2 research outputs found
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 Ā± 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 Ā± 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 Ā± 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>āEicosanephosphonic 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 ā2.
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>āPO<sub>3</sub>H<sub>2</sub>,
C<sub>20</sub>H<sub>41</sub>āPO<sub>3</sub>H<sup>ā</sup>, and C<sub>20</sub>H<sub>41</sub>āPO<sub>3</sub><sup>2ā</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 phosphonicāphosphonic
hydrogen bonds (|<i>Z</i><sub>av</sub>| = 0ā0.03)
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>| ā 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>| ā 1 fingering domains were identified. When <i>Z</i> ā 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>| ā 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