Poly(ethylene oxide) Is Positively Charged in Aqueous Solutions

Abstract

There have been controversies about the binding of cations to poly(ethylene oxide) (PEO) chains in aqueous solutions. In the current study, single molecular evidence of charging PEO chains by cation binding in aqueous solutions is provided. From the adoption of the photon-counting histogram method, it is discovered that the local pH value at the vicinity of the PEO chain is higher than the bulk solution, showing that the PEO chain is positively charged. Such a situation exists with and without the presence of salt (NaCl) in the solution, presumably due to the binding of cations, such as hydronium and sodium ions. Single molecular electrophoresis experiments using fluorescence correlation spectroscopy demonstrate that the PEO chains are weakly charged with a charging extent of ~5%. In comparison to the salt-free condition, the addition of external salt (NaCl) at moderate concentrations further charges the chain. The charging causes the PEO chains to expand and a further increase in the salt concentration causes the chain to shrink, exhibiting a polyelectrolyte-like behavior, demonstrated by the hydrodynamic radii of a single PEO chain. The effect of ion identity is discovered with alkali cations, with the order of the charging capacity of Li+ < Na+ < Cs+ < K+.

Keywords: cations; charged; polyethylene oxide; single molecule fluorescence.

Figure 1

Auto-correlation functions of the fluorescence-labeled (a) PEO2k and (b) PEO40k in aqueous solutions under different salt concentrations: the salt-free condition, and salt concentrations of 1.0 × 10⁻⁴ and 1.0 M, respectively. The solid lines denote the result of the numerical fitting using a three-dimensional Brownian motion model. The fast dynamics around 10⁻⁶ s is due to the triplet state relaxation.

Figure 2

Values of the hydrodynamic radius ($R_h$) of the PEO single chain as a function of the salt concentration in aqueous solutions: (a) PEO2k and (b) PEO40k. The dashed lines denote the value under the salt-free condition of each case. (c) The normalized $R_h$ value (the ratio of $R_h$ over the value under the salt-free condition, $R_{h,0}$ ).

Figure 3

The auto-correlation function of PEO40k moving under the electric field. The values of the electric field strength are displayed accordingly in the figure. (a,b) are for the salt-free condition and salt concentration of 10⁻⁴ M, respectively. The solid lines denote the fitting results using a three-dimensional Brownian motion with directional motion. (c): the velocity of the PEO40k single chain ($v$) as a function of the strength of the electric field ($E$ ). The solid lines denote the fitting of linearity.

Figure 4

(a) Values of the molecular brightness (ε) of OG514 as a function of pH in the solution when it is free and attached to the chain-end of PEO2k under different salt concentrations. (b) Values of local pH at the PEO chain-end as a function of the pH value of the solution. The dashed line denotes the results of free OG514.