Monolayers of hexadecyltrimethylammonium p-tosylate at the air-water interface. 2. Neutron reflection

Neutron reflection has been used to determine the position of the p-tosylat e ion in monolayers of the hexadecyltrimethylammonium cation (C(16)TA(+)) a t the air-water interface. The monolayer was in equilibrium with a solution of composition 1 mM C(16)TAB and 3 mM sodium tosylate. The area per molecu le of the surfactant was found to be 0.45 and 0.52 nm(2) respectively in th e absence and presence of tosylate and 0.47 nm(2) in the presence of 3 mM N aBr. The ratio of tosylate to C(16)TA(+) in the layer was found to be 0.8. Five isotopic species of the C(16)TA(+) were labeled with deuterium in bloc ks of four atoms (C4D8, C4D9, or headgroup (CD3)(3)N), and the separation b etween the center of each of these blocks and the deuterated tosylate ion w as determined. The values show that the average position of the tosylate io n in the layer is at exactly the same level as the third block of four carb ons in the C(16)TA+ (the first block is defined as that containing the term inal methyl group); i.e., the tosylate ion is mainly in the hydrophobic reg ion of the monolayer. This conclusion is further confirmed from the separat ion of the average tosylate ion and the midpoint of the water distribution, which was found to be 0.7 +/- 0.3 nm. Taken in conjunction with sum-freque ncy spectroscopy measurements on the same system, which give the orientatio n of the tosylate ion in the interface, and the effect of added sodium tosy late on the surface tension of C(16)TAB, the tosylate is shown to be acting much more like a cosurfactant than like a normal counterion.