What decides the size and composition of clay nano-aggregates in water? Charged polymers and multivalent ions competing in the formation of clay tactoids.
Résumé
In this manuscript we address the formation of highly organized clay tactoids intercalated with a charged polymer (ionene) in aqueous environment. We report on an original route to achieve such tactoids by starting with pre-formed clay tactoids, held together by multi-valent 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. This hints at an "earthworm" mechanism of the charged chains exchanging with the multivalent atomic ions inside the tactoids, which is surprising. Our observations are based on small angle X-ray scattering, where a strong correlation peak, called the stacking peak, is an undisputable signature of the stack/tactoid formation and is distinct for the multivalent-ion clay stacks and charged-polymer clay stacks. The linear charge density of the polymer chains is a key parameter for the tactoid transition. For weakly charged polymer chains, the transition simply does not take place, even under conditions of strong excess of the charged polymer. The initial state of multivalent-ion clay tactoids is essential for the formation of highly organized final charged-polymer clay tactoids. Starting from individual delaminated clay layers (Na+ exchanged clays) does not lead to the same final charged-polymer clay stacks. Overall, the final structure of charged-polymer clay stacks strongly depends on the initial structural state of the host clay layers, and on the linear charge density of the guest polymer chains.
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