Carbon black, surfactant adsorption

Adsorption. Many studies have been made of the adsorption of soaps and synthetic surfactants on fibers in an attempt to relate detergency behavior to adsorption effects. Relatively fewer studies have been made of the adsorption of surfactants by soils (57). Plots of the adsorption of sodium soaps by a series of carbon blacks and charcoals show that the fatty acid and the alkaU are adsorbed independently, within limits, although the presence of excess aLkaU reduces the sorption of total fatty acids (58). No straightforward relationship was noted between detergency and adsorption. 

Figure 3.4 Surfactant adsorption on carbon black, T = 298 K, surface area = 1150m2g 1 (BET) (reproduced with permission [5]).

Of course, surface charge development has been well documented in the carbon black literature [36, where particle coagulation is the key issue. Thus, for example. Kratohvil and Matijevic [56] measured electrophoretic mobilities and concluded (without alluding to the work of Frampton and Gortner) that [c]arbon particles carry a positive charge at lower pH values." They also studied the pH dependence of the adsorption of a nonionic and an anionic surfactant as well as of electrolytes with counterions of varying charges. 

Several examples may be quoted from the literature to illustrate the adsorption of surfactant ions onto soHd surfaces. For a model hydrophobic surface, carbon black has been chosen [22, 23], and Figure 5.5 shows the typical results for the adsorption of sodium dodecyl sulphate (SDS) onto two carbon black surfaces, namely Spheron 6 (untreated) and Graphon (graphitised), which also describes the effect of surface treatment. 

The adsorption of ionic surfactants onto hydrophobic polar surfaces resembles that for carbon black [24,25]. For example, Saleeb and Kitchener [24] found a similar limiting area for cetyltrimethyl ammonium bromide on Graphon and polystyrene ( 0.4nm ). As with carbon black, the area per molecule depended on the nature and amount of the added electrolyte. This can be accounted for in terms of the reduction in head group repulsion and/or counterion binging. 

The adsorption of surfactants in fuel oil onto pulverized coal has been studied in connection with the development of coal-oil mixtures (COM), i.e., stable dispersions of finely pulverized coal in fuel oil. The stabilization of such dispersions by a cationic surfactant has been shown (Kosman, 1982) to involve adsorption of the cationic via its postively charged head group onto nucleophilic sites on the coal, with its hydrocarbon group oriented toward the oil phase. The adsorption of alkylaromatics on carbon black from w-heptane indicates adsorption in an orientation parallel to the interface, with the alkyl chains remaining mobile on the surface (van der Waarden, 1951). Increased length of the alkyl chains increases the degree of dispersion of the carbon. 

Adsorption layers of the same kind as at fluid interfaces are also formed at low-energy solid -water surfaces, as it was established on PE, polystyrene, paraffin, carbon black, and other related materials. The classical Langmuir or Frumkin adsorption isotherm is often applicable to describe this behaviour. Studies on surfactant adsorption at various solid surfaces have been summarised in a great number of reviews [2, 7, 8, 54, 98, 101, 111, 121, 126, 141, 144, 145, 177, 186, 190, 194-198]. The adsorption at the solid/liquid interfaces is governed by a number of factors ... 

In view of such applications, the adsorption of a grafted (rake-type) polymeric siloxane surfactant containing 48% PDMS, 39% PEO, and 13% PPO on carbon black particles dispersed in mixtures of water with polar organic solvents has been investigated [58]. The adsorption was foimd to obey the Langmuir isotherm below the critical micelle concentration and a sharp increase in the adsorbed amount was observed at higher surfactant concentrations. DLS and SANS data indicate that the structure of the adsorbed layer is similar to that of micelles. 

Figure 3.13. Surfactant adsorption on to carbon black T = 298 K surface area = 1150 g (BET) (11)...

Fig. 1.9 shows typical Langmuirian curves for the adsorption of a series of polyoxyethylated non-ionic surfactants on to graphitized carbon black... 

Addition of neutral electrolyte causes an increase in the adsorption of an ionic surfactant to a similarly charged surface, owing to a reduction in the repulsive electrostatic interaction, and a decrease in adsorption when the surface is of opposite charge. Because of the decrease in CMC of the surfactant with salt addition, the concentration at which the adsorption attains a maximum also decreases. This displacement of the isotherms to lower concentrations is noted for example with the adsorption of dodecylammonium chloride on to aluminium oxide [71] and sodium alkyl sulphates on to carbon black [72]. 

The adsorption of surfactants onto a clean nonpolar surface must occur with the hydrophilic group oriented outward into the aqueous phase. Adsorption, therefore, will always result in an increase in the hydrophilic character of the surface. Such action is responsible for the generally increased dispersibility of materials such as carbon black in aqueous surfactant systems, and the stability of aqueous latex polymers in paints. The action of surfactant adsorption onto colloidal surfaces can be useful to destabilize as well as stabilize systems. It may be useful, for example, to break an aqueous dispersion, to isolate the dispersed material, or to facilitate the process of separating dispersed solids in the sewage treatment process, although polymers and polyvalent cation salts are most commonly employed in such... 

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