Ionic-Group Dependence of Polyelectrolyte Coacervate Phase Behavior

Abstract

A complex coacervate is the polymer-rich phase resulting from liquid−liquid phase separation induced by two oppositely charged polyelectrolytes in an aqueous solution, which shows the considerable potential of functional materials and the substantial role of biological processes. In this study, the phase behavior of complex coacervates for differently charged moieties is investigated using well-defined polyether-based polyelectrolytes functionalized with ammonium, guanidinium, sulfonate, and carboxylate groups. Turbidity measurement and thermogravimetric analysis provide the stability and phase diagram of the complex coacervates, indicating a significant role of the pair of charged groups. In stark contrast to ammonium-based coacervates, guanidinium-based coacervates show considerable stability upon the addition of salt and enhanced dehydration with increasing salt at low salt concentrations. Together with all-atomistic molecular dynamics simulations, it is demonstrated that the chemical-specific interactions between charged molecules and the role of salts depending on the charged groups play essential roles in controlling the phase behavior of complex coacervates.

Ionic group-dependent structure of complex coacervate hydrogels formed by ABA triblock copolymers

“…† Note that the SAXS and SANS fitting results show close agreement and present qualitatively similar trends as functions of the salinity and the guanidinium fraction. At c NaCl = 0 M, the water fractions in the core, f core,water , coincide with thermogravimetrically measured values in our previous study: 24 f core,water = 0.6 and 0.4 for the A + S and G + S bulk coacervates which are analogous to G 0 + S and G 0.95 + S hydrogels, respectively. Furthermore, f core,water systematically decreases as x increases at constant c NaCl .…”

“…6(b) shows that c* increases with x, which is consistent with previous reports in which the greater salt resistance of coacervates at a higher guanidinium content is attributed to the hydrophobic nature of guanidinium groups. 22,24 Although the difference in the pK a of ammonium and guanidinium (pK a = 9.2 and 13.6 for ammonium and guanidinium chloride, respectively) was previously pointed out to account for the different salt resistance, 14 the pH B 7 in the present study ensures a negligible difference in the degree of ionization in both the ammonium and guanidinium groups. We note that increasing c* with enhanced hydrophobicity in homologous polyelectrolytes was also reported by others, where the hydrophobicity was controlled by the length of alkyl chains per charged repeat unit 43 or copolymerization of hydrophobic moieties with ionic monomers.…”

“…40,41 In addition, the formation of multiple hydrogen bonds was proposed as a possible reason for the stronger ion pair interaction involving guanidinium groups. 23,24 The inset in Fig. 6(a) shows a linear correlation between g core,0 and the polymer fraction in the coacervate core at the zero-salt limit, f core,pol,0 , which was obtained from the linear extrapolation from the data in Fig.…”

“…This behavior potentially indicates that the guanidinium group is more hydrophobic than the ammonium group (as discussed below). 24 With increasing c NaCl , the dissociation of the polyelectrolytepolyelectrolyte ion pairs led to the enhanced hydration of coacervate phases as signified by the increase in f core,water . This is accompanied by the simultaneous decreases in the core radius, R core , and the aggregation number, defined as the total number of end-blocks per core, N agg = (1 À f core,water )(4p/3) R core 3 /v core , where v core is the average molecular volume of the polyelectrolyte chains (see Fig.…”

“…Since the addition of salt reduces the interaction of the intrinsic ion pairs or even breaks them and allows more water molecules to swell and lubricate the coacervate phase, 30,31 the chain mobility of the coacervate phase and thus the hydrogel relaxation process are enhanced at higher salt concentrations. Consequently, the weaker salt responsiveness of guanidinium-rich C3Gs indicates that the guanidinium-sulfonate (G-S) ion pair are harder to break with salt than the ammonium-sulfonate (A-S) pair, 24 leading to slower relaxation. 19,22 A more detailed investigation of the ionic group effect on the relaxation kinetics will be discussed in a separate study.…”