The nature of water interactions and the molecular signatures of hydrophobicity

Despite being of utmost relevance in central fields ranging from biophysics to self-assembly processes in materials science, we still lack a precise comprehensive definition of hydrophobicity to replace the usual, merely qualitative descriptions that rely on water repellency or a lack of affinity. Building on recent findings regarding the structure and interactions of bulk water, we use a recently introduced water structural indicator to reach the following quantitative molecular elucidation: "hydrophobicity is the inability of a system to pay for the lacking hydrogen bonds (HBs) it produces in its hydration layer at least the same cost that this kind of defect imposes on bulk water, a defect interaction threshold whose magnitude is significantly lower than the HB energy." We will demonstrate that such a defect interaction threshold not only marks the transition to hydrophobicity that occurs at a contact angle of θ = 90° in surfaces with different polarity degrees (allowing for an absolute scale) but also accurately signals the onset of drying regimes in nanoconfined aqueous systems. This is relevant from a practical perspective, as the possibility of being (locally) wet or dry becomes crucial in many fields. Specifically, our approach allows for assessing local hydrophobicity with unprecedented resolution (suitable for regions of different sizes, even at the single-atom level), particularly for self-assembly processes in biology and materials science, which often entail patterned regions combining hydrophobic and hydrophilic sites, thus posing challenges (and opportunities) for rational design efforts.