Characterization of Surfactant Coatings in Capillary Electrophoresis by Atomic Force Microscopy

Abstract

This paper describes the adsorption mechanisms and aggregation properties of cetyltrimethylammonium bromide (CTAB) and didodecyldimethylammonium bromide (DDAB) surfactants that are used for dynamic coatings in capillary electrophoresis (CE). Atomic force microscopy is used to directly visualize surfactant adsorption on fused silica. It was found that the single-chained surfactant CTAB forms spherical aggregates on silica while the double-chained surfactant DDAB forms a bilayer. Aggregation at the surface occurs at approximately the same surfactant concentration in which EOF reversal is observed in CE. The nearest-neighbor distance between CTAB aggregates varies inversely with buffer pH and becomes constant at the point when the silanol groups are fully ionized. DDAB forms a flat, uniform coating independent of pH. Increasing the buffer ionic strength changes the morphology of the CTAB aggregates from spherical to cylindrical. The change in morphology can alter the surface coverage, which is related to the "normalized" EOF measured in identical buffers. The morphology of a surfactant coating is also shown to affect its ability to inhibit protein adsorption to the capillary wall. Specifically, the full surface coverage provided by DDAB proved superior in a head-to-head comparison with CTAB.

Preparative capillary zone electrophoresis using a dynamic coated wide‐bore capillary

“…Previously, it was demonstrated that bilayer coatings based on surfactants such as DDAB [31][32][33] and 2C 14 DAB [34] yield high efficiency separations of proteins under trace conditions. For instance, efficiencies in excess of 10 6 plates/m, have been reported for injections of 0.1 g/L of cationic proteins such as lysozyme, ribonuclease A, a-chymotrypsinogen and cytochrome c. Furthermore, protein recoveries from bilayercoated capillaries are essentially quantitative [31,34]. Experimental conditions: capillary length, 58 cm (48 cm to the detector), the injection plug length was kept constant at 6.2 mm, the analyte was dissolved in water at a concentration of 0.05 g/L, 50 mM lithium phosphate buffer at pH 7.0, voltage at 210 kV, l = 214 nm.…”

“…Coating of the capillary walls is the most effective approach to eliminate the electrostatic attraction between cationic proteins and the silica surface. Coatings can be broadly classified as: covalently bonded or cross-linked polymer coatings [25][26][27][28]; adsorbed cationic and nonionic polymer coatings [29,30]; and adsorbed surfactants (dynamic coatings) [3,[31][32][33][34]. Surfactant-based coatings are particularly attractive due to their simplicity, versatility and low cost.…”

“…Previously, it was demonstrated that double-chained cationic surfactants such as didodecyldimethylammonium bromide (DDAB) form semi-permanent wall coatings [31][32][33]. Specifically, the DDAB coating was sufficiently stable that free surfactant need not be present in the electrolyte buffer.…”

“…Rather, the coating simply needed to be regenerated between runs. The stability of the DDAB coatings resulted from the formation of a bilayer on the capillary surface [31,32]. However, the stability of the DDAB coating was influenced by a number of experimental factors, such that the coatings were unstable under some buffer conditions [33].…”

Fundamentos, produção e aplicações de marcas térmicas em eletroforese capilar

“…Esse fato se deve a Capítulo 4: Origem da marca térmica através da aplicação de um campo elétrico externo na direção radial ao capilar [43,44]. [46]. O polybrene é um exemplo de um polímero catiônico empregado como inversor de EOF [47].…”