Nonionic polymers and surfactants: Temperature anomalies revisited

In view of a rapidly increasing importance in applications, as well as significant fundamental observations, the temperature anomalies displayed by several oxyethylene compounds, including poly(ethylene glycol), and oxyethylene surfactants and copolymers continue to create interest but also controversies. Here we review a wide range of experimental observations and present the different theoretical approaches that have been suggested. We find that a model that attributes the temperature effects to conformational changes that make the oxyethylene groups less polar at higher temperature has a strong predictive power and gives a quantitative rationalization of phase diagrams and other observations. In line with observations, it also predicts that the behaviour is not restricted to aqueous solutions but also occurs in other solvents. Different NMR approaches as well as Raman spectroscopy directly demonstrate the conformational behaviour predicted. We also note that the relevant molecular segment also occurs in other compounds, like cellulose derivatives such as methylcellulose. In line with this, qualitatively the same type of temperature anomalies is found for these compounds as with oxyethylene-containing substances.