Interaction forces, colloid stability

Atomic force microscopy (AFM) [20] has recently been used to image interfacial aggregates directly, in situ and at nanometer resolution [21, 22], The key to this application lies in an unusual contrast mechanism, namely a pre-contact repulsive force ( colloidal stabilization force ) between the adsorbed surfactant layers on the tip and sample. In contrast to previous adsorbate models of flat monolayers and bilayers, AFM images have shown a striking variety of interfacial aggregates - spheres, cylinders, half-cylinders and bilayers - depending on the surface chemistry and surfactant geometry. I review the AFM evidence for these structures and discuss the possible inter-molecular and molecule-surface interactions involved. 

Colloidal Stabilization. Surfactant adsorption reduces soil—substrate interactions and faciUtates soil removal. For a better understanding of these interactions, a consideration of coUoidal forces is required. 

The surface force apparatus (SFA) is a device that detects the variations of normal and tangential forces resulting from the molecule interactions, as a function of normal distance between two curved surfaces in relative motion. SFA has been successfully used over the past years for investigating various surface phenomena, such as adhesion, rheology of confined liquid and polymers, colloid stability, and boundary friction. The first SFA was invented in 1969 by Tabor and Winterton [23] and was further developed in 1972 by Israela-chivili and Tabor [24]. The device was employed for direct measurement of the van der Waals forces in the air or vacuum between molecularly smooth mica surfaces in the distance range of 1.5-130 nm. The results confirmed the prediction of the Lifshitz theory on van der Waals interactions down to the separations as small as 1.5 nm. 

In this situation, the equilibrium thickness at any given height h is determined by the balance between the hydrostatic pressure in the liquid (hpg) and the repulsive pressure in the film, that is n = hpg. Cyril Isenberg gives many beautiful pictures of soap films of different geometries in his book The Science of Soap Films and Soap Bubbles (1992). Sir Isaac Newton published his observations of the colours of soap bubbles in Opticks (1730). This experimental set-up has been used to measure the interaction force between surfactant surfaces, as a function of separation distance or film thickness. These forces are important in stabilizing surfactant lamellar phases and in cell-cell interactions, as well as in colloidal interactions generally. 

From a technical standpoint, it is also important to note that colloids display a wide range of rheological behavior. Charged dispersions (even at very low volume fractions) and sterically stabilized colloids show elastic behavior like solids. When the interparticle interactions are not important, they behave like ordinary liquids (i.e., they flow easily when subjected to even small shear forces) this is known as viscous behavior. Very often, the behavior falls somewhere between these two extremes the dispersion is then said to be viscoelastic. Therefore, it becomes important to understand how the interaction forces and fluid mechanics of the dispersions affect the flow behavior of dispersions. 

We have already seen from Example 10.1 that van der Waals forces play a major role in the heat of vaporization of liquids, and it is not surprising, in view of our discussion in Section 10.2 about colloid stability, that they also play a significant part in (or at least influence) a number of macroscopic phenomena such as adhesion, cohesion, self-assembly of surfactants, conformation of biological macromolecules, and formation of biological cells. We see below in this chapter (Section 10.7) some additional examples of the relation between van der Waals forces and macroscopic properties of materials and investigate how, as a consequence, measurements of macroscopic properties could be used to determine the Hamaker constant, a material property that represents the strength of van der Waals attraction (or repulsion see Section 10.8b) between macroscopic bodies. In this section, we present one illustration of the macroscopic implications of van der Waals forces in thermodynamics, namely, the relation between the interaction forces discussed in the previous section and the van der Waals equation of state. In particular, our objective is to relate the molecular van der Waals parameter (e.g., 0n in Equation (33)) to the parameter a that appears in the van der Waals equation of state ... 

Our objective in this chapter is to establish the quantitative connections between interparticle forces and colloid stability. Before we consider this it is instructive to look at the role of interaction forces in a larger context, that is, the relation between interparticle forces and the microstructure of dispersions and the factors that determine such a relation. These aid us in appreciating the underlying theme of this chapter, namely, the manipulation of interparticle forces to control the properties of dispersions. 

Clearly, W is a function of any property of the dispersion that affects the strength of the interparticle forces and the energy barrier that slows down (or prevents) coagulation. A classical goal of colloid science has been to develop the equations necessary to predict the extent of stability of dispersions so that the results could be used in combination with the theories of interaction forces developed in previous chapters to promote or prevent the stability of dispersions. 

Chapters 11-13 of the second edition, which discussed van der Waals forces (old Chapter 11), electrical double layers (old Chapter 12), electrokinetic phenomena (old Chapter 13), and colloid stability (old Chapters 11 and 12), have been restructured and new materials on colloid stability and polymer/colloid interactions have been added. For example ... 

The question to be discussed is whether saturation of the electric field (asserted by Proposition 2.1) implies saturation of the interparticle force of interaction. Consider for definiteness repulsion between two symmetrically charged particles in a symmetric electrolyte solution. In the onedimensional case (for parallel plates) the answer is known—the force of repulsion per unit area of the plates saturates. (This follows from a direct integration of the Poisson-Boltzmann equation carried out in numerous works, primarily in the colloid stability context, e.g., [9]. Recall that again in vacuum, dielectrics, or an ionic system with a linear screening, the appropriate force grows without bound with the charging of the particles.)... 

Owing to their fundamental interest and their practical importance in issues such as colloid stability, much experimental effort has been devoted to the measurement of electric double layer and van der Waals interactions between macroscopic objects at close separations. Such measurements involve balancing the force(s) to be measured with an externally applied force. 

Israelachvilli and co-workers95 have measured directly the forces between molecularly smooth cleaved mica surfaces separated by organic liquids and have observed a corresponding periodicity of force with separation. The extent to which these short-range interactions (solvation or structural forces) may influence colloid stability and other related phenomena is not entirely clear. 

In this discussion of colloid stability we will explore the reasons why colloidal dispersions can have different degrees of kinetic stability and how these are influenced, and can therefore be modified, by solution and surface properties. Encounters between species in a dispersion can occur frequently due to any of Brownian motion, sedimentation, or stirring. The stability of the dispersion depends upon how the species interact when this happens. The main cause of repulsive forces is the electrostatic repulsion between like charged objects. The main cause of attractive forces is the van der Waals forces between objects. 

A pair of polysaccharide molecules approaching each other in water exerts an interaction potential ( ) that is the algebraic sum of the competing attractive and repulsive forces. integrated over all pairs of molecules, is . This principle is embodied in the Deijaguin-Verwey-Landau-Overbeek (DLVO) theory of colloidal stability (Ross and Morrison, 1988). The equilibrium distance between the molecules is related to c, the volume of the hydrated particles, ionic strength, cosolute, nonsolvent additions, temperature, and shearing. 

There are some qualitative difficulties when the specific ion effects are explained via the dispersion forces of the ions. Particularly the anions, for which the dispersion coefficients / , are large, affect the double layer interactions. However, experiments on colloid stability [6] or colloidal forces [11] revealed strong specific ion effects especially for cations. Furthermore, the ions which affect most strongly the solvating properties of the proteins are those from their vicinity, since they perturb mostly the structure of water near the proteins. However, the van der Waals interactions of ions predict that the cations remain in the vicinity of an interface, and the anions are strongly repelled, while Hofmeister concluded that anions are mainly responsible for the salting out of proteins. 

A quantitative treatment of the effects of electrolytes on colloid stability has been independently developed by Deryagen and Landau and by Verwey and Over-beek (DLVO), who considered the additive of the interaction forces, mainly electrostatic repulsive and van der Waals attractive forces as the particles approach each other. Repulsive forces between particles arise from the overlapping of the diffuse layer in the electrical double layer of two approaching particles. No simple analytical expression can be given for these repulsive interaction forces. Under certain assumptions, the surface potential is small and remains constant the thickness of the double layer is large and the overlap of the electrical double layer is small. The repulsive energy (VR) between two spherical particles of equal size can be calculated by ... 

Ninham and coworkers [12,13] very recently proffered an alternative explanation. They proposed that the shortcoming of traditional DLYO theory in predicting particle stability has arisen from its ab initio decomposition of forces into van der Waals and double-layer components. More specifically, they suggested that not accounting for dispersive interactions between colloid surfaces and dissolved ions is primarily responsible for the reported discrepancies in the traditional colloidal-stability modeling approach. 

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