Monosilicic acid polymerization

In the absence of a suitable soHd phase for deposition and in supersaturated solutions of pH values from 7 to 10, monosilicic acid polymerizes to form discrete particles. Electrostatic repulsion of the particles prevents aggregation if the concentration of electrolyte is below ca 0.2 N. The particle size that can be attained is dependent on the temperature. Particle size increases significantly with increasing temperature. For example, particles of 4—8 nm in diameter are obtained at 50—100°C, whereas particles of up to 150 nm in diameter are formed at 350°C in an autoclave. However, the size of the particles obtained in an autoclave is limited by the conversion of amorphous siUca to quartz at high temperatures. Particle size influences the stabiUty of the sol because particles <7 nm in diameter tend to grow spontaneously in storage, which may affect the sol properties. However, sols can be stabilized by the addition of sufficient alkaU (1,33). 

Monosilicic acid polymerizes in neutral or acidic solution, with a characteristic rate depending the pH [ref. 118-120]. Tarutani [ref. 118] suggested that exclusion chromatography is useful for studying the polymerization of silicic acid. The mechanism of the growth of the polymer particles was discussed on the basis of changes in the elution curves for polysilicic acids with time [ref. 118-121]. The polymerization of silicic acid is slowest at pH 2 in aqueous media [ref. 118, 119]. The eluent adjusted to pH 2 was used throughout the experiments. 

This proposed mechanism is based largely upon the fact that the maximum reaction rate is attained at pH 8 to 9, at which concentration of both species is reasonably high. On the other hand, monosilicic acid polymerizes at room temperature when the concentration exceeds approximately 120 ppm (McKeague and Cline [1963]). The reaction is catalyzed by OH", F , and silica gel, and a polymerization mechanism in which a six-coordination silicate ion H2O Si(OH)j combines with Si(0H)4 is envisaged by Iler [1955]. The hypothesis that condensation involves a temporary expansion in the coordination number of silica from four to six is also claimed by Okblerse [1961] from kinetic studies on submicroporous and macroporous silica. In a study of the polymerization and condensation process, Baumann [1958] showed... 

Solutions of monosilicic acid may also be obtained by carehil hydrolysis of tetrahalo-, tetraalkoxy-, or tetraacyloxysilanes by electrolysis or acidification of alkah sihcate solutions or by ion exchange (qv). By operating under carefully controlled conditions at low temperature and pH, solutions may be obtained that remain supersaturated with respect to amorphous sihca for hours at temperatures near 0°C. Eventually, however, polymerization reactions involving the formation of siloxane linkages occur, leading ultimately to the formation of coUoidal particles and further aggregation or gel... 

Condensation occurs most readily at a pH value equal to the piC of the participating silanol group. This representation becomes less vaUd at pH values above 10, where the rate constant of the depolymerization reaction k 2 ) becomes significant and at very low pH values where acids exert a catalytic influence on polymerization. The piC of monosilicic acid is 9.91 0.04 (51). The piC value of Si—OH decreases to 6.5 in higher order sihcate polymers (52), which is consistent with piC values of 6.8 0.2 reported for the surface silanol groups of sihca gel (53). Thus, the acidity of silanol functionahties increases as the degree of polymerization of the anion increases. However, the exact relationship between the connectivity of the silanol sihcon and SiOH acidity is not known. 

Hydrogen peroxide (H2O2) has shown to be an excellent source of oxygen in a plasma. The reaction has several steps beginning by the formation of a gaseous monosilicic acid, followed by polymerization and the removal of H2O, as follows 0 1... 

Silicic Adds. The behavior of silicate ions in solution, the dependence of various properties on pn, the nature of silica sols and gels, and the study of hydrated silicas constitute chapters in inorganic and colloid chemistry that go far beyond the scope of this review. Germane to the present subject, however, are certain observations on the formation of monosilicic acid and its stepwise polymerization. 

It is clearly established that, in these solutions, the silicon is in tetrahedral coordination (13). The simplest form of silica in solution is the monosilicic acid Si(0H)4. Polymerization results from the condensation of two silanol groups with the elimination of a water molecule. A whole series of silicate anions, of various polymerization and ionization degrees, are thus connected through dynamic equilibria. The equilibria are governed by the normal chemical parameters, namely the silica concentration, the pH and the cation type. 

Monosilicic acid is stable in aqueous solution only at low pH and very low concentration (Her, 1979, p. 209). The rate of condensation-polymerization of silicic acid is dependent on pH, concentration and temperature. At a certain stage, the polysilicic acid sol is converted into either a precipitate (i.e. a flocculated system) or a hydrogel. 

If the reaction follows the first route, the interaction of monosilicic acid with calcium hydroxide molecules appears to be a direct reason of polymerization and precipitation of hydrated calcium silicates. The scheme of the reaction can be written as follows [20] ... 

Polymerization of monosilicic acid has a minimum rate at pH about 2. The reaction is approximately second-order above that pH, and third below. In the latter region there is also fluoride catalysis From studies in methanolic solution it is clear that water, ammonia and amines all accelerate polymerization, probably as proton acceptors rather than as nucleophiles. The kinetics are in an early stage of development. 

According to Jones and Handreck (1967), silica in soil solutions is entirely in the monomeric form Si(OH)4 (monosilicic acid) and is present in concentrations generally ranging from 7-80 jug g , but always less than the saturation value (about 120 pgg" ). The concentration of dissolved silica in soils depends on those factors which control dissolution rates of polymeric silica and on those which control the rate of removal of monosilicic acid from solution. 

Strazhesko, Strelko, Vysotsky, and co-workers (49-51) investigated the polymerization of monosilicic acid. Polysilicic acid is more acidic than monosilicic acid, and the dissociation constant of the acidic centers on the surface of polysilicic acid is 2 to 3 orders of magnitude greater than that of the monomer (49-51). Strazhesko, Vysotsky, and co-workers (52-55) showed that the isoelectric point represents not only the minimal rate of the gel formation process but also its syneresis. As a result, mechanically strong silica gels were obtained that had a maximum specific surface area. 

Figure 4. Variation with time of concentration of individual species during polymerization of L5 M monosilicic acid at —13°C, initial pH 3.5. Vertical scale of lower part of figure is expanded five times to show changes in the less abundant species. Curve for cyclic tetramer is included in both parts of diagram.

Figure 5. Distribution of species during polymerization at two different pH values L5 M monosilicic acid at —13 C. Key , monomer 0, g,lc. volatile material (lower block represents species with 2- silicate units) polysilicate.

Fig. 14 shows the elution curves for silicic acid in solutions of pH 9.5 on a Sephadex G-lOO column. The peaks on the right are due to monosilicic acid and those on the left to polysilicic acid. The elution curves for the polysilicic acids indicate a symmetrical distribution of particle sizes, and the elution volume of the polymers obtained after standing the sample solution for 250 h is almost the same as that obtained after 100 h. This suggests that when the concentration of monosilicic acid is close to the solubility of amorphous silica, the growth rate of the particles of the polymers becomes very low. It was thus assumed that the growth of the particles is mainly due to the polymerization between monomer and polymer, and the polymerization between polymer species... 

The preparation and reactions, for example, polymerization, of dilute solutions of monosilicic acid are further described in Chapter 3. Meanwhile, some of its characteristics are noted, as follows, prior to discussing solubility ... 

Hydrated amorphous silica, here designated as a separate class in which most, if not all, of the silicon atoms each retains one or more hydroxyl groups in the silica structure. This type of polymeric structure is obtained if monosilicic acid or oligosilicic acids in water are concentrated and polymerized at ordinary or low temperature and in slightly acidic solution. It is now believed that under these conditions the silica polymerizes to extremely small spherical units less than 20-30 A in diameter, which, when concentrated, link together into a three-dimensional gel mass, trapping water in the interstices, which are of molecular dimensions and retain water which can be desorbed only above about 60 C. 

Kitahara (169) shows that the effect of pH, salt, and temperature have the same influence on the rates of polymerization of monosilicic acid as they have on the gelling of sol, showing that both phenomena, that is, the growth of particles and the cementing together of particles once they are in contact, are influenced by the same factors that is, those. that affect the rate of dissolution and deposition of monomeric silica. 

As will be discussed in detail in Chapter 3, this technique has been shown by Alexander and others to allow the formation of SKOH) from NajSi0,-9H,0. Salt-free solutions can be obtained by ion-exchange techniques. Since conditions have been found for converting monomeric silicate ions to monosilicic acid, which is extremely prone to polymerize, it is evident that higher polysilicates can likewise be converted to the more stable polysilicic acids with even less difficulty. 

The structure of the silicomolybdic acid is such that within the molecule there is a tetrahedron of four oxygen atoms in which only one silicon atom can fit (20, 24). Thus only monosilicic acid. Si(OH)4, can react directly. All polymeric species must fir.st depolymerize to monomer. The silicomolybdatc anion SiMoijO o apparently has a compact structure similar to that established for basic aluminum chloride in... 

Definitive data on nucleation of colloidal silica particles in brine solutions at pH 4.5-5.5 and 95°C have been obtained by Makrides and associates in a study related to the deposition of silica from hot geothermal waters (106d). Their work showed conclusively that a solution of monosilicic acid requires an induction period for the formation of nuclei that strongly depends on the degree of supersaturation. Under these conditions appreciable time is required for the early stages of polymerization to produce three-dimensional polymer particles of the type that can function as nuclei. With a supersaturation ratio of 2-3. the nucleation time ranged from a few minutes to several hours. 

Figure 3J8. Limited polymerization of monosilicic acid (0.8% SiOz) from hydrolyzed methyl orthosilicate in dilTerent hydrolysis and aging media at 25 C. Molecular weight number average as SiO. Curve 1. 10 N HCI Curve 2. 10 N HCI Curve 5, 10 N H S04. [From Schwarz and KnaufT(2l).]...

In earlier studies at 2S°C, Hoebbel and Wieker obtained basic data on the polymerization of a 0.4 M solution of monosilicic acid (2.4% SiO,) at the most stable pH of 2.0 as the solution aged from S min to 24 days. Measurements were also... 

Figure 3.44. Earliest stages in the polymerization of 0.5m monosilicic acid at -2 C and pH 2.0 /I, monomer B. probably cyclic trimer. may also be dimer and linear trimer C. cyclic tetramer D. higher polysilicic acids. (Hocbbcl and Wicker (84).J...

Figure 3.46. Relation between molecular weight of (SiOj..xHjO) and rate constant of reaction with molybdic acid k ., . It may be significant that the cubic octamer and cyclic hcxamcr. which are not on the linear plot, are also not found in the course of aqueous polymerization of monosilicic acid. [From Hoebhel. Wicker et al.. 1973 (84)-l...

---