The effect of electrolytes on the cloud point of mixed solutions of polypropylene glycol and ionic surfactants

Abstract

The cloud points of polypropylene glycol (PPG) in the presence of small amounts of ionic surfactants, including dodecyltrimethylammonium bromide (DTAB), cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and sodium cetyl sulfate (SCS) have been determined. The results show that the cloud point of PPG will be elevated if a certain amount of ionic surfactant is added. The longer the hydrocarbon chain length of the ionic surfactant homologues, the stronger the effect is, and the effect of anionic surfactants is stronger than that of cationic surfactants with the same hydrocarbon chain length. On the other hand, the cloud point remains unaltered on addition of the same amount of inorganic salt. However, when salts are added to the PPG solution in the presence of a certain amount of ionic surfactants, the cloud point decreases in a ratio depending on the concentration of salts, the nature of the ions and, particularly, the valency of the ions with charges opposite in sign to the charges of the surface-active ion. For systems of 0.01% PPG + SCS and 0.1% PPG + SCS, the valency of the cation of salts is decisive, and the order of the effect is: 1/3 LaCl 3 > 1/2 CaCl 2, > NaCl. For the system of 0.1% PPG + CTAB, the valency of the anion of electrolytes plays the dominant role, and the order of the depressing effect is: 1/3 K 3Fe(CN) 6 > 1/2 Na 2SO 4 > NaBr. For the system of 0.01% PPG + CTAB, a striking phenomenon was observed, i.e. the cloud point was enhanced rather than decreased by the addition of electrolytes. An explanation in terms of the enhancing adsorption of surface-active ions on the PPG chains due to the addition of electrolytes was suggested.