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

Abstract

It has been proposed that disk-like micelles may be precursors to the formation of lamellar liquid crystals. The possibility of obtaining n-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 n-Eicosanephosphonic Acid Monolayers at the Air/Water Interface. Experimental Determinations and Molecular Dynamics Simulations

“…These predominant distances correspond to the two first maxima of the phosphorus−phosphorus radial distribution function in Figure 5. In a previous work we have studied both experimentally and by MDS disk-like micelles of n-decanephosphonic acid (C 10 H 21 −PO 3 H 2 ) 3 where the first peak of the P−P rdf is also at 4.7 Å. In the C 20 H 41 −PO 3 H − monolayer there are fewer hydrogen bonded groups and there is electrostatic repulsion between the headgroups; thus, the two peaks corresponding to the first coordination layer are of almost the same magnitude and superimposed.…”

“…In the micelles, the Stern layer is connected by hydrogen bonds from charge near 0 up to an average charge of −1. 2,3 The phase behavior depends on the charge. Alkanephosphonic acids give rise to lamellar liquid crystals when solid is in contact with water.…”

Electrochemistry of Surfactants

“…Rod-like micelles are rigid if L is less than 100 nm, whilst longer micelles are flexible. The flexibility has been compared with that of a caterpillar [25].…”

“…Since a LIA can measure signals with different phase shifts separately, using Eq. (25), one can obtain the C d value from the real and imaginary current components [81]:…”

Spontaneous Vesicle Formation of Monododecenyl Phosphonic Acid in Water

“…The possible role of single-tailed phosphorus amphiphiles such as alkyl phosphates 11 13 and phosphonic acids 14,15 in protocell membranes has been debated. In contrast to simple organic amphiphiles such as fatty acids, they exhibit unusual phase behaviour, i.e., the formation of liquid crystalline phases over a wide pH range 11 13 .…”