A material can be noticeable near a blotter yet contribute little at a distance. Another can seem quiet at first but remain recognisable later. Describing both as “strong” hides the differences. Separate how material enters the air, how it travels and whether a person can perceive it.
Three ideas that are often bundled together
Volatility concerns the tendency to enter the vapour phase under stated conditions. Diffusion has a precise transport meaning in physical chemistry, while perfumers also use “diffusive” to describe a sensory impression of reach or presence. Tenacity is a practical description of persistence and needs a surface, environment and observation criterion.
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| Term | A useful question | Do not treat it as |
|---|---|---|
| Vapour pressure | What is the equilibrium vapour pressure at this temperature? | A direct number of hours on skin |
| Diffusion coefficient | How does molecular transport respond to a concentration gradient in this medium? | A supplier’s odour-strength rating |
| Sensory projection or reach | What is noticeable at the defined distance and time? | The laboratory diffusion coefficient |
| Tenacity | How long does the stated sensory feature remain detectable under the comparison conditions? | A permanent, context-free property of the bottle |
This vocabulary makes a formulation discussion more useful. “The woody character was still recognisable on the blotter at the final observation” says something different from “the sample filled the room immediately”. Neither observation should be rewritten as the other.
First: release into the air
The OECD vapour-pressure guideline defines a saturation pressure above a solid or liquid and treats temperature as part of the measurement. A pure substance’s vapour pressure is therefore not a standalone number detached from temperature or physical state.
A perfume is usually a mixture, and the gas above it need not have the same proportions as the liquid. Original headspace chromatography research measured changes in vapour composition during evaporation of perfumery mixtures. “Headspace” simply means the gas region being sampled above or around the source; it does not mean that every liquid component contributes equally.
As a mixture loses components, the remaining liquid can change. This is one reason an opening impression and a later impression should not be treated as two readings of an unchanged system. Do not infer an exact evaporation curve from the ingredient names alone.
On a narrow screen, scroll the diagram sideways to read every label.
Second: movement through air
The IUPAC diffusion coefficient relates molecular flux to a concentration gradient. It describes transport in a specified medium. It is not an odour descriptor, and a larger chemical diffusion coefficient is not a general definition of a more impressive fragrance.
Air movement also carries vapour. A room with a draught, an enclosed comparison space and the air close to a blotter are not equivalent environments. If the observation is about perceived reach, keep the observer’s position and surroundings consistent enough to make that comparison meaningful.
Research such as Engineering perfumes combines liquid-mixture behaviour, gas transport and odour thresholds in a model. The important lesson for a reader is the need for several inputs. It is not evidence that one material’s logP can replace all of them.
Third: the nose is not a concentration meter
Detectability depends on more than the amount in air. Fragrance-mixture transport and odour-performance research considers odour thresholds alongside physical properties. A component’s concentration and the concentration needed for detection answer different questions.
A noticeable change can therefore arise from a change in relative contributions, not simply a larger total amount of fragrance. Keep “more noticeable”, “more pleasant”, “more recognisable” and “more material present” as separate statements. None automatically proves the others.
“I cannot smell it now” also needs context: which feature, at what distance, compared with what earlier observation? Use the description to record an observation rather than declaring that no molecules remain. Conversely, detectable scent does not establish a safe exposure or an acceptable use level.
Persistence belongs to a surface and a time frame
Substrate matters. An original study of fragrance evaporation from skin and model surfaces used headspace analysis to compare release. Its specific experimental system should not be turned into a universal prediction for all perfumes, surfaces or people.
A blotter observation is a blotter observation. It should not be advertised as a guaranteed duration on skin, clothing or another surface. Similarly, a supplier’s tenacity description needs its stated test basis before it can support a precise comparison.
LogP uses an octanol/water system. It contains no air phase, test surface, elapsed time or sensory threshold. That is why “high logP means it stays on skin rather than entering air” is too strong as a general rule. A physical descriptor can be one input in an appropriate model; it is not the finished performance result.
Ask a sharper performance question
- Opening presence: compare the specified early impression, without relabelling it as persistence.
- Character over time: identify the feature being followed, rather than recording only “still there”.
- Reach: specify observer distance and environment, rather than switching between close and room observations.
- Substantivity: identify the surface and the meaning of “remaining”, whether analytical or sensory.
- Evidence quality: distinguish supplier descriptions, your observations and instrument-based studies.
These questions do not require a material-ranking chart. They help you decide which comparison would answer the actual creative problem. For a fuller sensory framework, use reading a fragrance over time. To understand the narrower physical descriptor that is often confused with performance, return to logP explained for perfumers.