Hydrofluoric acid Hydroxyl

Hydrated amorphous silica dissolves more rapidly than does the anhydrous amorphous silica. The solubility in neutral dilute aqueous salt solutions is only slighdy less than in pure water. The presence of dissolved salts increases the rate of dissolution in neutral solution. Trace amounts of impurities, especially aluminum or iron (24,25), cause a decrease in solubility. Acid cleaning of impure silica to remove metal ions increases its solubility. The dissolution of amorphous silica is significantly accelerated by hydroxyl ion at high pH values and by hydrofluoric acid at low pH values (1). Dissolution follows first-order kinetic behavior and is dependent on the equilibria shown in equations 2 and 3. Below a pH value of 9, the solubility of amorphous silica is independent of pH. Above pH 9, the solubility of amorphous silica increases because of increased ionization of monosilicic acid. 

The surface chemistry of coesite and stishovite was studied by Stiiber (296). The packing density of hydroxyl groups was estimated from the water vapor adsorption. More adsorption sites per unit surface area were found with silica of higher density. Stishovite is especially interesting since it is not attacked by hydrofluoric acid. Coesite is dissolved slowly. The resistance of stishovite is ascribed to the fact that silicon already has a coordination number of six. Dissolution of silica to HaSiFg by hydrogen fluoride is a nucleophilic attack. It is not possible when the coordination sphere of silicon is filled completely. In contrast, stishovite dissolves with an appreciable rate in water buffered to pH 8.2. The surface chemistry of. stishovite should be similar to that of its analog, rutile. 

Actual construction of a biosensor probe tailored for the detection of G4 is a multistep process. First, the exposed fused silica core of the probe is cleaned with concentrated hydrofluoric acid (HE), then the 200 im core is tapered to 65 im by concentrated HE. Tapering requires about 40 minutes. 3-Mercapto-proply-trimethoxy-silane reacts covalently with the surface hydroxyls to produce immobilized thiol groups. Next, the malimide reactive group of the heterobifunctional crosslinker N-succinimidyl-4-maleimidobutyrate is reacted with the immobilized thiol to yield a tethered succinimide capable of reacting with the primary amines of a protein. ... 

The hydroxyl ion is the unique catalyst in alkaline solutions and hydrofluoric acid in acid solutions. The structure of amorphous silica is an even more open arrangement than that of cristobalite, to which it is closely related. On the surface there are spaces between oxygen ions sufficiently large to accommodate hydroxyl ions. Such a... 

Reviews have also been published on the following chemical aspects of carbohydrates thermal decomposition, DMSO-dependent oxidation of hydroxyl groups, and reactions in liquid hydrofluoric acid."... 

For example, a mild HF etch (1 100) is used in cleaning glass for flat panel display production. Hydrofluoric acid solution is a common etchant for sihcon and can leave a sihcon surface either hydrogen- or hydroxyl-terminated. The silicon surface is hydrophobic ( wata--hating ) if hydrogen-terminated, and hydrophilic ( water-loving ) if hydroxyl-terminated. 

---