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

Amsterdam

2021

10 S.

Sonderdruck

40002233

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.
Keywords: Naphthenic acids; Interface water; Activated carbon nanopores; Adsorption; Molecular dynamics simulations