Hydrophobic interactions drive many important biomolecular selfassembly phenomena. However, characterizing hydrophobicity at the nanoscale has remained a challenge due to its nontrivial dependence on the chemistry and topography of biomolecular surfaces. Here we use molecular simulations coupled with enhanced sampling methods to systematically displace water molecules from the hydration shells of nanostructured solutes and calculate the free energetics of interfacial water density fluctuations, which quantify the extent of solute-water adhesion, and therefore solute hydrophobicity. In particular, we characterize the hydrophobicity of curved graphene sheets, self-assembled monolayers (SAMs) with chemical patterns, and mutants of the protein hydrophobin-II. We find that water density fluctuations are enhanced near concave nonpolar surfaces compared with those near flat or convex ones, suggesting that concave surfaces are more hydrophobic. We also find that patterned SAMs and protein mutants, having the same number of nonpolar and polar sites but different geometrical arrangements, can display significantly different strengths of adhesion with water. Specifically, hydroxyl groups reduce the hydrophobicity of methyl-terminated SAMs most effectively not when they are clustered together but when they are separated by one methyl group. Hydrophobin-II mutants show that a charged amino acid reduces the hydrophobicity of a large nonpolar patch when placed at its center, rather than at its edge. Our results highlight the power of water density fluctuations-based measures to characterize the hydrophobicity of nanoscale surfaces and caution against the use of additive approximations, such as the commonly used surface area models or hydropathy scales for characterizing biomolecular hydrophobicity and the associated driving forces of assembly.nanotube | curvature | chemical pattern | hydrophilicity | graphene H ydrophobic interactions drive many important biological and colloidal self-assembly processes (1-6). During such assembly, the hydration shells of the associating solutes are disrupted, replacing hydrophobic-water contacts with hydrophobic-hydrophobic ones. Characterizing how strongly water adheres to a given solute is, therefore, directly relevant to the strength of hydrophobic interactions between solutes. Macroscopically, surface-water adhesion is quantified by measuring the water droplet contact angle on a surface. However, such characterization does not translate usefully to proteins and other nanoscale solutes. Indeed, characterizing how strongly or weakly a protein surface or a specific patch on it adheres to water (i.e., its hydrophobicity) is incredibly challenging and has necessitated the use of simplifying assumptions.To this end, simple surface area (SA) models have been used to estimate the driving force for assembly, ∆G = γ∆A, where ∆A is the nonpolar SA buried upon assembly and γ is an appropriate surface tension (7-9). However, the value of γ used in popular models is nearly an order of magnitude lower t...

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