Krafft Points and Cloud Points of Polyoxyethylated Nonionic Surfactants Part 3 The Effect of Hydrotropes and other Organic Electrolytes

The effect of various organic electrolytes on the Krafft point (KP) of two and the cloud point (CP) of three polyoxyethylated nonionic surfactants is described. Additives that lower the KP or raise the CP salt the surfactants in. The short-chain organic electrolytes acetic acid and propionic acid lower the KP of polyoxyl 10 cetyl ether (C(16)E(10)) while sodium acetate, propionate and butyrate leave the KP largely unaffected. Sodium acetate and sodium propionate lower the CP of C(16)E(10) substantially. Sodium butyrate raises the CP, presumably by rendering the C(16)E(10) micelles partly ionic via mixed micelle formation. Hydrotropes (sodium benzoate, salicylate and xylene sulfonate, and the nonelectrolyte nicotinamide) lower the KP of polyoxyl 10 stearyl ether (C(18)E(10)) substantially at 3.0 m concentrations and raise the CP substantially even at 0.10 m concentrations. The KPs of mixtures of C(18)E(10) (KP=48 degrees) and hexadecylamine hydrochloride (HDCI, KP = 44 degrees) were measured at a constant total surfactant concentration of 2.0% based on the weight of water. The plot of KP versus the relative weight-percent HDCI is V-shaped with a eutectic minimum of 32 degrees at 33% HDCI. The left branch is entirely linear. It shows how increasing weight-percentages of HDCI lower the KP of C(18)E(10). The right branch is linear up to ca. 80% HDCI but becomes less steep at higher HDCI concentrations. It shows how decreasing weight-percentages of C(18)E(10) raise the KP of HDCI.