Size and Composition of Clay Nanoaggregates in Water: Charged Polymers and Multivalent Ions Competing in the...

In this paper, we address the formation of highly organized clay tactoids intercalated with a charged polymer (ionene) in an aqueous environment. We report on an original route to achieve such tactoids by starting with preformed clay tactoids, held together by multivalent inorganic atomic ions, as is the case in clay suspensions exchanged with Ca²⁺ or La³⁺ ions. Contrary to previously evoked mechanisms of disaggregation-aggregation or successive delamination of individual platelets ("peeling"), we observe clearly a reversible transition between the two types of clay tactoids at an almost constant number of clay nanoplatelets per tactoid. Our observations are based on small-angle X-ray scattering, where a strong correlation peak, called the stacking peak, is an indisputable signature of the stack/tactoid formation and is distinct for the multivalent-ion clay stacks and charged-polymer clay stacks. The initial state of multivalent-ion clay tactoids is essential for the formation of highly organized final charged-polymer clay tactoids. It is as if the preformed multivalent-ion clay tactoids provided a template into which the charged polymer chains get incorporated. Starting from individual delaminated clay layers (Na⁺ exchanged clays) does not lead to the same final charged-polymer clay stacks. The linear charge density of the polymer chains is another key parameter for the tactoid transition. For weakly charged polymer chains, the transition simply does not take place, even under conditions of a strong excess of the charged polymer. Overall, the final structure of charged-polymer clay stacks strongly depends on the initial structural state of the host clay platelets and the linear charge density of the guest polymer chains.