.[36]
Melting points of all gelators and polarized optical micrographs
(POMs) of gels were recorded on either a Leitz 585 SM-LUX-POL
microscope equipped with crossed polars, a Leitz 350 heating
stage with a full-wave plate, a Photometrics CCD camera interfaced
to a computer or an automated Nikon Eclipse Ti-E inverted micro-
scope, equipped with a Linkam PE100-NIF inverted Peltier heating/
cooling stage with a full-wave plate. The samples for POMs using
the Leitz instrument were prepared by pouring a hot solution/sol
into a 0.4 or 0.5 mm path-length, flattened Pyrex capillary tube (Vi-
troCom) which was then flame sealed. The samples were heated to
their liquid phase and then cooled by either the fast- or slow-cool-
ing process. The fast-cooled and slow-cooled gel samples em-
ployed with the Nikon instrument were placed between two glass
coverslips. Atomic force microscopic images were recorded at
room temperature on a NTEGRA Prima scanning probe microscope
(from NT-MDT) mounted on an inverted Nikon Eclipse Ti-S fluores-
cence microscope. An NSG30 tip (NT-MDT) was modified hydro-
phobically by dipping it in a 10 mm dichlorodimethylsilane in tolu-
ene solution for 1 h, followed by washing sequentially with chloro-
form, 2-propanol and water. The tip was then heated to 1508C for
30 min. Gels were deposited directly onto a glass substrate, reheat-
ed to their solution/sol phase, and (when necessary) liquid was
added to maintain the concentration after heating within 5 wt%.
Samples were scanned in an open system. Differential scanning
calorimetry (DSC) studies were performed on a TA DSC200 instru-
ment using hermetically sealed aluminum pans against an empty
reference cell. Transition temperatures from DSC, including gelator
melting, gel melting, gelator solidification, and gel formation tem-
peratures (Tm, Tgm, Tc, and Tgc, respectively), are reported at the
onsets of endotherms (on heating) and exotherms (on cooling)
with heating and cooling rates of 108CminÀ1. The enthalpies of
the transitions for the neat gelators were also measured. The en-
thalpies of the gelators in the gels were normalized to “neat” con-
centrations by dividing the observed heats by the effective SAFiN
concentrations (i.e., the total added wt% of gelator minus the
CGC).
Instrumentation and procedures
1H and 13C NMR spectra were recorded on a Varian Inova 400 MHz
(100 MHz for 13C) spectrometer operating at 400 MHz. MestReNova
v5.2.4-3924 software from Mestrelab Research was used to analyze
1
the spectra. Data from the H NMR spectra are reported as chemi-
cal shifts (d in ppm), referenced to tetramethylsilane (TMS), with
the corresponding integration values. Coupling constants (J) are re-
ported in hertz (Hz). Standard abbreviations indicating multiplicity
are: s (singlet), b (broad), d (doublet), t (triplet), q (quartet), and m
(multiplet). Data for 13C NMR spectra are reported in terms of d. IR
spectra of neat solids were obtained on a PerkinElmer Spectrum
One FT-IR spectrometer interfaced to a PC, using an attenuated
total reflection accessory (ATR). IR spectra of gels were obtained on
a Nicolet 380 FT-IR spectrometer using liquid cells having NaCl win-
dows and sealed with Teflon plugs. GC-MS measurements were ob-
tained on a Shimadzu GC-17A gas chromatograph connected to
a Shimadzu QP-5000 mass spectrometer using a 0.25 mm SGE BPX5
(15 m0.25 mm) column and a flame ionization detector. Elemen-
tal analyses were performed on a PerkinElmer 2400 CHN elemental
analyzer using acetanilide as a calibration standard. High-resolution
mass spectral data, in electrospray ionization mode, were obtained
on a Bruker-Daltronicsꢁ BioTof mass spectrometer in positive
(ESI+) ion mode.
Rheological measurements were obtained on a strain-controlled
Anton Paar-Physica MCR 301 rheometer using Peltier temperature-
controlled parallel plates (25 mm diameter) at a gap of 0.5 mm.
Data were collected using Rheoplus/32 Service V3.10 software.
Before data were recorded, each sample was heated between the
shearing plates to ensure that a solution/sol was present. It was
cooled to 08C (208CminÀ1), the temperature was increased to
258C, and the sample was incubated there for 15 min to reform
the gel. Both strain sweeps and angular frequency sweeps were
measured for three aliquots of each sample.
Hansen space correlations and spheres
In this method, the overall energy density (d) of a liquid or gelator
is separated into dispersive (dd), polar (dp), and H-bonding (dh) in-
teraction components [Eq (1)].[3a,7,45,46,54] These Hansen solubility
parameters (HSPs) for the neat liquids were taken from the litera-
ture (Table S3 in the Supporting Information).[48] The Hansen space
was plotted using MATLAB HSP 3D Plotting and Fitting Program
from UMD Complex Fluids and Nanomaterials Laboratory.[7] HSP
values for each gelator were calculated as the center of the
spheres constructed from the data points. For the purpose of
these analyses, the samples of gelators in different liquids were
Powder X-ray diffraction (XRD) patterns of the samples were ob-
tained on a Rigaku R-AXIS image plate system with CuKa X-rays
(l=1.54 ) generated by a Rigaku generator operating at 46 kV
and 40 mA with the collimator at 0.5 mm. Data processing and
analysis used Materials Data JADE (version 5.0.35) XRD pattern
processing software. Neat powders were sealed in 1.0 mm glass ca-
pillaries (W. Müller, Schçnwalde, Germany) and the diffraction data
were collected for 2 h. Hot solution/sols were placed in 0.5 mm
glass capillaries (W. Müller, Schçnwalde, Germany) that were flame-
Chem. Eur. J. 2015, 21, 8530 – 8543
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