Exponential Function

The plots of Rm versus cp resemble in their shape a decreasing exponential in the first part followed by an increasing one. Hence, one could describe this pattern empirically by the following equation:

The decreasing exponential in Eq. (5) expresses the contribution of hydrophobic interactions between the test compound, the stationary hydrophobic phase and the aqueous mobile phase to Ru, while the increasing exponential corresponds to the contribution of polar adsorption. Parameters A, B, C, and D were calculated by nonlinear regression. If A, B, C, and D are given, RMw is calculated by setting cp = 0:

Eq.(6) is calculated by an iteration programme, which in some cases calculates negative values for D, indicating that the programme fits the plot of Ru versus <p to two decreasing exponentials. Although most RMw, calculated according to Eq.(6), agree well with those obtained by linear extrapolation [24] this approach is not generally applicable.

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