An open coaxial re-entrant
microwave sensor has been used for the
non-invasive and continuous monitoring of the sol–gel transition of
physical gels characterized by different gelation mechanisms, solvents,
compositions, and stabilities. Comparison of measurements by
differential scanning calorimetry allowed the identification of the
phase transition by a change in the dielectric properties of the
material over time.
Self-assembled viscoelastic gels of organic solvents (organogels), water
(hydrogels) or water–organic solvent mixtures (aqueous gels) have been
recognized as promising materials for bottom-up nanofabrication tools in
various fields including biomedicine, sensors, cosmetics, food,
catalysis, and environmental remediation.As soft materials, gels are
continuous in structure and solid-like in rheological behavior. In
contrast to chemical gels, which are based on covalent bonds (usually
cross-linked polymers unable to redissolve), physical (also called
supramolecular) gels are made of either low-molecular-weight (LMW)
compounds or polymers – so called gelators – through extensive
non-covalent interactions, predominantly hydrogen-bonding, van der
Waals, dipole–dipole, charge-transfer, donor–acceptor, π–π stacking and
metal-coordination interactions. Furthermore, systems based on both
types of connections are also known. The solid-like appearance of these
gels is the result of the entrapment of the liquid (major component) in
the interstices of a solid 3D matrix of large surface area (minor
component), usually through surface tension and capillary forces.
Remarkably, many gels can immobilize up to 105 solvent molecules per
molecule of gelator and increase the viscosity of the medium by a factor
of 1010.
In the case of LMW gelators, the formation of the viscoelastic matrix is
a consequence of the entanglement of 1D supramolecular fibers
(typically of micrometer scale lengths and nanometer scale diameters),
which is usually induced by cooling their hot isotropic solutions to
room temperature (RT). However, it should be noted that gelation of
liquids at RT or induced by ultrasound treatment instead of
heating–cooling has also been described. Due to the weakness of the
non-covalent interactions that maintain the dynamic supramolecular
structure, physical gels are usually thermoreversible. Moreover, the
sol–gel (and/or gel–sol) phase transition could also be triggered by
other stimuli such as pH, light irradiation or ionic strength if the
gelator molecule possesses appropriate structural moieties for
recognition. It is also important to recognize that the metastable
nature of physical gels derives from an elusive equilibrium between
dissolution and crystallization, which has stimulated numerous studies
and applications in the field of crystal engineering during the last few
years.
Due to the brittleness of these materials, it is usually easier to
monitor the gel–sol transition rather than the sol–gel for the
construction of phase diagrams according to both the gel–sol transition
temperature (TGS) and the sol–gel transition temperature (TSG). Among
different techniques, rheology, NMR spectroscopy and conventional
differential scanning calorimetry (DSC) are the most common and accurate
methods used so far for this kind of study, albeit they normally suffer
from the disadvantages of being relatively time consuming and requiring
the use of very expensive equipments and trained personnel. Techniques
of higher specificity such as ESR, NIR and fluorescence spectroscopy
have also been used to characterize the sol–gel transitions of
colloids.17 On the other hand, dielectric measurements have also been
used to determine sol–gel transitions, usually below a few kHz. At these
frequencies the dielectric properties are normally related to the
conductive nature of the material and this quantity becomes (less)
sensitive to chemical changes that occur at gelation.Dielectric
measurements at microwave frequencies, however, are very sensitive to
the mobility of molecules in the gel (especially when some water dipoles
are involved). Therefore, the use of the mobility of the molecular
structure through dielectric properties provides a direct (and in situ)
measurement of the chemical and physical state of the matter.Changes in
dielectric parameters can be related to critical points in different
material processes, such as cure reaction onset, gelation, end-of-cure,
build-up of the glass-transition temperature, etc.For example, a
microwave system designed for adhesive cure monitoring has been
previously described by some of us where in situ dielectric measurements
correlate very well with conventional measurement techniques such as
DSC, combining accuracy and rate with simplicity and an affordable cost.
This communication presents a microwave non-destructive system for
monitoring the sol–gel transition process of supramolecular gels (Fig.
1A). A microwave sensor adapted to a standard pyrex vial containing the
precursor isotropic solution allows in situ measurements of dielectric
properties in order to distinguish the changes over time and
temperature.
Fig. 1B shows a picture of the portable microwave device used to conduct
the dielectric measurements. The system comprises a microwave sensor, a
microwave transmitter and receiver (from 1.5 to 2.5 GHz) and a control
unit to provide real-time information about the gelation progress
without interfering with the reaction. The precursor isotropic solution
is introduced in a pyrex vial and placed inside an open coaxial
re-entrant (microwave) cavity sensor. When the low-intensity
electromagnetic waves penetrate into the material, its molecules tend to
orient with the (applied) external field and the material gains certain
polarization, reflecting the back part of the microwave signal from the
sensor. This reflected signal is measured continuously to determine the
resonance frequency and quality factor of the sensor during gelation to
monitor the transition process. Fig. 1C and D show a typical response
of the reflected signal in the microwave cavity sensor in the imaginary
plane (Smith chart) or in magnitude representation of a gelation
experiment at a given temperature. We have reported elsewhere the
fundamental details of the microwave system with a different sensor
head.
Fig. 2 shows the library of known gelators that we prepared (ESI) to
test the ability of the microwave sensor to monitor the sol–gel
transition of physical gels. The library included single LMW gelators
(1–8) as well as bicomponent (9) and multicomponent gelator systems
(10). A number of gels with different solvents and compositions could be
easily obtained from this library at well-defined concentrations.
Moreover, N,N′-dibenzoyl-L-cystine (6) was included in this study for
the preparation of aqueous gels. Azobenzene-containing peptide 8 was
selected because its phase transition can be triggered either thermally
or photochemically. Besides the classical heating–cooling treatment
needed for the formation of thermoreversible physical gels made from
solid compounds 1–8, gelator systems 9 and 10 enable sol–gel phase
transitions at RT and well below RT, respectively. In the case of 9, DMF
stock solutions of oxalic acid dihydrate and copper(II) acetate
monohydrate were mixed at RT to form the corresponding organogel.
Multicomponent solution 10 constitutes a special system used to form
organogels at low temperatures upon addition of a small amount of this
solution to a suitable organic solvent (ESI). In contrast to the gels
obtained from 1–8, those derived from 9–10 are not thermoreversible
despite the non-covalent interactions involved in the gelation process.
Moreover, gels made from 10 eventually undergo subsequent transition to a
thermodynamically most stable crystallization phase This collection of
gelators offered a versatile scenario for the proof-of-concept of the
detection of the sol–gel transition in physical gels by continuously
monitoring the dielectric properties of the materials.
The isotropic solutions of the gelators were prepared as previously
reported (ESI). Preliminary experiments with solutions prepared at
different concentrations of a LMW gelator showed a response of the
microwave sensor to viscosity changes of the medium (ESI). On the basis
of this observation, the dielectric properties of the sol–gel transition
were continuously monitored at microwave frequencies and the obtained
profile was correlated with the actual temperature of the material
(ESI). Moreover, DSC thermograms were recorded separately for model
systems in order to draw meaningful comparisons between the change in
the dielectric properties of the material and the exothermic effect
associated with the sol–gel transition. The temperature profiles during
the sol–gel period were constructed independently by means of a
thermocouple probe (∅ 0.1 mm) centrally placed inside the mixture. We
confirmed that the use of this probe did not affect the gelation
kinetics. After each measurement, the state of the material was examined
by the “stable-to-inversion” test, and the gel condition of model
samples that did not show gravitational flow upon turning the vial
upside-down was also confirmed by oscillatory rheological measurements
(ESI).
The results indicated a good correlation between the different
techniques to recognize the sol–gel transition under different
conditions (e.g., solvent nature, concentration, and gelator structure).
Finally, preliminary experiments have shown that the microwave sensor
could also be used to detect the melting (gel–sol) transitions as we
could record the variation of the dielectric properties of the material
at single points (upon heating separately) and correlate marked changes
with the TGS determined by DSC or the inverse flow method (ESI).
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