Letters Molecular Dynamics Study of Temperature Dehydration of a C 12 E 6 Spherical Micelle Fabio Sterpone, † Carlo Pierleoni,* ,‡ Giuseppe Briganti, § and Massimo Marchi † Commissariat a ` l’Energie Atomique, DSV-DBJC-SBFM, Centre d’E Ä tudes, Saclay, 91191 Gif-sur-Yvette Cedex, France, INFM and Dipartimento di Fisica, Universita ` di l’Aquila, Via Vetoio, 67100 Coppito L’Aquila, Italy, and INFM and Dipartimento di Fisica, Universita ` di Roma “La Sapienza”, P.A.Moro 2, 00185 Roma, Italy Received October 22, 2003. In Final Form: March 17, 2004 Hydration of a spherical micelles of C12E6 in solution is studied by molecular dynamics simulation. The interface is found to be separated in an inner part composed of water and hydrophobic and hydrophilic moieties and an outer part with hydrophilic moiety and water only. Hydration numbers in the inner and in the outer parts are in excellent agreement with experimental data from various different methods. Temperature dehydration occurs in the inner region only and is related to the presence of water molecules directly in contact with the hydrophobic core at low temperature. Aggregation states, sizes, and phase transitions of the oligooxyethylene glycol C i E j family of nonionic surfactant are the result of a delicate balance between inter- and intraaggregate interactions, 1 the strength of which can be related to the hydration state of the hydrophilic part of the aggregates. A variety of experimental techniques have been devised to estimate the amount of water in the interfacial region of a micelle. Most of these methods, such as light scattering, dielectric and ultrasonic relaxation, small-angle neutron scattering, aggregate and heavy water diffusion by NMR, and 17 O magnetic relaxation, extract the hydration number from bulk measurements through the application of suitable models of water-solute interactions. Depending on the member of the C i E j family, the concentration of the surfactant, the temperature, and the experimental method, estimates of the hydration number ranging from 1 to 9 have been reported in the literature (see ref 2 for a collection of data and references). Jonstro ¨ mer et al. 3 first recognized that hydration, defined as the number of water molecules per oxyethylene (EO) units, is not uniform in the interface of a spherical aggregate but depends on the distance from the oil core. By a cell-diffusion model analysis of the D 2 O diffusion in a low concentration C 12 E 8 /D 2 O solution (e35% surfactant), they reported values of 2.8 and 14 for the hydration of the inner layer and the outer layer, respectively, at T ) 5 °C. They also reported dehydration with temperature in the inner layer (the hydration number decreases to 1 at T ) 66 °C) but not in the outer one. More recently, Romsted and Yao 2,4 introduced a new experimental approach, known as the chemical trapping method, which exploits the reactivity of a chemical probe to water and OH group and is able to provide a direct measure of the local hydration number, i.e., near the detector molecule itself. The probe, attached to a long hydrophobic tail, is integrated in the micelle and can react with water at the micellar interface. The technique has been applied to † Commissariat a ` l’Energie Atomique, DSV-DBJC-SBFM, Centre d’E Ä tudes. ‡ INFM and Dipartimento di Fisica, Universita ` di l’Aquila. § INFM and Dipartimento di Fisica, Universita ` di Roma “La Sapienza”. (1) Carlstrom, G.; Halle, B. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1049. (2) Romsted, L. S.; Yao, J. Langmuir 1996, 12, 2425-2432. (3) Jonstromer, M.; Jonsson, B.; Lindman, B. J. Phys. Chem. 1991, 95, 3293-3300. (4) Romsted, L. S.; Yao, J. Langmuir 1999, 15, 326-336. © Copyright 2004 American Chemical Society MAY 25, 2004 VOLUME 20, NUMBER 11 10.1021/la035964t CCC: $27.50 © 2004 American Chemical Society Published on Web 04/27/2004