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.