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Water solubility

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Description of key information

experimental value in the range between slightly soluble (0.1 - 100 mg/L) to moderately soluble (100 - 1000 mg/L)
QSAR in the range between insoluble and soluble (depending on mehtod used for calculation and fatty acid chain length)

Key value for chemical safety assessment

Additional information

Amides, C6-18, N-[3-(dimethylamino)propyl] represent a mixture containing C8-, C10-, C12-, C14-, C16-, and C18-alkyl side chains. Based on this and the variable composition of the compound (alkyl chain distribution dependent on origin of the coco fatty acid) an experimental value for water solubility of the test substance as such could not be derived due to technical difficulties with this type of substance.

 

The values derived with the slow stirring method are considered not to be reliable without restriction because the two tests performed with different concentrations clearly show the effects of fractionated dissolution and the interaction between the individual components resulting in water solubilitiesdepending on the start concentration.

 

Hence the slow stirring method is not applicable to Amides, C6-18, N-[3-(dimethylamino)propyl], as it is also the case with the OECD 105 methods: the column elution method, is applicable for pure substances only, the flask method does not work because of foam formation.

 

As the test substance represents a mixture and has a variable composition the QSAR for the mixture is also not feasible. To get a hint on the physico-chemical data, a calculation was conducted for the C8, C12 and the C18 derivates. Based on the log Kow values of 2.44 (C8 derivate), 4.4 (C12 derivate) and 7.35 (C18 derivate), water solubilities of 257 mg/L (C8 derivate), 2.623 mg/L (C12 derivate) and 0.0025 mg/L (C18 derivate, at 25 °C each) were calculated using EPIWIN v4.0, WSKOW v1.41. The calculation of the water solubilities using the fragment method yielded values of 4660 mg/L (C8 derivate), 41.269 mg/L (C12 derivate) and 0.032 mg/L (C18 derivate; at 25°C each).

 

According to the classification scheme, the C8 derivate can be regarded as moderately soluble (result based on log Kow estimate) and soluble (result obtained via the fragment method), the C12 slightly soluble and the C18 derivate as insoluble (results based on log Kow estimate and fragment method).

 

The obtained experimental data are in-line with the estimated water solubilities of the individually component calculated by the modelling program EPIWIN v4.00 (WSKOW v1.41) and is in the range between slightly soluble (0.1 -100 mg/L) to moderately soluble (100-1000 mg/L).

 

Substance

CAS

Weight %

WATERNT v1.01

Water solubility

(fragment caclulation)

EPIWIN v4.0, WSKOW v1.41(fragment calculation)

EPIWIN v4.0, WSKOW v1.41 based on log Kow values

C 8-Amidamine

22890-10-4

8.6

4659 mg/l

4660 mg/l

257 mg/L

C10-Amidamine

22890-11-5

6.3

441,14 mg/l

 

 

C12-Amidamine

3179-80-4

46.6

41,269 mg/l

41,269 mg/l

2.623 mg/L

C14-Amidamine

45267-19-4

17.6

3,8232 mg/l

 

 

C16-Amidamine

39669-97-1

8.6

0,13534 mg/l

 

 

C18-Amidamine

7651-02-7

9.6

0,032067 mg/l

0,032 mg/l

0.0025 mg/L

 

 

General remarks on the water solubility of technical surfactants:

Surfactants by principle tend to concentrate on surfaces and to form foams and /or emulsions. At lower concentrations surfactant molecules in the water phase can form different types of solutions (e.g. mono-molecular-, micellar-, or lamellar solutions) depending on concentration and temperature. The mono-molecular solubility of a surfactant is defined by the critical micelle concentration (cmc). 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. Micelles form an isotropic, homogeneous solution and there is a very rapid exchange between monomolecular dissolved and micellar dissolved surfactant molecules (ïsec scale). This contrasts to classical non-dissolved material, which has molecular lifetimes of several seconds or longer. Thus, the cmc underestimates the total solubility of a surfactant. Instead, in analogy to the classical definition of the water solubility, the surfactant concentration should be determined, at which a true liquid/solid or liquid/liquid two-phase systems becomes apparent.

 

Another problem arises from the fact that commercial surfactants generally are multi-component substances[1]and usually are considered to be complex substances1. Those substances behave differently compared to single constituent compounds when brought into contact with water. When adding incremental amounts of a complex substance to water a point will be reached where the solubility limit of the least soluble compound is exceeded. Further amounts of the substance added then will change the relative shares of the individual compounds in the waterphase, because the compound(s) exceeding the limit of solubility will be removed from the water phase (fractionated dissolution). Consequently, the composition of the dissolved product in the water phase will be different to the composition of the original product.

 

The situation becomes even more complicated by the fact that the solution behaviour of an individual component is not only a property of this component, but it will also be influenced by the amount of the other substances being present in the product.

 


[1]Consideration is given to the consistency with the definition of “multi-component substances” (or “complex substances”)in Chapter 1 of a draft “Guidance Document on the Use of the Harmonized System for the Classification ofChemicals which are Hazardous for the Aquatic Environment” (ENV/JM/HCL(2000)11).

 

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