Abstract
The evaporation of molecules from water–organic solute binary mixtures is key for both atmospheric and industrial processes such as aerosol formation and distillation. Deviations from ideal evaporation energetics can be assigned to intermolecular interactions in solution, yet evaporation occurs from the interface, and the poorly understood interfacial, rather than the bulk, structure of binary mixtures affects evaporation kinetics. Here we determine the interfacial structure of nonideal binary mixtures of water with methanol, ethanol, and formic acid, by combining surface-specific vibrational spectroscopy with molecular dynamics simulations. We find that the free, dangling OH groups at the interfaces of these differently behaving nonideal mixtures are essentially indistinguishable. In contrast, the ordering of hydrogen-bonded interfacial water molecules differs substantially at these three interfaces. Specifically, the interfacial water molecules become more disordered (ordered) in mixtures with methanol and ethanol (formic acid), showing higher (lower) vapor pressure than that predicted by Raoult’s law.
Highlights
Binary liquid mixtures typically deviate from ideal mixing in a thermodynamic context.[1,2] In the bulk liquid phase, such nonideal interactions can be trivially understood, as they only require, in a mixture of A and B species, that the interactions between A and B differ from A−A and B−B interactions
We found that the variations of the C−H stretching mode of organic species and the free O−H stretching mode of water upon the addition of the organic species do not show a significant difference between the aqueous binary mixtures of methanol, ethanol, and formic acid
We explore the interfacial organization of the organic components at the air−aqueous binary mixture interfaces by probing the C−H stretching mode ν(C−H) of methanol/ethanol/formic acid with sum-frequency generation (SFG) spectroscopy
Summary
Binary liquid mixtures typically deviate from ideal mixing in a thermodynamic context.[1,2] In the bulk liquid phase, such nonideal interactions can be trivially understood, as they only require, in a mixture of A and B species, that the interactions between A and B differ from A−A and B−B interactions. A textbook example for nonideal mixtures where A−B interactions are stronger than A−A and B−B interactions in water− formic acid mixtures.[3] As a result, the vapor pressure of water− formic acid mixtures is markedly lower than that predicted by Raoult’s law. Water−alcohol mixtures typically show excess vapor pressures.[4] These excess interactions impact the energetics of the evaporation, which is directly relevant to distillation and purification processes in industrial processes and to atmospheric processes.[5−11]
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