Overlapping double layers in electrokinetics of concentrated dispersions

The traditional model of the Isolated Double Layer (DL) becomes inadequate in concentrated dispersions when the distance between particles becomes comparable to the Debye length and DLs overlap. The notion of "overlapping DLs" was introduced to Electrophoretic theory 60 years ago by Prof. T. Overbeek's group. The theory posed in that paper and a subsequent publication by Long and Ross predicts a decline of electrophoretic mobility due to DLs overlap. There is also more recent theory that describes electrokinetics at the extreme case of complete DLs overlap. We call it "quasi-homogeneous DL theory". It predicts an increase of electrophoretic mobility. We conducted an experimental study to verify which theoretical prediction is correct. We used an equilibrium dilution protocol, which allowed us to maintain the same interface-bulk electrochemical equilibrium at all volume fractions, ranging from 1 %vl up to 36 %vl (56 %wt). Electroacoustics were applied for measuring electrophoretic mobility and zeta potential. They remain constant up to 15% vl. At the higher volume fraction, where DLs become overlapping, they increase. This confirms prediction of "quasi-homogeneous DL theory" at least on qualitative level.