Microheterogeneity in Aqueous Acetonitrile Solution Probed by Soft X-ray Absorption Spectroscopy

Area of Science:

• Solution chemistry
• Physical chemistry
• Spectroscopy

Background:

• Microheterogeneity (MH) describes microscopic inhomogeneity in seemingly uniform liquid mixtures.
• Understanding MH is crucial for explaining chemical processes in solutions.
• Aqueous acetonitrile is a simple yet representative MH system.

Purpose of the Study:

• To investigate the molecular interactions and structural changes in aqueous acetonitrile solutions.
• To elucidate the origins of microheterogeneity in this binary liquid system.
• To correlate spectroscopic observations with theoretical calculations.

Main Methods:

• Soft X-ray absorption spectroscopy (XAS) was employed to probe molecular interactions.
• C, N, and O K-edge XAS were used to analyze acetonitrile and water molecules separately.
• Ab initio quantum chemical calculations were performed for theoretical analysis.

Main Results:

• XAS spectra revealed distinct molecular interactions of acetonitrile and water at different concentrations.
• The C K-edge spectrum showed three characteristic concentration regions with phase-transition-like behavior.
• Analysis identified specific local structures corresponding to these concentration regions.

Conclusions:

• The dipole interaction between acetonitrile and water molecules is the primary factor driving microheterogeneity.
• Aqueous acetonitrile exhibits distinct structural arrangements across different concentration ranges.
• XAS combined with quantum chemical calculations provides detailed insights into solution microheterogeneity.