Cloud Point of Mixed Ionic-Nonionic Surfactant Solutions in the Presence of Inorganic Salts

Effects of various additives including inorganic salts, nonionic and ionic surfactants on the cloud point of the linear nonionic surfactant, Tergitol 15-S-7 were investigated. The cloud point of this surfactant is a function of its concentration, as well as the additive concentration. The cloud points of nonionic surfactant mixtures lay in between the cloud points of individual surfactant. The presence of two ionic surfactants, sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) in the system leads to a substantial increase in the cloud point of a 1 wt% Tergitol 15-S-7 micellar solution. The inorganic salts (<0.1 M) decrease the cloud point only slightly. However, when inorganic salts are added to the Tergitol 15-S-7 solution in the presence of SDS or CTAB, the cloud point decreases drastically. For SDS-Tergitol 15-S-7 mixed micellar solutions, the polyvalent cations of the added inorganic salts are more active than the univalent cation, the order of decreasing effect is LaCl3 > MgCl2 > KCl; while for CTAB-Tergitol 15-S- 7 mixed micellar solutions, the polyvalent anions of the added inorganic salts are more effective than the univalent anion, the order of decreasing effect is Na3PO4 > Na2SO4 > NaNO3.