Effect of inorganic additives on solutions of nonionic surfactants — XVI. Limiting cloud points of highly polyoxyethylated surfactants

As their degree of polyoxyethylation is increased, the cloud points (CPs) of three homologous series of nonionic surfactants (NSs) rise steeply at first but then level off, converging asymptotically to the temperature range of 114–118°C, which corresponds to the CP of polyethylene oxide with a molecular weight of 104. Six of the 14 surfactants investigated were octoxynols, five were nonoxynols and three were polyoxyethylene dodecyl ethers. Ten had an average number of oxyethylene units per molecule p>20 and a CP>100°C. Three of these CPs were measured directly on 2.00% surfactant solutions sealed into ampoules. To avoid additional measurements under pressure, the other surfactants had their CPs lowered below the normal boiling point of their 2.00% solutions by adding the salting-out, CP-lowering salt NaNO3 at various concentrations, measuring the depressed CPs and extrapolating them to zero salt concentration. The CPs decrease linearlv with increasing molality m of NaNO3 up to ca. 2.5 m and more steeply at higher concentration. For a given concentration of NaNO3, larger p values result in more extensive salting out: The more highly polyoxyethylated a surfactant is, the greater the reduction in CP it undergoes. As p increases, the curves of CP in water versus p for the octoxynols and nonoxynols rise steeply and nearly linearly, reaching 100 and 103°C at p=16, and then begin to level off. The sharpest inflections are at p=22–25. The nonoxynol curve becomes horizontal at p≅40 and 115°C while the octoxynol curve rises gently from 3°C below the nonoxynol curve to merge with it at p=90 and 114°C. The CPs of the three polyoxyethylene dodecyl ethers are slightly higher than those of the polyoxyethylated alkylphenols with comparable p values, peaking at 118°C. The most highly polyoxyethylated surfactant of each series contains 95% polyoxyethylene. Their CP range of 114–118°C equals that of polyethylene oxides with molecular weights between 8000 and 20 000. Published quantitative CP–p relations for polyoxyethylated NSs cover only CP values below 100°C and p values below 16. They fail to predict the levelling off of the CPs as the p values rise above 16.