Nanoscale Inhomogeneity Controls Hydrolysis of Hydroperoxides in Macroscopically Homogeneous Solutions

Area of Science:

• Environmental Chemistry
• Physical Chemistry
• Aerosol Science

Background:

• Miscible solvent mixtures can exhibit microheterogeneity, influencing chemical reaction mechanisms.
• Understanding reaction kinetics in complex aqueous mixtures, like atmospheric aerosols, is crucial but challenging.

Purpose of the Study:

• To investigate the hydrolysis rates of alpha-hydroxyalkyl-hydroperoxides (α-HHs) in water/acetonitrile mixtures.
• To elucidate the role of microheterogeneity and nanodroplet formation in reaction mechanisms.

Main Methods:

• Electrospray mass spectrometry was used to monitor the decay of α-HHs.
• Dynamic light scattering experiments were performed to characterize nanodroplet formation and size distribution.

Main Results:

• Hydrolysis rates of C5, C7, and C12 α-HHs showed a nonlinear dependence on water content.
• α-HHs persisted longer in mixtures with lower water content (10 vol%) compared to higher content (20 vol%).
• Dynamic light scattering revealed the presence of nanodroplets (≤100 nm) whose size distribution varied with water concentration.

Conclusions:

• A proposed mechanism suggests α-HHs are sequestered within the hydrophobic cores of nanodroplets, hindering hydrolysis.
• Microheterogeneity and nanostructure formation significantly impact reaction kinetics in aqueous-organic mixtures.
• This finding has implications for understanding chemical processes in atmospheric aerosols and other complex environmental systems.