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

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 molal tetrahexylammonium bromide (THA+/Br-) and 0.0050 molal sodium dodecyl sulfate (Na+/DS-) in the absence and presence of 0.30 molal 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 A 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 A depth. Added NaBr similarly increases the interfacial concentration of DS- ions and broadens ...