Viscosification in Polymer−Surfactant Mixtures at Low Temperatures

Abstract

Interactions between the anionic surfactant sodium dodecyl sulfate (SDS) and hydroxyethylcellulose (HEC) or its hydrophobically modified analogue (HM-HEC) have been studied over an extended temperature region with the aid of turbidimetry, small-angle neutron scattering (SANS), and shear viscosimetry. Anomalous viscosity enhancements were observed for semidilute HEC/SDS and HM-HEC/SDS solutions at high SDS concentrations at temperatures far below the Krafft point for aqueous solutions of SDS. From the Arrhenius-Frenkel-Eyring (AFE) plots of the temperature dependence of the zero-shear viscosity, the activation energy of chain disengagement (DeltaE(vis)) was found to be on the order of 40 kJ mol(-1) for the HEC/SDS mixtures, whereas for the HM-HEC/SDS system, much higher values of DeltaE(vis) (up to 141 kJ mol(-1)) were reported, and the activation energy increased with an increasing level of SDS addition. Break points in the AFE plots were observed for both the HEC/SDS and HM-HEC/SDS systems at low temperatures and high SDS concentrations. Time evolutions of both the turbidity and the shear viscosity were monitored after quenching of the temperature from 25 to 1 degrees C. The turbidity results revealed in general a less pronounced transition for the HEC/SDS and HM-HEC/SDS systems than for the corresponding polymer-free SDS/water solutions. In the course of time, a significant viscosity enhancement was found for the HEC/SDS system at high levels of SDS addition, and a much stronger viscosification was observed for the HM-HEC/SDS system at the highest surfactant concentration. The overall results suggest that hydrated SDS aggregates act as cross-linkers of the network and generate the substantial viscosification of the systems at low temperature and high levels of SDS addition. For the HM-HEC/SDS system, further strengthening of the network occurs because of the contribution from hydrophobic interactions. The SANS data on HEC/SDS mixtures reveal that some structural reorganization takes place at low temperatures in the presence of high SDS concentrations, and this is ascribed to enhanced polymer-SDS interactions and the formation of clusters that strengthened the cross-links of the network.