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Physical & Chemical properties

Partition coefficient

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Reference
Endpoint:
partition coefficient
Data waiving:
study technically not feasible
Justification for data waiving:
the study does not need to be conducted because the substance has a high surface activity

Description of key information

JUSTIFICATION FOR DATA WAIVING

The purpose of this document is to describe the technical limitations of experimental techniques for determination of the octanol-water partition coefficient (typically expressed as its logarithm, or “log Kow”) of Sucrose Laurate (CAS 37266-93-6) (hereinafter the “Substance”) and to justify a waiver for this test under Annex XI, Section 2.  The Substance is comprised of sucrose esters (mono-, di, and triesters) of lauric acid. The substance is a UVCB.  The Substance is intended to be used as a surfactant.   The Substance acts as a surfactant by virtue of the combination of a hydrophilic carbohydrate group (the sucrose moiety) with hydrophobic, long-chain acyl group (the lauric acid moiety).  As described below in greater detail, log Kow is not a meaningful metric for surfactants such as the sucrose laurate and methods for experimental determination are not reliable.  

The word ‘surfactant’ is a combination of “surface-active agent.”  As this term suggests, surfactants are known to concentrate at surfaces, lending unique properties to this class of chemical.  However, this confounds typical methods for experimental determination of log Kow.  The most common method, the “shake-flask” method (OECD 107) is not feasible because a surfactant will localize to the solution interfaces.  This results in a disproportionate amount of analyte at the air-liquid interface, the liquid-liquid (i.e. oil-water) interface, and along the sides of the container (the liquid-solid interface).  Because a surfactant does not partition homogenously into the bulk of the water and octanol phases, the analytically determined concentrations of the individual phases are not representative of the substance’s tendency to dissolve into one or the other.  Indeed, these concentrations can be affected by a number of variables, including the solution preparation procedure and the size/shape of the test vessel.  Thus, direct measurement and comparison of water and octanol concentrations, as is conducted for the shake-flask method, does not provide accurate results.

A modified version of this test, the “slow-stir” method (see OECD 123), relies on repeated samplings of continuously stirred solutions.  The purpose of this test is to prevent the formation of microdroplets of octanol in the water phase, which can cause erroneous measurements for substances of high log Kow (>5) when using the shake-flask method.  In theory this method is preferred for surfactants, as the turbulence is thought to continuously disperse the interface-concentrated material back into the bulk solutions.  However, in our experience it has still not been demonstrated to be reliable.  Using slow stirring, one needs to demonstrate a consistent result when starting with the surfactant in either phase, not just in the octanol.  Even in an idealized case using a pure technical non-surface-active material, this can be prohibitively expensive and time consuming.  Current practices for this test consider dosing two ways as a starting point which doubles the associated time and costs.  

An additional complication for analysis of the Substance using the slow-stir method or any other technique requiring analytical monitoring (e.g. solubility), is quantitation.  Specifically, the Substance is a very complex mixture (the laurate group vary in number and position around the sucrose moiety) and would therefore require an analytical standard for quantitation of each significant component, as each has unique solubility and partition tendencies.  The capability to perform such analysis is not available.

Alternate methods, such as determination by HPLC (OECD 117) are also not practical.  These methods rely on comparison of the elution time with that of standards of various log Kow values, but the unique interaction of surfactants between the mobile and stationary phases prevents fair comparison to non-surfactant substances.

As described above, experimental methods for determination of log Kow are not amenable to analysis of surfactants.  Moreover, the very nature of surfactants, in terms of their tendency to concentrate at oil-water interfaces and other surfaces, renders the metric unsuitable for surfactants.  In essence, the log Kow parameter quantifies a particular behavior that surfactants do not exhibit and therefore cannot be accurately determined for surfactants such as the Substance at issue here.

Key value for chemical safety assessment

Log Kow (Log Pow):
1.43
at the temperature of:
20 °C

Additional information

The log Kow value used for the chemical safety assessment (1.434 at 20ºC) is the highest QSAR value among all isomers of the most abundant constituent, the monododecyl ester of sucrose.