Hydration Effects in a Guanidine Hydrochloride-Pyruvic Acid Low-Transition-Temperature Mixture: Physicochemical Insights

Abstract A low-transition-temperature mixture (LTTM) composed of guanidine hydrochloride (GUC) and pyruvic acid (PA) in a 1:2 molar ratio was prepared, and the effect of water on its physicochemical, electrochemical, and structural properties was investigated. To the best of our knowledge, this is the first report of a GUC-PA-based LTTM. Density (ρ), speed of sound (u), and conductivity (κ) were measured over the temperature range of 293.15–323.15 K. The excess molar volume (VE) of the pseudo-binary GUC-PA/water system was calculated and correlated using the third-order Redlich-Kister equation. Density decreased with increasing temperature, whereas the thermal expansion coefficient (αP) increased. The variation of conductivity with temperature and water composition was correlated using the VFT and Casteel–Amis equations. Water addition significantly enhanced conductivity, reaching a maximum at an LTTM mole fraction of 0.1170 ± 0.002, indicating improved ionic transport upon dilution. Cyclic voltammetry revealed that the neat GUC-PA LTTM and the system containing an LTTM mole fraction of 0.2314 ± 0.002 exhibited comparable electrochemical behavior. The neat LTTM displayed an electrochemical stability window of −0.45 to +1.35 V (vs Ag). Nile Red solvatochromic studies revealed that the neat LTTM provides a highly polar microenvironment than water, and water addition induces only a small hypsochromic shift (∼6 nm). SAXS measurements showed that water progressively weakens intermolecular correlations while preserving the local liquid structure. Density functional theory (DFT) analysis was performed to evaluate the interaction energies and frontier molecular orbitals (FMOs), providing valuable insights into the intermolecular interactions within the GUC-PA. Collectively, these results demonstrate that the GUC-PA LTTM retains its characteristic hydrogen-bonded structure up to an LTTM mole fraction of 0.2298 ± 0.002, beyond which the network progressively weakens. These findings provide fundamental insight into the effect of water on the structure and physicochemical properties of LTTM.