A database can show logP and logD beside the same molecule. That does not make the two values interchangeable, or mean the molecule has changed identity. The difference is which chemical forms are counted, and the conditions under which they are counted.
Two ratios answering different questions
For this guide, logP refers to the base-ten logarithm of the neutral species’ n-octanol/water concentration ratio at equilibrium. LogD refers to the base-ten logarithm of the ratio of total analytical concentrations: the relevant neutral and charged forms together in each phase. The IUPAC distribution-ratio definition distinguishes total concentration from a single-form partition ratio.
Scroll sideways to see every column when needed.
| Entry | What is counted? | What must remain attached? |
|---|---|---|
| logP | The specified neutral species | Solvent pair, identity, method and temperature |
| logD at a stated pH | Total forms analysed in each phase | The pH and other experimental conditions, as well as identity and method |
| Predicted logP or logD | The model’s defined species or distribution | The model, version and any assumed pH or chemical form |
The practical issue is comparability. An unqualified “2.0” tells the next reader very little. “Calculated logD at the stated aqueous pH, using the named model” is less compact, but it describes a claim that can actually be checked.
One ingredient name can involve more than one form
Some compounds can accept or lose a proton in an aqueous environment. The proportions of neutral and charged forms can change with pH. Those forms need not distribute in the same way between the phases. Original SAMPL8 measurement research explains why distribution measurements include both forms and records the methods used.
For a simple acid, increasing aqueous pH can favour its negatively charged form; for a simple base, decreasing pH can favour its positively charged form. This is a limited chemical explanation, not a direction to alter a fragrance’s pH. Multiple ionisable sites, ion pairing and other interactions can make a real system more complicated than that introductory picture.
Do not equate the word “chemical” with “ionisable”, or assume that every essential oil has one meaningful pH-dependent distribution value. A mixture contains components with different identities. A single compound’s property is not a collective score for everything in the product.
A hypothetical concentration example
The following invented numbers illustrate the bookkeeping only. They are not data for a perfumery material, a specified pH, a recommended preparation or a real experiment. Assume the phase analysis correctly distinguishes the forms. Every concentration uses the same arbitrary molar unit, so neutral and charged analyte amounts can be added consistently.
On a narrow screen, scroll the diagram sideways to read every label.
For the neutral form alone, 10 ÷ 0.1 = 100, so the illustrative logP is 2. For the total forms, (10 + 0) ÷ (0.1 + 0.9) = 10, so the illustrative logD is 1. Nothing contradictory has happened: the denominator changed because the question changed.
Suppose an analyst reported only the neutral concentrations in one record and the total concentrations in another. Comparing those records as though both were neutral-species logP could create an apparent tenfold disagreement. Naming the measured quantity is therefore as important as copying the digits accurately.
These numbers are concentration ratios. They do not establish how much total material occupies each phase without phase volumes. The separate phase-volume example explains that additional step.
Why “at what pH?” is a necessary question
A logD value for an ionisable compound is incomplete without its aqueous conditions. The University of Tartu’s research data page, for example, labels logD measurements with their pH, solvent system, temperature and uncertainty information. That is a useful model for how to retain context, rather than a source of universal values for your materials.
Do not assume a value reported at a physiological pH is specially appropriate to a perfume. The reported pH answers a study’s question; it is not a product-design instruction. Similarly, a pH reading in a water-containing alcohol mixture needs an appropriate method and interpretation. It should not be substituted casually for a defined aqueous-phase condition in a partition study.
Simple textbook curves often assume that only the neutral form enters the organic phase. More detailed partition research discusses concentration effects and the difficulty of separating neutral and charged contributions. Treat a simplified diagram as a model with assumptions, not a guarantee of the curve every compound must follow.
A fair way to compare two entries
- Confirm identity. Match the chemical structure, relevant isomer or salt form and the supplied product’s composition where known.
- Identify the quantity. Is it neutral-species logP, total-form logD, or a differently defined reported coefficient?
- Read the conditions. Retain the solvent pair, aqueous pH for logD, temperature and method. Mark missing information explicitly.
- Separate measurement from prediction. A model result can be useful, but it should remain a model result in your records.
- Keep the question proportionate. Does the entry help compare partitioning, or are you trying to infer an unrelated property?
If the form or conditions are unclear, “not comparable from the available record” is a useful conclusion. It is more informative than silently choosing whichever number looks more familiar. Ask for the underlying reference before using the value to support a formulation decision.
What this changes in perfumery practice
This distinction mostly improves how you read and discuss data. It does not require you to classify every bottle by pH, calculate a skin-retention score or modify an existing formula. Many formulation questions are better addressed by direct compatibility information for the actual supplied preparation.
Neither logP nor logD is a permitted use level, hazard classification or finished-product assessment. Nor does either one measure odour strength. Use solubility and carrier evidence for solvent questions, and release and sensory observations for performance questions. Keep these evidence streams connected, but separate enough that one cannot masquerade as another.