Non-DLVO Forces in an Aqueous Medium

The force is the sum of an exponentially decaying force plus a periodic contribution with an exponentially decreasing amplitude. 

Non DLVO forces in water deserve a special subchapter because they are important and far from being understood. They are important because water is the universal solvent in nature. Also, in more and more industrial processes water is used instead of organic solvent since it is harmless to the environment. 

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 

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