Validation of log P values Obtained from CPC

One of the main problems about CPC for log P measurements concerns solute partition equilibria in the chromatograph. Since a measurement usually takes from 10 minutes to 1 hour to complete, it is questionable whether the solute partition equilibrium is indeed reached in such a short period. Furthermore, different solvent systems possess rather distinct interfacial tensions (y = 8.5 mN m"1 at 20 °C) in comparison with alkane/water (y = c. 50 mN m"1 at 20 °C) [25] (see Table 1). This implies that the droplet size of emulsion formed at the inner part of the column close to the central axis would be much larger in alkane/water than in «-octanol/water under the same experi-

Table 1. Interfacial tensions (y in mN m'1) of some organic solvents against water at 20 °C [25]

Organic Solvent y

«-Hexadecane 54

w-Dodecane 53

Cyclohexane 51

Tetrachloromethane 45

Benzene 35

Chloroform 33

Oleic acid 16

Diethyl ether 11

n-Butanol 1.6

mental conditions. Consequently, the total contact surface area between the two phases would be smaller in alkane/water than in «-octanol/water, suggesting that partition equilibria take longer to be reached in alkane/water. This is certainly an important aspect when using the shake-flask method for partition coefficient measurements. It will be clear later that the mixing efficiency in the coil planet-type centrifuge seems to be sufficient to ensure that solute partition equilibria are reached in a relatively short time.

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