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Oscillations, Waves, and Interactions - GWDG

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384 U. Kaatze <strong>and</strong> R. Behrends<br />

Figure 17. Relaxation time distribution function of the Hill relaxation term of some nheptylammonium<br />

chloride aqueous solutions at 25 ◦ C.<br />

however, attract attention. Obviously, the low-frequency term, which within the<br />

micelle formation/decay scheme (Fig. 12) reflects the fast monomer exchange, is<br />

subject to a considerable distribution of relaxation times. Empirically this term can<br />

be adequately represented by a Hill relaxation function [77–79], defined by<br />

RH(ν) =<br />

AH(ωτH) mH<br />

[1 + (ωτH) 2sH ] mH +nH 2sH . (37)<br />

In this function AH is an amplitude <strong>and</strong> mH, nH, sH ∈]0, 1] are parameters controlling<br />

the shape <strong>and</strong> width of the underlying relaxation time distribution function<br />

GH(ln(τ/τH)). The principal relaxation time τH, according to [79]<br />

τH = τmax(mH/nH) 1/(2sH)<br />

is related to the frequency νmax = (2πτmax) −1 at which RH(ν) adopts its maximum.<br />

At some surfactant concentrations the relaxation time distribution function, defined<br />

by<br />

RH(ν) = AH<br />

� ∞<br />

(38)<br />

GH (ln(τ/τH)) ωτ<br />

−∞<br />

� 1 + (ωτ) 2�−1 d ln(τ/τH) (39)<br />

is shown in Fig. 17 for solutions of n-HepACl in water. The function GH (ln(τ/τH))<br />

has been calculated by analytical continuation [80] from the Hill spectral function<br />

(Eq. (37)) using the normalisation<br />

� ∞<br />

GH (ln(τ/τH)) d ln(τ/τH) = 0 . (40)<br />

−∞

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