Influence of non-ionic polymers on the rheological behaviour of Na+-montmorillonite clay suspensions—I Nonylphenol–polypropylene oxide–polyethylene oxide copolymers

The present experimental investigation relates the adsorption of non-ionic surfactants onto Na+-montmorillonite clay with the rheological behaviour of the concentrated clay suspensions. The non-ionic surfactants used were the commercially available nonylphenol(polypropyleneoxide)(polyethyleneoxide) surfactants of varying ethylene oxide chain length. Adsorption, electrokinetic, and phase analysis light scattering measurements gave evidence of the presence of surfactant micelles on the clay particles. Adsorption isotherms were of the Langmuir type; the maximum amount adsorbed and the affinity for the clay surface decreased with increasing number of ethylene oxide units. The zeta potential of the Na+-montmorillonite particles was calculated as a function of surfactant concentration. The surfactant layer thickness was estimated from the former measurements and was found to increase linearly with the number of ethylene oxide units. The rheological behaviour of the clay suspensions was obtained by shear stress–shear rate measurements within a wide range of shear rates (0.1–1000 s−1). The data was fitted to both the Bingham and Herschel-Bulkley models; from which the Bingham yield stress, plastic viscosity, Herschel–Bulkley yield stress, flow behaviour index and consistency coefficient were estimated. Dynamic oscillatory shear measurements enabled the determination of viscoelastic parameters; such as the complex, elastic and loss modulus. Results show that flocculation of the suspension occurs below maximum surfactant coverage of the clay particles, and occurs to a greater extent at approx. 50% surface coverage. Above this value, a reduction of the rheological properties takes place and a plateau value is reached as the maximum amount adsorbed is approached. Flocculation was attributed to the bridging of micelles between the particles, which promotes the formation of a strong gel-like network. Scaling of the elastic modulus with clay concentration gave a power law fit. The power exponent was related to the flocculation or stability of the Na+-montmorillonite suspensions, depending on the clay particle coverage.