Potassium isooctadecanoate

Water solubility

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

water solubility

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

supporting study

Study period:

1953

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Justification for type of information:

The water solubility study was conducted with potassium salts of stearic acid. Potassium soaps of long chain C18 fatty acids are to a certain degree considered structural analoguous with regard to their environmental fate taking into account the physico-chemical properties (surface active properties, logKow, water solubility) and the chemical structure of the test item (mainly potassium soap of iso branched long chain C17 fatty acid) and therefore the results of the source substance are considered relevant for risk assessment.

Reason / purpose for cross-reference:

read-across source

Qualifier:

no guideline available

GLP compliance:

no

Type of method:

other: Refractive index

Water solubility:

0 other: mM/l

Conc. based on:

test mat.

Temp.:

50 °C

Water solubility:

0.145 mg/L

Conc. based on:

test mat.

Temp.:

50 °C

Conclusions:

The critical micelle concentration of potassium stearate was reported at 0.00045 mg/l at 50°C.

Reference 2

Endpoint:

water solubility

Type of information:

experimental study

Adequacy of study:

supporting study

Study period:

2017

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 105 (Water Solubility)

Deviations:

no

Qualifier:

equivalent or similar to guideline

Guideline:

EU Method A.6 (Water Solubility)

Deviations:

no

GLP compliance:

no

Type of method:

other: solubility tests

Specific details on test material used for the study:

Product description: Potassium isooctadecanoate/potassium isostearate
Name: Isooctadecanoic acid, potassium salt
CAS No.: 66469-15-6
Physical state: off white to yellowish solid at 20 °C
Batch No.: PFS-755-169
Re-certification date of batch: 12 December 2017
Purity: 100 % (UVCB)
pH, 10% in DI water 8.0 - 9.0
Stability: stable under test conditions
Storage condition of test material: Room temperature, protected from light

Key result

Water solubility:

>= 200 g/L

Conc. based on:

test mat. (total fraction)

Incubation duration:

24 h

Temp.:

20 °C

pH:

10.41

Remarks on result:

other: completely miscible in DI water (gel formation)

Study results

 

With 0.5 g of test item being mixed into 2 g of water, a gel with a total weight of 2.5 g  has  been  formed.  

This is equivalent to 20% (w/w) of test item in solution/gel. Due to the fact, that the density of the gel is not known and that the formation of the gel is a continuous process and no discrete point can be identified at which dissolution in water switches to gelling, the water solubility can only be approximated to 200 g/L. It is likely that it is possible to create a gel with even higher concentrations of the test item

The pH value of a 2.5% solution of the test item in water was 10.41, the tests were performed at 20 °C.

 

Interpretation of results

 

Intuitively the values shown above are much higher than expected. As chain length of fatty acid soaps increases, one would expect solubility to decrease. For example, the sodium salt of C12 fatty acid 22000 mg/l (at 24 °C) is reported by Stephen and Stephen (1963). This is again not consistent with the value of 200000 mg/l reported. It is possible that the effect of the Krafft point (TK) and CMC values which explain some of these observations. The Krafft point is defined as the temperature at which the solubility of the surfactant is equal to the concentration of the micelle formulation (the CMC) and which is characterized by a rapid increase in solubility above this temperature. Krafft point values are tabulated. For C12 chain length, this effect would explain the high solubility observed since the solubility was obtained at a temperature above the Krafft point of 21.5 °C. The high solubility values observed, however, cannot be explained in this way since the Krafft points for these chain lengths are between 69 °C (C16) and 71 °C respectively. The presence of monovalent electrolytes such as NaCl will lower the Krafft point (Clint, 1992). It is possible that the presence of an excess of such an electrolyte when tested (possibly through the presence of potassium hydroxide in the tested soap, see pH of test solutions) resulted in a lowering of the Krafft point sufficiently to allow the large increase in observed solubility.

Besides the presence of significant amounts of divalent ions (e.g. Mg and Ca) has a significant impact on raising the Krafft point. This has been observed for C12 alkyl sulphate as follows (Hato et al, 1979):

Krafft point

 

[Na+]: 9 °C

[Mg2+]: 25 °C

[Ca2+]: 50 °C

 

This effect occurs for all anionic surfactants including fatty acid salts. Thus under environmentally relevant conditions of temperature and hardness, these high solubilities will not be observed. Data reported by other authors for the same chain lengths are also not consistent when measured at similar temperatures.

Reported values for calcium salts:

calcium salt of C16 solubilities of 28.1 mg/l and 30 mg/l at 20 °C (Seidell, 1958)

calcium salt of C18 oleate, solubilities of 66 mg/l at 20 °C (Stephen and Stephen , 1963) and 400 mg/l at 25 °C (Seidell, 1958)

calcium salt of C18 stearate, solubilities of 40.4 mg/l at 20 °C (Stephen and Stephen , 1963) and 1.6 mg/l at 27 °C (Seidell, 1958).

Laboratory measurements are also performed in conditions which are of lower hardness and higher temperature than found in the environment. Both these factors will lead to an increased solubility under laboratory conditions than in environmentally relevant conditions. Thus although the measured solubility values may be accurate, they can be considered accurate only for the conditions of hardness etc in which they were generated which are generally not consistent with environmental conditions.

Conclusions:

With 0.5 g of test item being mixed into 2 g of water, a gel with a total weight of 2.5 g has been formed. This is equivalent to 20% (w/w) of test item in solution/gel.

Executive summary:

With 0.5 g of test item being mixed into 2 g of water, a gel with a total weight of 2.5 g has been formed. This is equivalent to 20% (w/w) of test item in solution/gel.

Reference 3

Endpoint:

water solubility

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Study period:

2003

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Justification for type of information:

The fate of fatty acid salts in aqueous systems is complicated by the fact that there are a numbers of water soluble and water insoluble groups and combinations of these. In practice whilst the use of sodium or potassium salts are by far the most common use of soap in finished products, the
predominance of calcium and magnesium ions in waste water leads to rapid formation and predominance of relatively insoluble Ca and Mg salts [1,2]. Consequently the water solubility of calcium salts of stearic acid are considered a realistic scenario with regard to environmental fate. Taking into account the chemical structure of the test item (mainly potassium soap of iso branched long chain C17 fatty acids) the results of the source substance are considered relevant for risk assessment (environmental fate). See read across document for further details.

[1] Swisher R.D., (1987) Surfactant Biodegradation 2nd Edn. Marcel Dekker
[2] Prats D., Rodriguez M., Varo P., Moreno A., Ferrer J. (1996) Biodegradation of soaps in anaerobic digestors and on sludge amended soils.Proceedings of the 4th world surfactants congress, Barcelona, 3-7VI : 233-245. CESIO.

Reason / purpose for cross-reference:

read-across source

Qualifier:

no guideline followed

GLP compliance:

no

Key result

Water solubility:

0.002 mg/L

Conc. based on:

test mat.

Temp.:

20 °C

Remarks on result:

other: Soft water (0.3mmol [Ca])

Key result

Water solubility:

0.001 mg/L

Conc. based on:

test mat.

Temp.:

20 °C

Remarks on result:

other: Hard water (3mmol [Ca])

Solubility of saturated long chain fatty acid calcium salts

Irani and Callis [1] reported good correlation between solubility and the number of carbons in saturated fatty acid calcium salts according to the following relationship:

-log Ksp = -2.63 + 1.24C (Equation 1) where C = number of carbon atoms Ksp = solubility product = (Ca++)(RCOO)^2

This equation allows the calculation of solubility at any specified calcium concentration and overcomes the issue of using solubility measurements generated in hardness conditions unrepresentative of the environment. Environmental concentrations of calcium are reported to vary between 0.3mmol to 3mmol (BKH review, 1994). If these two values are considered as extremes then the possible range of solubilities expected in the environment can be calculated in mg/l for each chain length when considering molarity of calcium and the relevant fatty acid. In soft water (calcium concentration = 0.3mmol) the solubility varies from 130mg/l for C10 to 0.0023mg/l for C18 and 8.5E-6mg/l for C22. For harder water (calcium concentration = 3mmol) solubility ranges from 41mg/l for C10 to 0.0007mg/l for C18 and 2.7E-6 for C22.

[1] Irani R.R, Callis C.F. (1960) Metal complexing by phosphorus compounds II. Solubilty of calcium soaps of linear carboxylic acids. J. Phys. Chem. 64:1741

Conclusions:

The water solubility of calcium stearate in soft water (0.3 mmol Ca2+) is 0.0023 mg/l and 0.0007 mg/l in hard water (3 mmol/l Ca2+). Based on the similar chemical structure of calcium isostearate (to be formed from potassium isostearate in hard water) and calcium stearate the water solubility of 0.0007 mg/l is considered relevant to assess the water solubility of calcium isostearate.

Executive summary:

The water solubility of calcium stearate in soft water (0.3 mmol Ca2+) is 0.0023 mg/l and 0.0007 mg/l in hard water (3 mmol/l Ca2+). Based on the similar chemical structure of calcium isostearate (to be formed from potassium isostearate in hard water) and calcium stearate the water solubility of 0.0007 mg/l is considered relevant to assess the water solubility of calcium isostearate.

Description of key information

Water solubility and environmental fate

The fate of fatty acid salts in aqueous systems is complicated by the fact that here are a numbers of water-soluble and water-insoluble groups and combinations of these. In practice whilst the use of Na salts are by far the most common use of soap in finished products, the predominance of calcium and magnesium ions in wastewater leads to the rapid formation and predominance of relatively insoluble Ca and Mg salts.

Taking into account the artificially high solubility determined in the water solubility study of the test item and the fact that the fate of fatty acid salts is strongly influenced by the poor water solubility of the calcium and magnesium salts, water solubilities reported in HERA and by Irani et al. (calculated values) on calcium soaps of linear carboxylic acids will be used for risk assessment.

Key value for chemical safety assessment

Water solubility:

0.001 mg/L

at the temperature of:

20 °C