Determination of association constants of inorganic ions with C 12- and C 14-alkyldimethylammoniopropane sulfonate zwitterionic surfactants using capillary electrochromatography

Abstract

Association constants ( K ass) 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 K ass values of anions with C12N3S increased as follows: Cl − (4.2±0.1)< IO 3 − (4.6±0.1)< Br − (5.9±0.1)< NO 2 − (6.2±0.1)< MoO 4 2− (7.8±0.1)< NO 3 − (14±1)< ClO 3 − (26±1)< I − (68±7)< SCN − (310±30)< ClO 4 − (800±200) dm 3 mol −1. Similar K ass values were obtained for C14N3S micelles, but K ass values of these anions with a non-ionic surfactant and with a non-surfactant-type zwitterion (used for comparative purposes) were small. The K ass 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)< Mg 2+ (7.8±0.1)< Ce 3+ (12±1) dm 3 mol −1. The fact that the K ass 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).