Determination of the aggregation threshold of non–UV-absorbing, neutral or charged surfactants by frontal- and vacancy-frontal analysis continuous capillary electrophoresis

Supplementing our recent work on UV-absorbing anionic surfactants, new protocols based on frontal analysis continuous capillary electrophoresis (FACCE) were developed for the investigation of the aggregation threshold of non-UV absorbing anionic, cationic and neutral surfactants, and exemplified with sodium dodecyl sulfate (SDS), tetradecyltrimethylammonium bromide (TTABr) and Brij 35. Contrary to UV-absorbing surfactants, the critical micelle concentration (CMC) determination of non-UV absorbing surfactants requires the use of a marker providing adequate detection capabilities. UV-absorbing markers were selected, according to the charge of the studied surfactant (neutral for SDS and TTABr, anionic for Brij 35). In all cases, the free marker concentration was quantified as a function of the total surfactant concentration. In addition, a modified implementation of FACCE, that we called vacancy FACCE (VFACCE), was employed for the case of the neutral surfactant. VFACCE entails first filling the capillary with the system components to be studied in the background electrolyte, next continuously introducing the plain BGE electrokinetically. The salient theoretical features of FACCE and VFACCE were compared. These new protocols were successfully applied to yield reliable CMC values within short operational time and with low sample consumption.