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Oscillations, Waves, and Interactions - GWDG

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

Figure 21. Amplitude factor of the relaxation spectral function Rum(ν) (Eq. (53)) for<br />

aqueous solutions of tetra-n-propylammonium bromide at 25 ◦ C, displayed as a function of<br />

equilibrium mole fraction ¯x2 of salt [87,89]. The dashed line is the graph of the Romanov-<br />

Solov’ev ampitude factor (Eqs. (54,55)).<br />

For three series of solutes the maximum ξ values of aqueous systems are displayed in<br />

Fig. 22 as a function of the number n of alkyl groups per molecule of solute. Within<br />

the series of unbranched molecules the maximum correlation length ξmax increases<br />

significantly with length of the hydrophobic group of solute. Obviously, the nature<br />

of the hydrophilic group is of low significance for ξmax. In correspondence to the<br />

formation of micelles, the larger the hydrophobic part of the nonaqueous constituent<br />

Figure 22. Maximum value ξmax in the concentration dependence of a binary system,<br />

displayed as a function of the number n of alkyl groups per organic molecule<br />

for some aqueous solutions of urea derivatives (•), monohydric alcohols (◦), <strong>and</strong><br />

poly(ethylene glycol) monoalkyl ethers (△) at 25 ◦ C [89]. EtU, ethylurea; n-PrU, npropylurea;<br />

n-BuU, n-butylurea; EtOH, ethanol; n-PrOH, n-propanol; i-PrOH, 2-propanol;<br />

C2E1, 2-butoxyethanol; i-C3E1, isopropoxyethanol; C4E1, 2-butoxyethanol; C4E2, 2-(2butoxyethoxy)ethanol.

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