Registration Dossier

Physical & Chemical properties

Endpoint summary

Administrative data

Description of key information

Additional information

Discussion of physicochemical properties 

 

Substances in the lithium salts of monocarboxylic acids C14-C22 category are considered to be similar on the basis that they have common structures of a lithium ion, varying only by the length of the fatty acid chain and the presence of unsaturated and/or hydroxyl functional groups. As a result, due to the close structural similarity and the narrow range of carbon chain numbers covered in this category, the physico-chemical properties are expected to be predictable across the category. Where testing has been conducted to generate data in order to complete physico-chemical endpoints, testing of lithium myristate (C14), at the lower end of the category, lithium 12-hydroxystearate (C18), as a hydroxylated member, and lithium behenate (C22), at the upper end of the category, was considered the most efficient approach. The data for the longest and shortest carbon chain lengths and for a hydroxylated substance in the middle of the category were generated to bracket the possible range of properties across the category and to show that substance properties are consistent across the range of carbon chain lengths. The testing of three substances at strategic points in the category is designed to ensure that the most conservative results are identified and, where appropriate, provide a classification which covers all of the substances in the category. To support the existing data, studies on the remaining category members are currently ongoing for certain endpoints (Covance 2020).

 

Appearance/physical state/colour 

 

All of the substances in the lithium salts of monocarboxylic acids C14-C22 category are white solids at room temperature. The data on the substances are taken from substance identification information in GLP-compliant, guideline studies available as unpublished reports (Harlan 2012, 2013; Covance 2020). 

 

Melting point/freezing point 

 

All of the substances in the lithium salts of monocarboxylic acids C14-C22 category have similar melting points, in the range of 166 to 231°C. All category members slowly decompose after melting. 

 

The melting point of lithium myristate is 216 - 231°C and thereafter the substance slowly decomposes. The melting point of lithium myristate was determined in a GLP-compliant thermal analysis test, following OECD guideline 102 (Harlan 2012). This is supported by a melting point of lithium myristate of 223.6-224.2°C, taken from a peer-reviewed journal article, Shoeb (1999), citing a previous experimental study (Ralston 1948). No details are provided on the test methods or conditions and therefore the study has been assigned a Klimisch score of 4. 

 

Shoeb (1999), states that the melting point of lithium palmitate is 224 -225°C. The melting point of lithium palmitate has been taken from a peer-reviewed journal article, citing a previous experimental study (Ralston 1948). No details are provided on the test methods or conditions and therefore the study has been assigned a Klimisch score of 4. Although this substance is not being registered as a category member, it is a constituent present in other category substances and therefore available data on this structure have been included as supporting evidence.

 

The melting point of lithium stearate is 220 - 221.5°C. Although only a summary is available and there is no information available on the test methods or conditions, the results are taken from a published regulatory document (US EPA 2011) and are considered to be reliable and relevant for use. This is supported by Lide (2009), which states that the melting point of lithium stearate is ca. 220°C. No information on the primary source of the data or the methods used is available. However, this information is taken from a reliable peer reviewed handbook and can be considered reliable and relevant for use. This is also supported by a peer-reviewed journal article, Shoeb (1999), citing a previous experimental study (Ralston 1948), which states that the melting point of lithium stearate is 220.5 -221.5°C, though no details are provided on the test methods or conditions. 

 

The melting point of lithium 12-hydroxystearate is 200 - 216°C and of lithium behenate is 166°C; thereafter the substances slowly decompose. The melting points of lithium 12-hydroxystearate and lithium behenate were determined in GLP-compliant thermal analysis tests, following OECD guideline 102 (Harlan 2012, 2013). 

  

All of the substances in the lithium salts of monocarboxylic acids C14-C22 category are considered to have melting points in the range of 166 to 231°C. The data indicate that melting points for lithium salts of fatty acids C14, C16 and C18 are all similar, at around 220°C, with the melting point of C22 being slightly lower, at around 166°C. Thus, it can be justifiably expected that the melting points of remaining substances in the category with intervening carbon chain lengths would fall within this range and can be read across from the available data. To support this, studies on fatty acids C16 -18 lithium salts, fatty acids C16 -22 lithium salts, fatty acids C16 -18 (even numbered) saturated and C16 -20 (even numbered) unsaturated lithium salts and tallow lithium salt are currently ongoing (Covance 2020).

 

 Boiling point 

 

All of the substances in the lithium salts of monocarboxylic acids C14-C22 category decompose on melting to give a solid residue. The boiling point test is therefore not technically feasible on these substances. 

 

No determination of the boiling temperature was feasible for lithium myristate, lithium 12-hydroxystearate or lithium behenate as they were determined to decompose on melting to give a solid residue (Harlan 2012, 2013). As the substances with the longest and shortest carbon chain length in the category, as well as an intermediary substance all behaved in the same manner it is considered justified to expect that the remaining substances in the category with intervening carbon chain lengths would also decompose to solid residues on melting. 

 

 Density 

 

All of the substances in the lithium salts of monocarboxylic acids C14-C22 category have relative densities just greater than 1 at 20°C, with a range from 1.025 to 1.07. 

 

The relative density of lithium myristate is 1.07 at 20°C and of lithium 12-hydroxystearate and lithium behenate is 1.04 at 20°C. The relative densities of lithium myristate, lithium 12-hydroxystearate and lithium behenate were determined in GLP-compliant pycnometer tests following OECD guideline 109 (Harlan 2012, 2013). 

 

The specific gravity of lithium stearate is 1.025. No information on the test method used is available in the publication (CIR 1982). However, the data forms part of a clinical assessment prepared by an expert panel and are taken from a peer reviewed article. 

 

All of the substances in the lithium salts of monocarboxylic acids C14-C22 category are considered to have relative densities just greater than 1 at 20°C, with a range from 1.025 to 1.07. The relative densities of lithium myristate, lithium stearate, lithium 12-hydroxystearate and lithium behenate are all very similar, being just greater than 1. As all of these substances show similar densities, it can be justifiably expected that the densities of the remaining substances in the category would have similar relative density values as well. To support this, studies are currently ongoing for fatty acids C16 -18 lithium salts, fatty acids C16 -22 lithium salts, fatty acids C16 -18 (even numbered) saturated and C16 -20 (even numbered) unsaturated lithium salts, and tallow lithium salt.

 

Granulometry 

 

Where the substances are manufactured and usedin situ, such as for use as grease thickeners, the substances are not marketed or used in isolated solid or granular forms and so the particle size distribution test has been waived. Only lithium 12-hydroxystearate and fatty acids C16-C18 lithium salts are manufactured in an isolated form. Proprietary data are available, which indicate that lithium 12-hydroxystearate has a D10 of 1.1 µm, a D50 of 4.9 µm and a D90 of 31.5 µm and fatty acids C16-C18 lithium salts have a D10 of 6.4 µm, a D50 of 33.8 µm and a D90 of 91.8 µm. The data are taken from proprietary analytical data (Baerlocher 2011). 

 

For the two substances in the lithium salts of monocarboxylic acids C14-C22 category which are manufactured in an isolated form, proprietary analytical data are available. The data have not been read across to the other substances in the category, as these are all manufacturedin situin base oil and are therefore not marketed isolated substances in granular form. 

 

Vapour pressure 

 

The vapour pressure could not be determined for any of the substances in the lithium salts of monocarboxylic acids C14-C22 category. The vapour pressure test is not technically feasible for these substances as the predicted vapour pressures are below the limit of detection of the test methods. Substances in the lithium salts of monocarboxylic acids C14-C22 category have predicted vapour pressures of 3.88 E-09 Pa to 2.14 E-14 Pa using the Modified Grain Method in EPISuite, MPBVP v1.43 (US EPA 2009). Standard test methods, according to OECD guideline 104, are able to measure vapour pressure from 10 E-10 Pa to 10 E+05 Pa. As the predicted vapour pressures for the substances in the lithium salts of monocarboxylic acids C14-C22 category are around or below 10 E-10 Pa, the vapour pressures tests are technically not feasible. In order to support the QSAR predictions and confirm the feasibility of testing, a vapour pressure study is currently being conducted on lithium myristate (Covance 2020).

 

Water solubility 

 

The water solubility for lithium behenate (C22) was determined in a GLP-compliant test following EU method A6 (Harlan 2013) to be ≤0.000046 g/L. The GLP-compliant preliminary water solubility studies on lithium myristate and lithium 12-hydroxystearate (Harlan 2012) demonstrated that the substances have surface active properties and the water solubility could not be analytically determined because, after static equilibration, centrifugation and filtration, excess undissolved test item remained in the dispersion.

 

It is expected that the water solubility of the substances in the category decrease with increasing carbon chain length. The water solubility of lithium behenate, the longest carbon chain length substance, was determined to be ≤0.000046 g/L. However, the water solubility could not be experimentally determined for substances with a carbon chain length of less than C22 because they have surface active properties and formed stable dispersions in water rather than being truly soluble. The water solubilities of lithium myristate, the shortest carbon chain length substance, and lithium 12-hydroxystearate, an intermediate carbon chain length substance, could not be analytically determined because the undissolved test item remained in dispersion, even after filtration and centrifugation. All of the substances in the lithium salts of monocarboxylic acids C14-C22 category, other than lithium behenate, are considered to have surface active properties and therefore form stable dispersions rather than being truly soluble. Further investigations on the water solubility of lithium 12-hydroxystearate are currently ongoing (Covance 2020).

 

Partition Coefficient 

 

The partition coefficient could not be determined for any of the substances in the lithium salts of monocarboxylic acids C14-C22 category as the partition coefficient tests are not technically feasible on these substances. The substances are insufficiently soluble in n-octanol and water to determine the partition coefficient using the shake-flask method and the HPLC method is not suitable for salts of organic acids. Also, most substances in the lithium salts of monocarboxylic acids C14-C22 category are surface active and determination of partition coefficient using the shake-flask or HPLC estimation methods are not suitable for surface active substances. This is supported by the GLP-compliant, guideline n-octanol water partition coefficient studies which were attempted for lithium myristate, lithium 12-hydroxystearate and lithium behenate but were not feasible. As the tests could not be conducted on the substances with the longest and shortest carbon chain length in the category, as well as an intermediary substance, it is considered justified to expect that the remaining substances in the category with intervening carbon chain lengths would behave in the same manner and therefore cannot be tested for partition coefficient either. Further investigations on the partition coefficient of lithium 12-hydroxystearate are currently ongoing (Covance 2020). As additional supporting information, QSAR estimates have been provided for representative structures covering the substances in the lithium salts of monocarboxylic acids C14-C22 category, with Log Kow values ranging from 2.2 to 6.1.

 

The Environmental and Health Risk Assessment and Management (Erasm) taskforce has reviewed the common experimental (OECD 107 shake flask method, OECD 123 slow stirring method, OECD 117 HPLC method, and n-octanol/water solubility ratio method) and predictive methods (QSARs) for determination of partition coefficient of surfactants. The results of the investigations were published in Hodges et al. (2019) and all methods were identified as having limitations for determining accurate partition coefficient values for surfactants. The OECD 123 slow stirring method is considered to be preferred for surfactants (Hodges et al. 2019), though it requires a sensitive analytical method to analyse the test item in the water phase and the OECD 123 guideline clearly states under "applicability of the test" that the slow stirring method applies to pure substances which do not display significant interfacial activity. The HPLC method, which uses the retention time of a material to provide an indication of the partition coefficient, is not applicable as there is lack of reference surfactants with accurately determined log Kow values (Hodges et al. 2019) and, furthermore, the HPLC method is not suitable for salts of organic acids. Partition coefficient calculation based on the measured n-octanol and water solubilities is not always relevant or robust as the solubility of surfactants in water can be difficult to experimentally determine and may not be properly defined. Hodges et al. (2019) concluded that “there is little correlation between log Kow values generated using the slow-stirring and solubility ratio methods… [and the] available data suggest that the solubility ratio approach may underestimate the log Kow values compared to the slow stir method”. Hodges et al. (2019) concluded that “the solubility ratio method is not recommended as a robust or accurate method for the determination of log Kow values for the four classes of surfactants [non-ionic, anionic, cationic and amphoteric] assessed in this study”.

 

The octanol/water partition coefficient, Kow, is defined as the ratio of the equilibrium concentrations of a dissolved substance in each of the phases in a two-phase system consisting of octanol and water. It is a key parameter in studies of the environmental fate of organic substances, indicating the potential for bioaccumulation and soil absorption. It does not pertain to the lithium salts of monocarboxylic acid C14-C22 substances. Instead of the determination of a Kow value, the environmental fate and distribution of the dissociation products of the substances in water are better assessed according to the dissociation products in water as follows:

 

(i) the mechanisms for partitioning of Li+ in environmental media, including the adsorption and/or absorption by organic matter and living cells, are understood to be different from those traditionally attributed to carbon-based molecules and, thus, octanol/water partitioning has little relevance to ionic lithium. In order to measure an octanol/water partition coefficient, it is necessary to determine the concentration in each phase (as in OECD method 107), or to conduct an HPLC assay (as in OECD method 117). However, lithium is a metallic element that exists only in an ionic form in solution. The solubility of lithium cations in water is high and can safely be expected to be low in organic solvents such as octanol. Because of the unlikely partitioning of lithium cations into the octanol phase, it is not appropriate to determine the partition coefficient by direct quantification of lithium in both phases. Similarly, any aqueous HPLC mobile phase will cause dissociation of inorganic lithium compounds, and thus not allow the determination of a Kow by this method.

 

(ii) regarding the partitioning behaviour of fatty acid constituents (as reported in the respective registration dossiers): Palmitic acid (C16) has a log Kow of 7.17 (Sangster 1994); Stearic acid (C18) has a log Kow of 8.23 (Sangster 1994); and 12-hydroxystearic acid (C18-OH) has a log Kow of 5.7 (source not available).

 

Surface tension 

 

Substances in the lithium salts of monocarboxylic acids C14-C22 category have increasing surface tension with increasing carbon number. The surface tension of a saturated solution of 1.02 to 1.03 g/L lithium myristate is 34.0 mN/m at 21.5°C. The surface tension of a saturated solution of 1.00 to 1.01 g/L lithium 12-hydroxystearate is 51.9 to 52.0 mN/m at 21.5°C. The surface tension of a saturated solution of 1.00 to 1.07 g/L lithium behenate is 61.8 to 65.9 mN/m at 21.0°C. As the cut-off for being considered surface active is 60 mN/m and lithium behenate only slightly exceeds this, category members with carbon chain lengths of less than 22 are considered to have surface active properties. This is consistent with the observed characteristics of the substances in water and their structures, which are representative of the “soap” surfactant class. 

 

The surface tensions of lithium myristate, lithium 12-hydroxystearate and lithium behenate were determined in GLP-compliant ring balance tests following OECD guideline 115 (Harlan 2012, 2013). The studies deviated from the guideline because the test items formed stable dispersions from which the undissolved test item could not be removed but the deviations were not considered to have affected the integrity of the studies. Studies are currently ongoing for lithium stearate, fatty acids C16 -18 lithium salts, fatty acids C16 -22 lithium salts, fatty acids C16 -18 (even numbered) saturated and C16 -20 (even numbered) unsaturated lithium salts, and tallow lithium salt.

 

All of the substances in the category have a structure which is representative of the “soap” surfactant class (the lithium ion being the hydrophilic head and the fatty acid being the hydrophobic tail), however as the surface tension of the substances increases with increasing carbon chain length, only the shorter carbon chain length substances (<C22) meet the criteria to be considered surface active. 

 

Auto-flammability 

 

All of the substances in the lithium salts of monocarboxylic acids C14-C22 category have self-ignition temperatures above their melting points. Therefore, none of the substances in the category meet the criteria for classification as a self-heating substance. 

 

Lithium myristate, lithium 12-hydroxystearate and lithium behenate were determined not to have relative self-ignition temperatures below their melting points in GLP-compliant studies following EC 440/2008 A16 method (Harlan 2013). As the self-ignition temperature of the substances with the longest and shortest carbon chain lengths in the category, as well as one with an intermediary chain length, are above their respective melting points, it is considered justified to expect that the remaining substances in the category with intervening carbon chain lengths would also have self-ignition temperatures above their respective melting points. Therefore, none of the substances in the category meet the criteria for classification as a self-heating substance. Studies are currently ongoing for lithium stearate, fatty acids C16 -18 lithium salts, fatty acids C16 -22 lithium salts, fatty acids C16 -18 (even numbered) saturated and C16 -20 (even numbered) unsaturated lithium salts, and tallow lithium salt (Covance 2020).

 

Flammability 

 

None of the substances in the lithium salts of monocarboxylic acids C14-C22 category are considered to be highly flammable. Therefore, none of the substances in the category meet the criteria for classification as flammable solids. 

 

Lithium myristate, lithium 12-hydroxystearate and lithium behenate were determined to be not highly flammable as they failed to ignite in the preliminary screening tests of GLP-compliant studies following the method EC440/2008 A10 (Harlan 2013). As the substances with the longest and shortest carbon chain lengths in the category, as well an intermediary substance, are not highly flammable, it is considered justified to expect that the remaining substances in the category with intervening carbon chain lengths would not be highly flammable either. None of the substances in the lithium salts of monocarboxylic acids C14-C22 category are considered to be highly flammable. Therefore, none of the substances in the category meet the criteria for classification as flammable solids. Studies are currently ongoing for lithium stearate, fatty acids C16 -18 lithium salts, fatty acids C16 -22 lithium salts, fatty acids C16 -18 (even numbered) saturated and C16 -20 (even numbered) unsaturated lithium salts, and tallow lithium salt (Covance 2020).

 

Experience in manufacture and handling shows that the substances in the lithium salts of monocarboxylic acids C14-C22 category do not ignite spontaneously on coming into contact with air at normal temperatures and are stable at room temperature for prolonged periods of time (days). Experience has also shown that the substances do not react with water and, therefore, they do not meet the criteria for classification as pyrophoric substances or substances which in contact with water emit flammable gases. 

 

Dissociation constant 

 

No data are available on the dissociation constants for substances in the lithium salts of monocarboxylic acids C14-C22 category, therefore the dissociation constants have been read across from sodium palmitate (Kanicky et al 2000) and potassium stearate (Kanicky and Shah 2002). Kanicky et al. (2000) titrated sodium palmitate with sodium hydroxide and calculated the mean pKa from the pH at half the neutralisation volume for each of the five replicates. Kanicky and Shah (2002) titrated potassium stearate with hydrochloric acid and calculated the mean pKa from the pH at half the neutralisation volume for each of the five replicates. The studies are non-GLP, non-guideline experiments, available in peer-reviewed published journal articles. The experiments follow sound scientific principles and are considered adequate for assessment. 

 

Although the test items (source substances) vary from the lithium salts of monocarboxylic acids C14-C22 category (target) substances by the metal cation, lithium, sodium and potassium are all low-molecular weight metals and the fatty acid salts of these metals are expected to have very similar physico-chemical properties as the majority of the molecular weight of each substance is comprised of monocarboxylic acids. Fatty acid metal salts are expected to dissociate into free metal cations and fatty acid anions in water. The fatty acid would then be expected to achieve equilibrium with respect to the H+ ions in the water (depending on the pKa of the fatty acid) and, at neutral pH, there would be ionised fatty acids, unionised fatty acids, and free metal ions present. As the dissociation constants of metal salts of fatty acids are expected to be determined by the equilibrium of the fatty acids with respect to the hydrogen ions in the water, read across between different metal salts of the same fatty acids is considered to be justified. 

 

The measured dissociation constants for sodium palmitate of 8.8 (Kanicky et al. 2000) and for potassium stearate of 10.15 (Kanicky and Shah 2002) are both relatively high and thus the fatty acid would be expected to be unionised (protonated) at environmental pH. This would suggest that in solution the fatty acid and metal ions would be dissociated (although these substances do not tend to become truly dissolved) and would suggest that stearic acid would be almost entirely protonated at pH < 8. As such, the pKa of the analogous fatty acid substances are all expected to be above 8, with ionised fatty acids expected to become protonated at neutral pH.  

 

Stability in organic solvents 

 

The main use of lithium salts of monocarboxylic acids C14-C22 is as thickeners in formulated greases, so the stability of the substances in organic solvents is not considered to be critical. The substances are typically manufactured and usedin situin base oil and over the extensive history of the use of lithium soaps to thicken base oil in grease formulation, the stability of the lithium-based thickeners in different base oils has been well characterised. The estimated shelf-life of formulated greases is around 3 years (Chevron) and analysis of various thickener substances has indicated that the isolated substances would have a similar stability.

 

Other physico-chemical endpoints 

 

As the lithium salts of monocarboxylic acids C14-C22 are solids, the flash point and viscosity endpoints are not required. Based on the structure of the substances in this category, the explosiveness and oxidising properties studies have not been conducted as there are no structural alerts that would indicate explosive or oxidising properties. Where the substances are manufactured and usedin situin a base oil, the substances are not marketed or used in solid or granular forms and so the particle size distribution test has been waived and, for the two substances manufactured and used in an isolated form, proprietary data on particle size distribution have been included. 

 

Classification and labelling 

 

None of the substances in the lithium salts of monocarboxylic acids C14-C22 category are classified for physico-chemical hazards under the CLP. Based on the structure of the substances, they do not meet the criteria for oxidising or explosive properties and, based on experimental data, the substances do not meet the criteria for flammable solids or self-heating substances.