Surfactant adsorption measurements, mineral

Table VIIL Mineral Composition (Weight%) of Rocks Used in Surfactant Adsorption Measurements...

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 isotherms of various surfactants were measured on minerals with a different character of Ppis. The course of the isotherms on minerals, with H and OH on one hand and those with latice ions as PDIs on the other hand, is similar, with a maximum in a region close to the CMC. Some characteristic adsorp-... 

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. 

Anionic surfactants such as the sulfonates used in this study typically have lower adsorption than nonionic surfactants on silica minerals assuming neutral to high pH. This is because the negative sulfonate head of the surfactant is repelled by the net negative charge of silica and other typical minerals that make up alluvium aquifers at typical values of groundwater pH. Surfactant sorption experiments can be conducted in either batch experiments or soil column experiments. When the surfactant adsorption is very low and the surfactant concentration high as in our experiments, batch surfactant adsorption is very difficult to measure... 

Polar solvents may interact strongly with a mineral oxide surface. In principle, the adsorption of die solvent must be considered. Claesson [13] studied the adsorption of fatty acids by sihca from solvents of various polarities. The results show that polar solvents compete with the solute for available sites on the surface, while nonpolar solvents show little competition. The polarity of the solvent is often determined from the measured dielectric properties. Krishnakumar and Somasundaran [13] studied surfactant adsorption on to silica and alumina from solvents with various dielectric properties. The aim of the study was to look at the effect of adsorbent and smfactant acidities and solvent polarity on the adsorption properties of the surfactant molecules. They used anionic and cationic surfactants as adsorption probes. The results show that polar interactions control the adsorption from solvents of low dielectric properties while hydrocarbon chain interactions with the surface play an important role in determining adsorption from solvents of higher dielectric properties. It was also found that an acidic surfactant interacts strongly with a basic adsorbent, and vice versa. One should be aware that the polarity of a molecule as measmed from the dielechic properties is not always eorrelated with the ability of the molecules to form ion pairs. For example, dimethylformamide and nihomethane have almost equal dielechic constants. However, the extent of ion pairing in nihomethane is much greater than that in dimethylformamide. Thus, the solvent acidity and basicity are the physical properties which can best characterize the ability of the solvent to compete with the solute for available sites on the mineral surface. 

In reviewing a vast quantity of literature on the subject, it is possible to encounter some studies which are of little value because insufficient attention has been given to the control of important physical factors, to the potentiality of the experimental technique, or to the limitations of particular theoretical approaches. Therefore, Section 2 is exclusively devoted to description of the materials and experimental methods used in measuring the adsorption of ionic surfactant onto mineral substrates. Some experimental problems, encountered in the everyday laboratory practice, will be pointed out. Among different experimental methods, the adsorption calorimetry particularly deserves to be noted. 

Adsorption isotherms are habitually obtained using the solution depletion method, which consists of comparing the solute concentrations before and after the attainment of adsorption equilibrium. Electrokinetic or zeta potentials are determined by two techniques microelectrophoresis [12,14,17] and streaming potential [13,58,59]. The former is employed to measure the mobility of small particles of chemically pure adsorbents, whereas the latter is adopted to investigate the electrophoretic behaviour of less pure coarser mineral particles. A correlation between the adsorption and electrophoretic results is usually examined with the aim of sheding light on the mechanism by means of which the surfactants are adsorbed at the solution-solid interface. This implies the necessity of maintaining the same experimental conditions in both experiments. For this purpose, the same initial operational procedure is applied. 

Understanding of the structure of the adsorbed surfactant and polymer layers at a molecular level is helpful for improving various interfacial processes by manipulating the adsorbed layers for optimum configurational characteristics. Until recently, methods of surface characterization were limited to the measurement of macroscopic properties like adsorption density, zeta-potential and wettability. Such studies, while being helpful to provide an insight into the mechanisms, could not yield any direct information on the nanoscopic characteristics of the adsorbed species. Recently, a number of spectroscopic techniques such as fluorescence, electron spin resonance, infrared and Raman have been successfully applied to probe the microstructure of the adsorbed layers of surfactants and polymers at mineral-solution interfaces. 

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. 

Measurement of zeta potential ( ) is valuable in determining the properties of dispersions. In addition, it has many other applications in various fields Electrode kinetics, electro-dialysis, corrosion, adsorption of surfactants and polymers, crystal growth, mineral flotation and particle sedimentation. 

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