Thermal gelation of nonionic cellulose ethers and ionic surfactants in water

Abstract

A novel gel-forming system which shows an anomalous temperature dependence is reported. The system consists of a nonionic cellulose ether and an ionic surfactant in water (the effect is particularly strong for ethyl (hydroxyethyl)cellulose [EHEC]). When the system is heated (typically to 40–50°C), it undergoes a transformation from a low-viscous solution to a clear and stiff gel. The unusual behaviour is reversible and an apparently unlimited number of temperature cycles can be performed without loss of the gelling capability. The rheological properties were characterized by steady-flow and dynamic viscosity measurements, and it is found that the gel has many features in common with weak polymer gels. The cellulose ether could either be relatively hydrophobic (methyl-, hydroxypropyl- and ethyl(hydroxyethyl)cellulose), or contain a small number of long alkyl groups (e.g.,‘hydrophobically modified’ hydroxyethylcellulose). The mechanism behind the formation of this novel type of gel is discussed in terms of cooperative binding of the ionic surfactants to the EHEC polymer, leading to small micelle-like aggregates. These may act as a cross-links between hydrophobic segments in different polymer chains, giving rise to extended and rigid polymer networks. Moreover, we have found that the interaction has a reverse temperature dependence — an increase in temperature leads to a higher degree of surfactant binding — which thus may explain the temperature anomaly in the gelling process.