Determination of surface areas of carbon blacks by soap adsorption

Abstract

A method is presented for the determination of the surface area of carbon blacks by adsorption of laurate or myristate soaps from aqueous solution. It involves conductometric titration of the blacks in aqueous dispersion with the respective soaps. Comparison of results obtained by this method on eleven samples of various blacks with nitrogen adsorption data indicates that up to a nitrogen area of 50 m.2/g. the two methods yield identical results if areas of 23.2 and 35.2 Å2 are employed for the laurate and myristate molecules, respectively. However, for the finer blacks the two methods yield divergent surface areas, with those by nitrogen being the larger. These differences are ascribed to surface irregularities present in the higher area blacks, which can be expressed as a surface roughness factor, f, defined as the ratio of the area of a black as determined by nitrogen adsorption to that given by the soap titration. These roughness factors have a value of unity for the smaller area blacks, and increase for the finer blacks to values above two. Since a nitrogen molecule is small and can be accommodated in spaces inaccessible to larger molecules, nitrogen areas appear to be too large for correlation with rubber reinforcement. On the other hand, the areas given by larger molecules, such as soap, should be much nearer to those involved in the interaction of carbon black and rubber.