ORIGINAL ARTICLE How does b-cyclodextrin affect the aggregation of sodium perfluoroheptanoate in aqueous solution: a 19 F NMR study Se ´rgio Lima Æ Cristiano Andrade-Dias Æ Ana M. A. Dias Æ Isabel M. Marrucho Æ Joa ˜o A. P. Coutinho Æ Jose ´ J. C. Teixeira-Dias Received: 15 May 2006 / Accepted: 20 October 2006 / Published online: 20 January 2007 Ó Springer Science+Business Media B.V. 2007 Abstract 1 H and 19 F NMR spectra were recorded for D 2 O solutions of sodium perfluoroheptanoate with defined concentrations up to 200 mM, in the absence and presence of b-cyclodextrin (15 mM). Analysis of 1 H chemical shift data obtained by the method of continuous variations (Job’s method) confirms the formation of 1:1 inclusion complexes for the perflu- oroheptanote anion in b-cyclodextrin and leads to an estimate for the apparent inclusion constant (‡10 4 M –1 ). In addition, analysis of 19 F chemical shift data based on two simplifying assumptions (monodisperse perfluoroheptanoate solutions below the critical micellar concentration (CMC), and a single self-asso- ciated state after the CMC) enables to interpret all the experimental chemical shift data and allows to deter- mine CMC values for the absence and presence of b- cyclodextrin (104 and 116 mM). It is shown that the self-association of perfluoroheptanoate and its inclu- sion in b-cyclodextrin lead to shielding and deshielding of the fluorine atoms, respectively. Keywords Aqueous solution  b-Cyclodextrin  Critical micellar concentration  Inclusion  NMR chemical shifts  Self-association  Sodium perfluoroheptanoate Introduction Perfluorocarbons (PFCs) are well known for their hydrophobicity and rigid molecular skeleton [1–6]. They are powerful wetting agents and indispensable as emulsifiers in many industrial applications, including emulsion polymerization of chlorocarbons and fluoro- carbons, and in a variety of biomedical applications, including the development of oxygen-carrying fluoro- carbon emulsions, pulmonary drug and gene delivery [7, 8]. However, these compounds are strongly hydro- phobic, and for this reason their study in aqueous solutions is only possible if they are previously trans- formed to increase their solubility in water. A high surface tension can be added to the properties of PFCs when a hydrophilic group is bonded to a PFC chain originating an amphiphilic molecule [9]. Other at- tempts to solubilize highly fluorinated compounds in aqueous media may consist of their inclusion into na- tive cyclodextrins—cyclic oligosaccharides composed of six (a-cyclodextrin, aCD), seven (b-cyclodextrin, bCD) or eight (c-cyclodextrin, cCD) a(1-4)-linked glucopyranose residues [10–12]. Previous studies have concluded that the inclusion strongly depends on the cavity diameter, suggesting that the fluorocarbon chain cannot fit into the cavity of aCD, fits snugly inside the cavity of bCD, and loosely inside the cavity of cCD [13–19]. A recent study about oxygen solubility in aqueous solution of PFCs shows that oxygen prefer- entially interacts with the PFC surfactant molecules in the formed micellar aggregates [20]. If bCD is added to the aqueous solution containing the PFC surfactant, the PFC chain includes in the bCD cavity with the subsequent increase of the PFC surfactant critical micellar concentration (CMC) [20]. In addition, S. Lima (&)  C. Andrade-Dias  A. M. A. Dias  I. M. Marrucho  J. A. P. Coutinho  J. J. C. Teixeira-Dias Department of Chemistry, University of Aveiro, CICECO, 3810-193 Aveiro, Portugal e-mail: slima@dq.ua.pt 123 J Incl Phenom Macrocycl Chem (2007) 57:157–162 DOI 10.1007/s10847-006-9201-3