Attractive and “repulsive” forces between neutral particles in ionic solution

The electrical force between a pair of uncharged dielectric particles immersed in an electrolyte is studied. The free energy of interaction which is obtained using a double charging procedure separates into two parts: one, the Lifshitz–van der Waals interaction free energy and two, electrical terms associated with the electrolyte, the latter being calculated using a Debye–Huckel type approximation.The electrical terms consist of a screened attractive interaction and a long range unscreened repulsive interaction with Hamaker constant ∼3 × 10–14 erg for water–hydrocarbon systems. It is demonstrated that this repulsive force cancels identically with the zero frequency term in the Lifshitz–van der Waals part of the interaction free energy.