Oscillatory structural forces

Abstract. The stability of suspensions/emulsions is under consideration. Traditionally consideration of colloidal systems is based on inclusion only Van-der-Waals (or dispersion) and electrostatic components, which is refereed to as DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. It is shown that not only DLVO components but also other types of the inter-particle forces may play an important role in the stability and colloidal systems. Those contributions are due to hydrodynamic interactions, hydration and hydrophobic forces, steric and depletion forced, oscillatory structural forces. The hydrodynamic and colloidal interactions between drops and bubbles emulsions and foams are even more complex (as compared to that of suspensions of solid particles) due to the fluidity and deformability of those colloidal objects. The latter two features and thin film formation between the colliding particles have a great impact on the hydrodynamic interactions, the magnitude of the disjoining pressure and on the dynamic and thermodynamic stability of such colloidal systems. 

Emulsions and microemulsions are dispersions of liquid in liquid. Therefore, an important feature is their interfacial fluidity and deformability, which distinguishes them from suspensions of solid particles. The stability of the latter is usually treated in the framework of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, " which accounts for the electrostatic and van der Waals interactions between solid particles. During recent years it was shown that other types of interparticle forces may often play an important role for the stability of dispersions—hydrodynamic interactions, hydration and hydrophobic forces, oscillatory structure forces, etc. - It was proven both experimentally and theoretically that steric and depletion " interactions may sometimes be a decisive factor for the dispersion stability. 

In the presence of large amount of micelles, the total force between surfaces may oscillate due to the occurrence of oscillatory structural forces. Structural... 

PA. Kralchevsky and N.D. Denkov Analytical Expression for the Oscillatory Structural Surface Force. Chem. Phys. Lett. 240, 385 (1995). 

Kralchevsky P, Denkov ND (1995) Analytical expression for the oscillatory structural surface force. Chem Phys Lett 240 385-392... 

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... 

Oscillatory structural forces appear in two cases (1) in thin films of pme solvent between two smooth solid surfaces (2) in thin liquid films containing colloidal particles (including macromolecules and surfactant micelles). In the first case, the oscillatory forces are called the solvation... 

In all cases, the oscillatory structural forces appear when monodisperse spherical (in some cases ellipsoidal or cylindrical) particles are confined between the two surfaces of a thin film. Even one hard wall can induce ordering among the neighboring molecules. The oscillatory structural force is a result of overlap of the structured zones at two approaching surfaces. A simple connection between density distribution and structural force is given by the contact value theo-... 

The contribution of the oscillatory structural forces to the interaction free energy per unit area of the hhn can be obtained by integrating floj ... 

The experimental observations show that stratification is always observed when spherical colloidal particles are present in the film at a sufficiently high volume fraction therefore, a realistic explanation can be that the stepwise transitions are manifestations of the oscillatory structural forces. The role of the hard spheres this time is played by the colloidal particles rather than by the solvent molecules. The mechanism of stratification was studied theoretically in Reference 346, where the appearance and expansion of black spots in the stratifying films were described as being a process of condensation of vacancies in a colloid crystal of ordered micelles within the film. 

E. Long-Range (nonDLVO) Oscillatory Structural Forces... 

The adsorbed surfactant molecules counteract flic drop coalescence in two ways (1,2). The presence of surfactant gives rise to repulsive surface forces (of either electrostatic, steric, or oscillatory structural origin) between the drops, thus providing a thermodynamic stabilization of the emulsion see also Refs 3 and 4. Moreover, the adsorbed surfae-... 

In smectic hquid crystals it is again possible to observe both types of interactions. Overlap of pre-smectically ordered interfacial regions gives rise to the oscillatory structural force, presented in Sect. 3.1.6 and if the coupling energy between the smectic order and surface is very strong, the liquid crystal in the gap beteen the surfaces condenses into a smectic phase, as discussed in Sect. 3.1.7). 

Oscillatory structural forces appear in thin films of pure solvent between two smooth solid surfaces and in thin liquid films containing colloidal particles including macromolecules and surfactant micelles (Israelachvili 1992). In the first case, the oscillatory forces are called the solvation forces and they are important for the short-range interactions between solid particles and dispersions. In the second case, the structural forces affect the stability of foam and emulsion films as well as the flocculation processes in various colloids. At lower particle concentrations, the structural forces degenerate into the so-called depletion attraction, which is found to destabilize various dispersions. 

The Derjaguin s formula is applicable to any type of force law (attractive, repulsive, oscillatory) if only (1) the range of the forces, and (2) the surface-to-surface distance are much smaller than the surface curvature radii. This formula is applicable to any kind of surface force, irrespective of its physical origin van der Waals, electrostatic, steric, oscillatory-structural, depletion, etc. It reduces the two-particle interaction problem to the simpler problem for interactions in plane-parallel films. 

Special oscillatory structural forces appear in thin films containing small colloidal particles like surfactant micelles, polymer coils, protein macromolecules, and latex or silica particles [268,280-283]. For larger particle volume fractions, these forces are found to stabilize thin films and dispersions, whereas at low particle concentrations, the oscillatory force degenerates into the depletion attraction, which has the opposite effect see Sec. VI.C. 

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