Introduction
The behavior of fluids confined in
nanopores is significant for
understanding and resolving a suite of challenging problems such as
nanofluidic technology1, biochemical
flows2 and membrane separation3. The
structure and dynamics of confined fluids differ drastically from those
in bulk condition4–9 as a result of interactions with
the nanopore walls10–12. With charged surfaces, this
effect can be more pronounced through the creation of electric double
layer (EDL) structures 13,14 and the behavior of water
in the EDL15,16. These phenomena merit a deeper look
in to liquid transport through nanoporous structures with charged
surfaces.
There have been several studies focusing on nano-confined fluid
structures adjacent to charged surfaces. Urashima et al. discuss the
structure of water at a negatively charge silica surface and demonstrate
that the closest water molecules form hydrogen bonds with the negatively
charged silica surface17. Dobrynin et al. investigate
adsorption between a polyampholyte chain and a charged
surface18 while Zhang et al. demonstrate that the
charged surface can regulate molecular orientation and
interaction19. Tasca et al indicate that a positively
charged surface can enhance electron transfer20 and
Dreier et al. state that the alignment and transport of water molecules
are influenced by charged surfaces21. Ehre et al.
conclude that water molecules freeze differently on positively and
negatively charged surfaces22 and Lahann et al.
demonstrate that charged surfaces can switch interfacial properties,
such as wettability in response to an electrical
potential23. Lis et al. state that in addition to
causing water alignment at the surface, the charged surface can lead to
surface-charge screening24.
These contributions enhance our understanding of fluid confinement
within charged nanopore surfaces; however, a more complete picture
should encompass a discussion of transport. Clay minerals are one of the
most fundamental and abundant substances on earth 25and can be used as adsorbents26, carbon dioxide
storage and sequestration27,28 as well as water
purification29. Generally, clay minerals have negative
surface charges and nonbonded cations30,31 creating
negative and positively charged surfaces that make fluid transport quite
complex32. There is also strong evidence that the
cations in the fluid show impacts both on the structure and storage of
fluid15,33–36.
Mixture flow in nanopores, such as flow of red blood cells37, drug delivery38 and oil and gas
production from shales39 are quite common. In our
work, a dodecane and ethane mixture form the non-wetting hydrocarbon
phase and water the wetting fluid phase40–42. We use
equilibrium molecular dynamics (EMD) and nonequilibrium MD (NEMD) to
investigate the structure and transport of hydrocarbon-water mixtures in
clay-hosted nanopores with different charged surface chemistries.
Our work is prepared as following: Section 2 shows the construction of
different clay models with varying surface chemistries and clay-hosted
pores containing hydrocarbon-water mixtures in a molecular dynamics
simulation setup. In total, this section discusses 42 MD simulations
with varying pore size (5 nm, 10 nm and 15 nm), water concentration
(0-100%) and surface charges. Section 3 provides a thorough analysis on
fluid structure based on results from Section 2 and Section 4 discusses
transport of the hydrocarbon-water mixture based on the results from
Section 2 and 3. In Section 5, we show how the single-phase velocity
profiles are different for PH and HH pores and finally, we present our
conclusions in Section 6.