The materials studied in this investigation were aqueous solutions (0.02-25.0 mM) of the salts of alkali metal ion (Me(+)) and 2-nitrophenol (2-NP). In the investigation, small-angle X-ray scattering, wide-angle X-ray scattering, and membrane-pressure osmometry were used to study the 2-NP-Me(+) molecular salt structures and the onset of crystallization as a function of concentration and temperature. The experimental methods used to examine the 2-NP-Me(+) molecular salt complexes provided corroborative evidence for the existence of spherical clusters with hydrodynamic diameters between ∼12 Å (Li) and 14 Å (Cs). Guinier plots of the zero-angle scattering peak were characteristic of the scattering from lamellae-like shapes with thicknesses of ∼290 Å. Tetramer and pentamer 2-NP-Me(+) molecular clusters for Me(+) = Li, Na, K, and Rb were assembled from four or five 2-NP molecules bound to a central alkali metal ion. The coordination symmetry around the six coordinated Li(+), Na(+), and K(+) ions was that of a trigonal prism (D3h), with an octahedral arrangement (D2h). The Rb(+) also revealed six-coordinate geometry and the central Rb(+) ion adopted an octahedral arrangement (D2h). The eight-coordinated Cs(+) ions with six 2-NP ligands were characteristic of a square antiprism (D4d). The square antiprism was the outcome of leaving two o-nitro groups and two phenolic oxygens being left intermolecularly uncoordinated to the Cs(+) ion. The 2-NP residues were strictly planar and contained short non-bonded intramolecular distances. van der Waals forces were present between the adjacently stacked phenyl rings. No water molecules were involved as ligands for any of the 2-nitrophenol-Me(+) complexes.
