Effect of Ionization on the Behavior of <i>n</i>‑Eicosanephosphonic Acid Monolayers at the Air/Water Interface. Experimental Determinations and Molecular Dynamics Simulations

Abstract

Monolayers of <i>n</i>-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 (<i>Z</i>_av) from 0 to −2. Molecular dynamics simulations (MDS) results for |<i>Z</i>_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–PO₃^2–, depending on the subphase pH). The order and compactness of the monolayers decrease when increasing |<i>Z</i>_av|, while go from strongly interconnected by phosphonic–phosphonic hydrogen bonds (|<i>Z</i>_av| = 0–0.03) through an equilibrium between the total cohesive energy and the electrostatic repulsion between the charged polar groups (0.03 < |<i>Z</i>_av| < 1.6) to an entirely ionic monolayer (|<i>Z</i>_av| ≈ 2). MDS reveal for |<i>Z</i>_av| = 0 that the chains form spiralled nearly rounded structures induced by the hydrogen-bonded network. When |<i>Z</i>_av| ≈ 1 fingering domains were identified. When <i>Z</i> ≈ 2, the headgroups are more disordered and distanced, not only in the <i>xy</i> plane but also in the <i>z</i> 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 |<i>Z</i>_av| ≈ 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