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

Abstract

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 (CnDAB) and amine degree of diamines influence their relaxation dynamics of the resultant coacervates. To this end, we synthesized five CnDAB 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 CnDAB 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.