Adsorption of lauryl sulphonic acid in the surface of its aqueous solution, and the Gibbs theorem

Abstract

The absolute amount of adsorption per square centimetre of exposed surface of a solution may be measured with certainty and with moderate accuracy. In the microtome method (McBain and Humphreys 1932; McBain and Swain 1936; McBain, Ford and Mills 1940) the quiescent surface is cut from a solution in complete equilibrium with its vapour, collected, and analysed. In the interferometer method (Ford and McBain 1936; McBain, Mills and Ford 1940) a similar surface in equilibrium is compressed into the bulk of the liquid in the path of an interferometer beam which measures excess coming from the known surface destroyed. Both methods applied to solutions of lauryl sulphonic acid, C 12 H 25 SO 3 H, agree in showing strong positive adsorption over the whole range of concentrations. On the other hand, the curve of surface tension with concentration is of Type III, showing very great lowering in extreme dilution, leading immediately to a minimum, followed by a definite rise to a shallow maximum, with a slight decrease of this already low value with further concentration. The Gibbs theorem demands an initial positive adsorption of the acid, followed by no adsorption at all where the surface tension is most lowered, followed by strong negative adsorption, then zero adsorption, still in dilute solution with very low surface tension, followed by slight positive adsorption.