Interfacial Water Structure of Binary Liquid Mixtures Reflects Nonideal Behavior

Area of Science:

• Physical Chemistry
• Surface Science
• Atmospheric Chemistry

Background:

• Evaporation from water-organic mixtures is crucial for aerosol formation and distillation.
• Intermolecular interactions influence evaporation, but interfacial structure's role is poorly understood.
• Deviations from ideal evaporation energetics are linked to solution interactions.

Purpose of the Study:

• To determine the interfacial structure of nonideal binary mixtures of water with methanol, ethanol, and formic acid.
• To correlate interfacial structure with evaporation kinetics and vapor pressure.

Main Methods:

• Utilized surface-specific vibrational spectroscopy.
• Employed molecular dynamics simulations.
• Combined experimental and computational approaches to analyze interfacial properties.

Main Results:

• Identified indistinguishable free, dangling OH groups across different mixture interfaces.
• Observed distinct differences in the ordering of hydrogen-bonded interfacial water molecules.
• Found increased water disorder (methanol, ethanol) and order (formic acid) at interfaces, correlating with vapor pressure deviations from Raoult's law.

Conclusions:

• Interfacial water ordering, not just surface functional groups, significantly impacts evaporation kinetics.
• The study provides insights into the molecular mechanisms governing nonideal mixture evaporation.
• Findings are relevant for understanding and controlling processes like aerosol formation and distillation.