Experimental Depth Profiles of Surfactants, Ions, and Solvent at the Angstrom Scale: Studies of Cationic and Anionic Surfactants and Their Salting Out

Abstract

Neutral impact ion scattering spectroscopy (NICISS) is used to measure the depth profiles of ionic surfactants, counterions, and solvent molecules on the angstrom scale. The chosen surfactants are 0.010 m tetrahexylammonium bromide (THA+/Br–) and 0.0050 m sodium dodecyl sulfate (Na+/DS–) in the absence and presence of 0.30 m NaBr in liquid glycerol. NICISS determines the depth profiles of the elements C, O, Na, S, and Br through the loss in energy of 5 keV He atoms that travel into and out of the liquid, which is then converted into depth. In the absence of NaBr, we find that THA+ and its Br– counterion segregate together because of charge attraction, forming a narrow double layer that is 10 Å wide and 150 times more concentrated than in the bulk. With the addition of NaBr, THA+ is “salted out” to the surface, increasing the interfacial Br– concentration by 3-fold and spreading the anions over a ∼30 Å depth. Added NaBr similarly increases the interfacial concentration of DS– ions and broadens their positions. Conversely, the dissolved Br– ions are significantly depleted over a depth of 0–40 Å from the surface because of charge repulsion from DS– ions within the interfacial region. These different interfacial Br– propensities correlate with previously measured gas–liquid reactivities: gaseous Cl2 readily reacts with Br– ions in the presence of THA+ but drops 70-fold in the presence of DS–, demonstrating that surfactant headgroup charge controls the reactivity of Br– through changes in its depth profile.

Impact of Tetrabutylammonium on the Oxidation of Bromide by Ozone

“… 66 , 67 Salting out has been substantiated also by XPS for butanol in the presence of iodide 68 or by scattering for THA bromide in the presence of 0.3 M NaBr. 63 In the latter case, though in glycerol, a widening of the depth profile of THA has been observed, possibly beyond the thickness of a monolayer. In the present work, the photoemission signals of the carbons directly attached at the amine nitrogen changed in accordance with the aliphatic carbons, meaning that no substantial change in average orientation is apparent.…”

“…Such interactions have been found earlier for other surfactant systems 30 and specifically also for solutions containing TBA iodide. 38 , 39 , 60 − 62 Using He backscattering, Zhao et al 63 determined that bromide is distributed within a width of around one nanometer at the interface for tetrahexylammonium (THA) bromide in glycerol as a solvent. Note that this is compatible with our choice of Δ used to calculate local concentrations.…”

“…Taking the calibration for the aliphatic C 1s to water O 1s photoemission ratio for the TBA-Br solution, the 20% increase of the C/O ratio leads to a surface excess of 3.3 × 10 14 cm –2 of TBA in the presence of 0.55 M NaCl, thus an increase by about 10% compared to the 0.1 M TBA-Br solution. Salting effects of organics in the presence of inorganic electrolytes, with salting out being the more common case, are a well-described phenomenon, but predictive tools remain poorly constrained. , Salting out has been substantiated also by XPS for butanol in the presence of iodide or by scattering for THA bromide in the presence of 0.3 M NaBr . In the latter case, though in glycerol, a widening of the depth profile of THA has been observed, possibly beyond the thickness of a monolayer.…”

Vibrational exciton delocalization precludes the use of infrared intensities as proxies for surfactant accumulation on aqueous surfaces

“…In response to this apparent deviation from Hofmeister "saltingout" effects, 42,43 surface tension measurements were collected to further investigate PFOA interfacial adsorption and surface concentration for comparison to the IRRAS spectra. Although surface tension does not provide any molecule-specific information, it does yield robust quantitative measurements of surfactant adsorption to the air/water interface.…”

Infrared Hide-and-Seek: Vibrational Excitons Conceal Surfactants at the Air/Water Interface

“…Three main peaks are present in the C-F vibrational mode region of the IRRAS spectra of PFOA in In response to this apparent deviation from Hofmeister "salting-out" effects, 20,21 to the Szyszkowski equation, 23,24 130…”