Viscometric, thermodynamic and theoretical DFT studies of dl-2-aminobutyric acid in aqueous sodium nitrate solutions at different temperatures

Abstract

The dynamic viscosities of six pseudobinary systems consisting of dl-2-aminobutyric acid + pseudo-unary solvent (aqueous sodium nitrate solutions) were determined, in the temperature range of 283.15–318.15 K, each 5 K under pressure of 101.1 kPa, using a microviscometer automatic Anton Paar (AMVn). Absolute viscosities, relative viscosities, solvation numbers, viscosity coefficients A and B, the slope (dB/dT), activation free energy per mole of solute, the enthalpy of partial molar activation, and zwitterionic partial molar activation entropy were calculated at infinitesimal concentration. Also, the intermolecular structures and interactions of several water-sodium nitrate-dl-2-aminobutyric acid molecular complexes were theoretically modeled from DFT electronic calculations by using the B3LYP/6-31G(d) chemical model. It was found that the forces between zwitterion-pseudo-unary solvent species dominate the viscosimetric behavior of the six systems studied at all concentrations and temperatures. We also analyzed the activation parameters of the viscous flow with respect to the solute, and it was found that the zwitterion is more ordered in the transition state at the expense of a process of viscous endothermic flow and by predominance of zwitterion-solvent interactions.