Effect of bromide on the interfacial structure of aqueous tetrabutylammonium iodide: Photoelectron spectroscopy and molecular dynamics simulations
Introduction
The interactions between water molecules and dissolved ions are of crucial importance for many physical and chemical processes in biological systems, in the atmosphere, and in technological applications. Recent experimental and theoretical studies of the interfacial structure of aqueous solutions suggest that certain hydrophilic aqueous ions are located within the solution surface [1], [2], [3], [4], [5], [6], [7], which would contrast the commonly assumed thermodynamic picture of an interface depleted of ions [8], [9]. Specifically, non-polarizable ions, such as alkali metal cations or fluoride, have been shown by molecular dynamics (MD) simulations, using polarizable force fields, to be repelled from the interface [5]. Contrarily, soft, polarizable simple ions, such as the heavier halides Cl−, Br−, and I−, exhibit surface affinity, the effect scaling with the anion polarizability and size [7]. This mechanism is different from the hydrophobic interactions for ionic surfactants, containing for instance aliphatic chains, that also leads to the accumulation at the aqueous surface [10], [11], [12].
This study focuses on the effect of the counter-anions, iodide vs. bromide, on the surface behavior of tetrabutylammonium (TBA) which is a prototype of an ionic surfactant. TBA is in fact one of the most efficient and intensively investigated phase-transfer catalysts [13]. In a previous work it was shown how the iodide propensity for the surface is affected by the counter-cation [11], [14]. When Na+, which is not surface-active, is the counter-ion surface active iodide tends to be moved toward the bulk due to Coulomb attraction [14]. In contrast, when an ionic surfactant, such as TBA+, is the counter-ion iodide is rather dragged to the solution interface [11]. In the present work we contrast both photoemission spectra and MD simulations for TBAI dissolved in pure water vs. aqueous bromide solution. An important question is whether or not the addition of bromide counter ions, in excess compared to the iodide concentration, influences the surface behavior of a strong surfactant such as TBA+. Is there for instance a noticeable competition for surface sites of the different anions, and how would that affect the surface structure in terms of anion vs. cation distribution at the interface, or with respect to the number density of the completed surface monolayer?
Section snippets
Experimental
Photoelectron spectroscopy is combined with the liquid microjet technique. The 6-μm diameter liquid microjet is generated in a high-vacuum environment yielding nearly collisionless evaporation [15]. Briefly, the jet, having a temperature of 4 °C, is formed by injecting the liquid at 80 bar He pressure through a 10 μm diameter orifice [15], [16]. At the exit of the nozzle the beam contracts to 6 μm acquiring a final velocity of about 125 m s−1[15], [17]. For 3–5 mm downstream from the nozzle the beam
Photoemission measurements
Fig. 1 shows typical photoemission spectra of 0.02 m TBAI aqueous solution (top), of 0.02 m TBAI dissolved in 1 m aqueous NaBr (center), and of 1 m NaBr aqueous solution (bottom). The spectra were obtained for 100 eV photon energy. Electron binding energies are presented with respect to vacuum [14], [16], and relative intensities of the three traces are scaled to the synchrotron beam current. Lower intensities of the water photoemission signal (liquid water orbital emission 1b1, 3a1, 1b2, 2a1, is
Conclusions
Photoelectron spectroscopy and molecular dynamics simulations were employed to investigate the counter-anion specificity at the vacuum/solution interface of tetrabutylammonium iodide in aqueous sodium bromide. The most important result, emerging from both experiment and calculations is that iodide is more enhanced in the interfacial layer, covered by surface-active tetrabutylammonium cations, compared to bromide. The cations are surface-bound due to hydrophobic interactions of the butyl chains,
Acknowledgments
Support from the Czech Ministry of Education (Grant LC512) and from the US-NSF (Grants. CHE 0431312 and 0209719) is gratefully acknowledged. Part of the work in Prague was completed within the framework of the Research Project Z40550506. We thank the METAcenter Project for providing computer facilities for our work. We also thank Laserlab-Europe for support (via Grant mbi001063).
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