Spectroscopic studies of the perturbation of water at organic interfaces

Abstract

Fourier transform infrared spectroscopy (FTIR) has been employed to study the state of water and water-head group interactions in AOT microemulsions in n-heptane. Isotopic dilution of water (4% D(_2)O in H(_2)O) has been used to monitor the uncoupled v(OD) band of HOD as a function of W(_o) (the number of water molecules per head group). It is found that the v(OD) band always consists of a single band profile and there is no evidence of co-existing multiple water species in these systems (on the vibrational time scale). By detailed examination of the v(SO) and v(CO) bands of the head group it is found that up to W(_o) = 6 (severely perturbed) water molecules are bound closely to the sulphonate 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 further away from the first hydration shell. At higher values of W(_o) than 12 there is some evidence that the water molecules are similar to those in the bulk. However, up to twenty water molecules per head group are probably needed to observe spectra of bulk-like water. A two site model has been developed to predict the frequency of the decoupled v(OD) band as a function of W(_o). (^17)O spectroscopy and FTIR have been used to investigate the extent of water perturbation in the caesium pentadecafluoro-octanoate (CsPFO)/ water system. This system forms discotic micelles above the CMC across the whole composition range. In the nematic and lamellar phases, these micelles align in a magnetic field. Decoupled water HOD (4% in D(_2)O) was used to examine the infrared spectrum of water. The water is shown to be perturbed to the extent of W(_o) = 15, which accounts for the surfactant headgroup, the counter-ion and a small amount of water near the micellar surface. (^17)O NMR quadrupolar splittings were observed due to the anisotropic motion of water at the micellar interface, and the slow motions of the micelles. T(_1) relaxation measurements at 298 and 320 K have been made, and a range of correlation times for the water molecule derived on the basis of a two site model.