Oscillations, Waves, and Interactions - GWDG

Liquids: Formation of complexes and complex dynamics 383
Figure 15. Monomer concentration [N1] of sodium dodecylsulfate solutions as resulting
from the evaluation of the ultrasonic relaxation amplitudes (Fig. 14) shown as a function of
surfactant concentration C. The dashed line shows [N1] on the assumption of completely
dissociated surfactant molecules (αm = 1), the dotted line indicates [N1] = cmc [71].
76]. As an example, spectra for aqueous solutions of n-heptylammonium chloride
(n-HepACl) at surfactant concentrations slightly above and slightly below the cmc
(≈0.45 mol/l) are displayed in Fig. 16. Amphiphiles with such short alkyl chain do
not reveal a well defined sharp critical micelle concentration. Rather they display a
transition region. Obviously, even at surfactant concentrations somewhat below the
transition region, both relaxation ranges of the proper sodium dodecylsulfate micelle
system (Fig. 13) exist also in the spectra of the short chain n-HepACl solutions
with extraordinarily high cmc (Fig. 16). Some features in the spectra of the latter,
Figure 16. Sonic excess attenuation spectra for aqueous solutions of n-heptylammonium
chloride at 25 ◦ C and at two concentrations: ◦, 0.4 mol/l ≈ cmc; •, 0.5 mol/l [74]. The
dashed lines show the subdivision of the former spectrum in a Hill term (H) and a Debye
term (D). Full lines are the graphs of the sum of these terms, respectively.