title: Cholinium-amino acid based ionic liquids: a new method of synthesis and physico-chemical characterization.
abstract: In the present work we report the synthesis and physico-chemical characterization in terms of the viscosity and density of a wide series of cholinium-amino acid based room temperature ionic liquids ([Ch][AA] RTILs). 18 different amino acids were used to obtain 14 room temperature ILs. Among the most common AAs, only valine did not form an RTIL but it is a liquid above 80 °C. With respect to the methods reported in the literature we propose a synthesis based on potentiometric titration which has several advantages such as shorter preparation time, stoichiometry within ±1%, very high yields (close to 100%), high reproducibility, and no use of organic solvents, thus being more environmentally friendly. We tried to prepare dianionic ILs with some AAs with two potentially ionisable groups but in all cases the salts were solids at room temperature. All the ILs were characterized by (1)H NMR to confirm the stoichiometry. Physico-chemical properties such as density, viscosity, refractive index and conductivity were measured as a function of temperature and correlated with empirical equations. The values were compared with the data already reported in the literature for some [Ch][AA] ILs. The thermal expansion coefficient αp and the molar volume Vm were also calculated from the experimental density values. Due to the high number of AAs explored and their structural heterogeneity we have been able to find some interesting correlations between the data obtained and the structural features of the AAs in terms of the alkyl chain length, hydrogen bonding ability, stacking and cyclization. Some parameters were also found to be in good agreement with those reported for other ILs. We think that these data can give an important contribution to the understanding of the structure-property relationship of ILs because they focused on the structural effect of the anions, while most data in the literature are focussed on the cations.
authors.affiliation: Sapienza University of Rome, Department of Chemistry, Piazzale Aldo Moro 5, I-00185 Rome, Italy. giancarlo.masci@uniroma1.it.
authors.forename: Serena
authors.initials: S
authors.lastname: De Santis
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Masci
authors.forename: Francesco
authors.initials: F
authors.lastname: Casciotta
authors.forename: Ruggero
authors.initials: R
authors.lastname: Caminiti
authors.forename: Eleonora
authors.initials: E
authors.lastname: Scarpellini
authors.forename: Marco
authors.initials: M
authors.lastname: Campetella
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Gontrani
mesh_terms: D000596:Amino Acids / Q000737:chemistry*; D002627:Chemistry, Physical; D002794:Choline / Q000737:chemistry*; D052578:Ionic Liquids / Q000138:chemical synthesis* / Q000737:chemistry
chemical_list: D000596:Amino Acids; D052578:Ionic Liquids; D002794:Choline