Monte Carlo simulations of hydrophobic weak polyelectrolytes: Titration properties and pH-induced structural transitions for polymers containing weak electrolytes

Abstract

Monte Carlo simulation has been used to study titration and configurational properties of an isolated hydrophobic polymer containing weakly-ionizable groups. Using a cubic lattice, simulations were performed in the grand canonical ensemble to include the effect of the local charge environment on the ionization of weak electrolyte segments. Properties were studied as a function of polymer hydrophobicity, fraction of ionizable segments, solution ionic strength, and pH. The polymer segments experienced three types of competing interaction: excluded volume, attractive nearest-neighbor forces which account for the net balance of segment–segment, segment–solvent, and solvent–solvent interactions, and long-range electrostatic forces between ionized segments, calculated with a screened Debye–Hückel potential. Simulations show that the model chain expands with chain ionization, which depends on solution pH. As the chain becomes increasingly charged, the ionization process becomes more difficult because of rising local charge density around the ionizable segments. The effect of rising local charge density increases for larger fractions of ionizable groups, with increased hydrophobicity and with low ionic strength.

Monte Carlo simulations of a hydrophobic weak polyelectrolyte. Charge distribution as a function of conformation

“…As previously reported (19), there is a close relationship between chain conformation (chain stiffness) and a.. The same holds true for the shape of the q(r) curves.…”

“…The polymer is represented as a selfavoiding walk of N segments on a cubic lattice. The fraction of potentially ionizable beads, A., is 0.325 .for comparison to previous work (19). The interaction between ionized segments is described by a screened Debye-Hiickel coulombic potential:…”

“…The cot;tfigurations were obtained using the Metropolis Monte Carlo method and recorded every 3000 attempted moves. Details of the simulations are given in our previous paper (19).…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Simplified polyelectrolyte models, while retaining the essential features of the system, allow studies of larger regions of phase space than would be possible for more sophisticated descriptions.…”

“…The simplicity of the lattice model has made possible incorporation of the effect of hydrophobicity and observation of the concomitant structural transition in response to chain ionization (18). Most recently, we have used the lattice model in the grand-canonical Monte-Carlo simulation to study pH-induced structural transitions in hydrophobic, weak polyelectrolytes (19). For simulations in the grandcanonical ensemble, the ionization state of each of the ionizable segments is not constant but fluctuates in response to the local charge environment.…”