Structural properties of deprotonated naphthenic acids immersed in water in pristine and hydroxylated carbon nanopores from molecular perspectives

Abstract

We use molecular dynamic simulations to study the structural properties of deprotonated cyclohexanoic acid (DCHA) and heptanoic acid (DHA) immersed in water in pristine and hydroxylated carbon nanopores (PACNs and HACNs) in relation to NA removal by activated carbons (ACs). In PACNs, both NAs can aggregate on the pore surface by depleting water molecules, while water molecules accumulate in the area where there is no NA aggregation. The hydrophobic tails of NAs are generally in the interface water region (IWR), while the hydrophilic head groups prefer to be hydrated by water and form pairing with Na+ ions outside the IWR. The linear carbon tails of DHA tend to be parallel to the pore surface, while a slightly inclined configuration of the carbon ring in DCHA is observed. In HACNs, water forms a predominant interface layer by forming hydrogen bonding with -OH groups, while the hydrophobic tails of NAs are driven away from the surface. Both NAs have a more perpendicular orientation close to the pore surface with their hydrophilic head groups forming hydrogen bonding with -OH groups. The strong water film greatly reduces hydrophobic interactions as well as decreases the available adsorption sites for NAs.