MD Simulation Study of Polyelectrolyte Chain Collapse in Aqueous Mixed Solvent Due to Hydrophobicity: PAA–Water–Ethanol

Molecular dynamics (MD) simulations were performed to investigate conformational transitions, especially swelling and collapse, of poly(acrylic acid) PAA in dilute water-ethanol solution, as a function of ethanol concentration (leading up to phase separation level) and degree-of-ionization (i.e. charge density).  Results were compared to available experimental data from literature on this system. With increase in ethanol concentration, chain swelling (increase of radius of gyration) is observed for un-ionized chain (f=0), and on the contrary for partially and fully ionized cases chain collapse is observed.  Snapshot of ionized PAA chain with solvent molecules at 90 vol% ethanol at which significant reduction of <R_g> occurs is shown in figure 1, and the variation of <R_g> with charge density and ethanol concentration is shown in figure 2.  The addition of ethanol to the solution results in expansion of the compact-form for un-ionized chain due to hydrophobic effect. The inter and intra-molecular hydrogen bonds were analyzed. With an increase in ethanol concentration the number of hydrogen bonds between un-ionized PAA and ethanol molecules shows increase and that between PAA and water molecules shows a decrease. For the ionized PAA case, chain shrinkage is found to be influenced by extent of intra-chain and intermolecular hydrogen bonding with water as well as ethanol.  We observe that the number of intra-chain hydrogen bonds of PAA increases with addition of ethanol to the solution, in the entire range of ionization (charge density 0 < f < 1). The localization of ethanol molecules near the PAA backbone at higher levels of ethanol is  facilitated by a displacement of water molecules indicating presence of specific ethanol hydration shell, confirmed by results of the RDF curves and coordination number calculations.  This behavior, controlled by hydrogen bonding provides a significant contribution to such a conformational transition behavior of the polyelectrolyte chain. The canonical electrostatic potential energy contributions were analyzed.  The increase in overall ethanol content leads to formation of ethanol clusters near the PAA chain, thereby accompanying chain shrinkage. The interactions between counter-ions and charges on the PAA chain also influence chain collapse, by way of intermolecular attraction due to dipolar interactions leading to formation of multiplets and a decrease in the osmotic pressure of the chain given the decrease in number of mobile counter-ions.

Fig 1.  Snapshot of solution containing fully ionized PAA chain (ethanol: water=90:10 by vol.).

 

(a)                                                                                                                                           (b)  

                        

                                     

                                                    (c)

Fig 2. Radius of gyration <R_g> with increase in ethanol fraction for (a) f = 0, (b) f=0.75 and (c) f=1.