Nanoscale Inhomogeneity Controls Hydrolysis of Hydroperoxides in Macroscopically Homogeneous Solutions

Mixtures of water and miscible organic compounds sometimes show physical/chemical anomalies, referred to as microheterogeneity. Reaction mechanisms in multicomponent aqueous mixtures such as atmospheric aerosols are expected to be different from those in single-solvent systems but remain to be fully elucidated. Here, by using electrospray mass spectrometry, we showed that the hydrolysis rates of C₅, C₇, and C₁₂ α-hydroxyalkyl-hydroperoxides (α-HHs) derived from ozonolysis of nerolidol in water/acetonitrile solutions changed as a nonlinear function of water content. Notably, the mass spectral signals of C₅ and C₇ α-HHs at 10 vol % water persisted for at least a week, whereas they decayed to zero within a few hours at 20 vol %. Dynamic light scattering experiments indicated the presence of nanodroplets (D ≤ 100 nm) in these macroscopically homogeneous aqueous mixtures. The relative contribution of scattering intensity from small droplets (<10 nm) decreased as the water content increased from zero to 20 vol % water, whereas that of larger droplets (10-100 nm) increased. Based on these findings, we proposed a mechanism in which the α-HHs residing in the hydrophobic cores of the nanodroplets did not undergo hydrolysis because they were isolated from the water in the bulk phase.