Reversible state transition under nano-confinement
Reversible switch between the dispersion and aggregation states in nano-confined aqueous solutions was observed in MD simulation.
Whether solute molecules are in the dissolved or aggregated states has fundamental importance in a large variety of physical and biological processes, including the reaction efficiency of chemical catalysis, the functions of proteins, and the possible toxicity of nanoparticles. Systems at nanoscales usually display physical behavior qualitatively different from that of macroscale systems. Will the association behavior of the solute molecules in the confined geometries also show difference from that in macroscopic scale? Recently, Liang Zhao, Dr. Chunlei Wang, Prof. Haiping Fang and colleagues from the Shanghai Institute of Applied Physics, Chinese Academy of Sciences and Prof. Zuowei Wang from the Reading University in United Kingdom have observed the reversible state transition in the nano-confined aqueous solutions by molecular dynamic (MD) simulations, as shown in the Figure below. The system can switch between the dispersion state and the aggregation state as the time goes on. Besides, the confinement also leads to a significant increase of critical aggregation concentration. Based on the developed theoretical model, they find the reversible state transition is attributed to the low free energy barrier (of order kBT) comparable to the thermal fluctuation in between two energy minima corresponding to the dispersion and aggregation states, and the enhancement of the critical aggregation concentration results from the fact that at lower concentrations the number of solute molecules is not large enough to allow the formation of a stable cluster in the confined systems. These findings enrich the theory of the association behavior of the solute molecules to the nanoscale and may give insights into the fields relevant to the dissolution property of material in confined aqueous environment, such as the drug absorption, toxicity of nanomaterial, oil extraction and restoration of carbon element in soil. (Liang Zhao et al., Phys. Rev. Lett., in press)—Haiping Fang.