octanolwater

Octanol-water refers to a standard biphasic liquid system used in chemistry and related fields to estimate how substances distribute between a lipophilic (organic) phase and an aqueous phase. The system consists of n-octanol and water, which are immiscible under normal conditions, and serves as a simple model for biological membranes and environmental interfaces.

The central concept is the octanol–water partition coefficient, Kow, defined as the ratio of a compound’s equilibrium concentration in octanol to its concentration in water. It is often expressed as log Kow (log10 of Kow). Kow provides a measure of hydrophobicity or lipophilicity: higher values indicate greater tendency to partition into the organic phase, while lower values indicate greater aqueous solubility. Kow is temperature dependent and generally refers to the neutral (non-ionized) form of the compound; ionized species typically show much lower partitioning.

Measurement of Kow is commonly done by the shake-flask method, where a compound is equilibrated between octanol and water, with care taken to use water-saturated octanol or octanol-saturated water to minimize solvent loss. For ionizable compounds, log Kow reflects the neutral form; at different pH levels, the partitioning is more accurately described by log D, the distribution coefficient, which varies with pH.

Applications of Kow include environmental fate modeling, assessment of bioaccumulation potential, pharmacokinetics, toxicology, and quantitative structure–activity relationship (QSAR) studies. Kow correlates with membrane permeability and sorption to organic matter, making it a foundational descriptor in risk assessment and drug design.

Limitations include its simplicity and the fact that 1-octanol is an imperfect surrogate for biological membranes. It does not account for pH effects, ionic strength, temperature variations, or specific molecular interactions.