Direct observation of the transition to counterion condensation

Abstract

Direct observation of the transition to counterion condensation is demonstrated by measurement of the electrophoretic mobility of 6, 6-ionene, a polyelectrolyte of charge spacing 8.7 Å, under conditions of varying dielectric constant. In this way, the reduced charge density parameter ξ can be varied between 0.82 and 1.85. At precisely ξ=1, we observed a drop in the electrophoretic mobility of greater than a factor of 2. Independent viscosity experiments were used to verify that the polymer does not undergo a major conformational transition under these conditions. The reduction of electrophoretic mobility, presumably the result of the reduction of charge density by counterion condensation, is substantially greater in magnitude than current theories of counterion condensation theory would predict. It is suggested that the physical basis for the departure from theory is the fact that near ξ=1, the spacing of condensed counterions predicted by counterion condensation theory is greater than the Debye screening length.