How deprotonation changes molecular self-assembly - an AFM study in liquid environment

We study the influence of Alizarin Red S deprotonation on molecular self-assembly at the solid-liquid interface of the natural cleavage plane of calcite immersed in aqueous solution. To elucidate the adsorption details, we perform pH dependent high-resolution atomic force microscopy measurements. When Alizarin Red S is deposited onto calcite(10.4) in a liquid environment at an acidic pH of 5, weakly bound, ordered islands with a (3 x 3) superstructure are observed. A sharp structural transition is revealed when increasing the pH above 8. Above this pH, stable needle-like structures oriented along the [01.0] direction form on the surface. Comparing these results with potentiometric titration data allows for unambiguously assigning the two molecular structures to the single and two-fold deprotonated moieties of Alizarin Red S. Our work, thus, illustrates the decisive impact of the protonation state on molecular self-assembly.