Free energy approach, hydration forces

The previously described measurements have been performed on lipids in aqueous solutions, but lipid bilayers also swell in some other solvents (12) and the results of such measurements compare quite well with the aqueous case. In addition, hydration (solvation) forces act between DNA polyelectrolytes (13) and polysaccharides (14). These facts make the interpretation of the forces even more complicated and it is no wonder that different approaches to explain the nature of this solvation force exist. So far no truly ab initio theory has been proposed. The existing theories include models based on the electrostatic approach, the free energy approach, and an approach based on the entropic or protrusion model. 

Free Energy Approach. Nearly immediately after the publication of the first data on the hydration force (10), Marcelja and Radic (MR) proposed a very elegant theory to explain the nature of the observed strong force (20). According to MR theory the force is due to the modification of water structure near the membrane-water interface. The water molecules near the interface differ from the water molecules in the bulk they are more ordered. To describe this order, one can introduce an order parameter T (%) and perform a Landau-type expansion of the free energy density g(x) that is,... 

The treatment of the hydration interaction follows the model employed earlier by Schiby and Ruckenstein,7based on the mutual interactions of dipoles. In this model, it is considered that the dipoles of the surface polarize the water molecules of the first layer of water and the polarization propagates from layer to layer. When two surfaces approach each other, the polarized layers will increasingly overlap. As a result, the local dipole moment of the water molecules will be decreased. This increases the free energy of the system and thus generates a repulsive force. 

Experimental information on gas hydrate nucleation at a microscopic level is almost nonexistent or at best very limited. Most of the studies on the hydrate nucleation are based on a macroscopic approach. Although there are some differences, gas hydrate nucleation has similarities with salt crystal nucleation. For the nucleation to occur, supersaturation of the aqueous solution with the hydrate former gas is required. The supersaturation is necessary to overcome the free energy barrier for creating a new surface of a solid hydrate nucleus. The degree of supersaturation or the driving force for nucleation may be defined in terms of difference in the chemical potential or the fugacity of a hydrate former in the solution and that at the... 

To describe interfacial forces caused by orientation correlations, Landau theory in combination with a mean field approach has been applied. In Landau theory, the free energy density of a system is expanded in a power series of the order parameters and their derivatives. A description of the theory is beyond the scope of this book and the reader is referred to textbooks on the statistical physics of liquids. It was first applied to describe hydration forces (see below) [1135,1136]. De Gennes applied it to liquid crystals [1137], and sometimes the theory is referred to as Landau-de Gennes theory. 

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