Mechanism of attraction between like-charged particles in aqueous solution

Although it has been long known that like-charged particles attract one another in aqueous media, the mechanism underlying this counter-intuitive phenomenon has remained controversial. We tested the hypothesis put forth long ago by Langmuir and again by Feynman and by Ise, that the attraction between like-charged entities lies in an intermediate of unlike charges. Tests were facilitated by the observation that the attractive forces could be confirmed between widely separated particles of macroscopic size. Two approaches showed comparable results. In the first, pH-sensitive dyes showed intermediate zones of opposite charge: an accumulation of protons was found between negatively charged spheres, whereas between positively charged spheres the intermediate zone contained OH- groups. In the second and complementary approach, microelectrode measurements showed that in between negatively charged spheres, the electrical potential was relatively positive, whereas between positively charged spheres it was relatively negative. Hence, both approaches confirm theoretical expectations. The large number of unlike charges lying in between the like-charged spheres may come from the build-up of the recently reported "exclusion zone'' surrounding each particle.