Non-invasive and continuous monitoring of the sol–gel phase using a fast microwave sensor

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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