Effect of Na+ solvation behavior on evaporation processes in solar-driven interfacial evaporation systems

Abstract

Solar-driven interfacial evaporation (SDIE) systems, as energy-efficient and innovative water treatment technologies, face a critical challenge: their evaporation efficiency is markedly influenced by the microscopic state of water molecules in saline solutions, such as hydrogen-bond networks and ion hydration behavior. This study investigates the dissolution behavior of sodium ions (Na⁺) and its regulatory mechanism on SDIE performance through molecular dynamics (MD) simulations and density functional theory (DFT) calculations, revealing that Na⁺ ions form stable [Na(H₂O)₅]⁺ hydration clusters via coordination with five water molecules. Within these clusters, coordinated water molecules lose approximately 0.11 electrons, inducing structural distortions characterized by elongation of the OH bond length from 0.970 Å (free water) to 0.972 Å (coordinated water), expansion of the H-O-H bond angle from 102.59° to 103.47°, and reduction of the OH bond order from 0.872 (free water) to 0.841 (coordinated water), collectively weakening hydrogen bond strength and enhancing the chemical reactivity of coordinated water. Fourier-transform infrared (FTIR) spectroscopy further demonstrates that increasing salt concentration in NaCl solutions triggers a red shift in the OH stretching vibration peak (v(OH)), indicative of weakened hydrogen-bond networks. However, the strong binding energy between Na⁺ and water molecules (−25.79 kcal·mol⁻¹), significantly surpassing the intermolecular interaction energy between water molecules (−3.97 kcal·mol⁻¹), suppresses evaporation rates by approximately 18 % through a hydration‑hydrogen bond competition mechanism. By elucidating these molecular-scale regulatory effects of salt ions on SDIE systems, this study establishes a theoretical foundation for optimizing evaporation efficiency in high-salinity environments.