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.