(16c) Morphological and Rheological Transitions of Zwitterionic Surfactant Based pH-Tunable Dynamic Binary Complex Coacervates

The long chained amphiphilic structure of surfactants has made it a prime candidate for a wide variety of applications encompassing consumer goods, pharmaceutics, oil and gas, etc. Particularly, zwitterionic surfactants, which comprise of both a negative and positive head group, have received a lot of attention in recent years due to their relatively mild nature and diverse properties. Supramolecular chemistry involves exploring dynamic noncovalent interactions between molecules, which are reversible in nature and thus capable of interesting architectures and properties. Upon combining a synthesized zwitterionic surfactant 2-(dimethyl(octadecyl)ammonio)acetate) (stearyl betaine) with diethylenetriamine as a supramolecular complexing agent, stimuli-responsive dynamic binary complexes (DBCs) can be formed.

Herein, aqueous suspensions of the above constituents were prepared, keeping the total loading at 2 wt%. A pH-sensitive system with higher viscosities in the acidic regime was identified. The rheological changes that occur in the system were studied by measuring the viscosity and storage/loss modulus at various pH values. Differential scanning calorimetry(DSC) was used as a tool to characterize the thermal and phase transitions within the system. Also, the morphological changes were characterized by using atomic force microscopy (AFM) and small angle X-ray scattering (SAXS).

Long rod-like hollow nanotubular structures were identified as responsible for the high viscosities, which are quite different from wormlike micelles that have been popular in literature so far. Vesicles and lamellar 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.