Use of gemini surfactants as semipermanent capillary coatings in aqueous‐organic solvents for capillary electrophoretic separation of inorganic anions

Abstract

This paper proposes a new method for CE separation of inorganic anions based on the use of gemini surfactants as capillary coatings in mixed aqueous-organic solvents. The semipermanent gemini surfactant coatings were facilely prepared by rinsing the capillary with 18-s-18 solutions; they can keep be stable during the electrophoretic runs without surfactants in buffer. The coatings showed a good tolerance of methanol (MeOH) or ACN, e.g. at pH 8.0 and with 40% v/v MeOH or ACN, the EOF magnitude after 60 min of continuous electrokinetic rinsing only decreased by 2.9 or 6.0%, respectively. The coatings were successfully applied to the separation of inorganic anions. Adding organic solvents in buffer can effectively improve the resolution and efficiencies; however, it remarkably prolonged the analysis time due to the suppression of EOF. Interestingly, varying the spacer length of the gemini surfactants can also modulate (improve) the resolution but without any sacrifice of analysis time. This benefit was resulted from the unique chemical structures of gemini surfactants because it introduced a new variable, i.e. the spacer length, to the separation mechanism. IntroductionGemini surfactants are a relatively new type of surfactants, which have two hydrophobic chains and two polar headgroups linked by a spacer group [1][2][3]. Compared with the corresponding conventional (monomeric) single-chained surfactants, the gemini surfactants show better water solubility, lower critical micelle concentration, lower Krafft point, and better surface activity in aqueous solutions [2][3][4]. These features make the gemini surfactants very attractive in both academic and industrial worlds. In particular, the spacer group that can be hydrophilic or hydrophobic, flexible or rigid, heteroatomic, aromatic, etc., plays an important role in determining the solution properties of gemini surfactants [5][6][7][8][9]. In principle, the spacer group can connect any two identical or different surfactants among the available ones. Therefore, gemini surfactants are thought to be designable with an enormous variety of structures [3].Although gemini surfactants have recently generated much research interest, their applications in CE are still rare. Gemini surfactants can be good alternatives to other micelle-forming reagents, buffer additives and capillary coatings, thus it is rational to anticipate a bright prospect of application of gemini surfactants in CE. A few earlier reports described the use of gemini surfactants as micelleforming reagents in MEKC, and found that the gemini surfactants exhibit better resolution or wider migration time windows than the commonly used single-chained surfactants [10][11][12]. Warner and coworkers [13][14] and systematically studied the use of anionic gemini surfactants as pseudostationary phases in MEKC, and presented some advantages of anionic gemini surfactants compared with conventionally used SDS. Due to their lower critical micelle concentrations, the gemini surfactants can be used at much lower concentrat...

Nonionic surfactant enhanced semipermanent coatings for capillary electrophoresis of inorganic anions without use of organic additives

“…It has been found that the spacer length ( s ) of Gemini surfactant can influence the coating stability 15, 19. Figure 4 shows the variation of coating stability as s increases from 2 to 10.…”

“…In chromatographic analysis, it was found that Gemini coatings were superior to other cationic surfactant coatings, including didodecyldimethylammonium bromide coating, because Gemini surfactants provide a new tool, i.e. spacer length, to optimize the selectivity so that I − and SCN − show fine peak shapes but the peak tailing of some anions such as IO 3 − cannot be avoided anyway 19. However, with regard to its use in the CE separation of anions, the result was not so good.…”

“…Baryla and Lucy 18 reported capillary electrophoretic separation of inorganic anions using a capillary coated with a mixture of cationic surfactant didodecyldimethylammonium bromide (DDAB) and zwitterionic surfactant. Liu et al 19 reported recently that cationic Gemini surfactants were good capillary coating materials for the separation of inorganic anions in aqueous‐organic media. Addition of organic solvents in the buffer resulted in a diminished zeta potential of the silica surface and altered the electrophoretic mobilities of the inorganic anions, and thus could modify the selectivity for inorganic anions 20.…”

“…Second, the strongly reversed EOF rapidly sweeps the analytes toward the detector and overwhelms the mobility difference, which is necessary for the separation; thus, the resolution of the analytes was limited. In addition, organic solvents, which are harmful to health, were involved in the method 19, 21.…”

Synthesis and characterization of comb polycarboxylic acid dispersants for coatings

“…Polymeric dispersants have been available for recent years with improved performance over traditional surfactants 1–3. Surfactants typically have a two‐part structure: a hydrophilic chain for steric stability and a hydrophobic group for pigment wetting or anchoring.…”