The amphiphilic nature of surfactants has made it a sought-after commodity for a wide variety of applications encompassing consumer goods, pharmaceutics, oil and gas, etc. Supramolecular chemistry looks at dynamic noncovalent interactions between molecules, which are reversible in nature and thus opens the possibility of fine-tuning their architectures and properties. By combining surfactants with diethylenetriamine as a supramolecular complexing agent, stimuli-responsive dynamic binary complexes (DBCs) can be formed.<br/><br/>Herein, we examined 2 separate surfactant systems as the DBC: oleic acid and stearyl betaine, both with diethylenetriamine as the complexing agent. Aqueous suspensions of the above constituents were prepared, keeping the total loading at 2 wt%. We studied the rheological changes that occur in the system by measuring the viscosity and storage/loss modulus at various pH values. The high viscosity range for oleic acid based DBC was alkaline and was sensitive to the presence of NaCl as well. In the stearyl betaine based DBC, the working condition was acidic. The morphological changes were also characterized by using atomic force microscopy (AFM) and small angle X-ray scattering. We were able to identify long rod-like structures, different from that of the commonly reported wormlike micelle, that entangle and subsequently enhance the viscosity of the suspension. Vesicles and sheet-like morphologies were also observed in less viscous solutions. Upon gaining a comprehensive understanding from all the results, a transition mechanism between different suspension architectures was proposed.

Morphological and rheological transitions of pH-responsive dynamic binary complex suspensions with nanotubular architecture

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ACS Fall 2022

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