Atomistic simulations of liquid crystal mixtures of alkoxy substituted phenylpyrimidines 2PhP and PhP14

Abstract

We study liquid crystal mixtures of alkoxy substituted phenylpyrimidines 2-[4-(butyloxy)phenyl]-5-(octyloxy)pyrimidine (2PhP) and 2-[4-(tetradecyloxy)phenyl]-5-(tetradecyloxy)pyrimidine (PhP14) using molecular dynamics simulations at the all atom level. The molecular length of PhP14 is 1.8 times that of 2PhP, resulting in an interesting binary mixture phase diagram. Our simulations are composed of 1000-1600 molecules for a total of 80,000-130,000 atomic sites, with total simulation times of 60-100 ns. We first show that a pure 2PhP system self-assembles into isotropic, nematic, smectic A and smectic C phases, and a pure PhP14 system self-assembles into isotropic and smectic C phases. Binary mixtures of PhP14 and 2PhP display a stabilization of the smectic A phase at the expense of the smectic C and nematic phases. We determine that the concentration-induced phase transition from the smectic C to the smectic A phase in the mixture is driven by an out-of-layer fluctuation arrangement of the molecules. We also observe that the tilt angle in the smectic C phases formed in the mixtures is concentration dependent. The results of our simulations are in good agreement with the experimental findings of Kapernaum et al. [J. Org. Chem. 5, 65 (2009)], thus showing that atomistic simulations are capable of reproducing the phase behavior of liquid crystal mixtures and can also provide microscopic details regarding the mechanisms that govern phase stability.

Algorithms forGPU‐based molecular dynamics simulations of complex fluids: Applications to water, mixtures, and liquid crystals

“…The 80/20 mixture of n-hexane and 2-propanol has a total density of 0.660 g cm 23 and a temperature of 298 K. Benchmarking tests for the 2PhP liquid crystal are conducted at a density of 0.9 g cm 23 and a temperature of 350 K. The generation of distribution functions, illustrative snapshots, and energies requires a careful and lengthy equilibration to yield welldefined layers. Following Yan and Earl [8] and Daub and Cann, [42] we start with an initial temperature of 600 K and apply an ordering field. After simulating for 0.25 ns, the temperature is gradually lowered to 350 K, over 0.25 ns, while the strength of the ordering field decreasing with temperature.…”

“…It is imperative to carefully select a such that the Coulomb energy is well converged and the sums in eqs. (8) and (9) can be limited to the central cell, eliminating the need for summations over periodic images.…”

“…The erfc in eqs. (8) and (9) is calculated via a well-known [48] rational approximation with an absolute error less than 1.5 3 10 27 for all arguments. The contribution to the electrostatic force and energy from sites in the same molecule that are close must be removed.…”

“…As a result, longer MD simulations can be performed or larger system sizes examined, potentially leading to a better description of the system of interest. Liquid crystal simulations, in particular, can greatly benefit from the computing power of GPUs as an accurate description of molecular complexity, long simulations, and large simulation cells, are all essential to capture the relationship between subtle molecular changes and their impact on the phase characteristics . Similarly, chiral liquid crystals require a large number of molecules in the simulated fluid to investigate the connection between pitch and dopant characteristics.…”

“…Liquid crystal simulations, in particular, can greatly benefit from the computing power of GPUs as an accurate description of molecular complexity, long simulations, and large simulation cells, are all essential to capture the relationship between subtle molecular changes and their impact on the phase characteristics. [8] Similarly, chiral liquid crystals require a large number of molecules in the simulated fluid to investigate the connection between pitch and dopant characteristics. Smectic phases are especially challenging as the layer spacing can be impacted by the shape and size of the simulation cell.…”