April 10th – Presentation by J. Lyklema (Wageningen University): Overcharging by simple ions: observations and interpretations

April 10th – Presentation by J. Lyklema (Wageningen University): Overcharging by simple ions: observations and interpretations

We are glad to announce that J. Lyklema will give a presentation entitled Overcharging by simple ions: observations and interpretations.The presentation will be on April 10th at 13.30 in room ZH 286. The abstract reads:

As a whole, electrical double layers are electroneutral. For double layers around colloidal particles their surface charge σ is equal and opposite to their countercharge. The classical way of measuring that is by comparing σo with the electrokinetically measured charge, σek . As electrokinetically only part of the charge is measured, the general trend is that the two charges have the same signs although the absolute value of σek < σo.

In some cases the two charges have different signs, suggesting that the surface charge is overcompensated by the countercharge. It is a counterintuitive phenomenon and called “overcharging”. No surprise that overcharging has drawn much attention. Several mechanisms for overcharging have been proposed. Traditionally is that by specific adsorption: more than equivalent binding of counterions by non-electrostatic forces. A familiar illustration is that of a negatively charged latex particle onto which a superequivalent amount of cationic surfactant adsorbs (by hydrophobic bonding). But simple counterions, especially multivalent ones, can also have the propensity of overcharging.

Besides this “chemical” interpretation of overcharging also a physical one has been proposed, namely by so-called ion correlations. This luxury problem of having two different options for interpreting overcharging has given rise to an interesting debate in the literature that led to interesting new information but the issue is not yet finalized. The status quo will be discussed.