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EC number: 266-369-7
CAS number: 66469-15-6
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):
[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.
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.
Solubility of saturated long chain fatty acid calcium salts
Irani and Callis  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.
 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
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.
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.
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