Behavior of Hydrophobic Polyelectrolyte Solution in Mixed Aqueous/Organic Solvents Revealed by Neutron Scattering and Viscosimetry

Abstract

We investigate in this paper the influence of the improvement of the solvent quality on the structure and the viscous properties of solutions of an hydrophobic polyelectrolyte, poly(styrene-co-sodium styrenesulfonate): PSS. The solvent used is a mixture of water and an organic solvent, THF, which is also slightly polar. We use small angle neutron scattering in the semidilute regime and viscosimetry as a function of concentration in dilute and semidilute unentangled regime. The structure, namely the scattering from all chains, is characterized by a maximum ("polyelectrolyte peak"). Its position and amplitude depends, at a given sulfonation rate of PSS, on the solvent quality through the added amount of organic solvent (THF). These evolutions with the THF amount are more pronounced when the sulfonation rate f is low (more hydrophobic polyelectrolyte) and the amount of added THF is high. Adding THF to hydrophobic PSS (f = 0.50 or f = 0.38), diminishes also the "shoulder" visible in the log I - log q plot and associated with the pearl size. It is therefore proposed that when THF is added to aqueous polyelectrolyte solutions, the pearls are dissolved and the chain conformation evolves from the pearl-necklace shape already reported in pure water toward the string-like conformation in pure water for fully sulfonated PSS. An addition of THF also reduces the important low q upturn found with hydrophobic polyelectrolyte solutions: the large aggregates are dissolved by THF. The upturn can become for PSSNa f = 0.38, after adding enough THF (50%), even smaller than that for the charged hydrophilic case PSSNa f = 0.82, in water. This can mean that in the quasi-fully charged PSS at f = 0.82 there are still hydrophobic effects in water, which is disagreeing with our recent reports, or that the electrostatics contribution to the upturn is reduced due to a lower dielectric permittivity. Concerning the hydrophilic polyelectrolyte, poly(sodium-2-acrylamido-2-methylpropanesulfonate)-co-(acrylamide): AMAMPS, no evolution in structure occurs until 25% THF. The viscosimetry variation with THF fraction is in good agreement with the scattering one up to 25%: though little dependent on THF for AMAMPS, and for hydrophilic PSSNa, it increases for hydrophobic PSSNa in agreement with the chain expansion signaled by scattering. At 50% THF concentration, the hydrophilic polyelectrolyte shows new surprising behaviors: the scattering of PSSNa is no longer characteristic of polyelectrolytes, and AMAMPS solutions display an unexpected viscosity decrease.

Conformational transition of polyelectrolytes in mixed solvent by dielectric spectroscopy: Electrostatic and hydrophobic interactions

“…In general, electrostatic repulsion interaction between segments can be screened all or in part by counterions around chains, which is also considered as one of key factors to influence chains conformation . According to the research by Essafi et al, the degree of intermolecular aggregation is higher for stronger charged hydrophobic polyelectrolytes . It suggested that more counterions are distributed around a chain in the case of hydrophobic polyelectrolytes with high f i .…”

“…From the viewpoints of interactions, numerous studies showed that the solvent plays an important role by controlling monomers–solvent and monomers–monomers interaction . On the one hand, when the solvent qualify is low, hydrophobic attractive interaction is high, causing the collapse . On the other hand, few researches proposed that when the dielectric permittivity of the solvent is low, the effect of condensation of counterions onto chains is obvious, causing the decrease of electrostatic repulsive interaction between segments and the collapse of molecules .…”

“…[7][8][9][10][11] On the one hand, when the solvent qualify is low, hydrophobic attractive interaction is high, causing the collapse. 7,13,14 On the other hand, few researches proposed that when the dielectric permittivity of the solvent is low, the effect of condensation of counterions onto chains is obvious, causing the decrease of electrostatic repulsive interaction between segments and the collapse of molecules. 12,15,16 In order to clarify the effect of solvent qualify, it is necessary to explore how the electrostatic and hydrophobic interactions balance in the process of conformational transition.…”

Spectrophotometric study of the interaction of methylene blue with poly(styrene-co-sodium styrene sulfonate)

“…One can also note that THF addition causes a conformation transition from a collapsed conformation to a stretched one for PSSNa f = 0.37, however for the fully charged PSSNa f = 1, THF addition is not accompanied by transition conformation 37 and this difference may also affect the release of the dye from the two polyelectrolytes, being faster for PSSNa f = 0.37.…”

Study of the dynamical behavior of sodium alginate/myoglobin aqueous solutions: A dynamic light scattering study

“…At higher concentrations but still below the overlap concentration, c*, a transition regime of semiflexible polyions was identified. In this regime, beside a fast diffusion, that was found to follow a power law, [38][39][40][41] a slow mode was observed in many systems even if its origin is still under debate. According to the most accredited proposals, this slow mode is related with the presence of multichain domains [36,37,42,43] whose formation is highly influenced by many factors (such as polyelectrolyte molecular weight [36,37,44] and concentration [37,44]); notwithstanding some controversies, the slow diffusion coefficient was found to decrease with concentration [36,37,44].…”