The adsorption of calcium ions to surfactant films on Graphon

Previously, the adsorption mechanisms of sodium dodecyl sulfate (NaDS) and of sodium dodecylbenzene sulfonate (NaDBS) onto Graphon (a high area, graphitized carbon black) had been determined. Calorimetric heats of adsorption of the surfactants were also measured using a plastic calorimeter designed to avoid contamination by calcium ions. This work has now been extended to learn the effects of calcium on the Graphon/solution interface. Radiocalcium adsorption measurements showed that the adsorbed NaDS takes up limiting amounts of one calcium for two DS ions and the chains extend directly from the surface in close‐packed array. (In the absence of calcium, the NaDS takes up one of two arrangements with two different lengths of the hydrocarbon chain lying on the Graphon surface). Several lines of evidence indicate that the calcium is tightly bound at the interface. Above the cmc, the micelles compete for the calcium ions. For NaDBS, the arrangement on the Graphon is not altered by the calcium but other results are similar. Trace amounts of calcium strongly enhance the flocculation of the Graphon. This finding suggested that deposition of the surfactant‐coated Graphon onto cotton should be followed as a function of added calcium. It was found that traces of calcium greatly increased the deposition apparently by bridging surfactant on the Graphon to surfactant on the cotton.