Hydrophobicity is a chemical property that helps determine a compound’s physiological impact. This property is often quantified using the partition coefficient ( P ), which describes how soluble a compound is in aqueous and hydrophobic environments. In practice, chemical compounds consumed orally must be hydrophobic enough to be absorbed through the lipid bilayers of the small intestine, while still being hydrophilic enough to be soluble in the bloodstream. Additionally, the hydrophobicity of a chemical determines how it is distributed throughout the body. Highly hydrophobic compounds tend to accumulate in the tissues and take longer for the body to eliminate. Therefore, they are more likely to be toxic and have long-term adverse effects.

     A research group studying the toxicity of various components of cigarette smoke decided to measure the partition coefficients for several chemicals found in cigarette smoke condensate. The results are shown in Table 1.

Table 1 Partition coefficient data for components of cigarette smoke condensate

     To determine the partition coefficient for a chemical, its solubility is usually measured in a mixture of water and octanol, a solvent that mimics the hydrophobicity of the lipid bilayer. The most common method of determining the partition coefficient is the shake flask assay. In this assay, a known amount of a chemical compound is dissolved in a mixture of water and octanol, typically in a separatory funnel. The aqueous and hydrophobic layers are then allowed to separate, each solubilizing some portion of the compound, as shown in Reaction 1:

Reaction 1 The concentration of the chemical in each layer can then be measured to yield an equilibrium constant for this reaction, known as the partition coefficient ( P ):

     Hydrophobicity is a chemical property that helps determine a compound’s physiological impact. This property is often quantified using the partition coefficient ( P ), which describes how soluble a compound is in aqueous and hydrophobic environments. In practice, chemical compounds consumed orally must be hydrophobic enough to be absorbed through the lipid bilayers of the small intestine, while still being hydrophilic enough to be soluble in the bloodstream. Additionally, the hydrophobicity of a chemical determines how it is distributed throughout the body. Highly hydrophobic compounds tend to accumulate in the tissues and take longer for the body to eliminate. Therefore, they are more likely to be toxic and have long-term adverse effects.

     A research group studying the toxicity of various components of cigarette smoke decided to measure the partition coefficients for several chemicals found in cigarette smoke condensate. The results are shown in Table 1.

Table 1 Partition coefficient data for components of cigarette smoke condensate

     To determine the partition coefficient for a chemical, its solubility is usually measured in a mixture of water and octanol, a solvent that mimics the hydrophobicity of the lipid bilayer. The most common method of determining the partition coefficient is the shake flask assay. In this assay, a known amount of a chemical compound is dissolved in a mixture of water and octanol, typically in a separatory funnel. The aqueous and hydrophobic layers are then allowed to separate, each solubilizing some portion of the compound, as shown in Reaction 1:

Reaction 1 The concentration of the chemical in each layer can then be measured to yield an equilibrium constant for this reaction, known as the partition coefficient ( P ):

     Since the value of P is often very large, it is usually expressed as log( P ), as shown in Table 1.