Adsorption of insoluble surfactants

Abstract The adsorption of insoluble surfactants, spread as a monolayer at the gas-solution interface (GS), onto an electrified metal-solution (MS) interface of a gold single-crystal electrode has been investigated. 

A Langmuir trough converted into an electrochemical cell was employed in these studies and the adsorption of insoluble surfactants on the Au electrode surface was measured using electrochemical techniques. The results of these experiments have shown that the transfer of insoluble surfactants from GS onto the MS interface is strongly affected by the electrode potential and the transfer ratio is 1 1 only at the potential of zero charge (pzc). UV-Vis and light scattering experiments were employed to demonstrate that insoluble... 

Bizzotto D (1996) Characterization of the adsorption of insoluble surfactants onto an electrified interface. PhD Dissertation, University of Guelph, Guelph, ON. 238p... 

In the method developed by Exerowa, Cohen and Nikolova [144] the insoluble (or slightly soluble) monolayers are obtained by adsorption from the gas phase. A special device (Fig. 2.28) was constructed for the purpose a ring a in the measuring cell of Scheludko and Exerowa for formation of microscopic foam films at constant capillary pressure (see Section 2.1.2.). The insoluble (or slightly soluble) substance from reversoir b is placed in this ring. Conditions for the adsorption of the surfactant on either surface of the bi-concave drop are created in the closed space of the measuring cell. The surfactant used was n-decanol which at temperatures lower than 10°C forms a condensed monolayer. Thus, it is possible to obtain common thin as well as black foam films. The results from these studies can be seen in Section 3.4.3.3. 

Hence, a new method is developed for formation of NBF by adsorption of a surfactant from a gas phase. It offers new options for the production of such films from insoluble substances with biphilic structure. 

As already stated, the limiting form of the governing mass transfer problem for this limit of insoluble surfactant is (7-270). Thus, in this case, we do not need to consider either the bulk transport or surfactant adsorption-desorption processes and the problem is greatly simplified. The governing equation (7-270) requires that either us or T be zero at every point on the drop interface. To verify this fact, we may note that the surfactant interface concentration is axisymmetric so that the solution of (7-270) reduces to the form... 

Fig. 11-5. Displacement of the barrier due to the action of two-dimensional pressure of the adsorption layers of insoluble surfactants...

Such a technique, proposed by Pockels and extensively developed by Langmuir [6] (see Chapter II, 2), allows one to study the surface tension -adsorption dependence of insoluble surfactants. 

The studies of adsorption layers at the water/alkane interface give excess to the distribution coefficient of a surfactant, which is a parameter of particular relevance for many applications. Theoretical models and experimental measurements of surfactant adsorption kinetics at and transfer across the water/oil interface will be presented. The chapter will be concluded by investigations on mixed surfactant systems comprising experiments on competitive adsorption of two surfactants as well as penetration processes of a soluble surfactant into the monolayer of a second insoluble compound. In particular these penetration kinetics experiment can be used to visualise separation processes of the components in an interfacial layer. 

As mentioned in the Sec. 1, an important thermo-dynamic parameter of a surfactant adsorption monolayer is its Gibbs (surface) elasticity. The physical concept of surface elasticity is the most transparent for monolayers of insoluble surfactants, for which it was initially introduced by Gibbs (18, 19). The increments A a and AT in the definition of Gibbs elasticity ... 

Surfactants can react with the dissolved ionic species in the bulk to form insoluble complexes. It has been proposed that adsorption of the surfactant can take place through similar reactions with ions on the surface, or by surface precipitation, under conditions where no bulk precipitation takes place, but where the interfacial concentration is high enough to exceed the solubility product in the interfacial region (8). 

Gibbs and Insoluble Monolayers The adsorption of surfactant molecules at the surface of a liquid can be so strong that a monomolecular film (Gibbs monolayer) of unidirectionally ordered surfactants is formed (Fig. 5). Since the decrease in surface tension is directly related to the surface excess adsorption of the surfactant by the Gibbs adsorption equation (Eq. 6), the formation of the Gibbs mono-layer can be monitored by decrease of the surface tension. The maximum number of molecules filling a given area depends upon the area occupied by each molecule. 

The potential-induced adsorption and desorption of insoluble surfactants was investigated further using 12-AS, an insoluble surfactant-dye molecule adsorbed onto the... 

Flotation processes are an important part of water treatment technologies in modern water treatment plants. Flotation is based on the principle of adhesion of insoluble particles to air bubbles and adsorption of dissolved surfactants at the surface of air bubbles. Flotation allows for different kinds of admixtures to be removed from water bulk in a physical and chemical manner. In this way, suspended and colloidal particles, emulsions of oils and fats, the separate surfactant molecules and their micelles, complexes of surfactants with colloid rust, and multivalent ions of heavy metals can be removed. At present, the flotation processes and equipment for their realization are widely described in the literature [12]. Flotation involves the injection of small bubbles of air or other gas into the water bulk. Surface-active impurities are adsorbed at the bubble surface and transferred through the water bulk to its surface. As a result, the foam concentrate is formed on the surface of bubbling water. It contains surfactants, suspended solid particles (water impurities), emulsified substances, bacterial cells, etc. This foam is evacuated from the surface by means of special scrapers and other devices. 

Different mechanisms have been suggested for the adsorption of anionic surfactants on negatively charged surfaces [21]. Multivalent cations in solution such as calcium ions can improve the adsorption of anionic surfactants when the cations adsorb on the surface and do not form insoluble metal soaps with the surfactants. Sulfonate and phosphate group containing surfactants have proven to be particularly suitable [22]. 

The examples in the preceding section, of the flotation of lead and copper ores by xanthates, was one in which chemical forces predominated in the adsorption of the collector. Flotation processes have been applied to a number of other minerals that are either ionic in type, such as potassium chloride, or are insoluble oxides such as quartz and iron oxide, or ink pigments [needed to be removed in waste paper processing [92]]. In the case of quartz, surfactants such as alkyl amines are used, and the situation is complicated by micelle formation (see next section), which can also occur in the adsorbed layer [93, 94]. 

Depending upon the physical properties of a surfactant (component), removal from the mixed liquor is further possible through precipitation of insoluble salts and adsorption onto solids or bacterial floes, which, in turn, are subsequently withdrawn with the excess sludge [53]. In particular, intact or partly degraded low water-soluble surfactants are eliminated by this route. 

In the case of adsorption from solution, the surfactant layers are in equilibrium with the solution and will de-sorb on dilution. However, it would be very useful to produce adsorbed layers in both air and water, which will remain adsorbed. This can be achieved using the Langmuir-Blodgett deposition technique. The technique is based on the observation that if a surfactant, which is insoluble in water, is dissolved in a volatile, non-aqueous solvent and then spread on water, an insoluble monolayer of orientated surfactant molecules will remain at the air/solution interface. The effect of the spreading surfactant and its surface film pressure can be dramatically demonstrated by spreading hydrophobic talc powder on a clean water surface and then placing a... 

Effects of Surfactants on Solutions. A surfactant changes the properties of a solvent in which it is dissolved to a much greater extent than is expected from its concentration effects. This marked effect is the result of adsorption at the solution s interfaces, orientation of the adsorbed surfactant ions or molecules, micelle formation in the bulk of the solution, and orientation of the surfactant ions or molecules in the micelles, which are caused by the amphipathic structure of a surfactant molecule. The magnitude of these effects depends to a large extent on the solubility balance of the molecule. An efficient surfactant is usually relatively insoluble as individual ions or molecules in the bulk of a solution, eg, 10-2 to 10-4 mol/L. 

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