involved charge separation and is dependent on the concentration of the ions present
in the solution, the distance, and the dielectric constant of the surrounding medium.
In principle, the flux of free ions and their steady-state at and across interfaces can be
calculated based on chemical and electrostatic gradients using the Nernst equation. A
special case is the sorption of IOCs into biological membranes that possess a positive
dipole potential which favors the sorption of anions (Benz 1988). Free ions can also
partition between uncharged bulk phases together with their counter ions (as free
separate ions) and as ion pairs, e.g., partitioning between water and an organic
solvent like octanol (Ingram et al. 2011; Jafvert et al. 1990; Zhao and Abraham
2005). In this case the free separate ions and the ion pairs (both will be in equilibrium
with each other) can distribute throughout the complete bulk phases because there is
no charge separation and therefore no electrostatic force that keeps them in the
vicinity of the interface. Significant partitioning of ion pairs is only expected if the
stability constant of the ion pair is already high in water, which is typically the case if
both ions are relatively hydrophobic (e.g., an alkylamine cation combined with an
alkylacid anion) (Schunk and Maurer 2005). In the environment, the formation of
ion pairs is probably of minor importance, because it is unlikely that a hydrophobic
counter ion is available at sufficiently high concentration for relevant ion-pair
formation (Hallén et al. 1985). The partitioning of free organic ions together with
inorganic counter ions from water to an organic phase is also expected to be rather
insignificant, because the counter ions are often small ions with a high surface charge
density (Ingram et al. 2011). These ions tend to stay in water rather than to form ion
pairs, because of favorable dipole–charge interactions with the water molecules
(Westall et al. 1990).
The partitioning of neutral organic chemicals is dominated by van der Waals
interactions and H-bond formations. In contrast, sorption of IOCs is often driven by
interactions of their charge with complementary charges in the environment (e.g.,
with ion exchangers and in electric fields) followed by H-bond interaction and
interactions with strong dipoles like water (Collins et al. 2007). The relative importance of charge–charge and charge–dipole interactions decreases, as the hydrophobicity of an ion increases (Lund et al. 2008). Whereas small inorganic ions usually
have high surface charge densities, the charge density of organic ions can vary
widely. In fact, hydrophobic ions that show strong sorption from water to an
uncharged organic phase typically have low charge densities at their surface (Lund
et al. 2008). Low surface charge densities of ions are either a result of delocalization
of the charge (see bis(fluorosulfonyl)imid, Fig. 3b) or the charge is hidden in the
center of a bulky molecule (e.g., tetraphenyl and quaternary ammonium ions,
Fig. 3a). Another factor that has to be considered is the hydration shell that forms
around a charged molecule and that may also alter the effective surface charge
density if it stays in place when the molecule interacts with charged sorption sites
(Hammer et al. 2018; Hühnerfuss 1989).
Another difference between neutral organic chemicals and IOCs in terms of
sorption behavior is that the sorption of ions can be highly influenced by pH value
and salt concentration of the surrounding medium (Sassman and Lee 2005; ter Laak
et al. 2006). The pH value can affect the sorption of ions by two different
46
L. Henneberger and K.-U. Goss
in the solution, the distance, and the dielectric constant of the surrounding medium.
In principle, the flux of free ions and their steady-state at and across interfaces can be
calculated based on chemical and electrostatic gradients using the Nernst equation. A
special case is the sorption of IOCs into biological membranes that possess a positive
dipole potential which favors the sorption of anions (Benz 1988). Free ions can also
partition between uncharged bulk phases together with their counter ions (as free
separate ions) and as ion pairs, e.g., partitioning between water and an organic
solvent like octanol (Ingram et al. 2011; Jafvert et al. 1990; Zhao and Abraham
2005). In this case the free separate ions and the ion pairs (both will be in equilibrium
with each other) can distribute throughout the complete bulk phases because there is
no charge separation and therefore no electrostatic force that keeps them in the
vicinity of the interface. Significant partitioning of ion pairs is only expected if the
stability constant of the ion pair is already high in water, which is typically the case if
both ions are relatively hydrophobic (e.g., an alkylamine cation combined with an
alkylacid anion) (Schunk and Maurer 2005). In the environment, the formation of
ion pairs is probably of minor importance, because it is unlikely that a hydrophobic
counter ion is available at sufficiently high concentration for relevant ion-pair
formation (Hallén et al. 1985). The partitioning of free organic ions together with
inorganic counter ions from water to an organic phase is also expected to be rather
insignificant, because the counter ions are often small ions with a high surface charge
density (Ingram et al. 2011). These ions tend to stay in water rather than to form ion
pairs, because of favorable dipole–charge interactions with the water molecules
(Westall et al. 1990).
The partitioning of neutral organic chemicals is dominated by van der Waals
interactions and H-bond formations. In contrast, sorption of IOCs is often driven by
interactions of their charge with complementary charges in the environment (e.g.,
with ion exchangers and in electric fields) followed by H-bond interaction and
interactions with strong dipoles like water (Collins et al. 2007). The relative importance of charge–charge and charge–dipole interactions decreases, as the hydrophobicity of an ion increases (Lund et al. 2008). Whereas small inorganic ions usually
have high surface charge densities, the charge density of organic ions can vary
widely. In fact, hydrophobic ions that show strong sorption from water to an
uncharged organic phase typically have low charge densities at their surface (Lund
et al. 2008). Low surface charge densities of ions are either a result of delocalization
of the charge (see bis(fluorosulfonyl)imid, Fig. 3b) or the charge is hidden in the
center of a bulky molecule (e.g., tetraphenyl and quaternary ammonium ions,
Fig. 3a). Another factor that has to be considered is the hydration shell that forms
around a charged molecule and that may also alter the effective surface charge
density if it stays in place when the molecule interacts with charged sorption sites
(Hammer et al. 2018; Hühnerfuss 1989).
Another difference between neutral organic chemicals and IOCs in terms of
sorption behavior is that the sorption of ions can be highly influenced by pH value
and salt concentration of the surrounding medium (Sassman and Lee 2005; ter Laak
et al. 2006). The pH value can affect the sorption of ions by two different
46
L. Henneberger and K.-U. Goss