Silica/water interface, zeta potential

Experiment 6.1 Zeta potential measurements at the silica/water interface... 

The unusual nature of the silica-water system has been noted by J. A. Kitchener (7), who pointed out that the endless confusion in the literature concerning the silica-water interface has arisen because the hydration and solubility characteristics have not been understood. For example, there is the question as to why silica sols are extraordinarily stable at pH 2 where the zeta potential is zero and become increasingly sensitive to electrolytes at higher pH. where the potential is highest—in contradiction to the generally accepted electrical double layer theory. Another mystery is that crystalline quartz becomes coated with a film of amorphous silica even though the solution is undersaturated with soluble silica with respect to a surface of amorphous silica. 

Electroosmosis or electroendosmosis is the bulk movement of the solvent (electrolyte solution) in the capillary caused by the zeta (0 potential at the wall/water interface of the capillary. Any solid-liquid interface is surrounded by solvent and solute constituents that are oriented differently compared to the bulk solution. Figure 17.2 illustrates a model of the wall-solution interface of the widely applied capillaries. Owing to the nature of the surface functional groups, in silica capillaries the silanol groups, the solid surface has an excess of negative... 

For the electrostatic boundary condition, in this work we use the Basic Stem (BS) model developed by Behrens and Grier [9] in which the silica surfaces acquire charges in contact with water by the dissociation of silanol groups, SiOH SiO + H, so that the zeta potential (0 on the interface can be expressed as a function of the surface charge density (cr) ... 

It is well known that hydrophobic particles adhere to each other in water once they have established contact, so it is not surprising that silica particles rendered hydrophobic even only on certain limited areas on the particles are flocculated. Thus at low concentrations, ions such as dodecylammonium affect the zeta potential of quartz particles in the same manner as sodium, but with higher concentrations there is a critical point where the potential changes abruptly and the long-chain ammonium ion apparently associates into patches of ions on the interface, in much the same way as micelles are formed in bulk solution (294). ... 

It seems probable that the location of the particles at the oil-water interface is the key to their role. Kulkarni et al. [227] have emphasized the importance of long-range electrostatic repulsion forces between antifoam entities and bubbles in contributing to antifoam behavior with hydrophobed silica-PDMS antifoams. These authors present some evidence of diminished antifoam effectiveness accompanying increasing zeta potentials in sodium lauryl sulfate solution with increasing concentration. This work then suggests that the role of particles at the oil-water surface will concern the (often electrostatic) forces between bubbles and antifoam entities. 

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