Evaluating the strength of molecular interactions in a deep eutectic solvent (DES) by means of ionization mechanisms involved in cold-spray ionization mass spectrometry and by DFT calculations

Abstract

This paper regroups the results of mass spectrometry analysis of two categories of deep eutectic solvents (DESs) using electrospray (ESI) and cold-spray (CSI) ionizations. Mass spectrometry of octanoic based DES's is first presented by proposing an interpretation of the ESI and CSI mass spectra based on the knowledge of the theoretical principles of ion formation by these two ionization methods. Whereas in the case of the ESI-MS analysis of the menthol/octanoic based DES, only the cationized adducts of the ester derivatives are observed in positive ion mode, the CSI-MS analysis discloses the formation of protonated non-covalent adducts of menthol and ester. By means of the acid properties of the DES precursors such as octanoic acid and thymol, CSI was used in negative ion mode to evidence the formation of deprotonated non-covalent species that are characteristic of an octanoic based DES. The interpretation is based on the main difference between ESI and CSI which is the spray temperature that induces a lowering of the dielectric constant of the solvent and then the ability of the medium to separate and then disperse opposite charges. In this context, the fact that diagnostic ions of DES are observed only in CSI indicates that the charge of the non-covalent species, whether the proton in positive ion mode or the deprotonated group in negative ion mode, is likely localised inside the supramolecular assemblies participating both to the ionization and the network structuration. In the case of the choline-chloride based DESs, the interpretation of their mass spectra obtained in negative CSI and the gas-phase stability studies of their characteristic ions, is supported by DFT calculations. The association of mass spectrometry experiments and computational studies shows that as the size and composition of clusters increase, the DES clusters containing thiourea are more stable in the gas-phase than those associating urea and glycerol, due to a greater orientation of thiourea towards the cationic quaternary ammonium group of choline in the DES.