Intermediate Structures for Higher Level Arrangements: Catching Disk-Like Micelles in Decane Phosphonic Acid Aqueous Solutions

It has been proposed that disk-like micelles may be precursors to the formation of lamellar liquid crystals. The possibility of obtaining <i>n</i>-decane phosphonic acid (DPA) disk-like micelles in aqueous solution without the addition of a second ionic surfactant led us to study in detail the low-concentration range of this system by both a battery of experimental techniques and molecular dynamics (MD) simulations. The experimental results indicate that premicelles with some capacity to solubilize dyes are formed at 0.05 mM. The critical micelle concentration (cmc) was found to be 0.260 ± 0.023 mM, much lower than that previously reported in the literature. Spherical micelles, which immediately grow, leading to disk-like micelles, are probably formed at this concentration. At 0.454 ± 0.066 mM, disk-like micelles become unstable, giving rise to the formation of an emulsion of lamellar mesophase that dominates the system beyond 0.670 ± 0.045 mM. These experimental results were corroborated by MD simulations which, additionally, allow describing the structure of the obtained micelles at atomic level. The analysis of the MD trajectories revealed the presence of strong intermolecular hydrogen bonds between the surfactant headgroups, producing a compact polar layer with low water content. The formation of such H-bond network could explain the ability of this surfactant to form disk-like micelles at concentrations close to the cmc

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>_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