Protrusion model, hydration force

Another model [27] attempted to explain only the hydration interactions between neutral phospholipid bilayers, because those interactions appear to be quite different from the hydration forces determined in other systems they have a much shorter decay length and are almost independent of electrolyte concentration [11], The theory was based on a steric repulsion between lipid molecules, which protruding from one bilayer collide with the molecules of the opposite bilayer [27]. However, it was shown that the hydration interactions are not affected much by the polymerization of bilayers, which essentially exclude molecular protrusions [28]. 

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. 

Protrusion Model for the Hydration Force. Recently Is-raelachvili and Wennerstrom (IW) proposed that the origin of the hydration force is due to the head-group protrusion of the phospholipid molecules into the solvent region (27), and, therefore, the force is more akin to a steric force acting between polymer-covered surfaces. Because such an explanation of the nature of the hydration force does not involve electrostatic concepts, we will not present the IW theory here. A detailed description of a protrusion model is available in a recent review by Israelachvili and Wennerstrom (28), and a critique of IW theory of protrusions can be found in Parsegian and Rand (29). 

The bound part of a closed vesicle is typically several nanometers away from the substrate and still shows thermal fluctuations [45], These conditions require refinement of the model by paying due attention to the mesoscopic interactions between membrane and substrate. Various forces are involved in this interaction, such as van der Waals interaction, electrostatic contributions in the case of charged membranes, and short-range repulsive forces such as the hydration force or protrusion forces [52]. 

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