Solid-liquid interface alumina

Some surfactants aggregate at the solid-liquid interface to form micelle-like structures, which are popularly known as hemimicelles or in general solloids (surface colloids) [23-26]. There is evidence in favor of the formation of these two-dimensional surfactant aggregates of ionic surfactants at the alumina-water surface and that of nonionic surfactants at the silica-water interface [23-26]. 

Some essential discoveries concerning the organization of the adsorbed layer derive from the various spectroscopic measurements [38-46]. Here considerable experimental evidence is consistent with the postulate that ionic surfactants form localized aggregates on the solid surface. Microscopic properties like polarity and viscosity as well as aggregation number of such adsorbate microstructures for different regions in the adsorption isotherm of the sodium dedecyl sulfate/water/alumina system were determined by fluorescence decay (FDS) and electron spin resonance (ESR) spectroscopic methods. Two types of molecular probes incorporated in the solid-liquid interface under in situ equilibrium conditions... 

Flocculation of alumina suspensions obtained by the sequential addition of polystyrene sulfonate (M j, = 4600) and cationic polyacrylamide (M, = 4,000,000) at pH 4.5 is compared in Figure 7.33 with that obtained using single polymers. While the anionic polystyrene sulfonate had only a minor effect, cationic polyacrylamide did not produce any flocculation. However, when used together, both polymers adsorb completely. This coadsorption is attributed to the interaction of complexes between cationic polyacrylamide and the polystyrene sulfonate at the solid-liquid interface. The mechanism of the superior flocculation obtained with the dual polymer system is illustrated schematically in Figure 7.34. The anionic polystyrene sulfonate adsorbs on alumina surface and acts as an anionic anchor for the adsorption of the long-chain cationic polymer, which ultimately provides interparticle bridging and excellent flocculation. 

These forces and hence the stability of the dispersions can be altered/controlled by the adsorption of ions, surfactants, or polymers at the solid-liquid interface. Adsorption of surfactants and polymers at the solid-liquid interface depends on the nature of the surfactant or polymer, the solvent, and the substrate. Ionic surfactants adsorbing on oppositely charged surfaces exhibit a typical four-region isotherm. Such adsorption can alter the dispersion stability mainly by changing the double layer interaction, which depends on the extent of adsorption. Thus, it is seen that alumina suspensions are destabilized by the adsorption of SDS when the zeta potential is reduced to zero. At higher concentrations, bilayered surfactant adsorption can occur with changes in wettability and flocculation of the particles by altering the hydrophobic interactions. 

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). 

Pyrene and dinaphthylpropane (DNP) fluorescence and nitroxide ESR probes have been successfully used to investigate the structure of the adsorbed layer of sodium dodecyl sulfate at the alumina-water interface [11,12], The fluorescence fine structure of pyrene yielded information on the polarity of the microenvironment in the adsorbed layer. Intramolecular excimer formation of DNP was used to measure the microfiuidity of this environment. The results indicate the presence of highly organized surfactant aggregates at the solid-liquid interface, formed by the association of hydrocarbon chains. 

Adsorption experiments conducted with PEO and alumina particles showed no adsorption of the former on the particles. Because it is known that SDS adsorbs on alumina and that PEO interacts with SDS, PEO adsorption tests were repeated with alumina pretreated with SDS. The authors observed that the presence of a surfactant on the alumina surface caused near-complete adsorption of the polymer as a consequence of its interaction with surfactant aggregates. This result showed that it was possible to force the adsorption of a polymer on a solid surface on which it spontaneously does not adsorb. Instead of drastically modifying the solid-surface properties to force the adsorption of PEO, it seems, by far, more convenient to form surfactant aggregates at the solid-liquid interface and then to allow the polymer to adsorb. 

Solubilization in polymer-surfactant aggregates adsorbed at the solid—liquid interface has been recently presented by Esumi et al. as a promising tool in wastewater treatment research [46]. Their study concerns the adsolubilization of 2-naphtol into PVP-anionic surfactants adsorbed at the alumina-water interface. Two surfactants were used SDS and Aerosol OT. 

By adding an element into the liquid metal, it is possible to decrease both (Ts and a. The adsorption of the element at the solid-liquid interface generally improves both the adhesion and the wetting. On the contrary, its adsorption at the free liquid surface always destroys the adhesion. It improves the wetting only for 0 angles lower than 90°. These features have been checked on Al-Sn alloy-alumina interfaces (Eustathopoulos and Drevet, 1994). 

Figure 9 Effect of sodium dodecyl sulfonate on the equilibrium and liquid advancing and receding contact angle on alumina. (Reprinted from 7. Coll. Int. ScL, 256, D. W. Fuerstenau, Equilibrium and Nonequilibrium Phenomena Associated with the Adsorption of Ionic Surfactants at Solid-Water Interfaces, 79 2002 with permission from Elsevier.)...

It is possible to compare battery systems from the state of the three main components the electrode A, the electrolyte, and the electrode B (Figure 11.1). All these media can be liquid, plastic, or solid. This is cmcial because certain interfaces are difficult to handle. Common batteries have a solid-liquid-solid interface the liquid-solid-liquid system corresponds to the Na-S battery using p-alumina as the electrolyte, which permits relatively easy manufacture. On the other side, the all-solid system involves difficult interface problems with cmcial dimensional stability at each interface. These difficulties can be solved by using a polymer film as a plastic electrolyte also, polyethylene oxide (PEO) film electrolytes are the key to a new battery design. 

The NaAlCl4 is a liquid electrolyte at the normal operating temperature of 270 °C (maximum of 350 °C) of the cell, interfacing both with the porous Ni/ NaCl electrode and the "-alumina. After assembly the cell is charged, liquid sodium (melting point 98°C) being formed. Excess NaCl in the liquid electrolyte ensures that Na ions are not removed from the solid electrolyte which would otherwise cause an increase in internal resistance. 

---