Role of the Properties and Structure of Liquid Water in Colloidal and Interfacial Systems

Abstract

The interaction between water and solid surfaces is of great interest to geological materials, polymers, biomaterials, cells, microbes, other particles, etc. Yet little is still known about the structure, bonding, and function of the first few water layers in contact with solids or solutes, when these are immersed in water. Mineralogical and colloidal surface studies suggest how water may interact with both hydrophobic and hydrophilic surfaces. We know from surface‐thermodynamic analyses of the attractive interactions between apolar (hydrophobic) molecules or particles, and of the repulsive interactions between polar (hydrophilic) molecules or particles, all immersed in water, that the first ones are driven by the strong hydrogen‐bonding free energy of cohesion between the surrounding water molecules, whilst the second ones are mainly caused by the molecules of water of hydration when these are very strongly hydrogen‐bonded to polar molecules or particles. However, this knowledge does not necessarily give us a direct insight into the molecular‐scale changes occurring in the conformation of the water molecules in the first few layers of water of hydration closest to a solid/water or to a non‐aqueous liquid/water interface. At the interface with apolar (hydrophobic) surfaces the first one or two layers of water molecules of hydration are exclusively attracted via (apolar) Lifshitz–van der Waals (LW) forces, as a thin net of water molecule clusters of diminished polarity in a manner that resembles similar to the structure of the water molecules participating in clathrate hydrates. On the other hand, at the interface with polar (hydrophilic) surfaces the first one or two layers of water molecules of hydration are bound, in addition to LW forces, for up to slightly more than 50% by Lewis acid–base (AB) forces, which cause a significant part of the most proximal water molecules of hydration to be oriented with their H‐atoms hydrogen‐bonded to the polar surfaces, and with their O‐atoms protruding into the bulk liquid. Neither mode of conformation of water molecules of hydration bears any obvious resemblance to any of the forms of crystal formation occurring in ice. The different conformation gradients of the water molecules of hydration at the interface with apolar (hydrophobic) and with polar (hydrophilic) entities permit action at a distance well into the bulk water, in the case of hydrophobic attraction as well as of hydrophilic repulsion. In both cases, the interaction energies decay exponentially as a function of distance with a decay length of water of approximately 1.0 nm. This (experimentally found) value for the decay length of water agrees more with a radius of gyration for small clusters of 4 to 5 water molecules than for single molecules of water. Further consequences of these considerations are discussed in detail, with particular stress on the influence of temperature on the interaction of partly polar entities immersed in water, on the necessity of using the extended DLVO (XDLVO) theory [extended by the inclusion of Lewis AB interactions in aqueous media] and on the unusual and sometimes extreme properties of the water–air interface and its manifestations.

Keywords:

• Liquid water
• Hydration
• Hydrophobic attraction
• Hydrophilic repulsion
• Lifshitz‐van der Waals forces
• Lewis in acid‐base forces
• Hydrogen‐bonding forces
• Electrical double layer forces

Notes

^aΔG _iwi ^LW at that distance, equals 1.6 × 10⁻⁴ kT and is therefore entirely negligible; ΔG _iwi ^EL is unknown.