Adsorption of Surfactants and Contact Interactions

The data presented in the previous chapter clearly indicate the universal importance of investigating particle cohesion under various conditions for establishing the scientific basis for explaining (and controlling) the mechanical properties of disperse systans in various natural and industrial processes. An important aspect of such investigations is the use of surface-active substances (surfectants), which at a low bulk concentration accumulate at the interfaces and radically change their properties. Before addressing specific results pertinent to the studies of contacts between particles of various natures in various surfactant solutions, let us briefly summarize the concepts of the adsorption of surfactants, primarily of the thermodynamics of adsorption. 

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Hydrophobic interactions may exist naturally or be induced by the adsorbed hydrophobic species. Polar solvent molecules squeezed between two hydrophobic surfaces have reduced freedom to form structures in certain directions, because contact with the particle surfaces is essentially avoided. The hydrophobic surfaces therefore have a preference to associate with each other. It is found that hydrophobic interactions extend over a much longer range than the van der Waals force. Graphite and coal are a few of the solids possessing natural hydrophobicity, and their aggregation is observed in polar solvents. More often, surface hydrophobicity of the solid particles can be induced by the adsorption of surfactants. 

The laws governing the interfacial phenomena between condensed phases and their vapor (or air) in single- and two-component systems, described in previous chapters, are largely applicable to the interfaces between two condensed phases, i.e., between two liquids, two solids, or between a solid and a liquid. At the same time, these interfaces have some important peculiarities, primarily related to the partial compensation of the intermolecular interactions. The degree of saturation of the surface forces is determined by the similarity in the molecular nature of the phases in contact. When adsorption of surfactants takes place at such interfaces, it may substantially enhance the decrease in the interfacial energy. The latter is of great importance, since surfactants play a major role in the formation and degradation of disperse systems (see Chapters IV, VI-VIII). 

The adsorption of surfactants on solid-liquid interfaces is at the center of interest in colloid and surface science. The nature of interactions between the solid surface, surfactant and the solution phase need to be understood to control the adsorption of surfactants. Solid surfaces in contact with aqueous solutions, especially surfaces of clays or minertils, are frequently charged and this charge generates a potential at the solid-solution interface. The adsorption of surfactants on the solid-liquid interface quite often proceeds at an interface which is electrified. Unfortunately,... 

The contact of a lubricating substance with a solid is particularly significant from a tribological point of view. Oxyethylated alcohols are nonionic surfactants, and their interactions with the surface are basically quite specific (hydrogen bonds). The contribution of universal (electrostatic) interactions is considerably smaller, as these are very weak dispersion interactions. In the solution in contact with a solid, one can distinguish the surface phase and the bulk phase. Due to adsorption from solutions, the surface phase is enriched with the component that has a stronger affinity for the surface. It is a characteristic of adsorption from solutions on a solid surface that individual components compete for free sites on the surface. At this point, one should not confuse adsorption with absorption, the latter of which may lead to penetration of the components into the solid. 

It is weU known that the selective adsorption of surfactants at the solid-water interface imparts hydrophobicity to the surface of the solid. The relative hydrophobicity of the solid surface is responsible for various macroscopic properties observed experimentally. For example, in mineral separation, the hydrophobicity of the solid surface leads to selective bubble-particle attachment, which accounts for the selective flotation of minerals in large scale industrial plants. The relative measure of mineral surface hydrophobicity is usually quantified in terms of contact angle measurements and flotation experiments (Fuerstenau 1957, 1970, 2000 Fuerstenau and Herrera-Urbina 1989 Fuerstenau and Pradip 2005 Pradip 1988). Molecular-modeling tools can be successfully employed to compute the interaction energies and contact angle on both virgin and surfactant-covered mineral surfaces. The relative flotation efficacy of different surfactants can thus be related to their molecular structure and properties. 

The colloid probe technique was first applied to the investigation of surfactant adsorption by Rutland and Senden [83]. They investigated the effect of a nonionic surfactant petakis(oxyethylene) dodecyl ether at various concentrations for a silica-silica system. In the absence of surfactant they observed a repulsive interaction at small separation, which inhibited adhesive contact. For a concentration of 2 X 10 M they found a normalized adhesive force of 19 mN/m, which is small compared to similar measurements with SEA and is probably caused by sufactant adsorption s disrupting the hydration force. The adhesive force decreased with time, suggesting that the hydrophobic attraction was being screened by further surfactant adsorption. Thus the authors concluded that adsorption occurs through... 

The adsorption of mixed surfactants at the air—water interface (monolayer formation) is mechanistically very similar to mixed micelle formation. The mixed monolayer is oriented so that the surfactant hydrophilic groups are adjacent to each other. The hydrophobic groups are removed from the aqueous environment and are in contact with other hydrophobic groups or air. Therefore, the forces tending to cause monolayers to form are similar to those causing micelles to form and the thermodynamics and interactions between surfactants are similar in the two aggregation processes. 

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