Ion pair and larger ion cluster formation in water interfacial regions: An atomic force microscopy study

Interfacial regions attached to hydrophobic and hydrophilic surfaces have very low relative permittivities (ε ∼ 3–5), and these low values induce the formation of ion pairs. A detailed description of ion pairing will be possible only after the development of adequate experimental probing methods. The scheme described in this work detects the ion pair (or multistage ion association) formation in water solutions in the interfacial region. Forces acting on the tip when immersed in the interfacial region attached to hydrophilic substrates, such as mica where ε < 7, are attractive (AFM tip dielectric constant = 7). These attractive force steps with extensions equal or smaller than the Bjerrum length at hydrophilic substrates are associated with ion pair formation in interfacial regions. Measurements show that the interfacial water molecular arrangement and ion pairs form an ordered structure (ε < 7) induced by mica interfacial charges. The interfacial region attached to hydrophobic substrates, however, form separated arrangements of ion pairs and water molecular structures as observed by the repulsive steps (ε > 7) intercalated in between attractive regions (ε < 7).