Automated potentiometric titrations in KCl/water-saturated octanol: method for quantifying factors influencing...

Area of Science:

• Physical Chemistry
• Analytical Chemistry
• Computational Chemistry

Background:

• Determining accurate log P(I) values for ion pairs is difficult, limiting understanding of partitioning behavior.
• Existing computational methods for log P(I) often neglect crucial factors like hydrogen bonding and steric effects.

Purpose of the Study:

• To develop a reliable potentiometric titration method for measuring ion pair partitioning.
• To investigate the influence of structural factors on ion pair partitioning coefficients (log P(I)).
• To improve the accuracy of log D calculations and enable species-specific quantitative structure-activity relationship (QSAR) studies.

Main Methods:

• Developed a potentiometric titration procedure in KCl/water-saturated octanol.
• Utilized a thermodynamic cycle of ionization and partitioning for log P(I) determination.
• Incorporated a program for calculating pK(a)'' values, accounting for water ionization in octanol.

Main Results:

• The titration method offers high reproducibility (hundredths of a log P) and is independent of log P magnitude.
• Ion pair partitioning (log P(I)) is significantly influenced by hydrogen bonding and steric factors, more so than for neutral species.
• The difference between neutral and ion pair partitioning (diff (log P(N - I))) can vary by over 3 log units for amines, challenging existing assumptions.

Conclusions:

• The developed titration method provides accurate ion pair partitioning data.
• Factors like water and counterion accessibility, steric bulk, and hydrogen bonding critically affect log P(I).
• This work enables more accurate log D predictions and promotes species-specific QSAR, highlighting nature's stabilized ion pairs.