The Supramolecular Structure Modulates the Acidity of Amphiphiles

It is recognized that the ionization state of amphipathic molecules can affect the curvature and arrangement of the supramolecular structure of which they are a part due to changes in their shape and interactions with neighboring molecules. The pKa influences the overall charge of the molecule and its local environment, which in turn can cause it to pack into different structures, from planar lamellar membranes to curved micelles or reversed phases. It is also recognized, though less explored, that the supramolecular structure can, in turn, affect the pKa value of the molecule. We explored this possibility with oleic acid molecules and found that the apparent pKa changed by two pH units when the surfactant was forced to remain on a flat surface, compared to the value of the aggregate in suspension, where the molecule adopts the most stable supramolecular structure for each ionization state. The pKa shifted to higher values when fatty acid was forced to form planar structures, and the pH range in which neutral and ionic species coexist (conditions under which lamellar vesicles form spontaneously) increased. Thus, we propose that it is possible to control the ionization state of molecules adsorbed onto a surface, and consequently the surface charge, by modifying surface roughness.