Molecular Simulation of a Dichain Surfactant/Water/Carbon Dioxide System. 1. Structural Properties of Aggregates

Abstract

Molecular dynamics simulation of a dichain surfactant + water + carbon dioxide (solvent) system is performed to study the structural properties of reversed micelle-like surfactant aggregates formed in the system. The simulations use a detailed and realistic molecular model for the surfactant molecule and explicit representation of the water and solvent molecules to enable quantitative comparisons with a prior experimental (small-angle neutron scattering) study. The results of the simulation are found to be in reasonable agreement with experimental values. The simulations show that the size and shape of the surfactant aggregates depends on their water-to-surfactant ratio. A higher water-to-surfactant ratio results in larger and more spherical aggregates. The two distinct tails of the surfactant molecule exhibit different conformations in carbon dioxide indicating contrasting CO2-philic behavior. The perfluoroalkane tails assume more extended conformation than the alkane tails. The microstructure of the aqueous core reveals that the water molecules in the interfacial region are strongly oriented in response to the electric fields of the anionic headgroups and sodium counterions, while water near the center of the core approaches bulklike properties with the presence of a hydrogen-bonded network.

Molecular dynamics simulation studies of CO2 – [bmim][PF6] solutions: Effect of CO2 concentration

“…X-ray diffraction experiments have been carried out by Kanakubo et al 49 to examine the intermolecular structure of the solution and, in particular, the location and orientation of the CO 2 molecule with reference to the anion. Based on the analysis of experimental data, they have concluded that the CO 2 molecule may not be tangential to the PF 6 sphere as claimed in the computer simulation studies. 46,48,53 We have recently carried out ab initio MD simulations of the [bmim][PF 6 ] -CO 2 mixture at a CO 2 concentration of 70 mol % to study the intermolecular environment around CO 2 molecules in the solution.…”

“…53 It also predicts the diffusion coefficients of ions, density, and surface tension in excellent agreement with experimental data. For the CO 2 molecules, the EPM2 model 56 6 ] were simulated. The systems were first equilibrated under constant temperature and pressure (NPT) conditions, and later in the canonical (NVT) ensemble, with the volume determined as an average over the earlier NPT trajectory.…”

“…[33][34][35][36] The interaction between the solute and the solvent in the mixture can impart new properties which the individual components may not possess. 37 Although CO 2 is remarkably soluble in imidazolium based ionic liquids, 33,[38][39][40][41][42][43] ionic liquids such as [bmim][PF 6 ] do not dissolve much in CO 2 . 33,38 The separation of organic compounds and water from ILs using CO 2 has been demonstrated.…”

Interfacial phenomena at the compressed co2-water interface

“…Insight into the microstructure of the bare and surfactant modified C|W interfacial region was recently reported (da Rocha, et al, 2001, Salaniwal, et al, 2001, Senapati, et al, 2002, Stone, et al, 2003. Figure 3 is a snapshot of an equilibrium configuration of a perfluoropolyether ammonium carboxylate (CF 3 (OCF 2 CF(CF 3 )) 3 OCF 2 COO -NH 4 + ) (PFPE-COO-NH 4 + ) surfactant at the C|W interface (da Rocha, et al, 2002) obtained by MD computer simulations.…”

Modeling Solvation in Supercritical CO2

“…[152] Further MD simulation studies focused on the formationo fr eversem icelles, on the structure-property relationship for different surfactant molecules, [153] and on the CO 2 -philic behavior of the surfactant tails. [154] The structuralf eatures of surfactants at the water-CO 2 interface were investigated by Rossky's group, who showedt hat CO 2 can solvate the fluorinated tails as wellasthe hydrocarbon tails, leading to an ew orientationalo rdering that allows increasedw ater accessibility of the hydrophobic part. [155,156] Aj oint experimental and computational study by Berkovitz et al characterized am icroemulsion in as ystem containing a fluorinated surfactant, water and scCO 2 .…”