The Sears number as a probe for the surface chemistry of porous silicas: Precipitated, pyrogenic and ordered mesoporous silicas

In 1956, Sears had introduced a simple empirical method to determine the surface area of colloidal silicas. This technique consists in the titration of silanol groups using sodium hydroxide solution and relies on the assumption that the silanol group density is constant. When porosimetric methods became more easily accessible and automated, this simple method for the determination of surface areas appeared obsolete. The Sears number was then further developed by industry as an indicator for the hydrophilicity of a silica material. As a rather simple empirical method, the Sears number is directly related to practical aspects of the application of a silica material in an aqueous environment, as it determines the number of silanol groups accessible for chemical reactions. Here, we describe the adaptation of the Sears number concept to ordered mesoporous silicas (SBA-15, SBA-16, MCM-41 and MCM-48) and show that with the method presented here, reproducible and useful results can be obtained. Due to their high surface areas, mesoporous materials have very high Sears numbers when compared to industrially fabricated precipitated and pyrogenic silicas.