Oscillations, Waves, and Interactions - GWDG

378 U. Kaatze and R. Behrends
Figure 10. Amplitude of the sonic relaxation term reflecting the complex formation in
aqueous solutions of 0.1 mol/l α-cyclodextrin with potassium iodide at 25 ◦ C displayed versus
salt concentration CKI [60].
Here Γ is a stoichiometric factor, which for the above equilibrium is given by
Γ −1 = [α − CD] −1 + � I −� −1 + � α − CD · I − � −1 , (23)
and ∆VS denotes the adiabatic reaction volume
∆VS = Λ∞
∆H − ∆V (24)
ϱ Cp∞
with the limiting high-frequency values Λ∞ and Cp∞ of the thermal expansion coefficient
and the specific heat at constant pressure, respectively. Taking into account
that the concentrations of the uncomplexed and complexed species are related to one
another by the total cyclodextrin and iodide concentrations, the amplitude A of the
ultrasonic relaxation term reflecting scheme (17) can be well described by relations
(22) and (23) if the reasonable values K = 50 (mol/l) −1 and ∆VS = 5.4 cm 3 /mol are
assumed (Fig. 10).
Another interesting association mechanism is the formation of stacks from purine
bases, assumed to be relevant in biology, because it constitutes the dominant interaction
maintaining the secondary structure of biopolymers, such as DNA. Purine
bases are polyaromatic ring molecules with hydrophilic sites mainly at the periphery
and largely hydrophobic upper and lower faces. In order to prevent these faces from
contact with water, the disc shaped bases form stacks in aqueous solutions. As with
amphiphilic surfactants condensing to micelles, the self-aggregation is controlled by
an isodesmic (sequential) reaction scheme
k f
i
N1 − Ni ⇋ Ni+1
k r i
, i = 1, 2, . . . (25)