Application of Octanol/Water Partition Coefficients in Surfactant Science:  A Quantitative Structure−Property Relationship for Micellization of Anionic Surfactants

• Published in: Langmuir Vol. 18; no. 2; pp. 345 - 352
• Main Author: Roberts, David W
• Format: Journal Article
• Language: English
• Published: American Chemical Society 22-01-2002
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la0108050

The octanol/water partition coefficient, usually as its logarithm, log P, is widely used as a hydrophobicity parameter in modeling pharmacological and toxicological properties. Since log P values can be calculated from molecular structure by summation of fragment values, they are very useful in predicting various biological properties of chemicals. Although hydrophobicity is a key concept in surfactant science, to date log P has not found much application in this field. In this paper, a log P based quantitative structure−property relationship (QSPR) is derived for anionic surfactant micellization potential, quantified as pCMC, the negative logarithm of the critical micelle concentration. The micellization potential of anionic surfactants covering a diverse range of structures is found to be well modeled by a combination of two parameters, πh and L, the former being the log P fragment value for the hydrophobe (simply defined as the whole molecule minus the negatively charged fundamental fragment) and the latter being the length, in C−C single bond units, of the hydrophobe. These parameters are simple to calculate, providing a rapid and accurate “back of envelope” method for estimation of CMC values. For surfactants with branching in their alkyl chains, the best agreement between calculated and observed pCMC values is obtained when the position-dependent branching factor, normally used in the log P calculation to quantify the effect of alkyl chain branching on solvation energy in the aqueous phase, is not applied. This is interpreted in terms of alkyl chain branching reducing the free energy of the micelle by reducing the amount of water in the interior of the micelle. Some of the log P fragment values and factors used in the calculation of the πh values used here have been derived inferentially, in previously published work, from fitting aquatic toxicity data for surfactants to quantitative structure−activity relationship equations. These are now validated by their successful application in the QSPR for micellization.