Hexyl D-glucoside

Categories

Category name:

Alkyl Polyglycosides (APG)

Justifications and discussions

Category definition:

This category comprises alkyl polyglycosides of linear fatty alcohols (carbon chain range between C6 and C16) with glucose or glucose oligomers

Category description:

The category applies to all systemic and local mammalian toxicity endpoints as well as ecotoxicological and environmental fate endpoints.

Category rationale:

The alkyl polyglycosides category contains D-Glucopyranose monomers and oligomers with fatty alcohols C6 to C16 linear. Structural similarities of the category substances are reflected in similar physico-chemical properties and mode of action. Alkyl polyglycosides have a common metabolic fate that involves hydrolysis of the ß-glycosidic bond to the fatty alcohol and glucose. Glucose and glucose oligomers enter the carbohydrate metabolic pathway and are catabolised. Fatty alcohols, representing the main difference in the structure of different alkyl polyglycosides, are oxidized to the corresponding fatty acid and fed into physiological pathways like the citric acid cycle, sugar synthesis and lipid synthesis.

Physico-chemical properties
Alkyl polyglycosides in the category are monomers or oligomers of D-glucopyranose and fatty alcohol. All substances are marketed in an aqueous solution with an average water content of 24 – 50%.
The melting temperature of the freeze-dried substances ranges from > 150 °C to > 300 °C.
The boiling point is for all freeze-dried substances > 300 °C at 1013 hPa.
The vapour pressure of the category members is very low. All measurements are < 0.01 Pa at 20 °C.
Partition coefficient log Kow for all category members does not exceed 3 and ranges between ≤ -0.07 and 1.77. Because alkyl polyglycosides are surface active substances, the measurement of log Kow is not possible, so the determination was done by calculation from the solubility of the substances in n octanol and water. A negative correlation with the number of D-glucopyranose units is observed. With increasing numbers of D-glucopyranose units the influence of the fatty alcohol moiety on the log Kow values decreases.
As alkyl polyglycosides are surfactants the category members are characterized with a rather good water solubility of > 200 g/L or even 750 g/L (CAS-No. 54549-24-5).
The surface tension ranges from 29.5 mN/m to 35.52 mN/m, which proves the surface activity. An aqueous solution of alkyl polyglycosides decreases the surface tension of water (~73 mN/m at 20 °C) markedly.

Environmental fate and pathways
All substances within the category are readily biodegradable. Despite the good water solubility, they do not hydrolyse under environmental conditions (hydrolytically stable at pH = 4, 7 and 9) and their bioaccumulation potential is low due to the low log Kow (-0.07 to 1.77). The vapour pressure is also low and a low henry’s law constant indicates that they will not evaporate into the air phase from water. However, if emitted into the air the category members are susceptible to indirect photodegradation (DT50 < 24 h). Adsorption, e.g. on soil, is not expected as indicated by the low log Koc (1.7 at 25 °C, measured and 0.7 – 1.7 calculated).

Ecotoxicity
Acute toxicity studies show a potential to be harmful for aquatic organisms with the exception of microorganisms. A positive correlation with fatty alcohol chain length can be observed. D Glucopyranose, oligomeric, C10-16-alkyl glycosides shows the highest harmful potential (LC50 fish = 2.95 mg/L, EC50 daphnia = 7 mg/L, EC50 algae = 12.5 mg/L). With decreasing chain length of the fatty alcohol moiety the toxicity decreases. The trend is shown for acute daphnia, fish and algae. For microorganisms a toxicity level of EC50 > 560 mg/L as lowest value was measured.
For chronic toxicity to aquatic organisms a worst case scenario has to be adopted, because read-across was done with the most harmful substance (proved in acute aquatic toxicity studies) CAS-No. 110615 47 9, D-Glucopyranose, oligomeric, C10-16-alkyl glycosides. For fish a NOEC of 1.8 mg/L and for daphnia a NOEC of 1 mg/L was found.
Sediment toxicity, toxicity to terrestrial macroorganisms except arthropods and toxicity to terrestrial plants show a different pattern. For these organisms none of the category members shows any harmful effects (LC50 > 650 mg/kg for sediment toxicity, LC0 > 654 mg/kg for toxicity to terrestrial macroorganisms and terrestrial plants).
Despite the harmful effects to aquatic organisms there is no classification needed, because all category members are readily biodegradable and have a log Kow < 3.

Human toxicity
Alkyl polyglycosides have a common metabolic fate that involves hydrolysis of the ß-glycosidic bond to the fatty alcohol and glucose.
Glucose and glucose oligomers enter the carbohydrate metabolic pathway and are catabolised. Fatty alcohols, representing the main difference in the structure of different alkyl polyglycosides, are oxidized to the corresponding fatty acid and fed into physiological pathways like the citric acid cycle, sugar synthesis and lipid synthesis.
Since the category chemicals differ from each other only by the number of -CH2- units, the category is very homogenous and it is assumed that either each category member exhibits the same toxic mode of action or there will be an evident relationship between chain length and the toxicological properties.
Existing data indicate that the skin irritation potential increases with increasing chain length. An alkyl compound consisting of eleven carbon atoms thereby represents the breaking point for classification.
The eye irritation toxicity of the compounds in this category is predicted to require the labelling "risk of serious damage to eyes" (R41), since all tested alkyl polyglycosides display a severe eye irritating potential.
The potential to provoke skin sensitization is considered to be very low, based on the known properties of the predictable metabolites as well as on the results of all tested category members covering a carbon chain range from C6 to C16.
The systemic toxicity of the compounds in this category is predicted to be very low, based on the known properties of the predictable metabolites (fatty acids and glucose).