THE ADSORPTION OF SOAP BY CARBON BLACK

Abstract

The adsorption of the sodium soaps of lauric, myristic, palmitic, stearic, and oleic acids from aqueous solutions by a carbon black has been measured. The adsorption appears to be a combination of two more or less independent processes: (a) adsorption of neutral soap, and (b) adsorption of fatty acid resulting from hydrolysis. The adsorption of both the fatty acid and alkali components of the soaps studied is in agreement with the Freundlich adsorption equation over the concentration range investigated, the extent of adsorption increasing with increasing chain length of the saturated soaps examined. In every case there is a greater adsorption of fatty acid than of alkali, but this difference becomes greater with increasing chain length of the soap. Increase in temperature causes a slight decrease in adsorption, but the effect is small. On the basis of its adsorptive behavior, sodium oleate appears to have an effective chain length of about 15 carbon atoms. Excess of fatty acid in the initial soap solution results in an increased adsorption of total fatty acid, but does not influence the adsorption of neutral soap; on the other hand, excess of alkali in the initial solution not only results in an increased adsorption of total alkali, but also leads to a decrease in the adsorption of fatty acid and neutral soap. This decrease in adsorption is attributed to suppression of hydrolysis, but even with 100% excess alkali, where hydrolysis must be almost completely suppressed, there is still an appreciable adsorption of fatty acid which must be adsorbed in the form of neutral soap. The adsorbed material corresponds to an acid soap of variable composition, the ratio of excess acid to neutral soap depending on the composition of the initial solution, the temperature, and the particular soap used. The adsorption of soap from 95% alcohol solution and from absolute alcohol solution is essentially the same, both being considerably lower than the adsorption of the same soap from aqueous solution. In spite of the fact that hydrolysis of soap does not occur in alcoholic solution, the fatty acid and alkaline components are not adsorbed in equivalent amounts, and it is suggested that, in this case, splitting of the soap is brought about by alcoholysis, followed by preferential adsorption of one of the reaction products.