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) o f three homologous series of nonionic surfactants (NSs) rise steeply at fir st but then level off, converging asymptotically to the temperature range o f 114-118 degreesC, which corresponds to the CP of polyethylene oxide with a molecular weight of 10(4). Six of the 14 surfactants investigated were oc toxynols, five were nonoxynols and three were polyoxyethylene dodecyl ether s. Ten had an average number of oxyethylene units per molecule p > 20 and a CP > 100 degreesC. Three of these CPs were measured directly on 2.00% surf actant solutions sealed into ampoules. To avoid additional measurements und er pressure, the other surfactants had their CPs lowered below the normal b oiling point of their 2.00%, solutions by adding the salting-out, CP-loweri ng salt NaNO3 at various concentrations, measuring the depressed CPs and ex trapolating them to zero salt concentration. The CPs decrease linearly 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 i n more extensive salting out: The more highly polyoxyethylated a surfactant is, the greater the reduction in CP it undergoes. As p increases, the curv es of CP in water versus p for the octoxynols and nonoxynols rise steeply a nd nearly linearly, reaching 100 and 103 degreesC at p=16, and then begin t o level off. The sharpest inflections are at p = 22-25. The nonoxynol curve becomes horizontal at p similar or equal to 40 and 115 degreesC while the octoxynol curt e rises gently from 3 degreesC below the nonoxynol curve to merge with it at p = 90 and 114 degreesC. The CPs of the three polyoxyethyl ene dodecyl ethers are slightly higher than those of the polyoxyethylated a lkylphenols with comparable p values, peaking at 118 degreesC. The most hig hly polyoxyethylated surfactant of each series contains 95% polyoxyethylene . Their CP range of 114- 118 degreesC equals that of polyethylene oxides wi th molecular weights between 8000 and 20 000. Published quantitative CP-p r elations for polyoxyethylated NSs cover only CP values below 100 degreesC a nd p values below 16. They fail to predict the levelling off of the CPs as the p values rise above 16. (C) 2001 Elsevier Science B,V. All rights reser ved.