(556g) 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.