Molecular dynamics simulations of liquid crystalline phases of dodecyltrimethylammonium chloride

Abstract

The formation of a second phase in presence of a crack in a crystalline material is modelled and studied for different prevailing conditions in order to predict and a posteriori prevent failure, e.g. by delayed hydride cracking. To this end, the phase field formulation of Ginzburg-Landau is selected to describe the phase transformation, and simulations using the finite volume method are performed for a wide range of material properties. A sixth order Landau potential for a single structural order parameter is adopted because it allows the modeling of both first and second order transitions and its corresponding phase diagram can be outlined analytically. The elastic stress field induced by the crack is found to cause a space-dependent shift in the transition temperature, which promotes a second-phase precipitation in vicinity of the crack tip. The spatiotemporal evolution during nucleation and growth is successfully monitored for different combinations of material properties and applied loads. Results for the second-phase shape and size evolution are presented and discussed for a number of selected characteristic cases. The numerical results at steady state are compared to mean-field equilibrium solutions and a good agreement is achieved. For materials applicable to the model, the results can be used to predict the evolution of an eventual second-phase formation through a dimen-

Prediction of EPR Spectra of Lyotropic Liquid Crystals using a Combination of Molecular Dynamics Simulations and the Model‐Free Approach

“…This result is consistent with scattering and NMR studies and other reports on the elliptical profile for rod‐like micelles . On the other hand, a close contact among the rods in DTAC forces the individual rods into having a hexagonal profile, in agreement with the previous studies …”

“…in DTAC forces the individual rods into having ah exagonal profile, in agreement with the previous studies. [12] Our MD simulations confirmed that at high concentration of water DTAC rods becomeu nstable. For instance, an MD simulation on as ingle rod with 50 wt %r esulted in its rapid decompositioni ntoi rregular micelles at times < 5ns.…”

“…Micelles Figure 4a shows the equilibrated structures of 5DSA (purple) doped in SDS (left) and DTAC (right)m icelles composed of 60 (N60) [6,10,13,14] and 40 (N40) [4,12] molecules, respectively at 320 K. The structures clearly highlight the differences in packing between the two surfactant systems. Due to the larger size, many of the alkyl chains in the SDS micelle are solvente xposed.…”

“…detergents, lubricants, colloidal agents, drugd elivery vectors) in av ast range of processes, and have been the subject of intense study for many years. Sodium dodecyl sulphate (SDS) and dodecyltrimethylammonium chloride( DTAC) (Figure1), representingc lassical exampleso fa nionic and cationic surfactants, respectively,h ave been the target of awide range of experimental techniques including nuclear magnetic resonance (NMR), [1,2] electron paramagnetic resonance (EPR), [3][4][5][6] fluorescence, [7] X-ray [8] and neutron scattering, [9] as well as theoretical studies [10][11][12][13][14] in order to shed light on their structure and dynamics. Both systems are industrially ubiquitous, have well definede xperimentalp hase diagrams exhibitingm ultiple lyotropic phases including micellar,r od/hexagonal and lamellar phases,a sw ell as intermediate mixed structures.…”

“…The majority of previousM Ds imulation studies on lyotropic systemsh ave been directed towards the geometry and thermodynamicso ft heir aggregates, [10][11][12][13][14] with the predominant focus on the micellar phase. In the last decaden ovel approaches have been introduced that allow the prediction of motional EPR spectra from the generated MD data either indirectly [23,[43][44][45] or,m ore recently,b yd irect propagation of dynamical trajectories.…”