Highly viscoelastic films at the water/air interface: α-Cyclodextrin with anionic surfactants

Rings, Hexagons, Petals, and Dipolar Moment Sink-Sources: The Fanciful Behavior of Water around Cyclodextrin Complexes

“…Approximately 500-ns-long MD trajectories for the simulations based on the GROMOS force field and 150-ns-long trajectories for the simulations using AMBER, all of them at 283 K, were generated. This temperature was employed because both the adsorption and the formation of 2:1 complexes are much more favorable at 283 K than at 298 K [5,6,23]. The NPT ensemble was employed for the simulations in solution and in the NVT ensemble for the simulation in the presence of water/air interfaces.…”

“…Notably, the hydration level of the different GPU residues after the rotation was lower than the maximum values reached by some residues when the structure was parallel to the interface. The possible presence of these alternative orientations for the complex (parallel and perpendicular to the interface) has been discussed in the past [23] using a battery of experimental data. Our simulations clearly indicate that the most probable orientation is that with the symmetry axis parallel to the surface but the rotation that appears in some simulations suggests that it could also be a marginal population of the perpendicular orientation.…”

“…As a consequence, the increase of mass in AA force fields could reduce the sampling even if the time step is increased. The mass increase of just polar Hs from 1 to 2 Da is justified based on the experimental fact that only the polar H atoms are spontaneously deuterated (thus doubling their mass) when they are solvated in deuterated water [1,23]. In the present work, we propose to also test the mass increase of just the polar H up to 4 Da.…”

“…), van der Waals, and electrostatic interactions. This can be experimentally demonstrated by the fact that structural and thermodynamic properties of many deuterated systems are indistinguishable from those of the equivalent hydrogenated structures (this has been specifically verified for CD systems) [23]. The Feenstra method, also called the HMR (hydrogen mass repartitioning) method, has been tested for many systems providing good results [24,25].…”

“…CD-surfactant mixtures are especially interesting because they proved to exhibit interesting aggregation, mechanical, and adsorption properties under certain experimental conditions (concentration range and temperature) where the presence of 2:1 complexes dominate the solution [4][5][6][27][28][29][30] but the molecular interactions leading to such behavior are unclear. This can be illustrated by an specific example: The adsorption of α-CD 2 SDS 1 complexes at interfaces leading to a monolayer has been conclusively demonstrated by different methods including surface tension, rheology, ellipsometry, Brewster angle microscopy (BAM), atomic force microscopy (AFM), and neutron reflectometry (NR), [5,6,23] although the adsorption driving force of those structures at the interface are still unknown. Additionally, preliminary work from our group indicates that this system is extremely sensitive to the parameterization, as will be shown in the present work.…”