Your Compounding Questions Answered

JULY 11, 2011
Martin A. Erickson III, RPh

Q: We presently have a container of silicon dioxide, colloidal. What is the difference between this item and silica gel?

A: The terminology is a bit confusing, primarily because pharmacists’ jargon uses “silica gel” interchangeably with fumed silica or colloidal silicon dioxide NF.

Definitions from the National Formulary 27 provide some clarity: “Silicon dioxide…is obtained by insolubilizing the dissolved silica in sodium silicate solution. Where obtained by the addition of sodium silicate to a mineral acid, the product is termed silica gel; where obtained by the destabilization of a solution of sodium silicate in such manner as to yield very fine particles, the product is termed precipitated silica.” It also explains, “Colloidal silicon dioxide…is a submicroscopic fumed silica prepared by the vapor-phase hydrolysis of a silicon compound.”

Thus, synonyms of colloidal silicon dioxide NF are colloidal silica, fumed silica, anhydrous silicic acid, silicic anhydride, and silicon dioxide fumed. Brand names include Aerosil, Cab-O-Sil, and Wacker HDK.

The product is used to improve the flow properties of powders in tableting processes, to stabilize emulsions, and as a thixotropic thickening and suspending agent in gels and semisolid preparations. It can form transparent gels with ingredients of similar refractive index. In these gels, the degree of viscosity depends upon polarity of the liquid (polar liquids require higher concentrations of colloidal silicon dioxide than nonpolar liquids), and viscosity is very temperature dependent, but pH can also have an effect.

The material may also be used as a tablet disintegrant and as an absorbent dispersing agent for liquids in powders or suppositories. PT

Mr. Erickson is director of professional affairs at Gallipot Inc. 

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