Molecular Dynamics Simulations of Dioctadecyldimethylammonium Chloride Monolayers

Abstract

Molecular dynamics are performed on systems of two-dimensional periodicity composed of 64 ionic dioctadecyldimethylammonium chloride amphiphiles arranged in a monolayer at 298 K with surface coverages ranging from 57 A2 per amphiphile to 150 A2 per amphiphile. Bond lengths are constrained whereas valence and torsional angles interactions are described by conventional expressions. Nonbonded interactions are introduced through an anisotropic united atom method due to Toxvaerd. The amphiphiles adhere to the surface through electrostatic and nonbonded interactions between the amphiphiles and an underlying substrate. Results are presented for the density normal to the monolayer plane, order parameters, and conformational transition rates. The structure of the two-dimensional layer at the lowest headgroup area studied is quite disordered. Small islands of empty surface surrounded by amphiphiles are formed. As the headgroup area increases, we observed further translational disordering and an increase in the number of amphiphiles aligned with the surface. The g+g+tg+g+ defect observed in one of the chains is stable on a timescale of 250 ps. The equilibrium structures of the two chains are quite different but the conformational dynamics as observed by the transition rates are almost indistinguishable.

Memory effects on the interfacial characteristics of dioctadecyldimethylammonium bromide monolayers at the air–water interface

“…The intermediate region of lower slope, ascribed to the LE-LC phase transition, occurs at increasing surface pressures as the temperature increases and vanishes above 25 • C. These results agree with the reported data from the literature [6,7,9,11]. The molecular areas at the collapse region (54-56 Å 2 ) are in good agreement with data from X-ray crystal structures of DODAB revealing a head group area of 56.7 Å 2 [12]. Fig.…”

“…The liquid-expanded to liquid-condensed phase transition is dominated by electrostatic repulsion of the charged head groups [9]. Molecular dynamics of DODAC monolayers has been determined at 298 K [12], indicating that the structure of the two-dimensional layer at the lowest head group area studied is quite disordered and that small islands of empty surface surrounded by amphiphiles are formed.…”

Structure and dynamic properties of water saturated CTMA-montmorillonite: molecular dynamics simulations

“…Here it is considered that tilt angle as the statistic value of different segments along the alkyl chains to obtain quantitative information on the orientation of the individual molecule and spatial distribution. Therefore, on the basis of previous researches (Adolf et al, 1995;Smith et al, 1999;Zeng and Yu, 2008;Zeng et al, 2004), the tilt angle is defined as the angle between a unit vector perpendicular to the silicate surface and a vector connecting two methylene separated by two bonds along the alkyl chain (Fig. 4).…”

The Effect of Molecular Geometry on Boundary Layer Lubrication

“…The model of the dioctadecyldimethylammonium chloride has been described in detail elsewhere [21]. Briefly, the bond-lengths are fixed at their equilibrium values using constraint dynamics implemented within the SHAKE algorithm.…”