Ion specificity at a low salt concentration in water-methanol mixtures exemplified by a growth of polyelectrolyte multilayer.

By use of a quartz crystal microbalance with dissipation (QCM-D), we have investigated the specific ion effect on the growth of poly(sodium 2-acrylamido-2-methylpropanesulfonate)/poly(diallyldimethylammonium chloride) multilayer at a salt concentration as low as 2.0 mM in water-methanol mixtures. QCM-D results demonstrate that specific ion effect can be observed in methanol and water-methanol mixtures though it is negligible in water. Moreover, the specific ion effect is amplified as the molar fraction of methanol (xM) increases from 0% to 75% but is weakened again with the further increase of xM from 75% to 100%. Nuclear magnetic resonance measurements reveal that the counterion-polyelectrolyte segment interactions may not account for the observed ion specificity. By extending the Collins' concept of matching water affinities to methanol and water-methanol mixtures, we suggest that the ion-solvent interactions and the resulted counterion-charged group interactions are responsible for the occurrence of the specific ion effect. The conductivity measurements indicate that water and methanol molecules may form complexes, and the change of relative proportion of complexes with the xM causes the amplification or weakening of the specific ion effect.