Electrochemical Impedance Spectroscopy for Determining the Solid-Liquid to Liquid-Liquid Phase Transition for Lignin-Acetic Acid-Water Mixtures at Elevated Temperatures
Lignin is unique among biopolymers in having significant aromatic character, which makes it potentially useful for a wide range of applications, including composites and carbon fibers. Unfortunately, most of the commercial-grade lignins available today (primarily Kraft lignins) have a high metals and ash content, eliminating them from contention for most high-value applications.
Hot acetic acid-water mixtures have been shown to partition Kraft lignins between two liquid phases: (1) a solvent-rich phase containing most of the metal salts and the lower mol wt lignin species and (2) a much denser and more viscous lignin-rich phase, containing the higher mol wt lignin species and reduced by almost 10-fold in metal salts. The lignin-acetic acid-water system exhibits solid-liquid equilibrium (SLE) at ambient temperatures and the desired liquid-liquid equilibrium (LLE) at higher temperatures, as described above. However, visual observation of this phase transition can be difficult, especially at acetic acid-water weight ratios above 50/50.
In order to establish the SLE-to-LLE phase transition for the lignin-acetic acid-water system, a detection method based on electrochemical impedance spectroscopy (EIS) was developed. The sum of the solution resistance and charge transfer resistance of the system was found to exhibit a maximum at the phase-transition temperature. EIS data was used to determine the SLE/LLE phase-transition temperature as a function of solvent composition, with that temperature exhibiting a minimum (~40 °C) near solvent mixtures of 90% acetic acid (AA)/10% water and then monotonically increasing to temperatures over 100 °C as the solvent approached pure water compositions. The most useful region for carrying out LLE phase splits has been found to be from 30/70 AA/water to 70/30 AA/water. We have also shown that the EIS method can be applied to other polymer-solvent systems in which standard techniques such as differential scanning calorimetry (DCS) fail.