(271d) The Effect of Free Surfactant and Grafted Surfactant Surface Coverage on the Rheology and Microstructure of Organoclay Dispersions

This work uses mechanical rheology to probe the solid-like network formed in organically modified montmorillonite clay dispersions. As the natural Na-montmorillonite is hydrophobic, a cation exchange reaction with a ditallow quaternary ammonium surfactant is necessary to render the clay organophilic. Here, we vary the quantity of surfactant relative to the cation exchange capacity of the clay in order to alter the grafted surfactant surface coverage. Unreacted free surfactant can be removed via hot alcohol refluxing. Hence, we may create samples in which the amount of free surfactant and surfactant surface coverage are independently altered. These quantities are assessed using thermogravimetric analysis (TGA). Samples of different organoclay loading are subjected to oscillatory and steady shear experiments in a controlled-stress rheometer. Experimental data show that all dispersions exhibit a solid-like response at low applied stress followed by yielding above an apparent yield stress. The presence of free surfactant is shown to significantly weaken the organoclay network, while a modest increase in surfactant surface coverage leads to a considerably more solid-like dispersion. The influence of free surfactant and surfactant surface coverage on organoclay exfoliation, tactoid size, and tactoid interactions are discussed in light of rheology and related novel wet scanning electron microscopy images.