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Thermal Scanning Conductometry (TSC) as a General Method for Studying and Controlling the Phase Behavior of Conductive Physical Gels
Chapters
Summary January 23rd, 2018
The kinetics of the cooling process defines the properties of ionic gels based on low molecular weight gelators. This manuscript describes the usage of thermal scanning conductometry (TSC), which obtains full control over the gelation process, along with in situ measurements of the samples' temperature and conductivity.
Transcript
The overall goal of this experiment is to develop a reliable and easy method to investigate the dynamically changing state of ionogels and to obtain information about subtle changes of their conductive properties during heating and cooling. This method can help answer key questions in the ionogels field, such as how the dynamics and conductive properties change when transitioning between the liquid and gel states. The main advantage of this technique is that it can follow subtle changes of the conductive and thermal properties of a sample during the gelation process and distinguish between the phases.
Most laboratories have the equipment for a thermal scanning conductometry set up. At its core is the measurement chamber. Nitrogen gas flows into a gas dewar with a heater.
The nitrogen passes through a gas mixer that is just below a sample in a polypropylene tube. The tube is attached to a conductometry sensor that is immersed in the sample. For an experiment, set up the apparatus under a fume hood.
Surround the sample and sensor with thermal insulation. A sense of the complete setup is provided by this schematic. A liquid nitrogen tank provides gaseous nitrogen for a heating and cooling medium.
The nitrogen passes through a sample cooler, and its temperature is regulated by a variable temperature controller. In the experimental chamber, the conductometer measures the conductivity and the temperature in the middle of the sample. A computer records the conductivity, temperature and time for each measurement.
At this point, prepare the experiment sample. To hold the sample, use a polypropylene vial with a screw cap and a rubber ring for tight closing. Begin with a cap then drill a hole to accommodate the conductive sensor, as with this example.
Next, take the cap to the sensor where it will be mounted. Orient the cap so that the vial can be screwed onto it before sliding the cap along the sensor. Position the cap so that the sensor will be approximately at the center of the vial.
Once in place, secure the cap with Teflon tape. Ensure the cap is tightly mounted and secured before continuing. Preparing the electrolyte requires some equipment.
There should be a scale, a heating block at 100 degrees Celsius and a mixer. Obtain the solvent and solute for the electrolyte solution. Employ the scale to weigh the required amount of compounds for the desired concentration, here, a one-molar solution.
Mix the two compounds in a glass vial that can be tightly closed. After mixing, close the vial and heat it at 100 degrees Celsius for 15 minutes. Then remove the vial from the block and place it on the mixer for one minute.
Heat the vial again at 100 degrees Celsius for five minutes to ensure the mixture is homogenous. When done, the electrolyte can be stored at room temperature. Preparation of the gels requires the previously made electrolyte solution.
It also requires low-molecular-weight gelator. For equipment, be ready to heat the sample at 130 degrees Celsius. Also, have a dry cooling block at 10 degrees Celsius.
Begin with four millimeters of the electrolyte in a glass vial. Add 178.6 milligrams of the gelator to create a 4%by weight ionic gel sample. Heat the vial at 138 degrees Celsius for 20 minutes.
During the 20 minutes, occasionally stir the contents of the vial to assist dissolution of the gelator in the electrolyte then continue with heating the sample until it is homogenous. When the sample is homogenous, quickly move the vial to the dry cooling block. After cooling, the result will be physical gelation to a homogenous gel phase.
For the measurement, set the nitrogen pressure to two bars and the flow to 10 liters per minute. Check that the data acquisition system will record the conductivity, temperature and time of each measurement. Next, move to the bench to work with the sample.
Have a polypropylene vial pre-cooled to 10 degrees Celsius. Take a gel sample and place it on the heater block. Raise the sample temperature above the gel-sol transition temperature.
Once the gel is in the sol phase, retrieve its container and transfer the gel to the pre-cooled vial. The fast cooling of the sol will produce the gel phase. Next, get the conductivity sensor with the vial cap.
Push the sensor into the vial and gel so that the vial can be screwed into the cap. Mount the sensor and sample in the thermal scanning conductometry set-up using a viewport to check for correct positioning. First, do a heating-cooling cycle without performing measurements.
This video tracks the changes of a sample as it rises from its gelation temperature of 10 degrees Celsius with a heating rate of two degrees Celsius per minute. The sample reaches the sol phase and then a temperature of about 100 degrees Celsius before it is cooled at a rate of seven degrees Celsius per minute back to 10 degrees Celsius. As it cools, gelation begins, and the sample ends in the transparent ionogel phase.
This cycle improves electrode contact and removes imperfections. Hold the sample at 10 degrees Celsius while setting up the conductometer to perform measurements. When ready, perform measurements using the same cycle parameters.
The sample temperature as a function of time is displayed here as it rises from its gelation temperature of 10 degrees Celsius to 100 degrees Celsius and back. Also plotted are the evolution of the conductivity as a function of temperature and as a function of time over the course of the cycle. The inset video tracks the sample changes.
This is an example of the final transparent gel phase of the sample. For the next heating-cooling cycles, start at 10 degrees Celsius and set both the heating and cooling rate to two degrees Celsius per minute. This experiment records begins as the sample cools from the sol phase at a temperature of about 100 degrees Celsius to its gelation temperature of 10 degrees Celsius.
As the sample reaches the gelation temperature, it is in a mixture of the transparent and opaque gel phase. The final transparent and opaque gel mixed phase is clearly seen here. For the final heating-cooling cycles, start the sample at 10 degrees Celsius, maintain the heating and cooling rates at two degrees Celsius per minute and use a gelation temperature of 60 degrees Celsius.
As the sample cools from the sol phase at about 100 degrees Celsius, stop the cooling when it reaches the gelation temperature of 60 degrees Celsius. Maintain the gelation temperature for 20 minutes. For this cycle, the final result is an opaque, white gel phase.
To perform another cycle, first reduce the temperature to 10 degrees Celsius and hold for 20 minutes. These data are for a heating rate of two degrees Celsius, cooling rate of seven degrees Celsius and gelation temperature of 10 degrees Celsius. The heating curve is in red.
The cooling curve is in blue. Identify the phase transition from transparent gel to sol phase by analyzing the first derivative. Similar analysis for this sample, with a mixed transparent and opaque gel phase, identifies two phase transitions, one for each phase.
These data are for a heating and cooling rate of two degrees Celsius and a gelation temperature of 10 degrees Celsius. A sample with just an opaque gel phase has one phase transition. In this case, the heating and cooling rates were both two degrees Celsius, and the gelation temperature was 60 degrees Celsius.
This technique paves the way for the researchers studying the ionogels as alternatives for electrolyte solidification to explore thermal and conductive properties of systems, which is crucial for future applications. Once mastered, this technique can deliver not only reliable and reproducible results in an easy and straightforward way but can be used to manufacture ionogels with targeted properties with easy-to-do characterization. After watching this video, you should have a good understanding of how to build your own experimental site for the thermal scanning conductometry method and how to perform the measurements.
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