partition constant of the neutral species (K ow,neutral ), assuming that the neutral
fraction of the chemical ( f neutral ) is dominating the sorption, because the ionized
fraction does not partition from water to the organic phase (Schmitt 2008).
D ow pH
ð Þ ¼ f neutral ∙ K ow, neutral
ð1Þ
This model works for ionizable chemicals, if the fraction of the ionic species
is small and if the ionic species is not very hydrophobic. If the ionic fraction of
the chemical ( f ion ) is contributing to the sorption, Eq. 2 is used to calculate D ow
(pH). For this calculation the octanol-–water partition constant of the ionic species
(K ow,ion ) is required in addition.
D ow pH
ð Þ ¼ f neutral ∙ K ow, neutral þ f ion ∙ K ow, ion
ð2Þ
In principle, Eq. 1 is not applicable to permanently charged ions (e.g., quaternary
ammonium compounds and ionic liquids, Fig. 1c) and compounds that show strong
dissociation (e.g., perfluorinated acids, Fig. 1b). Furthermore, for the calculation of
D ow (pH), it has to be considered that there is not a single value for K ow,ion . To keep
charge neutrality, the ionic species of the chemical is always sorbing to octanol
together with a counter ion (either as free ions or as ion pairs). Hence, K ow,ion must be
dependent on the concentration of the available counter ions (Johnson and Westall
1990; Westall et al. 1985). Given the complexity of the sorption processes (see Fig. 2
and corresponding discussion) to natural matrices like proteins, phospholipids,
humic matter, minerals, and others, it comes as no surprise that octanol, and
consequently D ow (pH), has little value as a surrogate for predicting sorption to
natural sorbents (Bittermann et al. 2016; Droge and Goss 2012b; Henneberger et al.
2016a).
As stated above, for charged environmental and biological phases, like soil
organic matter and proteins, ion sorption definitely needs to be taken into account.
Several models that explicitly consider the sorption of ions have been developed.
However, these models are often either lacking sufficient mechanistic background or
have a very limited domain of applicability. Polyparameter linear free energy
relationships (PP-LFERs) have been successfully used to model various sorption
processes of neutral organic chemicals (Endo and Goss 2014), including biological
phases such as proteins and lipids (Endo et al. 2011, 2012; Endo and Goss 2011;
Geisler et al. 2012). Abraham and coworkers have extended the original PP-LFER
approach by adding two new descriptors for ionic interactions (J
+ and J
À , Eq. 3)
(Abraham 2011; Abraham and Acree 2010a, b, c, d, 2015, 2016).
log K 1=2 ¼ c þ e ∙ E i þ s ∙ S i þ a ∙ A i þ b ∙ B i þ v ∙ V i þ j
þ
∙ J
þ
i þ j
À
∙ J
À
i
ð3Þ
In this equation the logarithmic partition constant of an ion between phase 1 and
2 (log K 1/2 ) is calculated using two different sets of descriptors. The small letters
(e, s, a, b, v, j+, j
À ) represent the properties of the sorption system. The properties of
52
L. Henneberger and K.-U. Goss
fraction of the chemical ( f neutral ) is dominating the sorption, because the ionized
fraction does not partition from water to the organic phase (Schmitt 2008).
D ow pH
ð Þ ¼ f neutral ∙ K ow, neutral
ð1Þ
This model works for ionizable chemicals, if the fraction of the ionic species
is small and if the ionic species is not very hydrophobic. If the ionic fraction of
the chemical ( f ion ) is contributing to the sorption, Eq. 2 is used to calculate D ow
(pH). For this calculation the octanol-–water partition constant of the ionic species
(K ow,ion ) is required in addition.
D ow pH
ð Þ ¼ f neutral ∙ K ow, neutral þ f ion ∙ K ow, ion
ð2Þ
In principle, Eq. 1 is not applicable to permanently charged ions (e.g., quaternary
ammonium compounds and ionic liquids, Fig. 1c) and compounds that show strong
dissociation (e.g., perfluorinated acids, Fig. 1b). Furthermore, for the calculation of
D ow (pH), it has to be considered that there is not a single value for K ow,ion . To keep
charge neutrality, the ionic species of the chemical is always sorbing to octanol
together with a counter ion (either as free ions or as ion pairs). Hence, K ow,ion must be
dependent on the concentration of the available counter ions (Johnson and Westall
1990; Westall et al. 1985). Given the complexity of the sorption processes (see Fig. 2
and corresponding discussion) to natural matrices like proteins, phospholipids,
humic matter, minerals, and others, it comes as no surprise that octanol, and
consequently D ow (pH), has little value as a surrogate for predicting sorption to
natural sorbents (Bittermann et al. 2016; Droge and Goss 2012b; Henneberger et al.
2016a).
As stated above, for charged environmental and biological phases, like soil
organic matter and proteins, ion sorption definitely needs to be taken into account.
Several models that explicitly consider the sorption of ions have been developed.
However, these models are often either lacking sufficient mechanistic background or
have a very limited domain of applicability. Polyparameter linear free energy
relationships (PP-LFERs) have been successfully used to model various sorption
processes of neutral organic chemicals (Endo and Goss 2014), including biological
phases such as proteins and lipids (Endo et al. 2011, 2012; Endo and Goss 2011;
Geisler et al. 2012). Abraham and coworkers have extended the original PP-LFER
approach by adding two new descriptors for ionic interactions (J
+ and J
À , Eq. 3)
(Abraham 2011; Abraham and Acree 2010a, b, c, d, 2015, 2016).
log K 1=2 ¼ c þ e ∙ E i þ s ∙ S i þ a ∙ A i þ b ∙ B i þ v ∙ V i þ j
þ
∙ J
þ
i þ j
À
∙ J
À
i
ð3Þ
In this equation the logarithmic partition constant of an ion between phase 1 and
2 (log K 1/2 ) is calculated using two different sets of descriptors. The small letters
(e, s, a, b, v, j+, j
À ) represent the properties of the sorption system. The properties of
52
L. Henneberger and K.-U. Goss