Diminished Fluid Transport through Carbon Nanochannels Induced by COOH Functionalization: Implications for Nanofiltration and Oil Recovery

Fluid transport on confined systems is a subject of theoretical and technological interest. For instance, remarkable high flow rates have been obtained in carbon nanochannels (CNs) which cannot be predicted by standard macroscopic theories. Natural rocks, which are at the heart of the shale hydrocarbon revolution, may also exhibit similar properties as the porous structure is mainly at the nanoscale and is carbon rich. Among other differences with CNs, the surfaces are not atomically smooth, as they have organic functional groups anchored on their inner porous surfaces. In this work, we assess the effects of carboxylic functionalization of the nanochannel surfaces on fluid transport. We consider water and methane as representative cases for polar/non-polar fluids and also mixtures of them. We find that the presence of only a few carboxylic groups on the CN causes a large reduction of flow rates for all fluids considered due to the associated geometrical distortion. However, for water, the hydrophilicity induced by the carboxylic functionalization causes not only a dramatic reduction in flow rates but also structural changes in which COOH groups act as nucleation centers for water droplets. Implications of our results show that the flow rates depend on the O/C ratio of the nanochannel, which is a measure of kerogen maturity. The relationship between rock permeability and maturity may provide a way to identify high conductive zones for hydrocarbon recovery. Another application is the possible use of chemical additives to enhance hydrocarbon flow on kerogen-rich rocks. The presence of a small amount of scattered organic functional groups in the nanochannel helps to distribute water molecules along the nanochannel walls, opening a path for hydrocarbon to flow.