1 research outputs found
Effect of Ionization on the Behavior of <i>n</i>‑Eicosanephosphonic Acid Monolayers at the Air/Water Interface. Experimental Determinations and Molecular Dynamics Simulations
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><sub>av</sub>) from 0 to −2.
Molecular dynamics simulations (MDS) results for |<i>Z</i><sub>av</sub>| = 0, 1, and 2 were compared with the experimental
ones. EPA monolayers behave as mixtures of mutually miscible species
(C<sub>20</sub>H<sub>41</sub>–PO<sub>3</sub>H<sub>2</sub>,
C<sub>20</sub>H<sub>41</sub>–PO<sub>3</sub>H<sup>–</sup>, and C<sub>20</sub>H<sub>41</sub>–PO<sub>3</sub><sup>2–</sup>, depending on the subphase pH). The order and compactness of the
monolayers decrease when increasing |<i>Z</i><sub>av</sub>|, while go from strongly interconnected by phosphonic–phosphonic
hydrogen bonds (|<i>Z</i><sub>av</sub>| = 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><sub>av</sub>| < 1.6) to an entirely ionic monolayer (|<i>Z</i><sub>av</sub>| ≈ 2). MDS reveal for |<i>Z</i><sub>av</sub>| = 0 that the chains form spiralled nearly rounded
structures induced by the hydrogen-bonded network. When |<i>Z</i><sub>av</sub>| ≈ 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><sub>av</sub>| ≈ 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