Solvation, structural and depletion forces

So-called solvation/structural forces, or (in water) hydration forces, arise in the gap between a pair of particles or surfaces when solvent (water) molecules become ordered by the proximity of the surfaces. When such ordering occurs, there is a breakdown in the classical continuum theories of the van der Waals and electrostatic double-layer forces, with the consequence that the monotonic forces they conventionally predict are replaced (or accompanied) by exponentially decaying oscillatory forces with a periodicity roughly equal to the size of the confined species (Min et al, 2008). In practice, these confined species may be of widely variable structural and chemical types — ranging in size from small solvent molecules (like water) up to macromolecules and nanoparticles. 

In the case of biopolymer molecules residing in the space between colloidal particles or droplets, the force associated with the deep energy minimum at contact is often referred to as the depletion force because the intervening biopolymer species are depleted from the narrow gap between the pair of neighbouring particles. This attractive interparticle interaction underlies the phenomenon of reversible depletion flocculation in oil-in-water emulsions (see equation (3.41) in chapter 3). 

In addition to the considerations mentioned above, it is rather important to keep in mind that a common aspect in determining the overall thermodynamic behaviour of a biopolymer solution/dispersion is the necessity of taking into account all of the component interactions. This includes the interactions of the biopolymer(s) with the water molecules, including both hydration (attraction) and dehydration (release), as well as the interactions amongst the water molecules themselves. 

Akhtar, M., Dickinson, E. (2007). Whey protein-maltodextrin conjugates as emulsifying agents an alternative to gum arabic. FoodHydrocolloids, 21, 607-616. 

Cantor, C.R., Schimmel, P.R. (1980). Biophysical Chemistry, Part I, San Francisco Freeman. 

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