Interfacial Behavior of Solid- and Liquid-like Polyelectrolyte Complexes as a Function of Charge Stoichiometry

Abstract

We systematically investigate in this work the surface activity of polyelectrolyte complex (PECs) suspensions as a function of the molar charge ratio Z (= [-]/[+]) from two model systems: the weakly and strongly interacting poly (diallyldimethylammonium chloride)/poly (acrylic acid sodium salt) (PDADMAC/PANa) and poly (diallyldimethylammonium chloride)/poly (sodium 4- styrenesulfonate) (PDADMAC/PSSNa) pairs, respectively. For both systems, the PEC surface tension decreases as the system approaches charge stoichiometry (Z = 1) whenever the complexation occurs in the presence of excess PDADMAC (Z < 1) or excess polyanion (Z > 1) consistent with an increased level of charge neutralization of PEs forming increasingly hydrophobic and neutral surface-active species. The behavior at stoichiometry (Z = 1) is also particularly informative about the physical nature of the complexes. The PDADMAC/PANa system undergoes a liquid–liquid phase transition through the formation of coacervate microdroplets in equilibrium with macroions remaining in solution. In the PDADMAC/PSSNa system, the surface tension of the supernatant was close to that of pure water, suggesting that the PSSNa-based complexes have completely sedimented, consistent with a complete liquid–solid phase separation of an out-of-equilibrium system. Besides, the high sensitivity of surface tension measurements, which can detect the presence of trace amounts of aggregates and other precursors in the supernatant, allows for very accurate determination of the exact charge stoichiometry of the complexes. Finally, the very low water/water interfacial tension that develops between the dilute phase and the denser coacervate phase in the PDADAMAC/PANa system was measured using the generalized Young–Laplace method to complete the full characterization of both systems. The overall study showed that simple surface tension measurements can be a very sensitive tool to characterize, discriminate, and better understand the formation mechanism of the different structures encountered during the formation of PECs.

Complexation in Aqueous Solution of a Hydrophobic Polyanion (PSSNa) Bearing Different Charge Densities with a Hydrophilic Polycation (PDADMAC)

“…However, the out-of-equilibrium conditions of the complexation or inhomogeneity of mixing probably explain why the complete charge neutralization was obtained at Z slightly different from 1. The ZP measurements also highlighted an abrupt reversal of the charge of the complexes around the point of zero charge (pzc), which confirms that PEC particles remained highly charged and rather stable even in the presence of a small excess of PE [10] (Figure 8). Furthermore, there was no significant difference in the absolute values of ZP as a function of the sulfonation rate, suggesting that the complexation mechanism and stoichiometry were similar for all P(Stco-SSNa).…”

“…However, other interactions, such as hydrogen bonding or hydrophobic ones, may play an additional part [2]. Two types of complexes can be distinguished on the basis of their physical nature, i.e., solid or liquid, the latter belonging to the broader family of coacervates [10]. Intermediate physical states can be observed as gel-like states, evidencing the existence of a continuum of morphology depending on the interaction strength between the two oppositely charged PEs or the complexation conditions used (temperature, ionic strength, solvent) [11].…”

Concentration-dependent characteristics of a complex coacervate

“…with PE concentration. 14 It is therefore conceivable that such an increase in ionic strength is also responsible for the suppression of coacervation. The concentration of free counterions is known to increases linearly as 𝐶𝐶 𝑐𝑐 = 𝐶𝐶 𝑃𝑃𝑃𝑃 𝑓𝑓 𝑒𝑒𝑒𝑒𝑒𝑒 , with feff the effective charged fraction of polymer units (SI).…”

“…[8][9] When the oppositely charged PE mixture is not at charge stoichiometry, complexation typically results in colloidal polyelectrolyte complexes where the excess PE forms an electrosterically stabilizing shell around the neutral complexed cores 10 with a solvation, charge, hydrophobicity that vary with the molar charge ratio Z([-]/[+]). 9,[11][12][13][14] At charge stoichiometry, when the amount of anionic and cationic charges are equal, two very different situations can occur depending on the strength of the interaction. When the charged chains are in strong interaction (binding constant kb>10 6 M -1 ), the system undergoes a liquid-solid transition generating aggregates that eventually sediment with time as in the case of the well-known PDADMAC/PSSNa system.…”

Wetting behavior of polyelectrolyte complex coacervates on solid surfaces

“…1,[16][17][18] In the past decade, there has also been significant progress in characterizing the interfacial properties of the coacervate-supernatant interface-notably, the interfacial tension. 19,20 Experiments and theory of symmetric mixtures of polyelectrolytes have characterized the scaling of interfacial tension with the salt concentration relative to the critical salt concentration [21][22][23][24][25][26][27][28][29] (g B [f crit AE À f AE ] 3/2 ) and with the chain length. 22,24,30 Simulations and theory also give access to additional interfacial properties, such interface thickness and excess adsorption.…”