Probing the Inhomogeneous Charge Distribution on Annealed Polyelectrolyte Star Polymers in Dilute Aqueous Solutions

Abstract

The conformational structure of a polyelectrolyte chain in dilute aqueous solution is strongly coupled with its surrounding electrostatic environment. With the introduction of branched topology, the distribution of counterions in the vicinity of a polyelectrolyte chain becomes highly inhomogeneous, giving rise to complex structures of branched polyelectrolytes in dilute aqueous solution. To directly probe the local electrostatic conditions near a branched polyelectrolyte in aqueous solutions, star-shaped poly(2-vinylpyridine) (P2VP) polymers with precise labeling of one single fluorophore at different locations, for example, the star center or the terminal group of one arm, were synthesized using reversible addition− fragmentation chain transfer (RAFT) polymerization of vinyl-terminated P2VP macromonomers. Using fluorescence correlation spectroscopy (FCS) combined with photon counting histogram (PCH) analysis, the conformational structures and local electric potential of P2VP star polyelectrolytes were investigated in dilute aqueous solutions of varied pH at a single molecule level. Despite the same hydrodynamic radius of P2VP stars, pH-sensitive fluorophores labeled at different locations sensitively differentiated the higher electric potential at the star center from the lower electric potential at the periphery in dilute aqueous solutions.

Explicit Ion Effects on the Charge and Conformation of Weak Polyelectrolytes

“…Indeed, we do not expect it, as it is entropically unfavorable for sufficient numbers of co- and counter-ions to occupy the region around the polyelectrolyte and meaningfully screen charged monomer interactions. The behavior of explicit-salt simulations is thus more similar to what has been observed in experiment [49,66,89].…”

“…In particular, for single-chain systems, it has been demonstrated both experimentally [49,50,51,52] and computationally [46,53,54,55,56] that the well-known transition of a poorly solvated titratable polymer from a collapsed (globule) state to an extended (coil) state (the coil–globule transition, or CGT) as a function of pH can be strongly dependent on the salt content of a solution. The CGT in particular, is of great interest in drug delivery, as the transition from a loaded globule state to an expanded coil can release drug at a specific site of action defined by the local pH [24].…”

Absorbed weak polyelectrolytes: Impact of confinement, topology, and chemically specific interactions on ionization, conformation free energy, counterion condensation, and absorption equilibrium

“…S4) neatly demonstrated the influence of vicinal charges on ionization when ξ = 0, even though these are quantities averaged over the whole polymer. Single monomers may, instead, behave somewhat differently depending on their location along arms or chains due to the difference in the local electrostatic potential . That this is the case, it has already been shown by previous SCF or MC investigations on linear and starlike species; these have suggested that chain ends are more ionized than monomers located around the chain midpoint or attached to the central branching point in starlike polyelectrolytes.…”

“…Thanks to improved synthetic approaches, it has nowadays become possible to generate weak polyelectrolytes with structures differing from the standard linear one such as treelike, starlike, or brushes. The change in geometrical disposition and, hence, local density of ionizable monomers compared with linear species should obviously impact on the electrolytic properties of the macromolecules, so that the behavior with respect to global or local ionization, conformations in solution, the thermodynamics of confinement, as well as the relative distribution of ions and ionized polymers may be markedly affected, as it neatly emerges titrating starlike poly(acrylic acid) (PAA), poly(dimethylamino ethyl methacrylate) (PDMAEMA), and poly(diethyl amino ethyl methacrylate) . For instance, the osmotic coefficient φ = π meas / π theo in the mentioned cases suggests that the amount of CIs acting as independent entities markedly decreases upon increasing the number of arms in the starlike polymer.…”

“…We do so with the intent of gauging the impact of confinement on the ionization behavior, especially with respect to the degree of branching, as well as the impact of the degree of ionization on the change in free (Helmholtz) energy associated with polymer encapsulation. Improving on the modeling approach employed previously, we explicitly included the presence of CIs in our in silico MC titration simulations to better mimic experiments . The polyelectrolytes are therefore contained in a cavity permeable to explicitly treated pH‐defining or neutralization deriving ions; this choice, more general than enclosing all particles inside the same cavity, allows us to seamlessly connect investigations dealing exclusively either with neutral chains or strong polyelectrolytes in capsids.…”