LINEAR ALKYLBENZENE SULFONATE (LAS) - UNEP Chemicals

OECD SIDS LINEAR ALKYLBENZENE SULFONATE (LAS)
defined in which CP is found by pairing off carbon atoms along the two
branches up to the terminus of the shorter branch. Regression analysis
correlating log (1/LC50) to goldfish with a combination of ALP [representing
log P calculated without a branch factor] and log (CP + 1) [representing the
water sharing function]:
Log (1/LC50) = 0.78ALP – 1.13 log (CP + 1) + 2.06 (EQ 2)
(for LC50 tests on guppies; n = 20, r = 0.997, s = 0.041)
Further, dividing the first two terms on the right of EQ 2 by 0.78 gave,
assuming the role of the second term to be solely that of the branching factor,
the following equation:
Log P = ALP – 1.44 log (CP + 1) (EQ 3)
Thus, the PDBF was defined as -1.44 log (CP + 1). Further details on this
method are described in Roberts (1989).
Log P values calculated using EQ 3 were found to give good correlations
with published river sediment sorption partition coefficients for LAS
compounds, supporting the applicability and validity of the PDBF. Log P
values calculated using EQ 3 were also used successfully in regression of
toxicity data for pure LAS homologues and isomers to Daphnia magna and
Gammarus pulex. The basic equation is similar to EQ 1 and has the general
form:
Log (1/LC50) = alog P + b (EQ 4)
with the values for a, b and regression data shown in the following table:
Daphnia Daphnia Gammarus Gammarus
(H) (S) (H) (S)
Value of a 0.7 0.64 0.76 0.71
Value of b
Regression
data
2.23 2.44 2.46 2.27
n 9 12 9 11
r 0.987 0.955 0.966 0.950
s 0.07 0.15 0.13 0.16
F 263 103 98 83
Notes: H = hard water (250 mg/L CaCO3); S = soft water (25 mg/L CaCO3). Strongly
negative outliers omitted.
The log P coefficients and intercepts for goldfish, Daphnia, and Gammarus
are all intermediate between those of Könemann’s QSAR equation (EQ 1),
suggesting that a narcosis (possibly polar) mechanism applies to LAS acute
toxicity.
Results: Agreement between observed and calculated toxicities for LAS is good. The
fact that QSARs derived from compounds whose log P values are calculated
with the PDBF give good predictions for unbranched compounds supports
the validity of the PDBF for the type of branching encountered in LAS.
Remarks: The analyses presented in this paper indicates that the problems of
calculating log P for surfactants can be overcome. The case for the
applicability of the PDBF appears compelling, although it is based on
indirect evidence. In terms of acute aquatic toxicity, anionic surfactants like
LAS do not seem greatly different from unreactive non-surfactant organic
chemicals. Anionic surfactants of various types have log (1/LC50) values
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