Interplay of ion-specific and charge-density effects in aqueous solutions of weakly charged ionenes as revealed by electric-transport measurements.

In 3,3-ionenes, one quaternary nitrogen is bonded to a chain of three methylene groups on each side, and in 6,9-ionene, it is bonded to a chain of six on one side and nine on the other. We examined how the solution properties of several ionenes changed with increased hydrophobicity of the polyion, depending on the nature of the counterion. We determined the electrical conductivities of aqueous solutions of 3,3-, 4,5-, 6,6-, and 6,9-ionene fluorides and bromides in the range of concentrations from 5 x 10(-3) to 1 x 10(-1) M and for the temperature interval 5-35 degrees C. Over these ranges, the conductivities of the ionenes were found to decrease with increasing concentration and increase with increasing temperature. The conductivity of 3,3-ionene bromide was lower than that of its fluoride analogue throughout the whole range of concentrations, whereas for the 6,9-ionenes, the trend was reversed. For 4,5- and 6,6-ionene, we observed a crossover in the concentration dependence of conductivity. The conductivity data were compared with the predictions of Manning's theory and scaling theory. Separately determined transport-number values were combined with the conductivity data to obtain the fractions of so-called "free" counterions, f. For bromide samples, f increased from 3,3- to 6,9-ionene. In the case of fluoride counterions, the fraction of free counterions was the lowest for 3,3-ionene and, within the experimental uncertainty, approximately constant for the other less charged ionenes.