Force repulsive hydration

Israelachvili J N and Pashley R M 1983 Molecular layering of water at surfaces and origin of repulsive hydration forces Nature 306 249-50... 

The surface potential can play an important role in the behavior of liposomes in vivo and in vitro (e.g.. Senior, 1987). In general, charged liposomes ai e more stable against aggregation and fusion than uncharged vesicles. However, physically stable neutral liposomes have been described (e.g.. Van Dalen et al., 1988). They are sufficiently stabilized by repulsive hydration forces, which counteract the attractive van der Waals forces. 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

These various relationships between force and particle separation imply that the attractive force between particles will become infinite when they touch. In reality, other short-range forces will modify this relationship when r is very small, in particular the repulsion from overlap of atomic orbitals. The van der Waals attraction will then be balanced by this overlap repulsion. At these short distances (a few tenths of a nanometer), the van der Waals attraction will be strong enough to hold the particles fairly strongly together. This balance between van der Waals forces of attraction and overlap repulsion forces is shown schematically in Fig. 1.4, where the very steep repulsive interaction at atomic distances is due to the overlap repulsion. Hydration forces (see section 1.3.3) may also result in repulsion between surfaces at somewhat greater separations. 

The invention and refinement of the SFA have been among the most significant advances in experimental colloid science and have allowed researchers to identify and quantify most of the fundamental interactions occurring between surfaces in aqueous solutions as well as nonaqueous liquids. Attractive van der Waals and repulsive electrostatic double-layer forces, oscillatory (solvation or structural) forces, repulsive hydration forces, attractive hydrophobic... 

In practice, van der Waals forces appear within a mix of forces. Measured between bilayers in free suspension, they are mixed with lamellar motions as well as with repulsive hydration forces. 

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

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

For typical membranes interacting via an exponential repulsion (hydration force) and a Van der Waals interaction, it was shown that at small separations the undulation interaction has an exponential behavior [16,22,23], However, the exponential repulsion cannot be valid at large separation, because it cannot predict the unbinding of the membranes (an exponential is always of shorter range that the van der Waals attraction). In general, the undulation repulsion is treated as an additive inter-... 

Fig. 7. Force normalised by radius as a function of surface separation between layers of DHP deposited onto hydrophobised mica surfaces measured in NaCl solutions of 2x 10 4M ( ), 1 x 10 3M ( ), 1 x 10 2M (o), at PH 5.2-5.1. At pH 9.5 no attraction was observed at any separation. The solid lines are force curves calculated from DLVO theory. The dashed line represents the part of the force curve that cannot be explained by the DLVO theory. This is a repulsive hydration force. Reprinted with permission from Ref. [95]. 1989, American Chemical Society.

Using a surface forces apparatus, Israelachvili determined the force law for two molecularly smooth charged mica surfaces immersed in an aqueous solvent (see Israelachvili and Marra, 1986, and references cited therein). The repulsive hydration force is oscillatory (Fig. 8A). It is understood to reflect the geometry and local structure of the solvent and... 

J. N. IsraelachvUi, R.M. Pashley, Molecular Layering of Water at Siufaces and Origin of Repulsive Hydration Forces , Nature, 306,249 (1983)... 

Horn RG, Smith DT, Haller W. Repulsive hydration forces between two silica surfaces. Chem Phys Lett 1989 162 404—408. 

Figure 2.5.2 Sterochemical model of a short-lived micelle. There are no binding interactions between the monomers. All headgroups are separated by hydration spheres (repulsive hydration forces).

Noncovalent spherical assemblies of amphiphilic lipids, on the other hand, are either short-lived already in aqueous solution (micelles) or collapse immediately upon drying (vesicles).This behavior is due to the character of the forces which form them. Curvature is retained by repulsive hydration forces. If the hydration sphere is removed from the head groups, the amphiphiles will pack together and form crystalline sheets and 3-D crystals. Neither crystallites nor micelles and vesicles can be considered as noncovalent polymers, because they change their molecular arrangement drastically when going from the dissolved to the dry state. They do not have material properties. 

Repulsive hydration forces These include steric-hydration forces between amphiphilic surfaces and repulsive hydration forces between hydrophilic... 

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. 

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