Comparative Study of the Effects of Temperature and Pressure on the Water-Mediated Interactions between Apolar Nanoscale Solutes

Different Temperature- and Pressure-Effects on the Water-Mediated Interactions between Hydrophobic, Hydrophilic, and Hydrophobic-Hydrophilic Nanoscale Surfaces

Water-mediated interactions (WMI) are responsible for diverse processes in aqueous solutions, including protein folding and nanoparticle aggregation. WMI may be affected by changes in temperature and pressure and hence, they can alter chemical/physical processes that occur in aqueous environments. Traditionally, attention has been focused on hydrophobic interactions while, in comparison, the role of hydrophilic and hybrid (hydrophobic-hydrophilic) interactions have been mostly overlooked. Here we study the role of T and P of the WMI between nanoscale (i) hydrophobic-hydrophobic, (ii) hydrophilic-hydrophilic, and (iii) hydrophilic-hydrophobic pairs of (hydroxylated/non-hydroxylated) graphene-based surfaces. We find that hydrophobic, hydrophilic and hybrid interactions are all sensitive to P. However, while hydrophobic interactions [case (i)] are sensitive to T-variations, hydrophilic [case (ii)] and hybrid interactions [case (iii)] are practically T-independent. An analysis of the entropic and enthalpic contributions to the PMF for cases (i)-(iii) is also presented. Our results are important in understanding T- and P-induced protein denaturation, and the interactions of biomolecules in solution, including protein aggregation andphase separation processes. From the computational point of view, the results presented here are relevant in the design of implicit water models for the study of molecular and colloidal/nanoparticle systems at different thermodynamic conditions.

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.