Titration of hydrophobic polyelectrolytes using Monte Carlo simulations

Abstract

The conformation and titration curves of weak (or annealed) hydrophobic polyelectrolytes have been examined using Monte Carlo simulations with screened Coulomb potentials in the grand canonical ensemble. The influence of the ionic concentration pH and presence of hydrophobic interactions has been systematically investigated. A large number of conformations such as extended, pearl-necklace, cigar-shape, and collapsed structures resulting from the subtle balance of short-range hydrophobic attractive interactions and long-range electrostatic repulsive interactions between the monomers have been observed. Titration curves were calculated by adjusting the pH-pK(0) values (pK(0) represents the intrinsic dissociation constant of an isolated monomer) and then calculating the ionization degree alpha of the polyelectrolyte. Important transitions related to cascades of conformational changes were observed in the titration curves, mainly at low ionic concentration and with the presence of strong hydrophobic interactions. We demonstrated that the presence of hydrophobic interactions plays an important role in the acid-base properties of a polyelectrolyte in promoting the formation of compact conformations and hence decreasing the polyelectrolyte degree of ionization for a given pH-pK(0) value.

Conformational transitions of a weak polyampholyte

“…[8][9][10] The overall shape of the titration curve depends on the nature of the charged polymer that is titrated. Fig.…”

“…12 The specific properties of weak polyelectrolytes have attracted considerable interest in experimental, 11,13 theoretical, 7,10,14,15 and simulation studies. 8,9,[16][17][18][19][20][21] On the other hand, strong or quenched polyelectrolytes, for example, poly(sodium styrene sulfonate), where the distribution of charges along the backbone is solely imposed by polymer synthesis dissociate completely in the total pH range accessible by experiment. The rich behavior of strong polyelectrolytes is qualitatively different from that of uncharged polymers.…”

“…Polyelectrolytes are more strongly stretched in the middle than at the ends, 14,24 which yields a weak logarithmic correction in Eqs. (7) and (8).…”

“…(1) because of electrostatic interaction between charges. The difference between the apparent and intrinsic dissociation constants, e.g., of an acid is obtained as [8][9][10][11] pK = pH − pK A 0 − log(f/(1 − f )).…”

“…30 A similar trend in Monte Carlo (MC) simulations of titration of hydrophobic polyelectrolytes have been observed where depending on the solvent quality for polymer backbone and pH − pK 0 value, a polyelectrolyte chain could be in five different conformational states: coil, globule, pearl necklace, sausage-like conformation, and fully stretched chain. 8 Raphael and Joanny have analyzed the behaviour of weakly charged quenched polyelectrolytes in a salt-free poor solvent using Khokhlov blob model. 7,27 The crucial point is that the electrostatic contribution to the chemical potential has a nonmonotonic variation with the degree of ionization.…”