Charge Inversion at High Ionic Strength Studied by Streaming Currents

Abstract

We report charge inversion, the sign reversal of the effective surface charge in the presence of multivalent counterions, for the biologically relevant regimes of divalent ions and mixtures of monovalent and multivalent ions. Using streaming currents, the pressure-driven transport of countercharges in the diffuse layer, we find that charge inversion occurs in rectangular silica nanochannels at high concentrations of divalent ions. Strong monovalent screening is found to cancel charge inversion, restoring the original surface charge polarity. An analytical model based on ion correlations successfully describes our observations. DOI: 10.1103/PhysRevLett.96.224502 PACS numbers: 47.57.jd, 66.90.+r, 68.08.ÿp Screening by counterions is of fundamental importance in mediating electrostatic interactions in liquids. For multivalent counterions (Z ions, where Z is the ion valency including the sign), a counterintuitive phenomenon is observed: Screening not only reduces the effective surface charge, but it can also actually cause it to flip sign. This socalled charge inversion (CI) has been proposed to be biologically relevant in, e.g., DNA condensation, viral packaging, and drug delivery [1]. CI is not explained by conventional mean-field theories of screening. Recently, an analytical model was proposed that assumes that Z ions form a two-dimensional strongly correlated liquid (SCL) at charged surfaces [2]. This effect is particularly strong for high Z, and was confirmed experimentally for Z 3 and 4 [3]. Experimental evidence has remained inconclusive for the cases Z 2 and mixtures of Z ions with monovalent ions [4], both of which are biologically relevant given that K , Na , and Mg 2 are the most abundant cations in the cell. The main difficulty is that existing experimental probes become unreliable at high concentrations (*10 mM): Electrophoretic mobility measurements suffer from increasingly low signal to noise at higher salt, whereas surface force measurements are complicated by short-range forces.In this Letter, we investigate CI in individual silica nanochannels at high ionic strength by employing streaming currents as a new method. A streaming current is an ionic current that results from the pressure-driven transport of counterions in the diffuse part of the double layer [5], as illustrated in Fig. 1(b). The Stern layer, where the SCL is formed, is generally accepted to be immobile [6]. Consequently, streaming currents provide a direct measurement of the effective surface charge at the diffuse layer boundary. The well-defined rectangular channel geometry allows for straightforward interpretation. Contrary to other methods, streaming currents remain a reliable probe of the surface charge at high salt, up to 1 M in our experiments. We report unambiguous CI by divalent ions at concentrations above 400 mM. Additionally, we resolve the effect of screening by monovalent salt. We find that monovalent ions reduce CI by high-Z ions, and even cancel CI entirely at sufficiently high monovalent ion concentrations. We succ...

Effects of electrostatic correlations on electrokinetic phenomena

“…In Figure 6 we compare the model to the nanochannel electrokinetic data of van der Heyden et al [54] as was done by Gillespie et al [52]. In the experiment a nanochannel with a characterized surface charge is driven by a pressure driven flow and the streaming current is measured.…”

Theory of volume transition in polyelectrolyte gels with charge regularization

“…For the case of a polyelectrolyte gel immersed in a solution with divalent salts, our theory predicts the phenomenon of overcharging, 18,44,[84][85][86][87][88][89][90] which corresponds to an effective charge reversal of the polymer gel backbone. This effect can be seen in Fig.…”

Charge inversion accompanies DNA condensation by multivalent ions

“…Increasing the TRIS buffer concentration to 10 mM hinders charge inversion, causing c 0 to increase to 1 mM. Further adding 50 mM monovalent KCl salt causes charge inversion to disappear entirely for c ≤ 3 mM, reminiscent of the disappearance of charge inversion at glass surfaces with increasing monovalent salt 16 . This is probably why DNA charge inversion by multivalent ions has not been reported earlier as most DNA studies are carried out at physiological (high) salt concentrations.…”