Conventional tools for the measurement of hydrophobicity (e.g., experimental determination of K_ow or contact angle) typically offer a single bulk or average value to describe the hydrophobicity across an entire surface or the whole population of molecules in the corona of nanomaterials. These average values may be representative enough when the samples are relatively homogenous (i.e., when molecules are mostly identical in a corona). However, organic molecules scavenged by nanomaterials can be highly heterogenous. Bulk hydrophobicity values may not be sufficient in capturing the varying hydrophobicity within the organic corona.
In this study, the molecular formulas of organic molecules in the corona were obtained from Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). These formulas were used as input for K_owWIN (a program developed by the USEPA) to provide a more granular view of the hydrophobicity by 1) enabling the computation of molecule-specific log K_ow (octanol-water partition coefficient) and 2) taking the relative abundance of each molecule in the corona into account. Weighting these log K_ow values by relative abundance not only enhances the statistical significance but also ensures that the analysis accurately reflects the contribution of more prevalent molecules, offering a realistic representation of the corona’s overall behavior.
For the first time, this study revealed the changes of hydrophobicity in the organic corona by examining the distribution of molecule-specific and abundance-weighted log K_ow values derived from FT-ICR MS data. We uncover subtle changes in hydrophobicity that are commonly obscured by conventional bulk measurements. Our study highlights the importance of the heterogeneity of organic molecules when monitoring the continuous evolution of nanoparticle surfaces, as such molecular-level differences can influence the behaviors of nanoparticles in environmental and biomedical systems

Salimar Cordero

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