Complexation between Oppositely Charged Polyelectrolytes in Dilute Solution: Effects of Charge Asymmetry

Abstract

We use dissipative particle dynamics to study polyelectrolyte complexation in dilute solutions under conditions of either chain-length or concentration asymmetry between the polycation and the polyanion, characterized by the macromolecular charge ratio of the minor component to the major component. Our results show that generally the systems form clusters carrying net macromolecular charges. The stoichiometry of these net-charged macromolecular clusters depends on the overall charge asymmetry, and clusters with certain stoichiometry can dominate the cluster population under a wide range of chain-length and concentration asymmetry. The morphology and stability of these net-charged clusters are closely related to their stoichiometry. When the charge ratio reaches a threshold value, the polyions condense into a single large coacervate cluster, signaling the onset of macroscopic coacervation. The threshold value is less for concentration asymmetry systems than for chain-length asymmetry systems at the same salt concentration. This threshold value decreases with added salt, as salt ions cause merging of small clusters into larger ones, eventually leading to the dominance of a single large cluster. This “salting-out” phenomenon is followed by a “salting-in” behavior, as further addition of salt dissolves the large cluster. The preferred size and composition of the net-charged clusters observed in the simulation highlight the significant role of length and concentration asymmetry on polyelectrolyte complex coacervation and hint at the possibility of a microstructured liquid.

Driving force and pathway in polyelectrolyte complex coacervation

“…This electrostatically induced LLPS underpins a number of important biological phenomena such as membraneless organelles in cells ( 3 – 6 ) and ocean life adhesion ( 7 – 11 ) and is also being exploited in novel biomedical and biomimetic applications such as drug delivery ( 12 – 14 ) and underwater adhesion ( 10 , 15 – 19 ). In the 6 decades since the pioneering theoretical work by Overbeek and Voorn ( 20 ), significant progress has been made on both the theory/simulation and experiment fronts in understanding the many effects on this LLPS, such as chain connectivity ( 21 – 30 ), excluded volume ( 22 , 25 , 31 – 35 ), charge sequence ( 36 – 40 ), ion pairing ( 22 , 41 , 42 ), charge asymmetry ( 43 – 47 ), temperature ( 48 – 53 ), pH ( 54 – 57 ), and solvent quality ( 58 – 60 ). We refer readers to several excellent recent reviews ( 61 – 65 ).…”

Wetting behavior of polyelectrolyte complex coacervates on solid surfaces

“…In reality, the oppositely charged polyions in the supernatant phase are likely to form dispersed clusters made up of two or more polyions. [67][68][69][70][71] Such a distinction undoubtedly influences the behavior of the supernatant near a solid surface. We recognize this as a limitation of our study; however, using a uniformly mixed assumption allows for the complete phase diagram (Fig.…”

Polyelectrolyte-multivalent molecule complexes: physicochemical properties and applications

“…By studying both charge- and concentration-asymmetry in PEC systems, Zhen-Gang Wang found that monovalent salts produce a salting-out phenomenon followed by a salting-in phenomenon due to electrostatic screening. 77,88 These results are not in agreement with most of PEMMC systems (salting out was not observed for PEMMCs), indicating that it is not enough to study only one type of asymmetry but to study both types of asymmetries and considering bulky systems. At the moment, the similarities and differences between PEMMCs and asymmetric PECs is far from being understood.…”

“…25 On the other hand, if the concentration of charges associated with the multivalent molecule is less than the concentration of charges associated with the polyelectrolyte (non-stoichiometric conditions), the aggregation process results in the formation of clusters that are superficially charged. 88 While the first stage of PEMMC formation is very fast and occurs in the range of seconds, a further slower size-increasing regime takes place, dominated by coalescence or precipitation in a similar way to the formation process of PEC coacervates or precipitates. 29,85 Under specific experimental conditions (highly non-stoichiometric), PEMMCs aggregates display a defined size in the range of a few hundred nanometers and remain relatively stable (do not undergo precipitation) in colloidal suspension (PEMMC nanocomplexes).…”

“…The reason why it is so, is that PEMMCs can be considered as complexes formed by interaction between a longchain polyion and a short-chain polyion. 88,110 In other words, the system is, essentially, a highly asymmetric PEC. It is not surprising, then, that PEMMCs had the tendency to dissolve at elevated ionic strength solutions.…”