Determination of association constants of inorganic ions with C12- and C14-alkyldimethylammoniopropane sulfonate zwitterionic surfactants using capillary electrochromatography

Abstract

Association constants (Kass) of 10 inorganic anions (chloride, bromide, iodide, nitrite, nitrate, chlorate, thiocyanate, iodate, perchlorate, and molybdate) and seven inorganic cations (lithium, sodium, potassium, rubidium, caesium, magnesium, and cerium(III)) with micelles of two zwitterionic surfactants 3-(N,N-dimethyldodecylammonio)propane sulfonate (C12N3S) and 3-(N,N-dimethylmyristylammonio)propane sulfonate (C14N3S) were determined using capillary electrochromatography (CEC). These Kass values of anions with C12N3S increased as follows: Cl− (4.2±0.1)<IO3− (4.6±0.1)<Br− (5.9±0.1)<NO2− (6.2±0.1)<MoO42− (7.8±0.1)<NO3− (14±1)<ClO3− (26±1)<I− (68±7)<SCN− (310±30)<ClO4− (800±200) dm3 mol−1. Similar Kass values were obtained for C14N3S micelles, but Kass values of these anions with a non-ionic surfactant and with a non-surfactant-type zwitterion (used for comparative purposes) were small. The Kass values of cations with the C12N3S micelle in 10 mM imidazole–20 mM 2-morpholinoethanesulfonic acid (MES) buffer of pH 6.2 increased as follows: Rb+ (1.4±0.1)=Cs+ (1.4±0.1)=K+ (1.4±0.1)<Na+ (3.2±0.1)<Li+ (4.5±0.1)<Mg2+ (7.8±0.1)<Ce3+ (12±1) dm3 mol−1. The fact that the Kass values for polarisable anions, such as iodide and thiocyanate, were larger than those of cations of +2 and +3 charge, agrees well with the relative retentions for those ions in electrostatic ion chromatography (EIC).