Surface hydroxide groups

Charging Hydroxylated Surfaces Definition of IEP(s). A hydroxyl-ated surface should be expected on all oxidic materials which have had a chance to come to equilibrium with an aqueous environment. Charge can develop on a hydroxylated surface through amphoteric dissociation of the surface hydroxide groups. Dissociation reactions can be written as follows, where underscored symbols refer to species forming part of the surface. Symbols not underscored refer to species assumed aqueous unless otherwise specified. 

Predicting IEP(s). To correlate and predict IEP(s) on the basis of solid composition, a useful simplification can be made by taking advantage of the close analogy between dissociation reactions of surface hydroxide groups and of mononuclear hydroxyl complexes. For example,... 

Specific electrolyte adsorption can occur on oxides by ion exchange with structural cations, with hydrogen or hydroxyl of the surface hydroxide groups, or with impurities (92, 94). Ions which can form insoluble compounds or undissociated complexes with a component of the solid crystal lattice adsorb more strongly than those which cannot (2). This does not imply or require that such complexes or compounds do or do not form. The question may be left open. It does imply that, of a series of species which form insoluble compounds with components of the solid, that which forms the least-soluble compound will be adsorbed most strongly. Thus any generalization which can be used to predict solubility or complexing tendency can be extended to predict adsorba-bility, at least qualitatively. 

It may be assumed that hydration of an oxide surface results in surface hydroxide groups bound to metal ions (MOH), and that these groups have amphoteric character [4,20.22,23]. The following equilibria are assumed surface protonation (reaction p)... 

The adsorption mechanism of H" and OH has long been attributed to the amphoteric reaction of the surface hydroxide groups -M-OH [4]. This is schematically represented as... 

An alternative derivation of Eq. (16), based on the amphoteric reactions of the surface hydroxide groups, given as Eq. (1), has been proposed by Levine and Smith [16]. Using the random mixing approximation for the distribution of the three types of surface sites [i.e., neutral, positive, and negative sites, as shown in Eq. (1)], these authors were able to derive an expression for the dependence of cm on the oxide surface. The derivation has also been outlined by Smith [2], and the modified Nemst equation can be written as... 

Oxides, especially those of Si, Al and Fe, are abundant components of the earth s crust. Hence a large fraction of the solid phases in natural waters, sediments and soils contain such oxides or hydroxides. In the presence of water the surface of these oxides are generally covered with surface hydroxyl groups (Fig. 2.1). 

These functional groups contain the same donor atoms as found in functional groups of soluble ligands i.e. the surface hydroxyl group on a hydrous oxide has similar donor properties as the corresponding counterparts in dissolved solutes, such as hydroxides, carboxylates, e.g., (S-OH is a surface group)... 

The Stober method can be used to form core-shell silica nanoparticles when a presynthesized core is suspended in a water-alcohol mixture. The core can be a silica nanoparticle or other types of nanomaterials [46, 47]. If the core is a silica nanoparticle, before adding silicon alkoxide precursors, the hydroxysilicates hydrolyzed from precursors condense by the hydroxide groups on the surface of the silica cores to form additional layers. If the core is a colloid, surface modification of the core might be necessary. For example, a gold colloid core was modified by poly (vinylpyrrolidone) prior to a silica layer coating [46]. 

Hydroxide and carbonate typically form insoluble precipitates with polyvalent cations in natural waters. The activity of both of these species increases with pH. The presence of surface functional groups that are capable of exchanging a proton creates pH dependent-charge, whereby the ionic character of the surface increases with pH [158,284,285]. 

Since the surface silanol groups react weakly acidic, neutralization with strong bases can be used for their direct determination. However, care must be taken that no dissolution of silica takes place. Greenberg (1ST) found that the adsorption of calcium hydroxide was roughly... 

Sears 189) and Heston et al. 190) used the adsorption of sodium hydroxide for the determination of the surface area of colloidal silica. An empirical factor was used for the conversion of alkali consumption into surface area. This is permissible provided the packing density of surface silanols is constant. The determination was performed in concentrated sodium chloride solution in order to keep down the dissolution of silica. Using the same technique, it was found in my laboratory that all surface silanol groups as determined by other methods are neutralized at pH 9.0. At higher pH, siloxane bonds in the surface were opened. A maximum in the sorption of Na+ ions occurred usually at pH 10.5-10.6 which corresponded to a packing density of ca. 5 OH/100 A. On further addition of alkali, silicate ions H3Si04 went into solution. 

Very few direct measurements of the reaction of surface silanol groups on quartz have been reported. This is apparently caused by the small effects due to the limited surface areas available. Adsorption of sodium ions on quartz was measured by radioactive tracer techniques by Gaudin et al. (293). Saturation was achieved at high pH (>10) and sodium ion concentrations above 0.07 Jlf. The calculated packing density of silanol groups was 4.25/100 A. Goates and Anderson (294) titrated quartz with aqueous sodium hydroxide and alcoholic sodium ethylate. The occurrence of two types of acidic groups was reported. 

The yields obtained after 10 min in a batch reactor with MgO, CaO, or SrO exceeded 92%, whereas with BaO the yield was lower (72%), probably because of its low surface area (2m /g). When alkaline earth hydroxides were used as basic catalysts, the yields were lower than for the corresponding oxides. The most active hydroxides were Sr(OH)2 8H2O and Ba(OH)2 8H2O, which gave the additional compound in yields of 75% and 70%, respectively, whereas carbonates were characterized by very poor activity. As observed for other reactions, the catalytic activity of MgO strongly depends on the pre-treatment temperature. A maximum in activity was observed when MgO was pre-treated at 673 K. At this temperature, decomposition of Mg(OH)2 to MgO is not complete, and Mg(OH)2 remains in the catalyst. It was suggested that the surface OH groups act as active sites, as for the Michael addition reactions described above. 

Finally, even if these criteria are satisfied, there remains the question of whether the product will adhere to form a film or just precipitate homogeneously in the solution. This is the most difficult criterion to answer a priori. The hydroxide and/or oxy groups present on many substrates in aqueous solutions are likely to be quite different in a nonaqueous solvent (depending on whether hydroxide groups are present or not). Another factor that could conceivably explain the general lack of film formation in many organic solvents is the lower Hamaker constant of water compared with many other liquids this means that the interaction between a particle in the solvent and a solid surface will be somewhat more in water than in most other liquids (see Chapter 1, van der Waals forces). From the author s own experience, although slow precipitation can be readily accomplished from nonaqueous solutions, film formation appears to be the exception rather than the rule. The few examples described in the literature are confined to carboxylic acid solvents (see later). 

Based on surface chemistry arguments, the double layer structure of metal oxide surfaces is effected by the solution pH (17,18), and hydroxide groups of the surface becomes less abundant with a decrease in solution pH. The process may be represented by the following equation (18) ... 

The conversion of the surface layers of many oxides into surface hydroxide layers is the result of the chemisorption of water on these oxides. On heating, H20 desorbs and the OH groups are converted into 0 ions again. 

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