A Fourier transform infrared study of the states of water included
in reverse micelles made from Aerosol-OT
(AOT) in isooctane is carried out for the whole sequence of alkali
metal counterions as a function of the
water content. The decomposition of the aqueous pool of these
micelles into “bound” and “free” water is
assumed. Two independent methods are applied for the purpose of
interpreting the shape and the intensity
of the water OH stretching vibration band. The modelization of the
OH stretching band by three Gaussian
components is retained as the most efficient decomposition and as an
adequate smoothing method. To
determine the number of water molecules bound per AOT polar head, three
water adsorption models are
tested for their ability to fit of the experimental data, to reproduce
the effect of the AOT concentration, and
to indicate the influence the counterion forming the ion pair with AOT.
A Langmuir adsorption isotherm
model satisfies best these criteria. The results suggest that the
hydration process concerns exclusively the
AOT molecule without any marked interference depending on the
counterion. The maximum hydration number
of AOT determined by this treatment is between 2 and 3. The
estimated standard free energy of the AOT
polar head hydration is about 2.5 kJ/mol.
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