Influence of the hydrophilic layer structure on the behavior of self-assembled supramolecular aggregates

ERICA PATRICA SCHULZ; ÁNGEL PIÑEIRO GUILLÉN; JOSÉ MIÑONES CONDE; JOSÉ MIÑONES TRILLO; MARISA A. FRECHERO; OLGA I. PIERONI; PABLO CARLOS SCHULZ

Villa Carlos Paz, Córdoba, Argentina

Congreso; 13th Latin American Conference on Physical Organic Chemistry; 2015

Universidad Nacional de Córdoba

The phosphonic acids have multiple applications in many technology and research fields, especially in the elucidation of the origin of life. A thorough knowledge of their behavior and the control variables of phosphonates monolayers is essential to move forward into cutting-edge technology and new theories, with alluring prospects such as their use in protocells. The charge of the hydrophilic layer can be tunned as 0 (R-PO3H2), -1 (R-PO3H-) or -2 (R-PO3=) by changing the medium, changing from connected by hydrogen bonds to entirely ionic. This versatility gives rise to dissimilar behaviors in monolayers at the air/water interface and atypical structures, such as disk-like molecules and inter-molecular regular order in monolayers. We have employed a full scope of techniques, with complementary experimental and theoretical results, to study the self-assembly of supramolecular surfactant systems with a multi-scale insight. We have studied the formation of n-decane phosphonic acid disk-like micelles in aqueous solution without the addition of a second ionic surfactant (dx.doi.org/10.1021/jp312713e | J. Phys. Chem. B 2013, 117, 6231−6240). Disk-like micelles are seldomly reported in literature and have been proposed as precursors to the formation of lamellar liquid crystals.We have also studied the whole range of ionization states of n-eicosanephosphonic acid monolayers on water subphases with different pH values using a Langmuir balance and Brewster angle microscope and molecular dynamic simulations. When the charge of the polar headgroups (Z) is 0 at low pH subphase values, spiraled near rounded structures are induced by the hydrogen-bonded network. When Z ≈ 1 fingering domains are identified. A rough layer is formed when Z ≈ 2 responding to compression with a large plateau in the isotherm. Therefore, there is a transition from a solid-like response at low pH subphases to a fluid-like response at high pH subphases, with consistent collapse behaviors. The computational results not only confirm the experimental findings but offer a nanoscopic perspective on the structures and interactions, allowing the analysis of the orientation of the headgroups and tails, of the hydrogen bonds, of the hydration of the different parts of the molecules and of the density profiles. (DOI 10.1021/la5050283| Langmuir 2015).All the simulations were performed with GROMACS 4.5.1. The surfactant molecules were modeled by using the G53a6 parameterization of the GROMOS96 force. It is worth mentioning that all the molecular dynamics simulations depart from surfactant molecules at random positions and orientations.