Unexpected binding mode of gemini surfactants and c-cyclodextrin: DOSY as a tool for the study of complexation A. Guerrero-Martı ´nez, M.A. Palafox, G. Tardajos * Departamento de Quı ´mica-Fı ´sica I, Facultad de Ciencias Quı ´micas, Universidad Complutense de Madrid, 28040 Madrid, Spain Received 25 September 2006; in final form 18 October 2006 Available online 25 October 2006 Abstract Diffusion-ordered NMR measurements for aqueous solutions at 298 K containing oxyethylene spacer gemini surfactants (12-EO i -12), in the absence and presence of c-cyclodextrin ( c-CD), were analyzed to determine the binding mode of the complex, and the effect of the complexation on the aggregation. The analysis of the self-diffusion coefficients reveals the formation of complexes of 1:1 stoichiometry (12-EO i -12:c-CD). This complexation is strong enough to destroy aggregates in an extension that confirms the 1:1 stoichiometry. The structure of surfactants and ROE enhancements point to a complex in which both hydrophobic chains thread one c-CD. Ó 2006 Elsevier B.V. All rights reserved. 1. Introduction Understanding the complexation of natural and syn- thetic hosts of various architectures is fundamental for the research on the field of supramolecular chemistry and molecular recoginition [1]. Prime candidates of complexa- tion in aqueous media are macrocycles that consist of sev- eral a-D-(+)-glucopyranose residues so called cyclodextrins (CDs), being the most well-known representatives a-, b-, and c-CD, with six, seven, and eight glucose units, respec- tively [2]. They have a toroidal truncated cone shape with a hydrophobic cavity and two hydrophilic rims formed by the primary (narrower rim) and secondary (wider rim) OH groups [3]. Their ability to form inclusion complexes renders interesting applications, and there are many reports of CD complexes with a wide variety of substrates in which an array of experimental methodologies was employed [4–6]. The conditions required to have an inclusion complex with CDs is the fit, total or partial, of the guest in the cav- ity, and a favorable energetic balance, which depends on the specific host–guest interactions, and different hydration states of the isolated and complexed molecules [7,8]. These non-covalent processes are responsible for the binding con- stants and stoichiometries of the complexes, and are used to modify the molecular properties of guests in water solu- tion. Of particular interest is the effect of CDs on the aggre- gation of surfactants. For example, surfactants quite different from the structural and nature point of view form inclusion complexes with CDs strong enough to displace the critical micelle concentration (cmc) in a certain exten- sion [9,10]. Gemini surfactants are made of two hydrophobic tails and two hydrophilic headgroups linked by a spacer chain, and have gained recent attention because of their excep- tional amphiphilic properties, such as low cmc and a stronger efficacy in decreasing the surface tension of water [11–13]. The use of these double-chained surfactants as tar- get molecules offers certain advantages to understand the complexation process of CDs, since it is easy to modulate their properties to study the effect that a determined factor may have in the inclusion (e.g. the number, hydrophobic nature, and flexibility of the hydrocarbon chains and spac- ers). We report herein the results of the analysis of diffu- sion-ordered NMR (DOSY) experiments, together with 1 H NMR and 2D rotating-frame Overhauser enhancement 0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.10.091 * Corresponding author. Fax: +34 913944135. E-mail address: tardajos@quim.ucm.es (G. Tardajos). www.elsevier.com/locate/cplett Chemical Physics Letters 432 (2006) 486–490