Influence of organic additives on the cloud point of PONPE-7.5

The effect of the addition of different organic compounds on the critical temperature (cloud point) of micellar solutions of a non-ionic surfactant was studied. In particular, the addition of aliphatic alcohols and some hydrocarbons to polyoxyethylene(7.5) nonylphenyl ether (PONPE-7.5) solutions was investigated. It was found that the short-chain alcohols (methanol, ethanol, propanol and butanol) increased the cloud point temperature, while for the longer-chain alcohols, the temperature decreased. The effect of the first group can be explained by considering that these species tend to remain in solution and promote the formation of expanded water structures, favoring micelle hydration and, in consequence, increasing the cloud point. This influence is more significant as the concentration of the additive increases, and can be negligible at very low concentrations. The effect of longer-chain alcohols suggests a preferential solubilization of these compounds in the inner core of the aggregate, inducing changes to the shape and size of the micelles. Since the volume-to-area ratio in a micelle increases with these additions, there is a general increase of the micelle size and the cloud point decreases. The effects of branching and cyclization were studied using two series: n-butanol, isobutanol and tert-butanol, and n-hexane, cyclohexane and benzene, respectively. For both series, the addition of the compound provoked a marked decrease in the cloud point temperature. These effects can be explained in view of the site of solubilization inside the micelle. Benzene molecules seem to interact preferentially with the ethylene groups, leading to strong dehydration, while the aliphatic compounds are solubilized in the inner core.