Effect of Inorganic Additives on Solutions of Nonionic Surfactants

The salt effects of 10 transition metal cations on nonionic surfactants were investigated by the action of their salts (nine nitrates, two sulfates) on the cloud point (CP) of octoxynol 9. All cations investigated raised the CP of octoxynol 9 by complexation with its ether groups, which represents salting in. However, aside from Ag+, these CP increases were no larger than those caused by Li+ and by di- and trivalent cations not belonging to the transition metal group, even though the transition metal cations have a greater capacity for forming complexes. This unremarkable, average salting-in capacity of most transition metal cations is attributed to competition for their coordination sites between the ether groups of the surfactant and water. With the exception of silver, the solid nitrates of all transition metals examined form stable solid hydrates containing three to nine molecules of water of crystallization, which indicates a high affinity of the cations for water. Only AgNO3 forms no stable solid hydrates. The low affinity of Ag+ for water results in a commensurately higher capacity for binding the ether groups of the surfactant. This was shown by the facts that AgNO3 produced the largest CP increases and that it was the only salt to reversibly precipitate an insoluble adduct with octoxynol 9 from concentrated solutions. The only salt that oxidized the surfactant when its solution was heated in a nitrogen atmosphere and illuminated during CP measurements was Fe(NO3)3. Oxidative degradation produced small but progressive CP reductions. Copyright 1997Academic Press