Effect of Ionization on the Behavior of n-Eicosanephosphonic Acid Mono layers at the Air/Water Interface. Experimental Determinations and Molecular Dynamics Simulations

Monolayers of n-eicosanephosphonic acid, EPA, were studied using a Langmuir balance and a Brewster angle microscope at different subphase pH values to change the charge of the polar headgroups (Z(av)) from 0 to -2. Molecular dynamics simulations (MDS) results for |Z(av)| = 0, 1, and 2 were compared with the experimental ones. EPA monolayers behave as mixtures of mutually miscible species (C20H41-PO3H2, C20H41-PO3H-, and C20H41-PO32-, depending on the subphase pH). The order and compactness of the monolayers decrease when increasing |Zav|, while go from strongly interconnected by phosphonic-phosphonic hydrogen bonds (|Z(av)| = 0-0.03) through an equilibrium between the total cohesive energy and the electrostatic repulsion between the charged polar groups (0.03 < |Z(av)| < 1.6) to an entirely ionic monolayer (|Z(av)| approximate to 2). MDS reveal for |Zav| = 0 that the chains form spiralled nearly rounded structures induced by the hydrogen-bonded network. When |Z(av)| approximate to 1 fingering domains were identified. When Z approximate to 2, the headgroups are more disordered and distanced, not only in the xy plane but also in the z direction, forming a rough layer and responding to compression with a large plateau in the isotherm. The monolayers collapse behavior is consistent with the structures and domains founds in the different ionization states and their consequent in-plane rigidity: there is a transition from a solid-like response at low pH subphases to a fluid-like response at high pH subphases. The film area in the close-packed state increases relatively slow when the polar headgroups are able to form hydrogen bonds but increases to near twice that this value when |Z(av)| approximate to 2. Other nanoscopic properties of monolayers were also determined by MDS. The computational results confirm the experimental findings and offer a nanoscopic perspective on the structure and interactions in the phosphonate monolayers.