Amines, C12-18-alkyldimethyl, N-oxides

Partition coefficient

Description of key information

The log Pow  is estimated conservatively to be < 2.7.

Key value for chemical safety assessment

Log Kow (Log Pow):

2.7

at the temperature of:

20 °C

Additional information

The log Pow for C12 -18 AO has been estimated using the approximate solubility in n-octanol and water [Younis, S]. The value obtained was -0.04. The same author also measured the log Pow using an HPLC method, resulting in a log Pow of 1.05. This result has been disregarded as the HPLC method is not appropriate for surfactants.

The log Pow of C12 -14 AO has been estimated on the basis of experimental data for the solubility of this substance in n-octanol and literature data for the critical micelle concentrations of the C12 and C14 alkyl dimethylamine oxides. The result was log Pow < 2.7. This is considered to be a worse case value for the following reasons:

Surfactants can form different types of solutions in water (e.g. mono-molecular-, micellar-, hexagonal or lamellar solutions) depending on concentration and temperature. The mono-molecular solubility of a surfactant is defined by the critical micelle concentration (cmc). If cmc data are available, they may be used as a worst-case approach in cases where the resulting log Pow is not critical. However, from a physico-chemical point of view, the cmc is not a good descriptor for the water solubility, as the micelles themselves completely dissolve in water. The rationale behind using the octanol/water solubility ratio as a measure for the octanol/water partitioning coefficient is that material insoluble in octanol would be “pushed” into the water phase while material insoluble in water would be “pushed” into the octanol phase. With micelles, this is definitely not true. Micelles form an isotropic, homogeneous solution in water and cannot partition into an organic phase due to their hydrophilic nature. Further, the typical lifetime of surfactant micelles is only a few milliseconds, and the lifetime of micelle/bulk exchange of surfactant molecules is only a few µs, i.e. there is a very rapid exchange between mono-molecular dissolved and micellar dissolved surfactant molecules. This is in stark contrast to classical undissolved material, which has molecular lifetimes of several seconds or longer. Thus, the cmc underestimates the total solubility of a surfactant and hence overestimates the partition coefficient.

Taking all the available evidence together it has been concluded that it is valid to use the same limit value of log Pow < 2.7 for C12 -18 AO, as this is still likely to overestimate the partition coefficient.