Marcelja-Radic theory

The MR theory predicts that the decay constant is independent of the nature of the surface, but the experimental data show that this is not the case and that the decay constant does depend on the nature of the surface (3). Recently, Komyshev and Leikin (KL 22) extended the Marcelja-Radic theory and demonstrated how this dependence can be explained. They replaced the homogeneous boundary conditions used in Marcelja-Radic theory by the inhomogeneous boundary conditions. According to Komyshev and Leikin, the description of the inhomogeneous character of the bound-... 

Figure 1. Pictorial representation of the Marcelja-Radic theory.

This equation is very similar to eq (24), the main result of Marcelja-Radic theory. The difference between the eqs 24 and 30 is that eq 30 clearly shows that the decay constant of the hydration force depends on the nature of the interface. Equation 30 is obtainable from MR theory if the constants a and c in eq 21 are assumed to be interface specific. Equation 30 justifies this assumption. 

Marcelja-Radic theory and its extensions (20-22) and Jonsson-Wennerstrom theory and its extensions (15-19) do not address the problem of the hydration force on the level of a detailed molecular Hamiltonian. Due to the complexity of the problem perhaps only a computer simulation technique can provide a description on this level. The tremendous usefulness of computer methods in the study of DNA and protein molecules (30, 31) inspires the hope that simulations will also play an important role in the study of biomembrane molecules. The first computer simulations to study the problem of hydration force on phospholipid molecules were performed by Scott (32, 33) and by Kjellander and Marcelja (34, 35). Because these studies were performed seven to eight years ago, the researchers were limited by the computer power available. As a result, the simulations were performed with... 

A simplified version of the theory was presented by Cevc and Marsh.12 They started from the Marcelja—Radic phenomenological treatment, assumed that the polarization constitutes the order parameter, and used the Gruen—Marcelja model to explain the various contributions to the free energy density. 

Up to now the origin of hydration forces is not clear and several effects are discussed. Certainly the fact that one layer of water molecules is bound to the solid surfaces is important. The hydration force, however, extends over more than only two water layers. Israelachvili and Wennerstom point out that the effect of the first water layer should not even be called a hydration force because it is caused by the interaction between water molecules and the solid surface and not by water-water interactions [175], In a classical paper Marcelja and Radic proposed an elegant theory to explain the short-range repulsion by a modification of water structure near hydrophilic surfaces [178], Modern theories take additional effects into account. In fact, short-range monotonically repulsive forces observed between inorganic surfaces are probably not only due to structured water layers propagated away from the surfaces, but to the osmotic effect of hydrated ions which are electrostatically trapped between two approaching surfaces [179], This is supported by the observation that the hydration force is... 

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,... 

Different theories have been proposed to explain hydration forces and to derive quantitative equations. Marcelja and Radic assumed that the water molecules in direct contact with the walls are fixed and preferentially oriented [1135]. This preferred orientation is coupled to the adjacent layers of water molecules. Using Landau-de Gennes theory and introducing a water-order correlation length they derived a force per unit area 1... 

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