Partition Coefficients in Alkane Water Systems

The measurement of alkane/water partition coefficients has become a necessity due in part to the emerging important Alog P parameter [19]. With the coil planet-type CPC,

«-heptane/water was first selected for the determination of log Paik. Excellent correlations in log P values were again obtained from the CPC and SF methods for the model compounds studied [10]:

The maximal volume of the stationary phase with a flow rate of 0.5-1 ml/min is usually > 95 %, leading again to a measurable log P range of -3 to 3. Due to the volatile nature of the heptane phase, small air bubbles were formed in the column during operation and eluted with the mobile phase, perturbing the detection of eluates. Its high volatility also interfered with the regulation of flow-rate which appeared to vary during the measurement. These two main drawbacks led us to switch to the less volatile n-dodecane for log P3lk determination [26], It should be noted that the partition coefficients in different al-kane/water systems such as «-heptane/water, n-dodecane/water and n-hexadecane/water are almost identical for the same compound [27]. Their differences in structural information are indistinguishable according to solvatochromic analyses. The n-dodecane/water [26] has circumvented the problems of «-heptane/water and been satisfactorily applied to the studies of structure-activity relationships in our laboratory.

6.5.3 Partition Coefficients in di-ra-Butyl Ether/Water and Chloroform/Water Systems

While the partition coefficients from di-n-butyl ether/water (DBE/water) and chloroform/water (CHF/water) are expected to encode different structural information from log Pocl and log Palk, there is yet no substantial applications of these parameters to structure-activity relationship studies. We have, however, undertaken the partition coefficient measurements of DBE/water and CHF/water systems using the CPC method.

The difficulties with DBE/water are similar to those encountered in heptane/water, irregular flow rates and air bubbles elution being frequently observed. Despite these problems, the log PDBE values of ca. 60 compounds were measured and their structural information analyzed using solvatochromic parameters [28].

The measurements of log Pchf are not straightforward as expected, in particular when chloroform is used as the mobile phase. A continuous loss of stationary phase was observed in the eluent. Thus far, we have not yet ventured to solve the problem.

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