Relation Between Rate of Dissolution and Particle Size

Relation Between Rate of Dissolution and Particle Size 

It is logical to assume that the rate of depolymerization or dissolution of colloidal silica particles would be proportional to the specific surface area of the silica particles. Thus different solvents such as dilute hydrofluoric acid, molybdic acid, or dilute alkali, all of which convert the monomeric silica permanently to other silicon com-. 

In a patent issued to Balthis (212b) a method is described whereby a sample of deionized silica sol is put into an excess of 0.01 IV NaOH solution at 30 C and over a period of 90 min samples of the solution are removed and the amount of monomeric silica is determined by reaction with the molybdic acid reagent. The rate of dissolution of the silica to form monomeric silicate in the alkaline solution was related to A, the specific surface area of the silica (in square meters per gram) determined by other means, by the following equation  

For the particular type of silica particles involved, which was highly porous, the value of K was 155 at the point where 20% of the silica was converted to monomer. For a dense silica particle prepared at elevated temperature by the buildup process according to Bechtold and Snyder (213), the value of K was about 8 at the same stage of dissolution. The porous and dense particles compared were of about the same size, being 18 and 16 nm in diameter, respectively. 

It is therefore obvious that the rate of dissolution in alkali cannot be used as a dependable method for determining specific surface area unless it is certain that all the samples being evaluated have the same particle structure, or at least have been made under the same process conditions from the same materials. 

---