Thermo-rheological characterization of Malic Acid based Natural Deep Eutectic Solvents

The rheological characterization for a series of Malic Acid based Hydrogen Bond Donor Natural Deep Eutectic Solvents (NADES) is studied in this work for their potential usage as sorbents for CO₂ capture. Three different NADES combinations were synthesized based on B-Alanine, Betaine and Choline Chloride as Hydrogen Bond Acceptors. The work provides insights on the rheological behaviors of Malic Acid-based NADES at temperature ranges from 25 to 105 °C and shear rates from 0.01 to 1000 s^-1, which shows the impact of altering the Hydrogen Bond Acceptor in a NADES system. All Malic Acid-based systems showed non-Newtonian, shear thinning behaviors and diverse viscoelastic flow behavior ranging from as low as 3 × 10² up to 4 × 10⁷ mPa stress requirements showing viscous liquids to solid-like gel structures. The different NADES combinations showed strong temperature dependence behavior, where the density at different temperatures dropped from 1.42 to 1.37 g/cm³ for B-Alanine: Malic Acid. This behavior fits on the Bingham model revealed that the yield stress for all Malic Acid-NADES decreased with increasing temperature as expected for the shear thinning materials. The differences in the yield stress magnitudes of approximately 7 × 10² to 6 × 10⁶ mPa in the case of B-Alanine: Malic Acid for example was attributed to the changes in the nature and the numbers of the interaction forces between the Hydrogen Bond Acceptor and Hydrogen Bond Donor of the NADES and the molecular weight. The viscoelasticity of these NADES systems demonstrated the fundamental differences between the ways the different Hydrogen Bond Acceptor interacts with the Hydrogen Bond Donor. The Linear Viscoelastic Region (LVR) was set to 0.1%-10% according to the type of NADES under a frequency range of 0.1-100 rad/s. The hole theory was used as a theoretical approach to describe the structural differences behind the flow behaviors.