Silica water interface, adsorption

The adsorption of Co(II) at the silica-water interface has been studied as a function of pH, ionic strength, and total Co(II) concentration. The adsorption data, together with electrophoretic mobility and coagulation data suggest that the free aquo Co(II) ion is not specifically adsorbed without participation of surface hydroxyls. Evidence for polymeric Co(OH)2 at the quartz surface is presented together with evidence of mutual coagulation of the quartz and precipitated cobalt hydroxide. 

The adsorption of Co (II) at the silica-water interface can be separated into three parts ... 

Esumi, K. Goino, M. Adsorption of poly(amidoamine) dendrimers on alumina/water and silica/water interfaces. Langmuir 1998,14, 4466-4470. 

The nature of the silica-water interface is determined by adsorption/desorption of the species in the water. When a silicon oxide, e.g., quartz, is fractured, the initial surface is composed of dangling silicon and oxygen bonds (Fig. 4.30a) which are not stable and hydroxylate easily with available waterThe hydroxylated surface is dominated by SiOH groups (Fig. 4.30b). The initial adsorbed water adjacent to the surface is oriented and has properties different from the bulk water. As this adsorbed water layer increases to more than three monolayers, its properties become more like bulk water. The surface potential changes as a result of the adsorption of the ionic species in the water. °... 

The coagulation-dispersion behavior of aqueous silica sols is central to almost all processes requiring their unique adsorption, dispersion, gelation, and sol-gel properties. Aqueous silica sols are of particular interest in colloid science because their coagulation-dispersion behavior is said to be anomalous , that is, their stability in terms of electrolyte-pH control does not follow the pattern followed by almost all other oxide and latex colloidal materials. This chapter examines aqueous silica sol coagulation effects in light of studies of macroscopic silica-water interfaces and in particular the electrical double layer at such interfaces. 

An explanation of the anomalous stability of Iler s silica sols in terms of steric stabilization effects requires that oligomeric or polymeric silicate species are present at the silica-water interface and that steric repulsion results during overlap of such layers. This mechanism is appealing in that soluble silicates, usually sodium silicates, are universal dispersants of many electrostatic colloids. Again, well-hydrated silicas [2] and other colloids exposed to aqueous silicate [18] acquire high adsorption densities of aqueous silica. 

Another relevant system involves oleic acid (OA) adsorption at the silica-water interface. This method was first demonstrated by Ding et al. [45] and was next used by Mahdavian and coworkers to encapsulate very small silica nanoparticles [46]. In the latter case, a core-shell structure with a core composed of aggregated silica particles and a shell made of MMA, styrene and acrylic acid (AA), was formed. The authors suggest that the polymerization proceeds through oligoradical entry into the OA admicelles. 

Malin, J. N., J. G. Holland, S. A. Saslow, and F. M. Geiger. 2011. U(VI) adsorption and spe-ciation at the acidic silica/water interface studied by resonant and nonresonant second harmonic generation. The Journal of Physical Chemistry C 115, no. 27 13353-13360. doi 10.1021/jp203091x. 

At a relatively early stage in the polymerization it is possible to characterize the polymeric silica, or silica particles in terms of the specific area of the silica-water interface. This is done by measuring the adsorption of hydroxyl ions in the pH range 4.00-9.00 (Beckman Type E electrode) in a nearly saturated salt solution which permits the surface charge denstly to approach a maximum. This method was developed by Sears (85) to determine the specific surface areas of colloidal particles and gels. Then it was found that if carried out rapidly it could give reproducible... 

Titratio 4 Method. Adsorption of base on the surface of sol particles provides a rapid estimate of the area of the silica-water interface. The sol rather than a dried powder is used in the Sears (182) method of titrating the silica surface with alkali in strong salt solution between pH 4.0 and 9.0. As discussed in Chapter 3, it provides the simplest way to follow the change in the specific surface area and thus particle size when the silica particle are smaller than 5-10 nm. However, it is equally useful for larger particles, up to a micron or more in diameter, provided it can be shown that the particles are not microporous by comparing results with the area determined by nitrogen or water vapor adsorption, or from electron micrographs. 

Pagac, E. S., Prieve, D. C. and Tilton, R. D., Kinetics and mechanism of cationic surfactant adsorption and coadsorption with cationic polyelectrolyte at the silica-water interface, Langmuir, 14, 2333-2342 (1998). 

As one example, the force between a hydrophilic silica particle and an air bubble at different concentrations of dode-cyltrimethylammonium bromide (DTAB) is shown in Fig. 10. Without surfactant, the particle is repelled by the air bubble. At distances above 5 nm, the electrostatic repulsion dominates. The reason being the negative surface charges on the silica surface and at the water-air interface [187-190]. Even at close distance, a stable water remains on the particle surface and no three-phase contact is formed. Adding even small amounts of the cationic surfactant DTAB changes the interaction drastically. At concentrations between 0.1 mM and typically 5 mM DTAB (critical micellar concentration is >= 16 mM), no repulsion was observed. When the particle comes into contact with the air-water interface, it jumps into the bubble and a three-phase contact is formed. Such a behavior can be explained with the strong adsorption of long-chain alkyltrimethy-lammonium ions to silica [191]. At a concentration of 0.1 mM, DTA+ forms a monolayer on the silica surface. This... 

Arnebrant, T. and Ericsson, B., Adsorption of arginine vasopressin and desamino-8-D-arginine vasopressin on to silica and methylated silica surfaces, and at the air/water interface, J. Colloid Interface ScL, 150, 428-435 (1992). 

More recently, an interesting technique has been used by Clark and Ducker [70] to measure kinetics, in the form of attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. They found that total surface exchange of a cationic ammonium bromide surfactant on a silica surface occurred in slightly less than 10 s (fig. 19.6). This technique had been used previously by Couzis and Gulari [71, 72] to look at the adsorption kinetics of anionic surfactants at the alumina-water interface with apparent timescales in the region of tens of hours. 

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