Ion Specificity at Low Salt Concentrations Investigated with Total Internal Reflection Microscopy

We have investigated the ion specificity at low salt concentrations in view of surface force by use of total internal reflection microscopy (TIRM). The polystyrene (PS) particle−glass slide interaction potential energies in different salt solutions have been measured as a function of the concentration of each salt, and the separation distance (hm) corresponding to the minimum potential energy as well as the Debye length (κ−1) are obtained from the potential profiles. In the case of cation specificity, a V-shaped cation series is observed in terms of hm, which is attributed to the difference in the direct interactions between the cations and the sulfate groups on the particle surface on the basis of the law of matching water affinities. The cation specificity of κ−1 indicates that the effective double layer thickness increases with the effective size of hydrated counterions. In the case of anion specificity, the anion sequence in terms of hm also exhibits a V-shaped series due to the accumulation or exclusion of anions near the particle surface. Moreover, the anion specificity of κ−1 is more complex than the cation specificity because the distribution of anions in the electrical double layer is influenced by both the anion−surface and anion−cation interactions.