Hydrogen Bonds Structure and Phase Transitions in the N-Octyl-N,N-Diethylammonium Tetrafluoroborate (N228BF4) Protic Ionic Liquid Electrolyte Probed by Solid State 2H NMR

Protic ionic liquids (PILs) are promising candidates for electrochemical applications due to their wide liquid-state range, tunable physicochemical properties, and the possibility of controlling hydrogen-bond networks via molecular design. In this work, we report the synthesis and characterization of N,N-diethyl-octan-1-ammonium tetrafluoroborate selectively deuterated at the N–H site, N228DBF4. The phase behavior and local hydrogen–bond structure were investigated by combining solid–state 2H nuclear magnetic resonance (NMR) spectroscopy and differential scanning calorimetry (DSC). DSC revealed the absence of a glass transition in the –120 °C to +150 °C interval, instead identifying multiple solid–solid transitions and a melting event via intermediate mesophases. Temperature-dependent 2H NMR spectra demonstrated the coexistence of two distinct hydrogen bonding environments in the solid phases (DQCC = 151-177 kHz and 129-140 kHz) at low and intermediate temperatures, with the transition to an isotropically mobile phase above 283 K. The 2H NMR evidenced the presence of the plastic phase above 283 K and enabled quantitative determination of transition thermodynamics (ΔH = 59 ± 8 kJ/mol; ΔS0 = 162 ± 25 J/(mol·K) to the state with mobile cations. The observed intermediate DQCC values reflect hydrogen-bond strengths in BF4– PILs and evidences multiple hydrogen–bonding geometries despite anion symmetry. These findings contribute to the understanding of structure–dynamics–phase behavior relationships in asymmetric cation PILs and may inform the design of optimized electrolytes for energy–related applications.