Journal of Chromatography A, 1157 (2007) 437–445 Linear solvation energy relationships of anionic dimeric surfactants in micellar electrokinetic chromatography I. Effect of the length of a hydrophobic spacer Geert Van Biesen, Christina S. Bottaro ∗ Memorial University of Newfoundland, Prince Philip Drive, Department of Chemistry, Chemistry/Physics Building Room 4000, St. John’s, Nfld A1B 3X7, Canada Received 26 January 2007; received in revised form 10 April 2007; accepted 12 April 2007 Available online 19 April 2007 Abstract The influence of the length of a flexible hydrophobic spacer on the selectivity of anionic dimeric surfactants was investigated. Disodium 1,-bis(decyloxymethyl)-dioxa alkane-1, disulfates with a spacer containing an ethylene, butylene, hexylene, octylene, decylene or dodecylene group were synthesized, and four of these were evaluated for use in micellar electrokinetic chromatography (MEKC) via linear solvation energy relationships (LSERs). There were no significant differences in the system constants of these surfactants, indicating that their micelles all have a very similar interface with the aqueous phase, regardless of the length of the hydrophobic spacer. Compared to sodium dodecylsulfate (SDS), these dimeric surfactants are slightly more cohesive, interact better with polarizable compounds, and are somewhat better hydrogen bond acceptors and worse hydrogen bond donors, while there is no difference in dipolarity. The critical micelle concentrations (CMCs) of these surfactants were in the order of 1 mM, except for the dimeric surfactant with a spacer containing an ethylene group, which had a CMC <0.03 mM. © 2007 Elsevier B.V. All rights reserved. Keywords: Critical micelle concentration; Dimeric surfactants; Gemini surfactants; Linear solvation energy relationships; Micellar electrokinetic chromatography; Spacer 1. Introduction Separation in micellar electrokinetic chromatography (MEKC) is based upon the differential partitioning of analytes between an aqueous phase and a pseudostationary phase of micelles of an ionic surfactant at a concentration higher than its critical micelle concentration (CMC). The selectivity of the separation can be manipulated by adding organic modifiers [1] or cyclodextrins (particularly for enantiomeric separations) [2], or by using different types of surfactants or mixtures of surfac- tants [3–5]. Generally, the choice of surfactant seems to be the most important variable in optimizing the selectivity [6]. Of the numerous surfactants that have been evaluated for MEKC, the majority are ‘conventional’ amphiphiles, i.e. they consist of one long hydrophobic chain with one ionic head group; sodium dode- cylsulfate (SDS) being the most widely used. At the other end ∗ Corresponding author. Tel.: +1 709 737 8088; fax: +1 709 737 3702. E-mail address: cbottaro@mun.ca (C.S. Bottaro). of the spectrum are polymeric surfactants, which have multiple hydrophobic chains and ionic head groups. They have gained a lot of attention the past few years for a number of reasons [7–9]. They have a zero CMC (i.e. they do not require self-assembly) and thus can be used at low concentrations, typically ≤1%, and with relatively high concentrations of organic solvents. This is of particular importance for the analysis of very hydropho- bic compounds, which are difficult to separate because of their high affinity for the pseudostationary phase, and for which the addition of an organic solvent is often necessary to decrease their partition coefficient. Polymeric surfactants also seem to be more compatible with mass spectrometric detection, presum- ably because of their lower surface activity [10]. However, it can be problematic to obtain monodisperse solutions, and mass- transfer kinetics may be slower than with conventional micelles, both of which can lead to lower plate counts [8]. Between these two extremes (monomeric and polymeric sur- factants) are dimeric (or gemini) surfactants, which are made up of two amphiphilic moieties connected at, or close to, the head groups by a spacer [11] (Fig. 1). If the spacer is further 0021-9673/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2007.04.026