Molecular Scale Hydrophobicity and Adsorption Thermodynamics on Hydrophobic-Charged Surfaces

Molecular-scale hydrophobicity, which governs many important phenomena, such as aggregation, repulsion, or separation of molecules, is determined largely by the chemical composition of the functional groups exposed near the surface-water interface. However, the contributions of these groups to water-mediated interactions are nonadditive, making it challenging to understand how chemical patterning influences hydrophobicity. To address this challenge, we examined a series of model alkanethiol self-assembled monolayers (SAMs) functionalized with 1) nonpolar methyl head groups and 2) polar (hydroxyl) and positively charged (guanidinium and ammonium) head groups separated at short, intermediate, and large spacings. Using molecular dynamics (MD) simulations and enhanced sampling tools, we quantified hydrogen bonding and ordering of local hydration water molecules as a function of the hydrophilic group spacing and hydrophilic group type. Additionally, we quantified the dewetting thermodynamics of interfacial water near patterned surfaces, along with the binding strength of two model hydrophobic solutes: an alkanethiol-functionalized gold nanoparticle (GNP) and a hydrophobic protein, hydrophobin. We found that the interface dewets less readily near charged groups compared with uncharged hydrophilic groups due to their tendency to impede cavity growth at the interface. We also found that different positively charged groups influence hydrophobicity in different ways due to variations in the geometry, partial charge distribution, and local hydrogen bonding network. Furthermore, the spacing between charged groups plays a major role in modulating hydrophobicity, with specific 'sweet spot' distances maximizing hydrophilicity. This work conceptually bridges dewetting and adsorption thermodynamics, elucidating how surface chemistry and patterning govern hydrophobic behavior.