Molecular dynamics simulation of pyrene solubilized in a sodium dodecyl sulfate micelle.

In the present work, the structural and dynamical aspects of the solubilization process of pyrene within a sodium dodecyl sulfate micelle were studied using molecular dynamics simulations. Our results showed that free pyrene as the fluorescence probe can be spontaneously solubilized into the micelle and prefers to be located in the hydrophobic core region. As the local concentration of pyrene increased, two molecular probes could enter into the core hydrophobic region and the excited dimer of pyrene molecules was formed, showing a stacking mode of π-π conjugation. Since the π-π stacking interaction between the two pyrene molecules was very weak, formation of the excimer was a dynamic process with the two pyrene molecules alternately separating and associating with each other. In this case, the two pyrene molecules were found to be mainly distributed in the palisade layer of the micelle due to the balance between the weak π-π stacking interaction and the hydrophobic interaction of probe molecules with the surfactant tails.

Microscopic wetting of self-assembled monolayers with different surfaces: a combined molecular dynamics and quantum mechanics study.

Molecular dynamics simulations are used to study the micronature of the organization of water molecules on the flat surface of well-ordered self-assembled monolayers (SAMs) of 18-carbon alkanethiolate chains bound to a silicon (111) substrate. Six different headgroups (-CH(3), -C═C, -OCH(3), -CN, -NH(2), -COOH) are used to tune the character of the surface from hydrophobic to hydrophilic, while the level of hydration is consistent on all six SAM surfaces. Quantum mechanics calculations are employed to optimize each alkyl chain of the different SAMs with one water molecule and to investigate changes in the configuration of each headgroup under hydration. We report the changes of the structure of the six SAMs with different surfaces in the presence of water, and the area of the wetted surface of each SAM, depending on the terminal group. Our results suggest that a corrugated and hydrophobic surface will be formed if the headgroups of SAM surface are not able to form H-bonds either with water molecules or between adjacent groups. In contrast, the formation of hydrogen bonds not only among polar heads but also between polar heads and water may enhance the SAM surface hydrophilicity and corrugation. We explicitly discuss the micromechanisms for the hydration of three hydrophilic SAM (CN-, NH(2)- and COOH-terminated) surfaces, which is helpful to superhydrophilic surface design of SAM in biomimetic materials.

Molecular dynamics study of the effect of calcium ions on the monolayer of SDC and SDSn surfactants at the vapor/liquid interface.

The effect of Ca(2+) ions on the hydration shell of sodium dodecyl carboxylate (SDC) and sodium dodecyl sulfonate (SDSn) monolayer at vapor/liquid interfaces was studied using molecular dynamics simulations. For each surfactant, two different surface concentrations were used to perform the simulations, and the aggregation morphologies and structural details have been reported. The results showed that the aggregation structures relate to both the surface coverage and the calcium ions. The divalent ions can screen the interaction between the polar head and Na(+) ions. Thus, Ca(2+) ions locate near the vapor/liquid interface to bind to the headgroup, making the aggregations much more compact via the salt bridge. The potential of mean force (PMF) between Ca(2+) and the headgroups shows that the interaction is decided by a stabilizing solvent-separated minimum in the PMF. To bind to the headgroup, Ca(2+) should overcome the energy barrier. Among contributions to the PMF, the major repulsive interaction was due to the rearrangement of the hydration shell after the calcium ions entered into the hydration shell of the headgroup. The PMFs between the headgroup and Ca(2+) in the SDSn systems showed higher energy barriers than those in the SDC systems. This result indicated that SDSn binds the divalent ions with more difficulty compared with SDC, so the ions have a strong effect on the hydration shell of SDC. That is why sulfonate surfactants have better efficiency in salt solutions with Ca(2+) ions for enhanced oil recovery.

Effect of Ca2+ and Mg2+ ions on surfactant solutions investigated by molecular dynamics simulation.

The effect of Ca(2+) and Mg(2+) on the H-bonding structure around the headgroup of the surfactants sodium dodecyl sulfate (SDS) and sodium dodecyl sulfonate (SDSn) in solution has been studied by molecular dynamics simulation. Our results show that binding between the headgroup of the surfactant and Ca(2+) or Mg(2+) is prevented by a stabilizing solvent-separated minimum formed in the potential of mean force (PMF) between the interacting ion-pair. Among the contributions to the PMF, the major repulsive interaction is due to the rearrangement of the hydration shell after the ions enter into the original H-bonding structure of water around the headgroup, leading to a decrease in the number of H-bonds and an increase in their lifetimes. In the second hydration shell around the headgroup, additional water molecules are bound to the headgroup oxygen atoms either directly or bridged by Ca(2+) and Mg(2+). The PMF shows that the energy barriers to ion-pairing between the headgroup and Ca(2+) and Mg(2+) in the SDSn system are higher than those in the SDS system, and the water coordination numbers for Ca(2+) or Mg(2+) in SDS solution are lower. This result indicates that SDS binds the ions easily compared with SDSn, and the ions have a strong effect on the original hydration structure. That is why sulfonate surfactants such as SDSn have better efficiency in salt solution with Ca(2+) and Mg(2+) for enhanced oil recovery.

Surface behavior of a model surfactant: a theoretical simulation study.

A quantum mechanics (QM) method has been used to calculate molecular properties of sodium dodecylbenzenesulfonate (SDBS) in vacuum and in solution. Furthermore, molecular dynamics (MD) simulations have been used to determine the dynamic behavior of SDBS moving from the bulk solution to the air/water interface. QM calculations suggest that two head-group oxygen atoms on each surfactant molecule interact with a Na(+) ion, despite the availability of three oxygen atoms in the head group. MD simulations showed that the Na(+) ion must overcome the energy barrier between two solvent layers around the head group to form stable ion pair in solution, which is consistent with experimental results. In the simulation, in moving from the bulk to the interface, SDBS can aggregate in a short time, and the adsorption adopts a preferred orientation. The results indicate that formation of favorable hydrophobic interactions of the surfactant alkyl chains is the origin of interfacial adsorption of SDBS.

Mesoscale simulation on patterned nanotube model for amphiphilic block copolymer.

Self-assembly of AB diblock copolymer confined in concentric-cylindrical nanopores was studied using MesoDyn simulation. Our calculation shows that in this confined geometry a zoo of exotic structures can be formed. These structures include bicontinuous phases like carbon nanotube, imperfect single helixes and double helixes. Moreover, the dependence of the chain conformation on the volume fraction, concentration, the interactions between blocks and the diameter of the cylindrical pore are investigated. The results of these simulations can be used to predict the diblock copolymer morphologies confined in concentric-cylindrical nanopores and should be helpful in designing polymeric nanomaterials in the future.

Effects of shear and charge on the microphase formation of P123 polymer in the SBA-15 synthesis investigated by mesoscale simulations.

Mesoscale simulation was performed to investigate the dynamical structural behavior of the pluronic P123 block copolymer in the synthesis of mesoporous SBA-15. Shear is introduced to represent stirring in the actual experiment, and a weak charge is included to simulate the acidic conditions in the synthesis. Under shear, with the increase in weak charge in the PEO [poly(ethylene oxide)] block, the template forms more ordered hexagonal phases, and the pore sizes of the cylindrical hydrophobic PPO [poly(propylene oxide)] blocks decrease. The structural factor shows three types of water molecules in the mesoscale aggregates, including bulk water in the solution, bound water around the hydrophilic PEO corona, and trapped water in the hydrophobic PPO core. When 1,3,5-trimethyl-benzene (TMB) is added to the system as a swelling agent, expanded hexagonal phases are formed, and the density mapping of TMB shows that the TMB molecules are mainly located in the hydrophobic PPO cores. In configurations with spherical micelles, although bimodally dispersed spheres are observed, the face-centered cubic (fcc) packing of the micelles hardly changes with the addition of TMB. In agreement with experimental results, the simulations show that the shear and the weak charge are essential to the formation of hexagonal templates in the copolymer. Mesoscopic simulations complement experimental investigations on the morphology changes of amphiphilic polymer in template syntheses and can provide important guidance for further experiments.

Mesoscopic simulation study on a weakly charged block polyelectrolyte in aqueous solution.

In this paper, we use density functional theory to study the effect of the charge of solvophilic beads and concentration on the mesoscale structures of polyelectrolyte solution. The polyelectrolyte A6B12A6 was selected as the triblock polymer, and the solvophobic B blocks have no charges, while the solvophilic A blocks are charged. The simulation results showed: at higher concentration (above 50% systems), relatively small charges on the solvophilic block do not alter the bicontinuous phase inherent to uncharged solution, but at moderate concentrations (50% system), even though the charge per solvophilic bead is very small, the order lamellar structures become disturbed. [Figure: see text].

The study of chiral discrimination of organophosphonate derivatives on pirkle type chiral stationary phase by molecular modeling.

Molecular modeling and molecular dynamics (MD) have been used to study the chiral discrimination and interaction energy of organophosphonate in N-(3,5-dinitrobenzoyl)-S-leucine chiral stationary phase (CSP). The elution order of the enantiomers can be predicted from the interaction energy. Quantitative structure-retention relationship (QSRR) has also been used as an alternative method to confirm the elution order of enantiomers. Molecular mechanics (MM), molecular dynamics and QSRR proved to be useful methods to study chiral discrimination.