Colloidal Systems in Concentrated Electrolyte Solutions Exhibit Re-entrant Long-Range Electrostatic Interactions due to Underscreening.

Surface force measurements have revealed that at very high electrolyte concentrations as well as in neat and diluted ionic liquids and deep eutectic solvents, the range of electrostatic interactions is far greater than the Debye length. Here, we explore the consequences of this underscreening for soft-matter and colloidal systems by investigating the stability of nanoparticle dispersions, the self-assembly of ionic surfactants, and the thickness of soap films. In each case, we find clear evidence of re-entrant properties due to underscreening at high salt concentrations. Our results show that underscreening in concentrated electrolytes is a general phenomenon and is not dependent on confinement by macroscopic surfaces. The stability of systems at very high salinity due to underscreening may be beneficially applied to processes that currently use low-salinity water.

Re-entrant swelling and redissolution of polyelectrolytes arises from an increased electrostatic decay length at high salt concentrations.

HYPOTHESIS: A detailed understanding of the influence of electrolytes on the conformation of polyelectrolyte chains is an important goal made challenging by the strong coupling between electrostatic interactions and chain conformation. This challenge is particularly evident at moderate to high salt concentrations where mean-field theories of electrolytes are no longer applicable and are therefore unable to predict the interactions between neutral or like charged surfaces that leads to re-entrant swelling of DNA and other polyelectrolytes at high salt concentrations. Recent developments arising from studies of surface forces in ionic liquids that have been extended to include a wide variety of monovalent electrolytes reveal a hitherto unknown increase in the electrostatic decay length at high electrolyte concentrations. We hypothesise that the re-entrant behaviour of polyelectrolytes is driven by an increasing electrostatic decay length with increasing electrolyte concentration. EXPERIMENTS: We survey numerous experiments in the literature on re-entrant swelling and calculate the effect of ion pairing on the electrostatic decay length in concentrated electrolytes. FINDINGS: Re-entrant solubility is driven by an increasing electrostatic decay length at high salt concentrations and is universal across all polyelectrolytes.