The Dependence of Hydrophobic Interactions on the Shape of Solute Surface

Abstract

According to our recent studies on hydrophobicity, this work is aimed at understanding the dependence of hydrophobic interactions on the shape of a solute's surface. It has been observed that dissolved solutes primarily affect the structure of interfacial water, which refers to the top layer of water at the interface between the solute and water. As solutes aggregate in a solution, hydrophobic interactions become closely related to the transition of water molecules from the interfacial region to the bulk water. It is inferred that hydrophobic interactions may depend on the shape of the solute surface. To enhance the strength of hydrophobic interactions, the solutes tend to aggregate, thereby minimizing their surface area-to-volume ratio. This also suggests that hydrophobic interactions may exhibit directional characteristics. Moreover, this phenomenon can be supported by calculated potential mean forces (PMFs) using molecular dynamics (MD) simulations, where different surfaces, such as convex, flat, or concave, are associated with a sphere. Furthermore, this concept can be extended to comprehend the molecular packing parameter, commonly utilized in studying the self-assembly behavior of amphiphilic molecules in aqueous solutions.

Keywords: hydrogen bonding; hydrophobic interactions; solute shape; water.

Figure 1

Hydration free energy at 293 K and 0.1 MPa. It is the sum of the Gibbs energies of the bulk (∆G_{Water–water}) and interfacial (∆G_{Solute–water}) water. With reference to Rc, it is divided into initial and hydrophobic solvation processes, respectively.

Figure 2

Simulated systems used to investigate the dependence of hydrophobic interaction on the geometric shape of the solute surface. A C₆₀ is constrained to move to the concave (a,b), flat (c,d), and convex (e,f) surfaces of the target solute. Both initial and final configurations are shown.

Figure 3

(a,c,e) The PMFs as the fullerene are associated with various surfaces of graphite in water and under vacuum at 300 K and 0.1 MPa. (b,d,f) The corresponding water-induced PMFs as solutes are aggregated in water.

Figure 4

The water-induced PMFs as the C₆₀ is aggregated with the concave surface of graphite at 300 K and 0.1 MPa. It is fitted as ΔG_{Water−induced} = a + b/(r − 1.75). In reference to R_H (hydrophobic radius), the hydrophobic interactions are divided into H1w and H2s hydrophobic processes. During the H1w process, γ = 1. During the H2s process, solute surfaces begin contact in water, and γ < 1.

Figure 5

The changes in the interfacial and bulk water as C₆₀ is associated with the concave (a,b), flat (c,d), and convex (e,f) surfaces of graphite in water at 300 K and 1 bar. The dashed line represents the corresponding time of R_H.

Figure 6

The relationship between water-induced PMFs and expelled water molecules from interfacial to bulk water as C₆₀ is associated with various surfaces of graphite.

Figure 7

Hydrogen bonding number per water molecule in interfacial and bulk water (a) and total water (b) as the fullerene is aggregated with the concave surface of graphite at 300 K and 0.1 MPa.

Figure 8

(a) Owing to hydrophobic interactions, the solutes are aggregated to minimize their surface area-to-volume ratio. (b) Hydrophobic interactions, related to the water molecules transformed from interfacial to bulk water, may be dependent on the solute shape.