Charge Transport and Structural Dynamics in Carboxylic-Acid-Based Deep Eutectic Mixtures

Charge transport and structural dynamics in the 1:2 mol ratio mixture of lidocaine and decanoic acid (LID–DA), a model deep eutectic mixture (DEM), have been characterized over a wide temperature range using broad-band dielectric spectroscopy and depolarized dynamic light scattering. Additionally, Fourier transform infrared spectroscopy measurements were performed to assess the degree of proton transfer between the neutral parent molecules. From our detailed analysis of the dielectric spectra, we have determined that this carboxylic-acid-based DEM is approximately 25% ionic at room temperature. Furthermore, we have found that the characteristic diffusion rate of mobile charge carriers is practically identical to the rate of structural relaxation at all measured temperatures, indicating that fast proton transport does not occur in LID–DA. Our results demonstrate that while LID–DA exhibits the thermal characteristics of a DEM, its charge transport properties resemble those of a protic ionic liquid.