(55d) When Nano-Confined Between Mica Sheets, Does Dodecane Undergo a Phase Transition?

It
has been suggested that when dodecane is confined
between mica sheets separated by <5-10 molecular dimensions at ambient
conditions, it undergoes a phase transition to an ordered solid-like structure¹.
However, even a brief search of the literature shows that there is much debate
surrounding what occurs when dodecane is confined
between sheets of mica. This is particularly true in the case of experiment, in
which resolution at the scales required to observe such phenomena is extremely
difficult to attain and, as such, the structure of the confined molecules is
generally inferred from surface force apparatus experiments (SFA).
Unfortunately, even with this technique the results of different laboratories
differ, with some observing an order of magnitude increase in viscosity²,
suggesting a phase transition, and others not³. Conversely,
molecular simulation is ideally suited to the study of phenomena at the nanoscale and, as such, several techniques have been
applied to the study of nano-confined systems^1,4,5. With respect to dodecane,
most workers have observed the formation of a layered herringbone structure,
consistent with a many-order-of-magnitude increase in viscosity, but have done
so using fairly simplistic models (E.g. mica represented by an fcc lattice of Lennard-Jones
spheres), or have performed the simulations in a manner which
may be criticized as biased towards the formation of such structures. Recently
some workers, such as Jabbarzadeh et
al.⁴, have considered factors which may cause such structures
either not to form, or to break down once formed, e.g. an irregular/amorphous
surface structure in the former case and rapid shear of the surfaces in the
latter. However, they still use an artificially enforced system shape as well
as a simplistic mica model (see above). Thus, while their suggested causes may
explain the variation in observed behavior, the question remains as to whether
or not such behavior exists in a real mica system or whether they are merely
artifacts of the choice of model and simulation technique applied.

Given
the level of interest in this phenomenon, and the ongoing disagreement, we feel
that it is appropriate to revisit the molecular simulation of these systems. In
particular, rather than a simple fcc
lattice representation of the mica surfaces, we use the fully atomist model of
Heinz et al.⁶, leading to the most realistic simulations of this
system yet. Using this model, we have performed simulations similar to those of
Jabbarzadeh et al.⁴
and Cui et al.¹ in that we
consider dodecane confined between infinitely
periodic mica sheets. Since the periodic nature of this system, together with
the fixed number of molecules used in the simulations, may be considered to
bias the formation of an ordered structure we have also used the simulation
technique of Gao et
al.⁵ for nano-confined systems. In
this case our system consists of parallel mica sheets surrounded by bulk dodecane which is free to move in and out of the gap
between the mica sheets, thus removing the bias towards an ordered structure.

 Based on previously published work, we
discuss our results in terms of surface structure, surface energy and the
strength of mica-dodecane interaction and the roles
they play in the formation of herringbone ordered layering. We then explain
why, and to what extent, these features are reproduced in our atomistically detailed simulations before considering
other, neglected, factors, which may play an important role in these systems.

¹S. T. Cui, P. T. Cummings and H. D.
Cochran, J. Chem. Phys. 114,
7189 (2001)

²J.
Klein and E. Kumacheva, J. Chem.
Phys.
108, 6996 (1998); 108,
7010 (1998)

³A. L. Demirel
and S. Granick, J. Chem.
Phys.
115, 1498 (2001)

⁴A. Jabbarzadeh, P. Harrowell
and R. I. Tanner, J.
Chem. Phys.
125,
034703
(2006)

⁵J. Gao, W. D. Luedtke,
and U. Landman, J. Chem. Phys. 106, 4309 (1997)

⁶H. Heinz, H. Koerner,
K. L. Anderson, R. A. Vaia and B. L. Farmer, Chem. Mater. 17, 5658
(2005)