Surfactants adsorption density

Although the real origin of the hydrophobic force stiU remains a mystery, its effect on dispersion stabihty has long been recognized. Somasundaran et al. reported a clear relationship between the stabihty and the surfactant adsorption density as well as the chain length of the adsorbed surfactants. 

Water-.soluble alcohols present a slightly po.sitive function, i.e.. their addition increases the hydrophilidiy of the surfaciantfalcoho amphiphile. but very liiilc indeed. As a consequence isopropanol effect in the correlation is negative, i.e, the same as decreasing salinity, 5CC-Butanol and /en-pentanol have practically no tormulation effect since their functions are essentially nil whatever the concentration. These arc the alcohols that are the more interface-seeking ones, and their miiin effect is to dilute the surfactant adsorption density without changing the formulation. 

Even though the flotation process involves three phases and three interfoces, most research work has been solely on the behavior of foe solid-liquid interface. This is in S Mte of the fact that adsoiptioa at the solid-liquid interface, as shown in Fig. 16.3-10, is of a considerably smaller magnitude than that at the solid-gas or liquid-gas interface. It is to be notisd that exceHent correlation has been obtained recoitly between surfactant adsoiption at the liquid-gas interface and flotation for the hematite-rdeate system (see Fig. 16.3-11). It is also important to note that the migration of the surfactam at the liquid-gas inteifoce is fastn- than its diffusion from bulk to the interface, as least for this system. Such a migration at the inteifoce can hdp toward faster attainment of requited surfactant adsorption density at the solid-gas interface upon the contact of the bubble with the particle. 

For charged surfactants, the surface electrical potential is an important component of the surfactant chemical potential in the adsorbed state. Thus, the calculation of surfactant adsorption density or aggregate shape must include a calculation of the surface potential. This is a difficult problem because the binding of the surfactant, its counterions, and other salt molecules all affect the surface charge, and interactions among these adsorbed species also affect the binding constants. Zhu and Gu used their two-state adsorption model to examine adsorption of cationic surfactants to silica [8]. This method is reasonably successful if the surface potential is either fitted to the data or measmed experimentally. 

At equilibrium surfactant concentrations of less than 0.0003 M SDS where the hematite surface is still positively charged, adsorption of surfactant follows its normal pattern due to the electrostatic forces which provide the driving force for adsorption. Sufficient effective surface area must be available for this level of SDS adsorption density. As surfactant adsorption... 

Fig. 48 Surfactant aggregation numbers determined at various adsorption densities (average number at each adsorption density shown along the adsorption isotherm)...

At very low surfactant concentrations in solution, the adsorption density at the air—water interface is so low that individual adsorbed surfactants do not interact significantly with each other. At a certain... 

The flotation of minerals is based on different attachment forces of hydrophobized and hydrophilic mineral particles to a gas bubble. Hydrophobized mineral particles adher to gas bubbles and are carried to the surface of the mineral dispersion where they form a froth layer. A mineral is hydrophobized by the adsorption of a suitable surfactant on the surface of the mineral component to be flotated. The hydrophobicity of a mineral particle depends on the degree of occupation of its surface by surfactant molecules and their polar-apolar orientation in the adsorption layer. In a number of papers the relationship was analyzed between the adsorption density of the surfactant at the mineral-water interface and the flotability. However, most interpretations of adsorption and flotation measurements concern surfactant concentrations under their CMC. 

The adsorption density of surfactants was determined by calculating the difference in the surfactant concentration before and after the adsorption of a mineral, as described earlier (5). 

The adsorption densities ( r ) on minerals (C< CMC) of the salt type are in some cases higher because of precipitation of the ionic surfactant with multivalent cations in the bulk phase. Measurements were carried out to determine the fraction f the precipitated surfactant by divalent cations Ca and Ba " leading to a decrease in its equilibirum concentration. They showed a shift of the adsorption maximum towards lower values of r, even after a correction of the adsorption density due to the precipitation. On the other hand, a direct co-adsorption of the precipitated surfactant on a mineral surface cannot be excluded. 

C. ,) and their mixtures above the CMC. Tne d dtrease in flotabi1ity begins at a concentration corresponding to the adsorption density e s i, and reaches a minimum at the surfactant concentration 5 CMC (Fig. 6). Similarly as in Figs. 3 and 4, only arrows for e s 1 and CMC, resp., were used. The minimum flotability of barite in the mixtures of pure surfactants C.q + nd C.q +... 

Recently, surfactant adsorption and y have been measured at C02-water and C02-organic interfaces with a tandem variable-volume tensiometer (Harrison, 1996). A pendant drop of an aqueous or organic phase, saturated with C02, may be suspended in C02 or a C02-surfactant mixture and equilibrated. From the digitized droplet shape and density difference between the phases, y may be calculated from the Laplace equation. In Figure 8.1, y of the binary C02-water (da Rocha et al., 1999), -polyethylene... 

Several authors98-106) describe the relation between the adsorption density of different surfactants and the electrical nature of the mineral surface using the potential data. 

Fuerstenau") was the first who used the Stern-Grahame model of EDL to describe the adsorption of long-chain surfactants for the equilibrium in heterogeneous systems. The adsorption density in the Stem plane is given by the equation... 

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. 

The value of Qst can be determined from the slope of the plot (log c)r against T. Measurements in the system N-dodecylammonium acetate-quartz showed that -AG increased with increasing temperature over the whole concentration range whereas the adsorption density against temperature showed a minimum over the greatest part of the concentration range. In the range of equilibrium surfactant concentrations of 1 - 9 x 10"4 mol 1" the values of - AG 12.55 to 17.58 kJ/mol were found for temperatures between 5 and 45 °C. 

System 1. Under ideal conditions the adsorption of a surfactant into the EDL proceeds as described in Chap. 3. The border of efficiency of anionic and cationic surfactants is IP or PZC, as follows from the correlation of e.g. adsorption density, potential and notability are dependent on pH [e.g. 44,129,167,174-176]. The course of such an adsorption is shown in Fig. 15. If H+ or OH" react with the surface of one mineral the released ions or their hydrolytic products can adsorb on unequally charged surface of the other mineral and cause an activated adsorption of the surfactant, or they can inhibit the adsorption, as shown on the schemes ... 

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