PalmitateLuciferin: A Molecular Design for the Second Harmonic Generation Study of Ion Complexation at the Air-Water Interface Gaelle Martin-Gassin,* ,† Guilhem Arrachart, † Pierre-Marie Gassin, †,‡ Noë lle Lascoux, ‡ Isabelle Russier-Antoine, ‡ Christian Jonin, ‡ Emmanuel Benichou, ‡ Stephane Pellet-Rostaing, † Olivier Diat, † and Pierre-Francois Brevet ‡ † Institut de Chimie Sé parative de Marcoule, UMR 5257 CEA-CNRS-UM2-ENSCM, Bâ timent 426, B.P. 17171, 30207 Bagnols sur Ceze cedex, France ‡ Laboratoire de Spectrome ́ trie Ionique et Molé culaire, UMR 5579 CNRS, Universite ́ Claude Bernard Lyon 1, Bâ timent Alfred Kastler, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne cedex, France * S Supporting Information ABSTRACT: A molecular organic chromophore, Palmitate- Luciferin, has been synthesized for studying ion complexation at the air-water interface using second harmonic generation (SHG). This molecule was designed through the addition of a long hydrophobic palmitoyl alkyl chain to the aromatic π- electron system of Luciferin. We first demonstrate that this organic chromophore is a potential candidate for SHG studies of ion complexation with the measurement of its first hyperpolarizability in aqueous solutions by hyper Rayleigh scattering (HRS) with and without calcium ions. Then, we characterize the PalmitateLuciferin surfactant properties at the air- water interface combining surface tension measurements with a surface SHG study and Brewster angle imaging. These results allow us to build a molecular description of the chromophore at the interface and observe its molecular reorganization during the monolayer compression leading to the formation of aggregates. Finally, we show that the initial goal of the designing work is achieved since PalmitateLuciferin indeed exhibits a higher SHG response in the presence of calcium ions in the aqueous subphase as expected. ■ INTRODUCTION Solvent extraction is one of the most common and widely used processes to separate and concentrate substances in solution. 1 The phase transfer reaction occurring at the liquid-liquid (LL) interface is often facilitated by the formation of complex species using oil-soluble ligands, also called extractants. These extractants have some slight amphiphilic features and the complexation will affect their interfacial properties. Today, it is considered essential to understand the organization of these extracting molecular compounds at the LL interface both in absence and during the extraction process in order to get a clear molecular picture 2 of the extraction phenomena. Such a study often starts with an initial stage where the complexation reaction is investigated, and this may be performed at the air- water interface. The standard way to investigate adsorption layers of surfactants at liquid interfaces, and the air-water interface in particular, relies on the measurement of the equilibrium surface tension and the construction of the corresponding adsorption isotherm for the complexing agent. However, this analysis is usually restricted to the determination of equilibria and does not provide alone a molecular structure of the interface nor a dynamic view of the system. For this reason, surface analytical tools directly monitoring molecular adsorption and molecular organization at interfaces, this is even more crucial in the case of buried interfaces like the liquid- liquid one, are highly desirable for a future progress in the field. A particularly appealing technique is the nonlinear optical technique of second harmonic generation (SHG), the phenomenon whereby two photons at a fundamental frequency are converted into a single photon at the harmonic one. This technique is indeed inherently surface specific, therefore discriminating between molecules and ions adsorbed at the interface from those dissolved in the bulk phase. This surface specificity stems from the lack of centrosymmetry of the interface as compared to the adjacent bulk phases. Indeed, the SHG phenomenon is forbidden in centrosymmetric media within the electric dipole approximation. 3-5 In the past, SHG has proven to be a powerful method to investigate buried interfaces, 6 yielding for instance relative molecular densities 7 or molecular orientations. 8,9 Real time SHG can also be used to measure transport kinetics across and along membranes 10,11 whereas fluctuation correlation analyses give access to interfacial characteristic times. 12 In all cases, a preferential orientation of the nonlinear optical probe molecules at the Received: December 29, 2011 Revised: March 6, 2012 Published: March 9, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 7450 dx.doi.org/10.1021/jp2125697 | J. Phys. Chem. C 2012, 116, 7450-7456