A FOURIER-TRANSFORM INFRARED STUDY OF WATER-HEAD GROUP-INTERACTIONS IN REVERSED MICELLES CONTAINING SODIUM BIS(2-ETHYLHEXYL) SULFOSUCCINATE (AOT)

Fourier transform infrared spectroscopy has been employed to study the state of water and water-head group interactions in AOT aggregates or microemulsions in n-heptane. Isotopic dilution of water (4% D2O and in H2O) has been used to monitor the uncoupled ν(OD) band of HOD as a function of W0 (the number of water molecules per head group). It is found that the feature always consists of a single-band profile and there is no evidence of coexisting multiple water species in these systems (on the vibrational time scale). By detailed examination of ν(SO) and ν(CO) bands of the head group it is found that up to W0 = 6 (severely perturbed) water molecules are bound closely to the sulfonate and Na+ ions at the water/organic interface. Beyond this point all the water molecules seem to have a similar vibrational relaxation rate but there are still gradual structural and electronic changes as the water molecules move farther away from the first hydration shell. At values of W0 higher than 12 there is some evidence that the water molecules are similar to those in the bulk, but up to 20 water molecules per head group are probably needed to form well-defined reversed micelles in these systems. © 1992.