(H, Li)Br and LiOH Solvation Bonding Dynamics: Molecular Nonbond Interactions and Solute Extraordinary Capabilities.

Abstract

We resolved the O:H-O bond transition from the mode of ordinary water to its hydration in terms of its phonon stiffness (vibration frequency shift Δω), order of fluctuation (line width), and number fraction (phonon abundance), fx(C) = Nhyd/Ntotal. The fx(C) follows fH(C) = 0, fLi(C) ∝ fOH(C) ∝ C, and fBr(C) ∝ 1 - exp(-C/C0) toward saturation with C being the solute concentration. The invariant dfx(C)/dC suggests that the solute forms a constantly sized hydration droplet without responding to interference of other ions because its hydrating H2O dipoles fully screen its electric field. However, the number inadequacy of the highly ordered hydration H2O dipoles partially screens the large Br-. The Br- then interacts repulsively with other Br- anions, which weakens its electric field and the fBr(C) approaches saturation at higher solute concentration. The consistency in the concentration trend of the fLiBr(C), the Jones-Dole viscosity η(C), and the surface stress of LiBr solution clarifies their common origin of ionic polarization. The resultant energy of the solvent H-O exothermic elongation by O: ⇔ :O repulsion and the solute H-O endothermic contraction by bond-order deficiency heats up the LiOH solution. An estimation of at least 0.15 eV (160% of the O:H cohesive energy of 0.1 eV) suggests that the H-O elongation is the main source heating up the solution, while the molecular motion, structure fluctuation, or even evaporation dissipates energy caped at 0.1 eV.