Effect of inorganic additives on solutions of nonionic surfactants III: CMC's and surface properties

Continuing work on the interaction of inorganic additives with nonionic surfactants in aqueous solution dealt with their effect on the CMC and surface tension. The surfactants were octoxynol and polyoxyethylated oleyl alcohol, containing an average of 9.5 and 10 ethylene oxide units, respectively. Their CMC values were lowered by most electrolytes studied, representing salting out of the surfactants. The steepest reductions in the CMC were produced by the nitrates of sodium and potassium, which had been found to lower the cloud points of nonionic surfactants, salting them out because of the inability of their cations to form complexes with the ether oxygen linkages of the polyoxyethylene moieties. However, even electrolytes with cations such as hydrogen, lithium, calcium, nickel, lead, and aluminum capable of forming complexes with the ether oxygens, thereby increasing the cloud points of the surfactants, lowered their CMC values. In the presence of increasing concentrations of the latter electrolytes, the CMC values frequently went through minima and approached the CMC of the surfactant in the absence of additives. Increases in the CMC over the entire range of additive concentrations investigated were produced by cadmium nitrate for octoxynol, urea for polyoxyethylated oleyl alcohol, and magnesium nitrate for both. Net increases in the plateau or micellar surface tension of polyoxyethylated oleyl alcohol, i.e., in the constant surface tension of surfactant solutions above the CMC, were brought about by the nitrates of cadmium, aluminum, and magnesium at low concentrations only and by urea at all concentrations. This increase is interpreted as salting in. The area per surfactant molecule adsorbed at the air-water interface was reduced by all added electrolytes. Urea caused no such reduction.