Levon H. Arsenyan

Molecular Dynamics Study of Poly(diallyldimethylammonium chloride) (PDADMAC)/ Sodium Dodecyl Sulfate (SDS)/Decanol/Water Systems

We have performed a 50 ns of molecular dynamics study of poly(diallyldimethylammonium chloride) (PDADMAC)/sodium dodecyl sulfate (SDS)/decanol/water systems. The influence of the cationic polyelectrolyte on the anionic SDS-based lamellar liquid crystalline system was investigated. The main structural parameters have been calculated and compared with experimental data. We obtain two types of PDADMAC conformation, a more folded structure A and a structure B where the PDADMAC molecule is adsorbed at the anionic head groups of the surfactant molecules. The polyelectrolyte-induced coexistence of two lamellar phases at a concentration of 2-3% of PDADMAC is observed, which is in agreement with experimental findings.

Molecular dynamics simulations of inverse sodium dodecyl sulfate (SDS) micelles in a mixed toluene/pentanol solvent in the absence and presence of poly(diallyldimethylammonium chloride) (PDADMAC).

We have performed a 15 ns molecular dynamics simulation of inverse sodium dodecyl sulfate (SDS) micelles in a mixed toluene/pentanol solvent in the absence and presence of a cationic polyelectrolyte, i.e. poly(diallyldimethylammonium chloride) (PDADMAC). The NAMD code and CHARMM force field were used. During the simulation time, the radii of SDS inverse micelles changed and the radii of the water droplets have been calculated. The behavior of SDS hydrocarbon chains has been characterized by calculating the orientation order parameter and the chain average length. The water droplet properties (water flow, water molecules displacement) have been examined. In summary the MD simulations indicate a more rigid and ordered surfactant film due to the formation of a polyelectrolyte palisade layer in full agreement with the experimental findings, e.g. the viscosity increase and shift of the percolation boundary.

Molecular Dynamics Study of Intermediate Phase of Long Chain Alkyl Sulfonate/Water Systems

Using atomic level simulation we aimed to investigate various intermediate phases of the long chain alkyl sulfonate/water system. Overall, about 800 ns parallel molecular dynamics simulation study was conducted for a surfactant/water system consisting of 128 sodium pentadecyl sulfonate and 2251 water molecules. The GROMACS software code with united atom force field was applied. Despite some differences, the analysis of main structural parameters is in agreement with X-ray experimental findings. The mechanism of self-assembly of SPDS molecules was also examined. At T = 323 K we obtained both tilted fully interdigitated and liquid crystalline-like disordered hydrocarbon chains; hence, the presence of either gel phase that coexists with a lamellar phase or metastable gel phase with fraction of gauche configuration can be assumed. Further increase of temperature revealed that the system underwent a transition to a lamellar phase, which was clearly identified by the presence of fully disordered hydrocarbon chains. The transition from gel-to-fluid phase was implemented by simulated annealing treatment, and the phase transition point at T = 335 K was identified. The surfactant force field in its presented set is surely enabled to fully demonstrate the mechanism of self-assembly and the behavior of phase transition making it possible to get important information around the phase transition point.

Long-chain alkyl sulfonate micelle fission: a molecular dynamics study

In this study, we investigate micelle fission of long-chain alkyl sulfonate molecules using atomistic scale simulation. GROMACS software code with the united atom force field was applied. 0.5-μs parallel molecular dynamics simulation study was conducted for a surfactant/water system consisting of 192 sodium pentadecyl sulfonate and 40,553 water molecules. The large preassembled micelle was ruptured at Krafft above T = 323-K temperature, and we track two ellipsoid-like micelles over the course of the production run. To estimate the micelle shape, we determined the principal moments of inertia and the eccentricity, which proved that the micelles have a pronounced prolate spheroid shape, which agrees well with our previous experimental data. The mechanism of micelle fission was explored in detail. The aggregation number, ionization degree, and other parameters obtained from simulation were consistent with existing experimental finding. The determined parameters in addition to simple visual inspection of trajectories revealed monomer-micelle exchange—with the estimated relaxation time τ1 = 10− 9s. We assume that the exchange process is conditioned by the unequal size of micelles leading to adjustment of aggregation number.

Dynamic Features of Complex Systems: A Molecular Simulation Study

The main aim of the article is the molecular simulation study and detailed analysis of surfactant molecules of complex micellar systems [1-2] consist of long hydrocarbon chain surfactant. The GROMACS software package [3] designed for high-performance simulations of large complex systems is used for the simulations. The IBM BlueGene/P supercomputer [4] at Bulgarian National Centre for Supercomputing Applications, with 8,192 processor cores connected by multiple high-performance networks, enables to investigate a completely new class of problems. The initially random distributed surfactant molecules in aqueous solute hydration have been simulated using GROMOS united atom force field [5]. An extensive series of short benchmarks run for timing purposes with different number of cores show that the studied system achieves good scalability (0.5ns per day) in case of using up to 512 processor cores. Further increasing the numbers of cores (for instance, 1024 cores) does not lead any significant increase. In spite of the limitation of number of simulations, the qualitative statistical data gave some interesting results, which indicates that long hydrocarbon chain surfactant self-assemble into small oligomers since 50ns of simulations, meanwhile in our previous study with surfactant rich content shows that 43ns is enough for self-assembling of spherical micelle.

Molecular dynamics study of the structure and mechanism of formation of molecular associates in aqueous solutions of surfactants

The process of micelle formation in an aqueous solution of the surfactant was simulated by the computer experiment. It was established by the molecular dynamics method that micelles are formed through the formation of premicellar associates of the surfactant. The practical absence of an attraction between single molecules of sodium pentadecyl sulfonate (SPDS) and premicellar associates dissolved in water was shown for a SPDS—water system. The function of the radial distribution of Na+ counterions towards polar groups of SPDS molecules in water and on the surfaces of micelles and premicellar associates was studied by the molecular dynamics method. The presence of dissociated and non-dissociated polar groups of the SPDS molecular on the micelle surface was found. The data obtained are consistent with the existence concepts on micelle formation processes.