conference papers 562 # 2003 International Union of Crystallography ✏ Printed in Great Britain ± all rights reserved J. Appl. Cryst. (2003). 36, 562±567 Structural effects of macrocyclic compounds and their partition in sodium dodecylsulphate aqueous solutions Delia Chillura-Martino, Eugenio Caponetti* and Lucia Pedone Dip. Chimica-Fisica, Università di Palermo Viale delle Scienze Parco D’Orleans II 90128 Palermo, Italy. E-mail: caponett@unipa.it The partition of 1,4,7,10,13,16-esaoxacyclooctadecane (18C6), 4,7,13,16-tetraoxa-1,10-diazacyclooctadecane (2.2), 2,5,8,11,14,17- esaoxabicyclo[16.4.0]dicosane (B18C6) and 2,5,8,15,18,21- esoxatricyclo[20.4.0.0 9.14 ]esacosane (Cy218C6) in sodium dodecyl sulfate (SDS) aqueous solutions and their effect on the structure of surfactant aggregates has been investigated by Small Angle Neutron Scattering. Results from data analysis have shown that by increasing macrocycle concentration the SDS micelles dimensions reduce for all systems investigated. At the same time information on macrocycles partition between the micellar and the continuous phase have been obtained. It was found that an appreciable portion of macrocyclic compounds is located in micellar aggregates; in particular, the amount of B18C6 and Cy218C6 results larger than that of 18C6 and 2.2. It was found that 18C6 and 2.2 molecules interact with charged surface of SDS micelles only via complexes formation between the sodium ions and the macrocycles. B18C6 and Cy218C6 interact either via complexes formation with the charged surface or with hydrophobic region inside the micelle, as a consequence of the presence of hydrophobic substituents. It was concluded that Cy218C6 fraction present inside the micelles is located in the core, while the B18C6 fraction is located in the palisade. Keywords: micelles, additive partition, SANS, crown ethers, cryptands 1. Introduction It is well known that the addition of several molecules having different properties, strongly affects the structure of aggregates in solution (Evans et al., 1986; Baglioni et al., 1988). It is accepted that highly hydrophobic species tend to locate themselves in the hydrophobic interiors of the aggregate. Polar or ionic species tend either to stay in the polar shell of the aggregates, directly affecting the structure, or to localize in the aqueous phase affecting the micelle structure by the so called “solvent effect” (Gould et al., 1985). Macrocycles compounds are particularly interesting because they can form stable complex with metal ions. The stability of the complex and the selectivity toward cations shown by certain cyclic polyethers and polyamines, like crown ethers and cryptands, is well documented (Izatt et al., 1976) and constitutes one of interesting features which distinguish them from most noncyclic ligands. It has been shown that macrocycles, once they have formed complexes with anionic surfactants counterions, can interact with charged micelles. It follows that they are partially localized on the micellar surface or in the Stern layer (Evans et al., 1988; Gould et al., 1985) but, depending on their hydrophilic-hydrophobic balance, they can interact as non complexed compounds with the inner part of micelles. The effect of 1,4,7,10,13,16-esaoxacyclooctadecane (18C6) on both sodium dodecyl-sulphate (SDS) and dodecyl-trimethyl-ammonium bromide (DTAB) aqueous solutions has been explored by means of Small Angle Neutron Scattering (SANS) (Caponetti et al., 1995). The study was performed at fixed crown ether content as a function of surfactant concentration. It was found that the 18C6 slightly affects the DTAB micelle structure. This result excludes that interactions take place between crown ether molecules and the micelle core; hence the macrocycle substantially locates in the continuous phase slightly altering the solvent properties. On the other side, it was found that the structure of SDS micelles is strongly affected by the 18C6 presence and the interaction is a consequence of the partial substitution of the sodium counterion by its complex. In this paper the previous investigation related to SDS solution was extended to the cryptand and the crown ethers whose pictorial representation of molecular structures is shown in Fig. 1. Figure 1 Molecular structures pictorial representation of: A) 1,4,7,10,13,16-esaoxacyclooctadecane (18C6), B) 4,7,13,16-tetraoxa-1,10-diazacyclooctadecane (2.2), C) 2,5,8,11,14,17-esaoxabicyclo[16.4.0]dicosane (B18C6), D) 2,5,8,15,18,21-esoxatricyclo[20.4.0.0 9.14 ]esacosane (Cy218C6). Names reported in parenthesis follow the Izatt nomenclature (Izatt et al., 1976). These compounds, as a consequence of strong ion-dipole interaction, form stable complexes with SDS counterions (Gokel 1991). The complex stability depends on the matching between the ring dimension and the ion diameter. Being similar the ring size, the four macrocycles stability constant values are approximately the same (6.3), but the hydrophobic character and the steric hindrance differ. The study has been performed at 25 °C at fixed surfactant concentration varying the macrocycles content. 2. Experimental 1,4,7,10,13,16-esaoxacyclooctadecane, 18-crown-6 ether (18C6), Sigma product, 4,7,13,16-tetraoxa-1,10-diazacyclooctadecane, Kriptofix 22 (2.2), Merck, 2,5,8,11,14,17-esaoxabicyclo[16.4.0]- dicosane, benzo-18-crown-6 ether (B18C6), and 2,5,8,15,18,21- esoxatricyclo[20.4.0.0 9.14 ]esacosane, dicyclohexane-18-crown-6