Comparative Study of Water-Mediated Interactions between Hydrophilic and Hydrophobic Nanoscale Surfaces

Self-assembly processes in aqueous solutions, such as protein folding and nanoparticle aggregation, are driven by water-mediated interactions (WMIs). The most common of such interactions are the attractive forces between hydrophobic units. While numerous studies have focused on hydrophobic interactions, WMIs between hydrophilic moieties and pairs of hydrophilic-hydrophobic surfaces have received much less attention. In this work, we perform molecular dynamics simulations to study the WMI between nanoscale (i) hydrophobic-hydrophobic, (ii) hydrophilic-hydrophilic, and (iii) hydrophilic-hydrophobic pairs of (hydroxylated/nonhydroxylated) graphene-based surfaces. We find that in all cases, the potential of mean force (PMF) between the plates exhibits oscillations as a function of the plate separations r, up to r ≈ 1-1.5 nm. The local minima of the PMF, which define the stable/metastable states of the system, correspond to plates' separations at which water molecules arrange into n = 0, 1, 2, ... layers between the plates. In case (i), the stable state of the system corresponds to the plates in contact with one another. Instead, in cases (ii) and (iii), water is never removed between the plates. The free-energy barriers separating the stable/metastable states of the system vary with the hydrophilicity/hydrophobicity of the interacting plates. However, the effective forces between the plates are comparable in magnitude. This strongly suggests that hydrophilic-hydrophilic and hydrophilic-hydrophobic interactions can play a relevant role in self-assembly processes in aqueous solutions, alike hydrophobic interactions. Interestingly, we find that the WMIs between hydrophilic-hydrophilic and hydrophilic-hydrophobic plates are similar, suggesting that only one hydrophilic surface is sufficient to induce hydrophilic-like WMI. We also briefly discuss the role of surface polarity on the WMI. In particular, we show that depending on the surface polarity, WMI can exhibit mixed features characteristic of hydrophobic and hydrophilic interactions. Our results suggest that the forces between hydrophobic, hydrophilic, and hydrophobic/hydrophilic surfaces are all relevant in driving a self-assembly system toward its final state, but it is the hydrophobic interaction that provides stability to such a final state.