Room-Temperature Ionic Liquids Show Piezoelectric Effects!

^{University of Massachusetts-Amherst/Lovley}

^{By Dr. Inés Urdaneta, Physicist at Resonance Science Foundation}

A team from Michigan State University have reported for the first time the piezoelectric effect in liquid phase. This was totally unexpected because, as it will be further explained in this article, the effect was thought to come solely from changes in the shape of a sample due to mechanical stress or pressure, and therefore, could only concern solids (such as certain crystals) and biological samples (such as bone).  

The fact that the effect was found in ionic rather than the common neutrally composed liquids, is probably a main factor for such a discovery, that may very well provide new physics understanding of the liquid phase in a broader sense.  

Ionic liquids (ILs) are salts in liquid state, meaning that these salts are “melted” in its ionic or charge components. Overall, the liquid is neutral, but its units are not neutral; they are ions, which are atoms that have lost or gained negative charge. Cations are the positively charged ions and anions are the negatively charged ions. Because salt crystalline structures are due to ionic forces between charges, which are supposed to be stronger than other binding forces such as the Vander Waals forces, they have high lattice energies that manifest in high melting points. For instance, common salt Na⁺Cl^- -composed of the cations Na⁺ and the anions Cl^-- melts at almost 800 degrees Celsius, point at which it could be considered an ionic liquid.

Some types of salt, especially those with organic ions, have low lattice energies and thus are liquid at or below room temperature, reason why they are now named room-temperature ionic liquids (RTILs). ILs have many important applications; they are potential heat transfer and storage media in solar thermal energy systems, and they help the recycling of synthetic goods, plastics, and metals, offering the specificity required to separate similar compounds from each other, such as separating polymers in plastic waste streams. Because of their low vapor pressure, ILs can also replace water as the electrolyte in metal-air batteries[1].

^{Figure 1: Room-temperature ionic liquids (RTILs) (a)BMIM+TFSI−  and (b) HMIM+TFSI−. In both cases, the cation is on the left side, and the anion on the right side.  We appreciate the remarkable structural complexity of the organic RTILs. when compared to the common inorganic salt Na+Cl-.}

• The piezoelectric effect

The direct piezoelectric effect concerns the production of charge in a material upon application of a mechanical force or pressure, and this is understood to occur based on the distortion of the material structure on the unit cell or molecular scale. The inverse process also holds true, an applied electric field results in the internal generation of a mechanical strain. To date, piezoelectric effects had only been observed in solid-phase materials, specifically in non-centrosymmetric crystalline materials, such as quartz, LiNbO3, BaTiO3. Though it had also been observed in ceramics and biological samples such as bone and DNA.  

Now, for the first time, chemists Iqbal Hossain and G. J. Blanchard at Michigan State University report the observation of the direct piezoelectric effect in (RTILs) [2]. The RTILs studied were the 1-butyl-3-methyl imidazolium bis(trifluoromethyl-sulfonyl)imide, abbreviated as (BMIM+TFSI−) and the 1-hexyl-3-methyl imidazolium bis(trifluoromethylsulfonyl) imide, abbreviated as (HMIM+TFSI−). Both compounds are depicted in Figure 1 above, where BMIM+TFSI− is on the left side.

These liquid piezoelectric materials were discovered as the researchers applied pressure with a piston to a sample of an ionic liquid in a cylinder, finding that this led to a release of electricity; they produced an electric potential upon the application of force when confined in the cell, with the magnitude of the potential being directly proportional to the force applied and the effect observed was one order of magnitude smaller than that seen in quartz.

^{Figure 1. Schematic of cell used to measure the direct piezoelectric effect in RTILs. The piston is non-conductive (Delrin) and contains an electrode along its center axis. The cylinder is made of steel. The system is sealed using an O-ring. Taken from [1].}

This outcome was a surprise for the scientists, a serendipitous observation. They also found that the optical properties of the RTILs changed, like the refractive index (in how the liquid bent light), when they released electricity. The discovery of such an effect and its side-effects has fundamental implications about the organization and dynamics in ionic liquids. Though the theoretical explanation of this mechanism must be found.

^{Figure 3: (a) Potential measured vs time raw experimental data for BMIM+TFSI− (a, b) and HMIM+TFSI− (c, d). The values indicated above each peak are the force applied. The scan time for the open circuit potential measurement shown was 400 s. in (b) and 200 s in (d). (b) and (d) depict the potential measured vs force applied for multiple time scans. The slope of the dependence is 16 ± 1 mV/N in (b) and 17 ± 1 mV/N in (d). The dependence of the open circuit potential on the force applied is presented (b and d) , revealing a linear dependence over the range studied. The control case, where the cell is filled with either neat ethylene glycol (red lines) or 1 M NaCl in ethylene glycol (black lines), in the absence of force (e) and with the application of force (f) in the same manner as that for the RTILs. The open circuit potential is 350 mV or less in all cases. Taken from [1].}

The piezo electric effect in a liquid remains a challenge for several reasons, the main being that the signature of the reciprocal piezoelectric effect is a change in the dimensions of a material upon the application of a potential across the material and by definition liquid is a material that takes the shape of its container, making difficult the characterization of such changes in shape.

Secondly, RTILs has demonstrated the existence of an induced charge density gradient that persists over multiple tens of micrometers, therefore, RTILs cannot be understood in the same conceptual framework as molecular liquids where their behavior is well-characterized and where the effects of charge separation and reorganization happens in a much smaller size or scale domain, of the order of nanometers.

Moreover, the idea that this piezoelectric effect could not happen in a liquid phase, when clearly it is not the case, is a strong indication that many thermodynamical mechanisms remain unclear. One wonders if we are witnessing a fluid electro-gravito-dynamics process, and if this discovery can bring a revolutionary understanding that hints into the relationship between charge and mass.

RSF in perspective –

In principle, ionic liquids wouldn't include liquids such as water or organic solvents (oil, etc) because these last are liquids presumably composed of electrically neutral molecules like the H₂0 units, in the case of water, that bind together through the Vander Waals forces and hydrogen bridges, though this picture is not totally clear.

We recall for instance the work by Mexican biochemist, Dr. Maria Esther del Rio, who found clusters of water molecules that she coined liquid crystal water [3], and this was much before the work from Gerard Pollack[4] and others. She discovered a very special type of water or Liquid Crystal Water, arriving to the conclusion that all the water that enters our body arranged in our cells as clathrates, which in this case are 37 molecules of H₂O, in a very stable geometry concerning dodecahedrons and octahedrons, with their formula being (H2O)₃₇. Liquid crystal water could have the ability to save and store information and more importantly the ability to retransmit, in the same way a computer microchip does.  

Salt diluted in water is also considered an electrolyte because the salt crystals dissolve by separating in its ionic forms Na⁺ and Cl^- by solvation, though the possibility of considering this combined system as an ionic liquid depending on its concentration of ions, is unfitting because their concentration would have to be very high with respect to the neutral water molecules and as we know, when water reaches its point of saturation, the salt excess precipitates as crystals, so it is no longer in liquid state. Presumably, this case could never become an ionic liquid capable in principle of showing the piezoelectric effect.  

Nevertheless, due to the extreme complexity of the intra- and inter-atomic/molecular interactions in liquids, one could wonder if we have really understood how to account for the internal charge distribution and configuration in liquid phase. For instance we could think that because EZ water (the exclusion zone in water[4]) shows charge separation in those EZ regions, it could be considered similar to a salt in a liquid state, capable of presenting piezo-electricity.

References:

[1] Anirban Paul et al. Review article Room-Temperature Ionic Liquids for Electrochemical
Application with Special Focus on Gas Sensors, J. Electrochem. Soc. 167, 037511(2020).

[2] Md. Iqbal Hossain et al, Ionic Liquids Exhibit the Piezoelectric Effect, The Journal of Physical Chemistry Letters (2023). DOI: 10.1021/acs.jpclett.3c00329

[3] https://www.toroidalfields.com/science/dr-esther-del-rio-we-humans-are-the-best-quantum-computers/

[4] Hwang, SG, Hong, JK, Sharma, A, Pollack, GH and Bahng GW: Exclusion zone and heterogeneous water structure at ambient temperature. PLoS ONE 13(4): e0195057. https://doi.org/10.1371/journal.pone.0195057  (2018).