Molecular Association of a Nonionic and an Ionic-Induced Surfactant: Cryptand (221D) NaCl in Water E. Caponetti,* D. Chillura Martino, and L. Pedone Dip. di Chimica Fisica, Universita ` di Palermo, Viale delle Scienze - Parco D’Orleans II, I-90128 Palermo Received May 30, 2000. In Final Form:October 6, 2000 The cryptand 5-decyl-4,7,13,16,21-pentaoxa-1,10-diazabicyclo-[8.8.5]tricosane [221D]strongly resembles a surfactant in itscharacteristics: it containsa hydrophilicheadgroup,thecryptandunit,anda hydrophobic unit, the decyl chain. It is insoluble in water, but in the presence of an appropriate amount of NaCl, as a consequence of the complex formation between 221D and Na + , it becomes soluble and forms aggregates. The aggregates, depending on the NaCl-221D molar ratio, can be considered as mixed ionic nonionic micelles or ionic micelles. The evolution of the aggregate dimensions and their shape has already been studied at two NaCl-221D molar ratios as a function of the 221D concentration. In the present work, to obtain information on the aggregation behavior of 221D in water on varying the NaCl concentration from onethirduptofourtimesthe221Dcontent,thesystemhasbeenstudiedbysmall-angleneutronscattering. Increasing the NaCl-221D molar ratio up to 1, the aggregation number rapidly falls, and then slowly increases at higher salt content; this behavior has been attributed to electrostatic interactions. After the micelle dimension behavior, the shape evolves continuously from oblate (axial ratio ≈ 0.67)toprolate(axial ratio ≈ 1.2) ellipsoid up to [NaCl]/[221D] ) 1, and tends to be spherical at higher ratios. Introduction Macrocyclic compounds having different cavity sizes and ring substituents and containing donor atoms such as oxygen nitrogen, sulfur, phosphorus, or their combina- tion have been synthesized and largely investigated. 1 Macrocyclic compounds having more than one cyclic ring form stable complexes with alkali, alkaline earth, and some transition elements. They have been studied, from the point of view, of both thermodynamics and kinetics, in different solvents such as water, methanol, and others. 1-3 The interest in these classes of compounds has been stimulated by their extensive and successful use in several fields where it is important tochoose a convenient cation and maintain it in a precise position in an organic moiety. 4 Cryptands are macrocyclic compounds having at least two basic bridgehead nitrogen atoms in the ring; they encapsulate ions by their cagelike structure and form metal complexes having 1:1 metal -ligand molar ratios. For alkali and alkaline earth the stability constant ofthe complex, called cryptate, is strongly related to the match- ing of the ionic crystal radius and the cavity radius. The cryptand 5-decyl-4,7,13,16,21-pentaoxa-1,10-diaza- bicyclo-[8.8.5] tricosane identified as 221D 5,6 accordingto the macrobicyclic ligands nomenclature is a derivative of thecryptand221obtainedintroducinga decylhydrocarbon chain in position 5 of the tricosane ring. Its structure is representedin Figure1.Consideringthe221cavityradius 6 andthealkaliandalkalineearthionsradii, 7 it is expected that the more stable complexes must be the ones with sodium and calcium ions. This is confirmed by the values ofthestabilityconstant,both in water andin nonaqueous media. 1 The alkyl substitution, as shown by Cox et al. in methanol, 8 is expected to reduce the cryptates stability and to increase their hydrophobicity. Whereas the 221 is very soluble in water, the 221D is practicallyinsoluble,but thepresenceofa certain amount of salt makes it soluble because of the complex formation between the ligand and the cation. Increasing the con- centration,thecomplexbehaveslikea cationicsurfactant and form micelles. 9 Similar behavior has been observed for other long-chain alkyl substitute macrocyclic com- pounds. 10-12 Surfactants are amphiphilic molecules, that is, mol- ecules made with a hydrophobic portion such as a hydrocarbon chain,and hydrophilicportions such as ionic * Corresponding author. E-mail: caponett@unipa.it. (1) Izatt, R. A.; Pawlak, K.; Bradshaw, J. S.; Bruening, R. L. Chem. R ev . 1991 , 91, 1721. (2) Izatt, R. M.; Bradshaw, J. S.; Nielsen, S. A.; Lamb, J. D.; Christensen,J. J.Chem.Rev . 1985 , 85,271;seealsoreferencestherein. (3) Cox, B. G.; Schneider, H. Coordination and Transport Properties of Macrocyclic compound in Solution ; Elsevier: Amsterdam, 1992; see also references therein. (4) Votgle, F., Ed. Topics in Current Chemistry; Springer-Verlag: New York, 1982 (5) Lehn, J. M. Struct. Bonding (Berlin ) 1975 , 16, 1. (6) Lehn, J. M.; Sauvage, J. P. Chem. Commun . 1971 , 440. (7) Shannon, R. D. Acta Crystallogr . 1976 , A32, 751. (8) Cox, B. G.; Firman, P.; Schneider, I.; Schneider, H. Inorg. Chim. Acta 1981 , 49, 153. (9) Caponetti, E.; Chillura Martino, D.; Floriano, M. A.; Triolo, R. J. Phys. IV Colloq. C8 1993 , 3, 173. Figure 1. Molecular structure of cryptand 5-decyl-4,7,13,16,- 21-pentaoxa-1,10-diazabicyclo-[8.8.5] tricosane (221D). 554 Langmuir 2003, 19, 554-558 10.1021/la000735h CCC: $25.00 © 2003 American Chemical Society Published on Web 01/07/2003