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Physical & Chemical properties

Surface tension

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Description of key information

Surface tension [2-(perfluorohexyl)ethyl]trimethoxysilane: 72.2 mN/m at 20°C (OECD 115)
Surface tension [2-(perfluorohexyl)ethyl]silanetriol: <22 mN/m at 20°C (OECD 115)

Key value for chemical safety assessment

Surface tension:
72.2
in mN/m at 20°C and concentration in mg/L:
10

Additional information

The surface tension of [2-(perfluorohexyl)ethyl]trimethoxysilane was read-across from the triethoxy analogue, [2-(perfluorohexyl)ethyl]triethoxysilane (CAS 51851-37-7). A measured surface tension value of 72.2 mN/m at 20°C and 10 mg/l was determined for [2-(perfluorohexyl)ethyl]triethoxysilane (CAS 51851-37-7) using a relevant test method method and in compliance with a known quality system (ISO 17025). The measurements were taken after a solution age of 30 minutes to minimise hydrolysis of the substance. The result is considered to be reliable and is selected as key study. Under the conditions of the study, the parent substance is considered as not surface active. Both substances are trialkoxysilanes with fluoroalkyl side chains, the surface tension study is therefore considered valid to be read-across to [2-(perfluorohexyl)ethyl]trimethoxysilane.

In contact with water, [2-(perfluorohexyl)ethyl]trimethoxysilane reacts rapidly to form [2-(perfluorohexyl)ethyl]silanetriol and methanol (half-lives of 0.4 h at pH 4, 0.4 h at pH 5, 8.8 h at pH 7 and 0.1 h at pH 9 and 20 -25°C). Therefore, the surface tension of the silanol hydrolysis product, [2-(perfluorohexyl)ethyl]silanetriol was determined using a relevant test method and read-across from octylsilanetriol.

A surface tension study was performed with the test substance [[2-(perfluorohexyl)ethyl]trimethoxysilane], under conditions in which the results would reflect the surface tension of the hydrolysis product. The surface tension study used a test concentration of 1 g/l at 20°C and 43 h solution age. At the age of the solution used during the study (43 h), hydrolysis would have occurred and the surface tension result is considered to reflect the silanol hydrolysis product(s) under conditions where the limit of solubility is significantly exceeded. Therefore, the study results are considered to indicate that the hydrolysis product, [2-(perfluorohexyl)ethyl]silanetriol has a measured surface tension value of <22 mN/m at 20°C and 1 g/l. This value is also supported by available measured data for the read-across substance octylsilanetriol.

The surface tension of octylsilanetriol was determined by the addition of octyltrichlorosilane in water. Octyltrichlorosilane (CAS No 5283-66-9) in contact with water reacts very rapidly with a half-life of <1 minute at pH 4, pH 7 and pH 9 and 25°C to form octylsilanetriol and hydrochloric acid. Both buffered (pH 7) and non-buffered solutions with solution age of 3 days and 4 hours respectively were used during the study. Surface tension of 53 mN/m at 20°C and 100 mg/l using EU Method A.5 was obtained for the buffered solution of octylsilanetriol. The result indicates that octylsilanetriol is slightly surface active.

Therefore, based on the available information and intended applications of the substance the silanol hydrolysis product, [2-(perfluorohexyl)ethyl]silanetriol, is considered to be surface active. The presence of fluorocarbon chains may mean that the condensation products could have some surface activity. For these reasons, the effective limits of solubility of both the parent substance and monomeric silanol hydrolysis products are very low and all forms of the substance may have a tendency to enrich at surfaces rather than forming stable evenly dispersed solutions.