Dodecylsulfonate on alumina

The study of adsorption kinetics of a surfactant on the mineral surface can help to clarify the adsorption mechanism in a number of cases. In the literature we found few communications of this kind though the adsorption kinetics has an important role in flotation. Somasundaran et al.133,134 found that the adsorption of Na dodecylsulfonate on alumina and of K oleate on hematite at pH 8.0 is relatively fast (the adsorption equilibrium is reached within a few minutes) as expected for physical adsorption of minerals with PDI H+ and OH". However, the system K oleate-hematite exhibits a markedly different type of kinetics at pH 4.8 where the equilibrium is not reached even after several hours of adsorption. Similarly, the effect of temperature on adsorption density varies. The adsorption density of K oleate at pH 8 and 25 °C is greater than at 75 °C whereas the opposite is true at pH 4.8. Evidently the adsorption of oleic acid on hematite involves a mechanism that is different from that of oleate or acid soaps. 

FIGURE 16.34 Adsoiption of dodecylsulfonate on alumina as a function of pH. (After Somasundaran and Fuerstenau courtesy of the American Chemical Society.)... 

The adsorption isotherm obtained for dodecylsulfonate (CDS) on alumina is given in Figure 2. This isotherm is similar to that obtained in the past for sulfonate/alumina systems (4). This isotherm behaves in an s-shaped manner (6) revealing its four characteristic regions of adsorption 1) Electrostatic interaction 2) Lateral association (hemimicellization), 3) Electrostatic hindrance and 4) micellization. 

Figure 17 Adsorption of sodium dodecylsulfonate from aqueous NaCl solution on alumina as a function of concentration (ionic strength = 0.002 M, pH = 7.2). (A) Slow rise in adsorption by simple exchange with surface ions (B) Enhanced cooperative adsorption of molecules at relatively high surface coverage (C) Concentration domain of unfavorable repulsive effects. (From Ref. 57.)...

Note these data also indicated a preference for DS over the competing chloride concentration of between 30 and 60. This implies the dodecylsulfonate was accumulated in the diffuse double layer surrounding the alumina relative to its bulk solution concentration more than an order of magnitude more preferentially than the inorganic chloride adsorbate. As a consequence, it is perhaps not surprising that Fuerstenau and Wakamatsu (1975) observed the accumulation of hemimicelles on the alumina at only about 400 /jM (pH 7.2) and about 7 fjM (pH 5.2) bulk DS-concentrations (see discussion of hemimicelles). 

In mineral-reagent systems, surface precipitation has been proposed as another mechanism for chemisorption. The solubility product for precipitation and the activities of the species at the solid-liquid interface determine the surface precipitation process. Under appropriate electrochemical conditions, the activity of certain species can be higher in the interfacial region than that in the bulk solution and such a redistribution can lead to many reactions. For example, the sharp increase in adsorption of the calcium species on silica around pH 11 has been shown to be due to surface precipitation (Somasundaran and Anan-thapadmanabhan, 1985 Xiao, 1990). Similar correlations have been obtained for cobalt-silica, alumina-dodecylsulfonate, calcite/apatite/dolomite-fatty acid, francolite-oleate and tenorite-salicylaldoxime systems. The chemical state of the surfactant in the solution can also affect adsorption (Somasundaran and Ananthapadmanabhan, 1985). 

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