Thermo-molecular orientation effects in fluids of dipolar dumbbells

Abstract

We use molecular dynamics simulations in applied thermal gradients to study thermomolecular orientation (TMO) of size-asymmetric dipolar dumbbells with different molecular dipole moments.We find that the direction of the TMO is the same as in apolar dumbbells of the same size, i.e. the smaller atom in the dumbbell tends to orient towards the colder temperature. The ratio of the electrical polarization to the magnitude of the thermal gradient does not vary much with the magnitude of the molecular dipole moment. We also investigate a novel second order TMO that persists even in size-symmetric dipolar dumbbells where molecules have a slight tendency to orient perpendicular to the gradient except very close to the hot region, where (anti-)parallel orientations are preferred. Finally, we investigate rotational correlation functions and characteristic rotational times in these systems in an attempt to model possible spectroscopic signatures of TMO in experiments. Although we cannot detect any difference in integrated rotational times between equilibrium simulations and simulations in a thermal gradient, more careful modelling of the anisotropic rotational dynamics in the thermal gradient may be more successful.

Polarisation of polar dumbbell fluids in thermal gradients: the importance of the treatment of electrostatic interactions

“…In recent work, we studied the effect of applied thermal gradients on molecular orientations using non-equilibrium molecular dynamics (NEMD) simulations [1][2][3][4][5][6][7]. In polar fluids, a tendency for the molecules to preferentially orient can cause significant electric fields to arise.…”

Molecular alignment in molecular fluids induced by coupling between density and thermal gradients

“…The same thermal gradient induces a TMO effect 2.4 times larger in the size asymmetric system studied here compared with the mass asymmetric system. The strength of the TMO signal varies both with the thermodynamic conditions as well as the model parameters, as has been noted previously [8,11,12,14,23,28]. …”

“…All the simulations were performed at temperatures and densities far from the coexistence region of the models investigated. In selecting the thermodynamic conditions, which correspond to a dense supercritical fluid, we used information on the critical point obtained in our previous work [8]. Due to the weaker orientational response of the mass asymmetric model, for this model we chose to use parameters at lower temperatures and higher densities relative to the critical point, but always avoiding simulations inside the coexistence region.…”

“…We have investigated a molecular fluid consisting of mass-or size-asymmetric diatomic molecules, following the models employed by our group in previous works [5,8,23]. The diatomic dumbbells interact via pairwise Lennard-Jones (LJ) interactions with well depth LJ and average atomic diameter σ LJ (not to be confused with the entropy production defined previously).…”

“…Diatomic molecules with slightly different geometries and atomic partial charges that lead to net molecular dipoles have also been studied [8]. In this case the orientation results in a concomitant polarization of the fluid.…”