title: Effect of ionization on the behavior of n-eicosanephosphonic acid monolayers at the air/water interface. Experimental determinations and molecular dynamics simulations.
abstract: 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 (Zav) from 0 to -2. Molecular dynamics simulations (MDS) results for |Zav| = 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-PO3(2-), 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 (|Zav| = 0-0.03) through an equilibrium between the total cohesive energy and the electrostatic repulsion between the charged polar groups (0.03 < |Zav| < 1.6) to an entirely ionic monolayer (|Zav| ≈ 2). MDS reveal for |Zav| = 0 that the chains form spiralled nearly rounded structures induced by the hydrogen-bonded network. When |Zav| ≈ 1 fingering domains were identified. When Z ≈ 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 |Zav| ≈ 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.
authors: Departamento de Química e INQUISUR, Universidad Nacional del Sur y CONICET , Bahía Blanca, Argentina.
authors: Erica P
authors: EP
authors: Schulz
authors: Ángel
authors: Á
authors: Piñeiro
authors: José
authors: J
authors: Miñones
authors: José
authors: J
authors: Miñones Trillo
authors: Marisa A
authors: MA
authors: Frechero
authors: Olga
authors: O
authors: Pieroni
authors: Pablo C
authors: PC
authors: Schulz
mesh_terms: D000388:Air; D056004:Molecular Dynamics Simulation; D010316:Particle Size; D013499:Surface Properties; D013729:Terpenes / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D013729:Terpenes; C000602730:n-eicosanephosphonic acid; D014867:Water