A computational study of polydimethylsiloxane derivatives as a semi-permeable membrane for in-field identification of naphthenic acids in water using portable mass spectrometry

A detailed computational study has been performed to assess the possibility of applying polydimethylsiloxane (PDMS) and its structurally similar derivatives as semi-permeable membranes to filter two naphthenic, cyclohexanecarboxylic and 2 norbornaneacetic, acids using a portable mass spectrometer. Several state-of-the-art levels of theory have been used to understand the interactions between PDMS membranes and naphthenic acids, which enabled us to consider membrane design in both liquid and gas phases. Molecular dynamics (MD) simulations have been performed to evaluate interaction energy between PDMS-based layers and naphthenic acids in the presence of water molecules, a scenario that corresponds to the most realistic conditions. Density functional tight-binding (DFTB) calculations were used to obtain interaction energies between PDMS-based chains and naphthenic acids, which allowed us to consider interaction in the gas phase. Density functional theory (DFT) calculations were used to investigate the electron density between the interacting molecular species, which was the basis to identify noncovalent interactions responsible for the interaction and binding between PDMS-based chains and naphthenic acids. Finally, the symmetry-adapted perturbation theory (SAPT) approach was used to decompose the interaction energies into meaningful physical components and identify the one that principally contributes to the attraction. Our calculations indicate that PDMS and some of its structurally similar derivatives have significant potential to be practically applied as semi-permeable membranes for filtration of cyclohexanecarboxylic and 2-norbornaneacetic acids.