Determination of the retention factor of ionizable compounds in microemulsion electrokinetic chromatography

Abstract

Determination of the retention factor of ionized compounds in microemulsion electrokinetic chromatography requires two mobility measurements at the same pH: one in the presence of the microemulsion and another in plain buffer. However, it has been observed that in some cases subtracting one mobility from another determined in a different medium leads to negative retention factors, which makes no sense from a chemical point of view. This indicates that there is some error in the process which has a direct impact when retention factors are used for further applications. Here, we evaluate how the components of the microemulsion confer different properties to the buffer medium, particularly varying the viscosity parameter (which is inversely related to mobility). Whereas sodium dodecyl sulfate, the surfactant used in the microemulsion, has little effect on the medium viscosity (only an increase of 5%-6%), the presence of 1-butanol, used as a stabilizer, increases it by around 30%. Meanwhile, heptane, which is used as an oil, provokes a slight decrease. Consequently, the mobilities obtained in the microemulsion system are shifted to higher values (less negative mobilities) compared to mobilities obtained in the aqueous buffer, and so one cannot be directly subtracted from the other. Since the microemulsion-buffer medium cannot be directly reproduced, we propose a correction that takes into account the variation of viscosities. This is determined from the electrophoretic mobility of the benzoate ion. As this ion does not interact with the microemulsion, the ratio of its mobilities (measured in plain buffer and microemulsion) is equivalent to the ratio of viscosities, and can be used as the correction factor for other measurements. Thus, mobilities in buffer and microemulsion media are placed on the same scale, overcoming the errors in retention factor determination.

Estimation of the octanol–water distribution coefficient of acidic compounds by microemulsion electrokinetic chromatography

“…However, the MEEKC medium contains not only the aqueous buffer but also the surfactant, the co-surfactant and the oil, which usually have viscosities very different from water. Thus, the presence of ME components changes the viscosity of the medium, leading to inaccurate k values, as demonstrated in previous work [18]. Note that the viscosity of the electrophoretic medium (ƞ) and the mobility of a compound are inversely related, according to Eq.…”

“…Experimental k-pH profiles of the compounds were taken from a previous study [18] and the log k-pH profiles were obtained by fitting the data to Eq. 11.…”

Comparison of the retention of basic compounds in anionic and cationic microemulsion electrokinetic chromatographic systems

“…In a previous work [14], we studied the effect of the ionization of acids in a MEEKC system with a ME composed of heptane, 1-butanol and sodium dodecyl sulfate (SDS, an anionic surfactant), a system which showed to be a good surrogate for the determination of octanol-water partition coefficients [6][7][8][9][10]. However, the study of the retention of partially protonated bases was not intended because additional interactions, other than partition of the unionized form of the base into the ME, were expected.…”

“…However, it is clear that micellar and microemulsion systems have different properties. For instance, we have demonstrated that the addition of the oil and cosurfactant, needed to form the microemulsion, change significantly the viscosity of the surfactant solution and the usual MEKC equation used to calculate the retention factor of partially ionized acids has to be corrected for this change of viscosity [14]. Also, the solvation properties of MEKC systems strongly depend on the surfactant used to form the micelle [15][16][17][18].…”

“…In a second instance, the retention of ionizable bases in two different MEEKC systems (anionic and cationic), and the retention behaviour of the unionized and ionized forms of the bases in the two systems will be also compared. The anionic system will be the same used previously [14] with a ME composed of heptane, 1-butanol and SDS (SDS-MEEKC system). The cationic system will have the same composition, but changing SDS by tetradecyltrimethylammonium bromide (TTAB; TTAB-MEEKC system).…”