Characterization of Counterion and Surface Influence on Micelle Formation Using Tapping Mode Atomic Force Microscopy in Air Lila Chaal, ²,‡ Franc ¸ oise Pillier, ² Boualem Saidani, ‡ Suzanne Joiret, ² Alain Pailleret,* ,² and Claude Deslouis ² UPR 15 CNRS Interfaces et Syste ` mes Electrochimiques, UniVersity P. et M. Curie, 4 Place Jussieu, 75252 Paris Ce ´ dex 05, France, and Laboratory Electrochimie et Corrosion, Department Ge ´ nie des Proce ´ de ´ s, UniVersity A Mira-Be ´ jaia, Algeria ReceiVed: March 15, 2006; In Final Form: July 7, 2006 Cylindrical micelles prepared in aqueous solutions from cationic surfactants octadecyl trimethylammonium (OTA + ) or cetyltrimethylammonium (CTA + ) and parachlorobenzoate (PCB) counterion were successfully imaged after evaporation of water using tapping mode atomic force microscopy (TM-AFM) onto very smooth gold and glass substrates. With the help of the obtained topography AFM images, it was shown that the micellar structures are preserved on gold substrates after evaporation of the solvent despite the new set of stresses due mainly to capillary forces and dehydration. The influence of the substrate on the resulting micellar morphology observed in air was investigated for these two materials: cylindrical micelles were evidenced as loosely adherent on gold surface in the presence of parachlorobenzoate (PCB) and identical, geometrically speaking, to those known to exist in aqueous solutions. In this situation, topographic AFM images allowed us to determine accurately their geometrical characteristics such as diameter and length in the nanometer range. On the other hand, AFM images obtained in air on glass surfaces revealed micellar structures that are different from those existing in the bulk of the solution. Indeed, bilayer-type micelles with a thickness close to twice the surfactant monomer expected length were observed, indicating that the well-established and strong influence of glass on micelle geometry at the glass/solution interface is maintained after evaporation of water. These results have been analyzed on the basis of positive charge of gold deduced from electrochemical impedance spectroscopy (EIS) and Raman spectroscopy measurements on one hand and of the negative charge of glass on the other hand. Although these results appeal to new theoretical considerations dealing with dynamics of evaporation of micellar solution drops and/or with counterion contributions to macromolecular interactions in aqueous solutions and in air, this new AFM imaging method appears to be the more adequate one to image and measure the micelles formed in the presence of water. Introduction Direct observation of micellar structures in the liquid phase or on surfaces is a challenging purpose for which only a few competitive techniques were proposed until now. To the best of our knowledge, two main techniques emerged from the literature over the last 10 years. The cryo-transmission electronic microscopy (cryo-TEM) technique is performed through the fast- freezing process of aqueous micellar solutions before analysis. This technique bears the disadvantage of subjecting the micellar samples to brutal although extremely short temperature treat- ments as well as to a blotting process that leads to very high shear rates. 1-2 Comparing the respective drawbacks of cryo-TEM in liquids with the technique we introduce in this work, namely, AFM in air, both suffer from a change in the thermodynamic conditions that results from a fast sweep through wide regions of the phase diagram plotted in the surfactant concentration and solution temperature coordinates. The cryo-TEM method obviously meets the latter one, whereas AFM in air involves progressive increase of both the surfactant and the counterion concentrations while evaporation proceeds. In addition, for gold, if specific micelle-substrate interactions are lessened, the evaporation process should just increase the concentration of cylindrical micelles instead of changing their morphology. On the other hand, in situ AFM appeared in the 1990s as a very powerful technique for the imaging of micelles adsorbed at the solid-liquid interface thanks to the pioneering work of Manne et al. 3-4 It appears from this latter paper 3,4 and the abundant bibliography citing it that most micellar structures based on alkyltrimethylammonium-type surfactants could be imaged on surfaces where the interactions of the surfactants with the surfaces were rather strong, leading to self-assembled layer- like structures in which each surfactant molecule undergoes strong interactions with other surfactant molecules and/or with the underlying surface depending on the surfactant concentra- tion. 5-13 The main types of surfaces used for such studies were mica, glass, gold, or graphite, for example. Consequently, it is now well-established that the surface micelle morphology observed on such substrates is frequently different from the micelle shape predicted or observed in the bulk solution, with the substrate thus playing a dominant role in determining the surface micelle structure. The strong interactions between the surface and the surfactants are thus also responsible for an * Corresponding author. Phone: 33 1 44 27 41 69; fax: 33 1 44 27 40 74; e-mail: paillere@ccr.jussieu.fr. ² University P. et M. Curie. ‡ University A Mira-Be ´jaia. 21710 J. Phys. Chem. B 2006, 110, 21710-21718 10.1021/jp061607q CCC: $33.50 © 2006 American Chemical Society Published on Web 10/07/2006