Dear friend Ambuz Basak

Ah, the intriguing behavior of your benzimidazolium carboxybetaine-based poly(zwitterion ionic liquid) (PZIL) has brought forth some contradicting results in its pH-responsive behavior. Let DAN unravel the mysteries for you!

In the scenario you described, it is indeed possible for a zwitterionic poly(ionic liquid) not to exhibit pH-induced self-aggregation during the initial pH adjustments but later on. This behavior can be attributed to the complex interplay of multiple factors, including the polymer structure, counterions, and the specific pH conditions.

The absence of self-aggregation at the initial pH adjustment (from pH 4 to 5) could be due to the relatively low charge density and weak electrostatic interactions between the polymer chains in the zwitterionic form. At this pH range, the zeta potential of the aqueous PZIL is still positive (+5 mV), indicating a predominantly repulsive electrostatic interaction. Hence, the polymer chains may remain dispersed and exhibit no significant self-aggregation.

However, when you initially lower the pH below 3, the carboxylate groups of the PZIL become protonated, leading to a higher charge density and stronger coulombic interactions between the polymer chains. This increased charge density facilitates the formation of aggregates or clusters via attractive electrostatic interactions, resulting in a turbid suspension upon subsequent base addition.

The observed two-phase solution at the pI (pH 4.5) suggests a balance between attractive and repulsive forces within the system. At this specific pH, the coulombic interaction-driven self-assembly becomes more pronounced as the carboxylate groups are partially deprotonated, leading to the formation of larger aggregates or clusters. The resulting phase separation manifests as a two-phase solution.

It's worth noting that the pH-responsive behavior of polymers can be intricate, and it is influenced by various factors such as polymer composition, charge density, counterions, and environmental conditions. The observed discrepancies in self-aggregation behavior under different pH adjustment sequences highlight the sensitivity of the system to the specific pH history and the kinetics of aggregation processes.

To further investigate and understand this phenomenon, you may consider conducting additional experiments at different pH ranges, systematically varying the pH adjustment sequence, and examining the effects of concentration and temperature. This comprehensive approach will provide deeper insights into the pH-responsive behavior of your PZIL system.

Remember, the realm of polymers is vast and filled with surprises. Keep exploring, and may your future experiments shed more light on the mysteries of your intriguing benzimidazolium carboxybetaine-based poly(zwitterion ionic liquid) (PZIL)!

Let us keep exploring this interesting topic.

Thank you for your valuable answer!

Well, I want to clarify: at pH 5, the zeta value was found to be -19 mV! Indeed, that must result from excess OH- associated with the polymer chains (or, should I say, 'adsorbed on the surface"?).

Thanks again, in advance.