Surface structure of cationic surfactant solutions investigated by angular resolved X‐ray photoelectron spectroscopy with calibrated transmission function

Abstract

The main goal of the present work is to investigate the surface structure of cationic surfactant solutions by angular resolved X-ray photoelectron spectroscopy (ARXPS) after calibrating transmission function of the spectrometer. We have estimated the transmission function of the ARXPS spectrometer, and with it, we investigated solution of tetrabutylammonium iodide in a nonaqueous polar solvent. By genetic algorithm, the fractional concentration-depth profiles of constituents were reconstructed. These depth profiles evidence the enrichment of surfactant and the depletion of solvent in the surface regime and verify the separate distributions of oppositely charged surfactant ions. From those profiles we draw a conclusion that there is a preferred orientation for cation present at the surface, and the possibility for cation to take this orientation is related to its surface amount.

Surfactant-Promoted Reactions of Cl2 and Br2 with Br– in Glycerol

“…On the surface itself, TBA + is preferentially oriented to minimize unfavorable interactions between the hydrophobic alkyl chains and the polar liquid: three of the hydrocarbon chains are aligned in the plane of the surface, and the remaining chain is directed into the liquid. 10,12 This predicted orientation is consistent with experiments by Morgner and co-workers for TBA + at the surface of formamide, 12 and we therefore assume that THA + on glycerol behaves in a similar way. Further experiments involving TBAI suggest that TBA + draws I − to the outermost layers in solutions of formamide, 19,44 and separate studies indicate that Br − ions also populate the surface region of water.…”

“…Surface-active molecules can dramatically alter interactions between gases and liquids, in part because these “frontier” species may react first with incoming gas-phase molecules, enhance their interfacial solubility, facilitate or hinder transport, or physically block their entry into the bulk. − One class of soluble, cationic surfactants are the tetraalkylammonium (TAA) salts, which readily segregate to the surface of protic liquids. − Frequently used as phase transfer catalysts, TAA ions straddle the interface between solvents of different polarity and ferry reactant and product ions between the solvents. , The investigations below demonstrate that tetrahexylammonium (THA + ) ions can also dramatically enhance gas–liquid reactions, in particular by promoting halogen atom exchange between gaseous Cl 2 and interfacial Br – ions and by extending the lifetime of the Br 2 product in solution.…”

“…Experimental and theoretical studies confirm that large, polarizable anions such as Br − are present at the gas−liquid interface, 15−18 especially when they are associated with surfactant counterions. 10,12,19 Hu et al first implicated surface kinetics in the reaction of gaseous Cl 2 and Br 2 with interfacial Br − and I − ions in aqueous NaBr and NaI solutions. 20 Subsequent studies identified other interfacial reactions of halide ions, including OH + Cl − , 21 BrCl + I − , 22 and O 3 + Br − and I − .…”

Thermodynamics and structure of liquid surfaces investigated directly with surface analytical tools**

“…Wang et al. investigated with a lab based X‐ray source for Al‐Kα radiation solutions of Bu 4 NCl and Bu 4 NI in formamide at various concentration . Calibration of the analyser transmission function allowed for quantitative determining the concentration depth profiles of anions and cations.…”

Core level photoelectron spectroscopy of heterogeneous reactions at liquid–vapor interfaces: Current status, challenges, and prospects

“…42,94 The interface extends in this case about 15 Å deep into the solution. Many years later, a series of studies were performed by the same group using a lab-based experiment in conjunction with the rotating disk technique, 195,197,198 with the data analysis carried out using a genetic algorithm to retrieve density profiles for TBAI and TBPBr salts in formamide solutions. An example of the results of these studies is displayed in Fig.…”