Electrophoretic mobilities of large organic ions in nonaqueous solvents: Determination by capillary electrophoresis in propylene carbonate, N,N-dimethylformamide, N,N,-dimethylacetamide, acetonitrile and methanol

Abstract

The mobilities of the monocharged permanent tertraphenylphosphonium cation and tetraphenylborate anion are determined by capillary zone electrophoresis in different organic solvents as a function of the ionic strength, I, of the background electrolyte. The nonaqueous solvents are propylene carbonate (PC), N,N-dimethylformamide (DMF), N,N,-dimethylacetamide (DMA), acetonitrile (MeCN) and methanol (MeOH). The ionic strength is between 5 and 50 mmol/L. The mobility as a function of I is in good agreement with the theory of Debye, Hückel and Onsager (DHO), extended by the ion size parameter as introduced by Falkenhagen and Pitts. The values of the limiting DHO slopes of the mobility vs. I curves (the slopes express the influence of the solvent on the reduction of the mobility with increase of I) decrease in the order MeCN > MeOH > DMF > DMA > PC. Absolute mobilities (obtained by extrapolation to I = 0) of a particular ion differ by a factor of about 7 between the solvents. However, constancy within 10% is observed for their Walden products (the absolute mobility multiplied with the solvent's macroviscosity). The role of dielectric friction on the mobility of the present monocharged, large analyte ions is discussed according to the theory of Hubbard and Onsager. Based on the radii of the ions, the static permittivity of the solvent and its permittivity at infinite frequency, and the relaxation time of polarization, an equal contribution of dielectric and hydrodynamic friction is predicted in MeOH as solvent. Experimental data are in contrast to this prediction, indicating the overestimation of dielectric friction, and the dominance of hydrodynamic friction on the migration of the analyte ions in all solvents under consideration.

Recent advances in nonaqueous capillary electrophoresis

“…Absolute mobilities of a particular ion differed by a factor of about 7 in the different solvents. Experimental data indicated the overestimation of dielectric friction in theoretical considerations based on the theory of Hubbard and Onsager, showing hydrodynamic friction according to Stokes law to govern ion migration of the analyte ions studied [24].…”

“…However, hydrolysis instability and reduced UV transparency in addition to the poor incompatibility with mass spectrometry cause some intrinsic drawbacks. Some recent papers also note the applicability of longer chain alcohols (C2-C4 alcohols) [20][21][22] and propylene carbonate [23,24] in the NACE studies.…”

“…Muzikár et al [24] also determined the electrophoretic mobilities of monocharged tetraphenylphosphonium cation and tetraphenylborate anion, which can be considered as relatively large organic ions, as function of the ionic strength of the BGE (acetate buffer at seven concentrations between 5 and 50 mM) in several solvents including propylene carbonate (PC). The other solvents were MeOH, ACN, DMF, and N,N-dimethylacetamide.…”

Affinity capillary electrophoresis employed for determination of stability constants of antamanide complexes with univalent and divalent cations in methanol

“…It is difficult to estimate the value of parameter a for ions with different sizes and nonspherical shapes. In this study, the ion size parameter 15.1 Å, determined by Muzikář et al . for tetraphenylphosphonium ion in methanol, was employed because similarly to the tetraphenylphosphonium ion, the studied AA‐ion complexes are large single or double charged ions with rather low electric charge density.…”

Affinity capillary electrophoresis and density functional theory study of noncovalent interactions of cyclic peptide [Gly6]‐antamanide with small cations

“…In the current study, the ion size parameter 15.1 Å, determined by Muzikar et al. for tetraphenylphosphonium ion in MeOH, was employed because similarly to tetraphenylphosphonium ion, [Gly 6 ]AA‐M + complexes are large singly charged ions with rather low electric charge density. Therefore, the value of a = 15.1 Å in Eq.…”