An equally good fit can be obtained with φ =
0; the experimental results are not sufficiently sensitive to
determine the phase accurately. The free energy of adsorption for the
benzocrown at the air/water interface is then obtained from K=1/55.5exp(-ΔabsG0/RT),
where 55.5 is the molarity of bulk water; the value are shown in
φ = 0o
Table 1 The Langmuir adsorption
equilibrium constant and the adsorption free energy for two different
assumptions of the phase difference between the SHG fields from the
crown ether and the bare air/water interface.
The difference between
the isotherm determined by SHG and the surface excess from surface
tension is puzzling. We therefore investigated if changes in the
orientation of the crown ether with concentration could be the cause
of this difference.