title: Characterization of surface confined ionic liquid stationary phases: impact of cation revisited.
abstract: Modification of the Linear Solvation Energy Relationship (LSER) equation to account for ionic interactions in the retention of ionizable compounds has enabled the elucidation in the effect of the imidazolium cation identity on retention. Three Surface Confined Ionic Liquid stationary phases were synthesized from an octylbromide phase on silica: 1-octyl-3-methylimidazolium bromide (MIM), 1-octyl-3-butylimidazolium bromide (BIM), and 1-octyl-3-benzylimidazolium bromide (BzIM). These phases were probed via a 35 analyte probe set, including 6 phenolic acids, 5 anilinic bases, and 2 pyridinic bases, and the resulting column parameters compared with previously reported interactions of ionic liquids or Surface Confined Ionic Liquids. The correlation between experimental and calculated retention for the conventional, 6-parameter LSER equation was very poor: r(2)=0.64 (MIM), 0.60 (BIM), and 0.62 (BzIM). By accounting for the ionic interactions between stationary phase and analytes, linearity for the modified, 8 parameter LSER equation was significantly improved to r(2)=0.997 (MIM), 0.996 (BIM), and 0.997 (BzIM). The primary difference between cation identities is within the retention of acids where BIM>BzIM>MIM. We conjecture that the accessibility of bulky, acidic analytes to the on-top interaction of the imidazolium ring is the major contributor to increased anion retention.
authors.affiliation: Irish Separation Science Cluster, Dublin City University, Dublin, IE.
authors.forename: Bradley J
authors.initials: BJ
authors.lastname: VanMiddlesworth
authors.affiliation: Irish Separation Science Cluster, Dublin City University, Dublin, IE. Electronic address: Apryll.Stalcup@dcu.ie.
authors.forename: Apryll M
authors.initials: AM
authors.lastname: Stalcup
mesh_terms: D000814:Aniline Compounds / Q000032:analysis; D000838:Anions; D002412:Cations; D002853:Chromatography, Liquid / Q000295:instrumentation*; D006863:Hydrogen-Ion Concentration; D007093:Imidazoles / Q000737:chemistry*; D052578:Ionic Liquids / Q000737:chemistry*; D010636:Phenols / Q000032:analysis; D011725:Pyridines / Q000032:analysis; D012822:Silicon Dioxide
chemical_list: C000593650:1-octyl-3-benzylimidazolium bromide; C000593649:1-octyl-3-butylimidazolium bromide; C528768:1-octyl-3-methylimidazolium; D000814:Aniline Compounds; D000838:Anions; D002412:Cations; D007093:Imidazoles; D052578:Ionic Liquids; D010636:Phenols; D011725:Pyridines; D012822:Silicon Dioxide
keywords: Acidic and basic solutes; Cation impact; Dissociation constants; Ionization; LSER; Methanol–water mixtures; Surface confined ionic liquid