Influence of chain length of amido betaines and amine degree of diamines on the binary supramolecular assembly and viscosity dynamics of amido betaine/diamine coacervates

Recently, there has been growing interest in the hierarchical assemblies of zwitterionic betaine amphiphiles across various fields due to their utility as stimuli-responsive materials. Herein, we systematically investigate the binary supramolecular assembly of zwitterionic amido betaines and diamines to determine how alkyl chain length of amido betaines (C_nDAB) and amine degree of diamines influence their relaxation dynamics of the resultant coacervates. To this end, we synthesized five C_nDAB molecules with systematically varying carbon chain lengths (n = 12, 14, 16, 18, and 20) and conjugated them with three different diamines (ethylenediamine, EDA; n,n'-dimethylethylenediamine, DMEDA; and n,n,n',n'-tetramethylethylenediamine,TMEDA). We employed rheology to compare the bulk properties and relaxation dynamics of these assemblies as well as to gain insight into their responsiveness to pH stimulus. All betaine/diamine co-assemblies for all pH values showed shear-thinning behavior while the onset of shear thinning behavior showed some variation for the shear rate inducing such an onset. By changing molecular architecture of co-assembling pairs, zero-shear viscosity values varied from ∼10⁻¹ Pa s to ∼10³ Pa s at a concentration of 100 mM C_nDAB and 50 mM diamine in water. Four-order-of-magnitude difference in viscosity with small changes in molecular architecture and pH indicates that precise tuning of the rheological properties is possible simply by controlling the self-assembly tendencies and nano-to-micro scale aggregation morphologies through bi-molecular design. Out of 15 different combinations of betaine and diamine pairs studied, the primary amine EDA conjugated with C₁₈DAB resulted in the highest degree of pH-controlled viscosity changes (i.e., highest pH-responsivity). Below 16-carbon alkyl chains on the betaines, pH responsiveness mostly disappeared. Overall, this systematic study brings new insights into the molecular structure-property relationships of amido betaine/diamine systems, which are widely used in diverse sets of applications and fields.