Hydrophobic and Ionic Interactions in Nanosized Water Droplets

Abstract

A number of situations ranging from protein folding in confined spaces, lubrication in tight spaces, and chemical reactions in confined spaces require understanding water-mediated interactions. As an illustration of the profound effects of confinement on hydrophobic and ionic interactions we investigate the solvation of methane and methane decorated with charges in spherically confined water droplets. Free energy profiles for a single methane molecule in droplets, ranging in diameter (D) from 1 to 4 nm, show that the droplet surfaces are strongly favorable as compared to the interior. From the temperature dependence of the free energy in D = 3 nm, we show that this effect is entropically driven. The potentials of mean force (PMFs) between two methane molecules show that the solvent separated minimum in the bulk is completely absent in confined water, independent of the droplet size since the solute particles are primarily associated with the droplet surface. The tendency of methanes with charges (M q+ and M q− with q + = |q − | = 0.4e, where e is the electronic charge) to be pinned at the surface depends dramatically on the size of the water droplet. When D = 4 nm, the ions prefer the interior whereas for D < 4 nm the ions are localized at the surface, but with much less tendency than for methanes. Increasing the ion charge to e makes the surface strongly unfavorable. Reflecting the charge asymmetry of the water molecule, negative ions have a stronger preference for the surface compared to positive ions of the same charge magnitude. With increasing droplet size, the PMFs between M q+ and M q− show decreasing influence of the boundary due to the reduced tendency for surface solvation. We 1 also show that as the solute charge density decreases the surface becomes less unfavorable. The implications of our results for the folding of proteins in confined spaces are outlined.

Interactions between amino acid side chains in cylindrical hydrophobic nanopores with applications to peptide stability

“…Hydrophobic solutes, such as Ala and Phe, are driven to the surface to maximize the solvent entropy (19). The solvent density plays a crucial role in determining whether the hydrophobic solute pair is thermodynamically stable in mutual contact or at large separations corresponding to the pore dimensions.…”

“…Therefore, in water-filled nanopores, both solutes will be relegated to the surface (Fig. 3) to maximize the solvent entropy (19). Fig.…”

“…In bulk water, the barrier has been identified with the free-energetic cost of squeezing out water molecules, thus creating a hydrophobic core in the folding of proteins (21). As noted before (19), the absence of the SSM and the associated barrier in the pores implies that such a model is not applicable in nanoconfinement.…”

Nanobubbles are not a Superficial Matter

“…[84,85] Given the tendency of ions to show either enhanced or reduced concentrations at the air-water interface, [15][16][17][18][19][20][21] it seems likely that nanobubbles will acquire a surface charge, which will then influence the energetics of their potential interactions with biological surfaces, just as is the case for nanoparticles. [86][87][88][89] The presence of nanobubbles at surfaces may also have hydrodynamic implications, especially if there is focused gas flow out of the bubble apex.…”

“…[5][6][7][8][9][10][11][12][13] These deviations from bulk properties can exacerbate the chemical inhomogeneities of interfaces, altering for example the dielectric or solvation characteristics there. [14] Furthermore, ions may be depleted or enriched at the solidwater or air-water interfaces relative to the bulk, [15][16][17][18][19][20][21] an effect that involves subtle balances of enthalpic and entropic factors. Even so apparently simple a question as whether the air-water interface is acidic or basic is still fiercely contested.…”