The Effects of External Interfaces on Hydrophobic Interactions I: Smooth Surface

Abstract

External interfaces, such as the air-water and solid-liquid interfaces, are ubiquitous in nature. Hydrophobic interactions are considered the fundamental driving force in many physical and chemical processes occurring in aqueous solutions. It is important to understand the effects of external interfaces on hydrophobic interactions. According to the structural studies on liquid water and the air-water interface, the external interface primarily affects the structure of the topmost water layer (interfacial water). Therefore, an external interface may affect hydrophobic interactions. The effects of interfaces on hydrophobicity are related not only to surface molecular polarity but also to the geometric characteristics of the external interface, such as shape and surface roughness. This study is devoted to understanding the effects of a smooth interface on hydrophobicity. Due to hydrophobic interactions, the solutes tend to accumulate at external interfaces to maximize the hydrogen bonding of water. Additionally, these can be demonstrated by the calculated potential mean forces (PMFs) using molecular dynamic (MD) simulations.

Keywords: hydrogen bonding; hydrophobic interactions; interface; water.

Figure 1

Hydration free energy at 293 K and 0.1 MPa. (a) Hydration free energy depends on the solute size. (b) Different dissolved behaviors are expected for solutes in initial and hydrophobic processes. As solutes aggregate in water, it is divided into H1w and H2s solvation processes.

Figure 2

The effects of external interfaces on hydrophobic interactions. (a) Hydrophobic interactions are related to water molecules transformed from interfacial to bulk water and Gibbs energy of DDAA hydrogen bonding. (b) The effects of external interface on hydrophobic interactions may be related to the surface molecular polarity, and the geometric characteristics of external interface, such as shape, surface roughness, etc.

Figure 3

The influence of smooth interfaces on hydrophobic interactions. (a) When the solute is associated with the interface, hydrophobic interactions may be related to the separation between them. (b) Owing to hydrophobic interactions, the solutes tend to be aggregated with the smooth interface. These may be related to the solute size (concentrations).

Figure 4

(a,c) The PMFs when two CH₄ molecules (two C₆₀ fullerenes) are associated in water and in vacuum. (b,d) Based on the MD simulations, the water-induced PMFs are determined.

Figure 5

(a,c) The PMFs when a CH₄ molecule (a C₆₀ fullerene) is associated with graphene sheet in water and vacuum. (b,d) Based on the calculated water-induced PMFs, they are used to understand the effects of external interfaces on hydrophobic interactions.

Figure 6

(a,c) The PMFs when a CH₄ molecule (a C₆₀ fullerene) is associated with graphene–CH₄ (graphene–C₆₀) in water and vacuum. (b,d) Based on the calculated water-induced PMFs, they are used to study the dependence of hydrophobic interactions on solute concentrations in the presence of graphene.

Figure 7

The water-induced PMFs when two CH₄ molecules (two C₆₀ fullerenes) are associated in water (a,c) and at the graphene surface (b,d).

Figure 8

Hydrophobic interactions when a C₆₀ fullerene is aggregated with graphene. Hydrophobic interactions are related to the separation between them. The fitted line is shown. When a C₆₀ fullerene is associated with graphene, with reference to R_H, it is divided into H1w and H2s processes.

Figure 9

The changes of interfacial (a) and bulk water (b) when CH₄ (C₆₀) is aggregated with graphene.

Figure 10

The density of water before (a,b) and after (c,d) a C₆₀ is associated with graphene sheet. (a,c) Only interfacial water layers are shown. (b,d) Various colors are used to show density distributions.

Figure 11

The hydrogen bonding number of interfacial and bulk water when C₆₀ fullerene is aggregated with graphene sheet.