EFFECT OF INORGANIC ADDITIVES ON SOLUTIONS OF NONIONIC SURFACTANTS .14. EFFECT OF CHAOTROPIC ANIONS ON THE CLOUD POINT OF OCTOXYNOL-9 (TRITON X-100)

The effect of salts on the nonionic surfactant octoxynol 9 was studied by the changes they produced in its cloud point (CP): CP increases in dicate salting in. The temperatures by which the sodium salts of water structure-breaking (chaotropic) anions increased the CP of 2.0% octox ynol solutions were measured as a function of salt concentration. The curves representing changes in CP versus salt molality rose to a maxim um in a parabolic fashion, followed by steep decreases. Their ascendin g branches, corresponding to salting in, were caused by a disruption o f the water structure due to the chaotropic effect of the anions combi ned with the effect of elevated CP temperatures. The descending branch es were due to salting out by Na+. The net CP increases due to the cha otropic effect of the anions were calculated at each concentration by subtracting the CP decrease due to Na+ from the observed CP increase o f the respective Na+ salts. With the exception of ClO4-, the plots of CP changes produced by the chaotropic anions rose in a nearly linear f ashion to a maximum and then levelled off. The levelling off occurred at the salt concentration and CP temperature leading to the maximum di sruption of the water structure of which each anion was capable. The c haotropic anions were ranked in the following order according to their capacity for increasing the CP: SCN- > I- > [Fe(CN)(5)NO](2-) > ClO4- > BF4-. Even though the thiosulfate anion is a very soft Lewis base, it lowered the CP in direct proportion to its molality; i.e., it enhan ced the structure of water and promoted salting out at all concentrati ons. (C) 1997 Academic Press.