Introduction
The behavior of fluids confined in
nanopores is crucial 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 carry
negative surface charges and nonbonded positive
cations30,31 creating negative and positively charged
surfaces that make fluid transport quite complex32.
There is also strong evidence that positive cations in the fluid can
also influence both the structure and distribution 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 paper is organized as following: Section 2 describes 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 P-H and H-H pores and
finally, we present our conclusions in Section 6.