Esters, silicate hydrolysis

Mono-, di- and triacylglycerols may all be measured by determination of the amount of glycerol released by hydrolysis. The lipid is first extracted into chloroform-methanol (2 1) and saponification is performed under conditions that will not affect any phosphate ester bonds, otherwise glycerol originating from phosphoglycerides would also be measured. Heating at 70°C for 30 min with alcoholic potassium hydroxide (0.5 mol l-1) has been shown to be satisfactory. However, the phospholipids may be removed prior to saponification either by extraction or by adsorption on activated silicic acid. 

Synthetic esters, (a) primarily esters of dibasic acids such as adipic and sebacic, though in Europe some monobasic acid esters have been prepared and used (h) organic esiers of phosphoric and silicic acid, which have some advantage of being more lire-resislant than Ihe other organic compounds bm which are subject to hydrolysis on exposure to water,... 

The tendency to hydrolyze by a relatively slow and controllable stepwise reaction is characteristic of all the silicon orthoesters. When ethyl silicate, for example, is poured into water it remains immiscible and hydrolyzes at the interface over a period of many days or even months. If both the ester and the water are dissolved in alcohol, however, a much faster reaction in the homogeneous medium becomes possible. If a drop of strong acid then is added to the solution, the hydrolysis is further accelerated to such an extent that the solution becomes warm and silica is precipitated as solid or gel. If less than the equivalent amount of water is used, there is formed a viscous nonvolatile liquid which may be stored for long periods. The addition of further water (as in mixing an ethyl silicate paint) then... 

The other silicon esters differ from ethyl silicate in their rates of hydrolysis, but all respond to catalysts and all give silica as end product ... 

Since the alcohol is regenerated and the net result is the same as though the original silicon halide had been hydrolyzed directly, it is obvious that the esters may be regarded as hydrolytic intermediates which allow the over-all process of hydrolysis to be controlled and directed. The commercial value of the esters therefore lies in their ability to deposit silica in a desired form and at a well-defined rate. Besides their use as stone preservatives, esters like ethyl silicate are used as sources of pure ion-free silica and as additives to organic paints and lacquers to provide a harder, more resistant film. A disperse sol of silica in n-propanol (possibly derived from propyl silicate) is available commercially for related applications. 

Besides hydrolysis, the silicic esters show many more of the reactions which have been described for the halides. For example, the alkoxy group reacts with the Grignard reagent to attach organic groups directly to the silicon atom in stepwise fashion ... 

CH3OH seems to be particularly difficult to purify. It has been shown by R. B. Porter, J. Phys. Chcm., 61, 1260 (1957), that CH3OH in pyrex or quartz will always contain small amounts of methyl borate and methyl silicate unless H2O is present to cause hydrolysis of those esters. Dry CH3OH in pyrex will thus contain II2O + (CIl30)8B in significant amounts. 

Compounds 3 and 18 - 20 have been prepared to combine luminescent semiconductors with curable units. The transformation of the monomeric compounds to siloxanes of higher molecular weight was performed by hydrolysis of the silicic esters in chloroform/ethanol solution, catalyzed by traces of hydrochloric acid for 2 days (Scheme 4). 

Silicic acid esters, Si(OR)4, are produced by the reaction of SiCl4 with the appropriate alcohols. The most important representative of this group is tetraethoxysilane (tetraethyl orthosilicate) Si(OC2H5)4, which is used directly, or after hydrolysis to ethylpolysilicates, as a binder for ceramic pastes, for inorganic zinc dust paints (corrosion protection), for the surface treatment of glass and for the modification of silicates. Silicic acid esters are further used for rendering polymer surfaces scratch-resistant. 

Extensive research on sol—gel processing of silicic acid esters, eg, tetraethoxysilane (TEOS), Si(OC2H5)4, in alcohol—water mixtures has ducidated silica polymerization in nonaqueous solvents (54,55). The relative rates of hydrolysis and condensation depend on the substrate, water, and catalyst (acid or base) concentrations these rates determine the polymer structure. For example, acid-catalyzed hydrolysis of TEOS at low water concentrations produces linear polymers. These solutions yield fibers upon spinning. Conversely, high H20 /TEOS ratios favor the formation of highly cross-linked polymers under... 

Graham (1861) is usually regarded as the founder of classical experimental colloid science. He classified all substances into two groups crystalloids and colloids. According to him the former could be easily crystallised, but not the latter. Colloids can be dissolved or dispersed and exposed to a semipermeable membrane the so called crystalloids pass through the membrane easily, but the colloids do not. This procedure is called dialysis. By 1864, silica were being prepared not only by the dialysis of gels but also by hydrolysis of silicate esters. 

In the case of the polyvinyl esters the alkaline hydrolysis caused by the mixing with the sodium silicate will tend to form in the already formed uniform film, polyvinyl alcohol, perhaps an even better adhesive than the ester itself. 

In 1853, a French researcher named Fremy [7] prepared a dilute colloidal silica from silicon sulfide. By 1864, silica colloids were being prepared by the dialysis of gels and by the hydrolysis of silicate esters [8]. All products were very dilute. 

In practice the ester is mixed with the required amount of water and a suitable solvent (e.g., alcohols, esters, ketones). To obtain 100 Vo hydrolysis of 1 mol of an orthosilicate, 2 mol of water must be added. By maintaining a constant pH (for ethyl silicate ca. 2) the condensation reaction can be suppressed to such an extent that hydrolyzates of ethyl silicate containing 20% SiOj and with a degree of hydrolysis of 80-90% remain stable for about one year. If this equilibrium is disturbed (e.g., by shifting the pH value or evaporating the solvent), condensation commences and the solution solidifies after a short time. This mechanism is used in two-pack zinc dust paints. In the simplest case the binder is a hydrolyzate in alcoholic solution. Zinc dust and, if necessary, extenders, antisettling agents, colorants, etc. are then added. Addition of zinc consumes acid and the pH value increases. 

On application, the paint layer hardens within a short time due to evaporation of the solvent, progressive hydrolysis of ester groups after uptake of atmospheric moisture, reaction of silanol groups with the zinc and substrate, and condensation of silanol groups to form sparingly soluble, cross-linked silica gel [2.186]. Conditions are generally chosen such that the film is hard enough to allow transportation after 4 h and a topcoat after 24 h. Alkyl silicate zinc dust paints are particularly suitable... 

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