Octanol/water partition of ionic species, including 544 cations

Partition coefficients of single ions in the octanol/water system (log P-oct) have been assigned on the (Ph4As+, Ph4P+) = Ph4B(-) assumption. The log P-oct values of Cl-, Br- and I- ions are then used to obtain the partition coefficients of cations from partition coefficients of the neutral combination of anion and cation. Partition coefficients of 544 cations derived from 585 organic salts from the MedChem database have been studied. The contributions of the aliphatic charge N+ and aromatic charge n(+) in these cations have been investigated. The results show that the contributions of N+ and n(+). in different homologous series are affected by the attached functional groups and are not constant, although the effect of the central cation is constant along any given homologous series. The latter can be accounted for qualitatively and semiquantitatively by the electrostatic theory of ionic solvation of Abraham and Liszi. A number of regression equations have been established between partition coefficients of ions and partition coefficients of the corresponding neutral species. These equations can be used to estimate octanol/water partition coefficients for additional cations to about 0.5 log unit.