Partition Coefficient

Detailed Description

The partition coefficient is a measure of how a substance distributes itself between two immiscible phases at equilibrium. The most commonly reported partition coefficient in Safety Data Sheets is the octanol-water partition coefficient (Kow or Pow), which describes the ratio of a chemical's concentration in the octanol phase to its concentration in the aqueous phase of a two-phase octanol/water system at equilibrium.

The octanol-water partition coefficient is typically expressed as its logarithm to base 10 (log Kow or log Pow) and is defined by the formula:

log Kow = log(Coctanol / Cwater)

Where:

log Kow is the logarithm of the octanol-water partition coefficient
Coctanol is the concentration of the substance in the octanol phase
Cwater is the concentration of the substance in the water phase

Key concepts related to partition coefficient include:

Lipophilicity: The affinity of a compound for lipid (fat) phases. Higher log Kow values indicate greater lipophilicity.
Hydrophobicity: The tendency of a compound to repel from water. Higher log Kow values indicate greater hydrophobicity.
Bioaccumulation potential: The tendency of a substance to accumulate in organisms, which is often correlated with log Kow.
Environmental partitioning: How a substance will distribute between different environmental compartments (water, soil, sediment, biota).
Membrane permeability: The ability of a substance to cross biological membranes, which is influenced by log Kow.

Importance in Safety Data Sheets

Partition coefficient information in an SDS is important for several reasons:

Environmental fate assessment: Indicates how a substance will distribute in the environment between water, soil, sediment, and biota.
Bioaccumulation potential: Helps predict whether a substance may accumulate in organisms and potentially biomagnify in food chains.
Regulatory classification: Used for PBT (Persistent, Bioaccumulative, Toxic) and vPvB (very Persistent, very Bioaccumulative) assessments under various chemical regulations.
Toxicokinetics: Provides insight into how a substance may be absorbed, distributed, and eliminated in biological systems.
Ecotoxicological assessment: Helps predict potential toxicity to aquatic organisms.
Transport modeling: Essential for predicting environmental transport and fate.
Waste management: Informs appropriate disposal methods and potential environmental impacts.

Measurement Methods

Several techniques are used to determine partition coefficients:

Method	Description	Applicable Log Kow Range
Shake-Flask Method	Direct measurement by equilibrating the substance between octanol and water phases, then analyzing concentrations in each phase	-2 to 4
Slow-Stirring Method	Similar to shake-flask but with gentle stirring to avoid emulsion formation	0 to 8.2
HPLC Method	Indirect measurement based on retention time on a reversed-phase HPLC column	0 to 6
Generator Column Method	Uses a column to generate saturated solutions for analysis	1 to 8
pH-Metric Method	Based on the shift in apparent pKa in the presence of octanol	0 to 6
Computational Methods	Estimation based on molecular structure (e.g., QSAR, fragment-based methods)	Wide range
OECD Test Guidelines	Standardized procedures (e.g., OECD 107, 117, 123)	Varies by method

Interpretation of Log Kow Values

Log Kow Range	Interpretation	Bioaccumulation Potential	Environmental Behavior
< 0	Highly hydrophilic	Very Low	Remains primarily in water phase; low adsorption to soil/sediment; rapid distribution in aqueous environments
0 to 3	Moderately hydrophilic	Low	Primarily in water phase with some partitioning to organic phases; moderate mobility in soil
3 to 4	Moderately lipophilic	Moderate	Significant partitioning to organic phases; moderate adsorption to soil/sediment; potential for bioaccumulation
4 to 5	Lipophilic	High	Strong partitioning to organic phases; strong adsorption to soil/sediment; high potential for bioaccumulation
> 5	Highly lipophilic	Very High	Predominantly in organic phases; very strong adsorption to soil/sediment; very high potential for bioaccumulation; limited bioavailability due to low water solubility

Note: Regulatory thresholds for bioaccumulation potential may vary by jurisdiction. For example:

EU REACH: log Kow ≥ 4.5 for B (Bioaccumulative) and ≥ 5 for vB (very Bioaccumulative)
US EPA: log Kow ≥ 3.5 for potential concern
Stockholm Convention on POPs: log Kow ≥ 5

Log Kow Values of Common Substances

Substance	Log Kow	Category	Notes
Glucose	-3.3	Highly hydrophilic	Simple sugar, very water-soluble
Ethanol	-0.31	Highly hydrophilic	Alcohol, miscible with water
Acetone	-0.24	Highly hydrophilic	Ketone, miscible with water
Acetic Acid	-0.17	Highly hydrophilic	Carboxylic acid, miscible with water
Caffeine	-0.07	Highly hydrophilic	Alkaloid, water-soluble
Acetylsalicylic Acid (Aspirin)	1.19	Moderately hydrophilic	Pharmaceutical, moderately water-soluble
Benzene	2.13	Moderately hydrophilic	Aromatic hydrocarbon, slightly water-soluble
Phenol	1.46	Moderately hydrophilic	Aromatic alcohol, moderately water-soluble
Toluene	2.73	Moderately hydrophilic	Aromatic hydrocarbon, slightly water-soluble
Atrazine	2.61	Moderately hydrophilic	Herbicide, slightly water-soluble
Naphthalene	3.30	Moderately lipophilic	PAH, very slightly water-soluble
Diazinon	3.81	Moderately lipophilic	Insecticide, very slightly water-soluble
DDT	6.91	Highly lipophilic	Insecticide, practically insoluble in water
PCB (Aroclor 1254)	6.5	Highly lipophilic	Industrial chemical, practically insoluble in water
Cholesterol	8.7	Highly lipophilic	Sterol, practically insoluble in water

Special Considerations

pH-Dependent Partition Coefficient

For ionizable substances, the partition coefficient can vary significantly with pH:

Acids: Become more lipophilic (higher log Kow) at lower pH where they are predominantly in the non-ionized form.
Bases: Become more lipophilic (higher log Kow) at higher pH where they are predominantly in the non-ionized form.
Distribution coefficient (log D): Describes the apparent partition coefficient at a specific pH, accounting for both ionized and non-ionized forms.

log D = log Kow (non-ionized) - log(1 + 10^(pH-pKa)) for acids
log D = log Kow (non-ionized) - log(1 + 10^(pKa-pH)) for bases

Temperature Effects

Partition coefficients are temperature-dependent, though the effect is often relatively small within the environmental temperature range. Higher temperatures generally lead to slightly lower log Kow values due to increased entropy.

Molecular Size and Bioavailability

Very large molecules with high log Kow values (typically > 6) may have limited bioavailability despite their lipophilicity, due to their size limiting membrane permeation. This is sometimes referred to as the "molecular size cutoff" effect.

Surfactants and Amphiphilic Compounds

Surfactants and other amphiphilic compounds (having both hydrophilic and lipophilic regions) may not exhibit conventional partitioning behavior, and their log Kow values may not accurately predict their environmental behavior or bioaccumulation potential.

Environmental and Safety Implications

The partition coefficient has significant implications for environmental fate and safety:

Bioaccumulation: Substances with log Kow values between 3 and 6 generally have the highest potential for bioaccumulation in aquatic organisms.
Biomagnification: Substances with high log Kow values (typically > 4.5) may biomagnify in food chains, reaching higher concentrations in predators than in their prey.
Soil adsorption: Substances with higher log Kow values tend to adsorb more strongly to soil organic matter, reducing mobility but potentially increasing persistence.
Dermal absorption: Moderately lipophilic substances (log Kow between 1 and 4) often have the highest potential for dermal absorption.
Blood-brain barrier penetration: Substances with log Kow values between 1 and 4 generally have the highest potential to cross the blood-brain barrier.
Volatilization: For substances with similar molecular weights, those with higher log Kow values generally have lower water solubility and higher Henry's Law constants, potentially leading to greater volatilization from water.

Examples of Partition Coefficient Descriptions in SDSs

"Partition coefficient (n-octanol/water): log Pow = 2.73 at 20°C"
"Log Kow: 3.30 (OECD 117)"
"Partition coefficient: log Pow = -0.31 (experimental)"
"n-octanol/water partition coefficient: log Pow = 4.2 (calculated)"
"Log Kow: Not applicable for inorganic substances"
"Partition coefficient: log Pow = 2.1 at pH 7; log Pow = 3.8 at pH 5"
"Log Kow: >6 (estimated by QSAR)"
"Partition coefficient: Not determined due to surface activity"

Regulatory Requirements

According to GHS and various regional regulations (EU CLP, US OSHA HazCom, etc.), the partition coefficient should be indicated in Section 9 of the Safety Data Sheet as part of the description of basic physical and chemical properties. This information is considered mandatory for substances and mixtures, though it may be reported as "not applicable" for inorganic substances or those for which determination is not technically possible.

For environmental hazard classification and PBT/vPvB assessment, the partition coefficient is a critical parameter under regulations such as EU REACH, US TSCA, and various national chemical control laws.

Best Practices

When reporting partition coefficient in an SDS:

Express the value as log Kow or log Pow (dimensionless)
Clearly indicate the temperature at which the measurement was made (typically 20°C or 25°C)
For pH-dependent partition coefficients, specify the pH at which the value applies
Indicate whether the value was experimentally determined or estimated
If estimated, specify the estimation method (e.g., QSAR, fragment method)
For experimentally determined values, specify the test method used (e.g., OECD 107, 117, 123)
For substances where partition coefficient is not applicable (e.g., inorganic substances, surfactants), clearly state this with a brief explanation
For mixtures, consider providing the partition coefficient of significant components rather than a single value for the mixture
Ensure consistency between partition coefficient information and other related properties in the SDS (e.g., water solubility, bioaccumulation potential)