Novel hydrophobicity ruler approach for determining the octanol/water partition coefficients of very hydrophobic compounds via their polymer/solvent solution distribution coefficients

A novel hydrophobicity ruler approach for determining the octanol/water partition coefficients of very hydrophobic compounds is proposed, which is an indirect method that measures the polymer/solvent solution distribution coefficients (log [K.sub.p/s]) of reference and unknown compounds. The log [K.sub.p/s] values of the unknown compounds can be calibrated to their log [K.sub.o/w] values via the correlation of the log [K.sub.p/s] values of the reference compounds with their log [K.sub.o/w] values. An organic solvent was used to increase the solubility of the very hydrophobic compounds in the aqueous solution, so that their concentrations and absorption amounts were high enough to be measured precisely. The solvent also reduced the hydrophobicity scale of the very hydrophobic compounds and controlled the amounts absorbed into the polymer phase, so that compounds spanning a very wide range of log [K.sub.o/w] values could be measured in a single measurement and the coexisting compounds would not interfere each other. Poly(dimethylsiloxane) (PDMS), aqueous methanol solutions, and a series of 21 PCB (polychlorinated biphenyl) compounds were used to demonstrate the principle of the hydrophobicity ruler approach. The PCB compounds with known experimental log [K.sub.o/w] values served as reference compounds, whereas the PCB compounds without known log [K.sub.o/w] values were determined. The log [K.sub./w] values determined for PCB126, PCB187, PCB197, PCB180, PCB170, and PCB195 were 6.94, 7.84, 8.33, 8.17, 7.92, and 8.49, respectively. The correlation of the log [K.sub.p/s] values of the reference PCB compounds with their log [K.sub.o/w] values was linear (log [K.sub.o/w] = 2.56 log [K.sub.p/s] + 1.08, [R.sup.2] = 0.95). The hydrophobicity ruler approach is also a valuable tool for validating the experimental and theoretical log [K.sub.o/w] values and identifying outliers in log [K.sub.o/w] databases.