Origin of the hydration force

Because the double layer force vanishes in the absence of surface charges, one expects the attractive van der Waals force to cause the coagulation of all neutral (or even weekly charged) colloids. The absence of such a behavior has been explained by the existence of an additional (non-DLVO) force, the hydration interaction, which is due to the structuring of water in the vicinity of hydrophilic surfaces. This chapter is devoted to the identification of the microscopic origin of the hydration force, and to the presentation of a unified treatment of the double layer and hydration forces, the Polarization Model. 

Recent molecular dynamics simulations of water between two surfactant (sodium dodecyl sulfate) layers, reported by Faraudo and Bresme,14 revealed oscillatory behaviors for both the polarization and the electric fields near a surface and that the two fields are not proportional to each other. While the nonmonotonic behavior again invalidated the Gruen—Marcelja model for the polarization, the nonproportionality suggested that a more complex dielectric response of water might, be at the origin of the hydration force. The latter conclusion was also supported by recent molecular dynamics simulations of Far audo and Bresme, who reported interactions between surfactant surfaces with a nonmonotonic dependence on distance.15... 

On the other hand, in support of the hypothesis that the polarization of water might play an important role in the hydration force between silica surfaces, one should note that the polarization model predicts an increase in the hydration force at higher ionic strength [30], which can be indeed observed in Fig. 5, by comparing the experiments at c =0.01 M (pH=3) with those at cE= 1 M. While both the gel-induced steric repulsion and the polarization model are consistent with the present experimental data, a final decision about the microscopic origin of the hydration force in the case of silica should be postponed until more accurate data or additional information regarding the nature of the silica surface will become available. 

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). 

ON THE ORIGIN OF REPULSIVE HYDRATION FORCES BETWEEN TWO MICA PLATES E. RUCKENSTE1N and D. SCHIBY... 

While the existence of the hydration force is undisputed, its origin is still a matter of debate. Marcelja and Radic showed that the exponential repulsion observed experimentally can be obtained if a suitable Landau free energy density, dependent on an unknown order parameter , is associated with the correlation of the water molecules in the vicinity of the surface.11 Later, Schiby and Ruckenstein12 and Gruen and Marcelja13 presented two different models, both involving the polarization of the water molecules. 

The various qualitative behaviors of the hydration force in different systems (either oscillatory [12] or monotonic [10], with various decay lengths (2—3 A [10] or about 10 A [13]), either independent of electrolyte concentration [10] or exhibiting strong specific ion effects [14]) appear to point out toward the existence of a number of different microscopic origins for the short range repulsions between surfaces immersed in water, in excess to those accounted by the DLVO theory. On the other hand, there are some striking similarities between the hydration forces in different systems. For example, the Molecular Dy-... 

The hydrophobic effect is related to the water structure adopted around nonpolar molecules. The strong inclination of water molecules to form H bonds with each other influences the interactions with nonpolar molecules that are incapable of forming H bonds. The nonpolar molecules affect the water structure around them by reorientation of the water molecules. Because of these phenomena, it is proposed that the hydrophobic effect has an entropic nature, but like the repulsive hydration force, the origin of the hydrophobic force is still unknown. The hydration and hydrophobic forces are not of a simple nature. These interactions are probably the most important, yet the least understood, of all the forces in aqueous solutions. 

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... 

E. Ruckenstein, D. Schiby On the origin of the repulsive hydration forces between mica plates, CHEMICAL PHYSICS LETTERS 95 (1983) 439-443. 

While the hydration force was associated with the structuring of water in the vicinity of a lyophilic surface [12], there is no consensus about its microscopic origin. This incertitude is probably due to the apparent contradictory experimental results for phospholipid bilayers, the hydration force is apparently independent of the electrolyte concentration and has a decay length of about 2 A [11], while for mica surfaces the hydration is strongly dependent not only on the electrolyte concentration, but also on the nature of the cation (cation-specific effects) [17] and has a decay length of about 10 A. 

We present experimental results on photophysical deactivation pathways of uracil and thymine bases in the gas phase and in solvent/solute complexes. After photoexcitation to the S2 state, a bare molecule is tunneled into and trapped in a dark state with a lifetime of tens to hundreds of nanoseconds. The nature of this dark state is most likely a low lying nn state. Solvent molecules affect the decay pathways by increasing IC from the S2 to the dark state and then further to the ground state, or directly from S2 to S0. The lifetimes of the S2 state and the dark state are both decreased with the addition of only one or two water molecules. When more than four water molecules are attached, the photophysics of these hydrated clusters rapidly approaches that in the condensed phase. This model is now confirmed from other gas phase and liquid phase experiments, as well as from theoretical calculations. This result offers a new interpretation on the origin of the photostability of nucleic acid bases. Although we believe photochemical stability is a major natural selective force, the reason that the nucleic acid bases have been chosen is not because of their intrinsic stability. Rather, it is the stability of the overall system, with a significant contribution from the environment, that has allowed the carriers of the genetic code to survive, accumulate, and eventually evolve into life s complicated form. 

Since the observed pressure cannot be due to the electrostatic interaction and since the value of A was close to the size of a water molecule, it was suggested that the origin of the force is due to the water. This is the reason that the force was named hydration force. Later it was shown that the values of X are distributed in a rather wide region (from 0.1 to more than 0.3 nm) [2] it was also realized that the range of dy, values depends on the definition of the membrane water boundary. For example, according to the definition adopted by McIntosh and Simon [3] the values of dy, are somewhere in the region of 0.5 to 1.5 nm. 

The origin of hydration force is not clear, and there are mainly two different points of view. At first, it was assumed that the hydration force originated from the energy needed to dehydrate interacting surfaces that contained ionic or polar species [22]. But in 1990 a completely different explanation for the origin of hydration force was proposed It is concluded that the short-range repulsion between amphiphilic surfaces is mainly of entropic origin [23]. 

A question is whether or not the hydration force should be considered as a solvation force. It was reported [24] that the forces between bilayers in a variety of non-aqueous solvents were very similar to those measured in water, while other data do not appear to support the modified solvent-structure origin of hydration forces [23]. 

Effects of different physical origin are termed hydration forces in the literature see Ref. 317 for a review. For that reason, from the very beginning we specify that here we call hydration force the short-range monotonic repulsive force which appears as a deviation from the DLVO theory for short distances between two molecularly smooth electrically... 

For the time being, there is no generally accepted theory of the repulsive hydration force. It has been attributed to various effects solvent polarization and H-bonding [323], image charges [324], nonlocal electrostatic effects [325], and the existence of a layer of lower dielectric constant, e, in a vicinity of the interface [326,327]. It seems, however, that the main contribution to the hydration repulsion between two charged interfaces originates from the finite size of the hydrated counterions [328], an effect which is not taken into account in the DLVO theory (the latter deals with point ions). 

The electrostatic forces are due to the fact that most particles are charged inside a medium, especially a polar one like water, which has a high relative permittivity. The origin of the surface charge can be complex and there are many mechanisms for this, e.g. adsorption of ions from the solution or dissociation of surface groups. Other repulsive forces, which help stability, exist especially at low interparticle distances (e.g. the so-called hydration or steric forces). 

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