title: Polyelectrolyte conformational transition in aqueous solvent mixture influenced by hydrophobic interactions and hydrogen bonding effects: PAA-water-ethanol.
abstract: Molecular dynamics simulations of poly(acrylic acid) PAA chain in water-ethanol mixture were performed for un-ionized and ionized cases at different degree-of-ionization 0%, 80% and 100% of PAA chain by Na(+) counter-ions and co-solvent (ethanol) concentration in the range 0-90vol% ethanol. Aspects of structure and dynamics were investigated via atom pair correlation functions, number and relaxation of hydrogen bonds, nearest-neighbor coordination numbers, and dihedral angle distribution function for back-bone and side-groups of the chain. With increase in ethanol concentration, chain swelling is observed for un-ionized chain (f=0) and on the contrary chain shrinkage is observed for partially and fully ionized cases (i.e., f=0.8 and 1). For un-ionized PAA, with increase in ethanol fraction ϕeth the number of PAA-ethanol hydrogen bonds increases while PAA-water decreases. Increase in ϕeth leads to PAA chain expansion for un-ionized case and chain shrinkage for ionized case, in agreement with experimental observations on this system. For ionized-PAA case, chain shrinkage is found to be influenced by intermolecular hydrogen bonding with water as well as ethanol. The localization of ethanol molecules near the un-ionized PAA backbone at higher levels of ethanol is facilitated by a displacement of water molecules indicating presence of specific ethanol hydration shell, as 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 interactions between counter-ions and charges on the PAA chain also influence chain collapse. The underlying origins of polyelectrolyte chain collapse in water-alcohol mixtures are brought out for the first time via explicit MD simulations by this study.
authors.affiliation: Macromolecular Modeling and Simulation Lab, Department of Chemical Engineering, Indian Institute of Technology (IIT) Madras, Chennai 600036, India.
authors.forename: Praveenkumar
authors.initials: P
authors.lastname: Sappidi
authors.affiliation: Macromolecular Modeling and Simulation Lab, Department of Chemical Engineering, Indian Institute of Technology (IIT) Madras, Chennai 600036, India. Electronic address: unatarajan@iitm.ac.in.
authors.forename: Upendra
authors.initials: U
authors.lastname: Natarajan
mesh_terms: D000180:Acrylic Resins / Q000737:chemistry*; D000465:Algorithms; D000431:Ethanol / Q000737:chemistry*; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D008958:Models, Molecular; D008968:Molecular Conformation; D012997:Solvents / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
chemical_list: D000180:Acrylic Resins; D012997:Solvents; D014867:Water; D000431:Ethanol; C006903:carbopol 940
keywords: Collapse; Hydrogen bonding; Hydrophobic interactions; Molecular dynamics simulations; Poly(acrylic acid); Swelling; Water–ethanol