Repulsion between hydrated surfaces

The increasing use of nonionic macromolecules as stabilisers, which has occurred since the development of the DLVO theory, has led to the awareness of other stabilising forces. The approach of particles with hydrated macromolecules adsorbed to their surfaces leads, on the interaction of these layers, to repulsion (Fig. 7.7), because of the consequent positive enthalpy change +AH which ensues. In more general terms, the approach of two particles with adsorbed stabilising chains leads 

Quantitative assessments of the steric effect depends on three parameters  

The steric effect does not come into play until H = 26, so the interaction increases suddenly with decreasing distance. There are many problems in applying such equations in practice, the main ones being the lack of an 

Chapter 7 Emulsions, suspensions and other disperse systems 

For particles with a hydrated stabilising layer of thickness 6, the volume of the overlapping region (1 ) is as derived in Fig. 7.9  

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Paunov, V.N., R.I. Dimova, P.A. Kralchevsky, G. Broze, and A. Mehreteab. 1996. The hydration repulsion between charged surfaces as an interplay of volume exclusion and dielectric saturation effects. J. Colloid Interface Sci. 182 239—248. 

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 number of refinements and applications are in the literature. Corrections may be made for discreteness of charge [36] or the excluded volume of the hydrated ions [19, 37]. The effects of surface roughness on the electrical double layer have been treated by several groups [38-41] by means of perturbative expansions and numerical analysis. Several geometries have been treated, including two eccentric spheres such as found in encapsulated proteins or drugs [42], and biconcave disks with elastic membranes to model red blood cells [43]. The double-layer repulsion between two spheres has been a topic of much attention due to its importance in colloidal stability. A new numeri-... 

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 plateau shown in Figure 3 corresponding to adsorbed and precipitated cobalt (II) is not amenable to analysis. However, the plateau of Figure 4 must correspond to saturation, or "monolayer adsorption. Depending on whether the Co (II) ion is unhydrated, hydrated to the extent of one sheath of water or hydrated to the extent of two layers of water then the fractions of a close packed monolayer for these three cases are 0.01, 0.16, and 0.5 respectively. Clearly this low coverage means that there must be considerable repulsion between adsorbed Co (II) ions on the adsorbed layer leading to a sparse population of the surface. 

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... 

Figure 5.12 Illustration offorce curves between silica surfaces in aqueous solutions of sodium chloride at various molar concentrations. At short range there is a strong repulsion which is not accounted for in the standard DLVO theory, due to hydration forces. Drawn based on data in Horn [284].

Ruckenstein and Schiby derived4 an expression for the electrochemical potential, which accounted for the hydration of ions and their finite volume. The modified Poisson-Boltzmann equation thus obtained was used to calculate the force between charged surfaces immersed in an electrolyte. It was shown that at low separation distances and high surface charges, the modified equation predicts an additional repulsion in excess to the traditional double layer theory of Deijaguin—Landau—Verwey—Overbeek. 

Another limitation of the Poisson-Boltzmann approach is that the interaction between two surfaces immersed in water might not be exclusively due to the electrolyte ions. For instance, water has a different structure in the vicinity of the surface than in the bulk and the overlapping of such structures generates a repulsion even in the absence of electrolyte [20]. In this traditional picture, the hydration repulsion is not related to ion hydration actually it is not related at all to electrolyte ions. However, as recently suggested [21], this hydration interaction can still be accounted for within the Poisson-Boltzmann framework, assuming that the polarization is not proportional to the macroscopic electric field, but depends also on the field generated by the neighboring water dipoles and by the surface dipoles. 

When the surfaces were not rigid, as in the case of lipid bilayers, the oscillations of the force were smoothed out and the interactions became monotonic. The short-range repulsion between neutral7 or weakly charged8 bilayers, often called hydration force, was exhaustively investigated experimentally and was found to have an exponential decay, with a decay length between 1.5 and 3 A, while the preexponential factor varied by more than an order of magnitude. 

The force between neutral surfaces (with a surface dipole density) depends on the electrolyte concentrations, as shown in Fig. 3b, particularly at large separations. However, at small separations, the interaction appears to be well described by an exponential with a decay length AH. For neutral lipid bilayers, the equilibrium is reached at a distance of about 20 A, at which the attractive van der Waals interaction balances the repulsive hydration and thermal undulation interactions [43], The experiments regarding the forces between neutral lipid bilayers [11] sample the interactions at separations smaller than 20 A, for which the dependence on ionic strength is much weaker. By adding to the total pressure a typical van der Waals disjoining pressure [12] ... 

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-... 

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