| Popis: |
Fractionation factors, ϕ M, of interfacial water at surfaces of anionic, cationic, and zwitterionic micelles were determined from the dependence of the 1H NMR chemical shift of H2O on the isotopic composition of the solvent (H2O plus D2O). Except for one surfactant, 3-dodecylamido-1-(N,N-dimethyl)-propane betaine, values of ϕ M are greater than unity, showing that interfacial water is more structured than bulk water, whose fractionation factor =1, by definition. Interfacial water structuring increases with increasing bulk of the headgroup of cationic surfactants, but is unaffected by increasing the length of the hydrophobic group of anionic surfactants (sodium tetradecylsulfate, versus sodium dodecylsulfate), or by the presence of two oxyethylene units at the interface (sodium dodecyl-dioxyethylenesulfate, versus sodium dodecylsulfate). The betaine surfactant does not affect the structure of interfacial water because of strong intermolecular interactions between the head ions (i.e.,-N+(CH3)2-CH2-CO 2 − ) of neighboring molecules in the micelles. Values of ϕ M are: sodium tetradecylsulfate, 1.06; sodium dodecyldioxyethylenesulfate, 1.06; dodecyl-dimethyl(n-butyl)-ammonium bromide, 1.08; dodecyltri-n-propylammonium bromide, 1.11; dodecyldimethyl(2-hydroxyethyl)ammonium bromide, 1.06; and 3-dodecylamido-1-(N,N-dimethyl)propane betaine, ≈1. |
