Loud on the blotter, gone two steps away — diffusion does not come from adding fixatives

A thread on r/DIYfragrance argued about something basic.

Asked how to get diffusion, one reply said the key is learning to work with fixatives — that they determine whether a perfume spreads around a room and how long it lasts — and suggested adding Iso E Super, Galaxolide or vanillin to an essential oil blend.

Another disagreed flatly: fixatives in the sense you are using the term are a myth. There is no such thing that one could learn to work with that will imbue a perfume with diffusion.

That argument can be settled with data.

A note about this thread. Several users identified the original post as AI-written; the top-voted reply (62 points) asks exactly that, citing "repeat threes" and "a negative followed by a positive" as the tells. So this article does not cite the post's account of its own experience, only the human replies and the disagreement. Incidentally, we have measured those two features ourselves: across 125 professionally written human articles, rule-of-three appears in 56% and contrastive framing in 38%. That detector would flag more than half of genuine human writing.

The short version

• Diffusion and longevity are two ends of one physical property, and you cannot have both. Across the 1,319 materials carrying both a vapour pressure and a longevity record, the log-log correlation is r = −0.625.
• The quartile ladder is perfectly monotone: the most volatile quarter lasts a median of 4 hours, the least volatile 216.
• And the strongest materials are also the most volatile. Materials needing dilution to 0.01% before evaluation have a median vapour pressure of 1.724 mmHg; those evaluated at 50% or below sit at 0.00265 mmHg — three orders of magnitude apart.
• So the argument resolves: adding fixatives cannot manufacture diffusion. What makes a material a fixative is precisely its low vapour pressure.

About a 9 minute read.

What vapour pressure buys, and what it costs

Vapour pressure is a material's tendency to leave the liquid and enter the air at a given temperature. For a molecule to reach a nose two metres away, it first has to get into the air.

The database holds 1,319 materials with a supplier that carry both a vapour pressure and a blotter longevity record. Correlating the logarithms:

r = −0.625

Negative, and not weakly. In quartiles:

Vapour pressure group	Median VP	Median longevity
Highest (most volatile)	1.163 mmHg	4 hours
Second	0.08 mmHg	24 hours
Third	0.011 mmHg	104 hours
Lowest (least volatile)	0.001 mmHg	216 hours

Monotone across all four, spanning a 54-fold difference in longevity.

This is why "add fixatives for diffusion" does not work physically. Choosing a material as a fixative means choosing the low-vapour-pressure end, and low vapour pressure is exactly why it will not enter the air in quantity. It can extend how long you smell something. Whether anything exists two metres away is a different question.

We measured this line once already using boiling point, where the question was whether boiling point can rank longevity. This uses vapour pressure and asks something else.

The counterintuitive half: strong things also move fast

Given only the table above, you might conclude that diffusion requires fast top-note material, which cannot last, so diffusion is doomed to be brief.

Not quite, because a second variable exists: potency. A material with modest vapour pressure but overwhelming strength needs only a trace in the air to register.

Using the database's "recommend smelling in a X% solution or less" field as a proxy for strength, grouped by vapour pressure:

Recommended evaluation dilution	Materials	Median VP
0.01% or less	9	1.724 mmHg
0.10% or less	44	0.311 mmHg
1.00% or less	195	0.123 mmHg
10.00% or less	498	0.061 mmHg
50.00% or less	12	0.00265 mmHg

The direction runs against intuition: the more a material must be diluted before you can evaluate it, the higher its vapour pressure. The strongest group's median sits roughly 650 times above the weakest group's.

The first row holds only 9 materials, so treat that 1.724 as indicative rather than precise. The middle three rows (44, 195, 498) carry the trend, and they point the same way.

I also checked how rare the ideal combination — slow and overwhelming — actually is. Among the 50 materials needing 0.1% or less that also carry a vapour pressure, only 3 sit below 0.001 mmHg (maple furanone, cortex pyridine, diisoamyl thiomalate).

So diffusion is volatility times potency, and in this data those two travel together. A fixative sits at the far end of both axes.

What the human replies got right

Beyond the argument, several concrete observations in that thread are worth keeping.

One: every sandalwood material — bacdanol, sandalore, even javanol — disappointed on a scent strip at every dilution tested, and all of them work well in the air.

Another: using lower doses, even diluted to 10%, diffusiveness went up.

A third supplied the vocabulary: separate "direct impact" (the strength you meet with your nose close to the strip) from "diffusion" (how it carries as it evaporates), and record both in your evaluation notes.

Those three together are the practical version of this article. A blotter measures direct impact, and direct impact and diffusion are different quantities. Loud on paper and absent in the room is not the paper lying. It is you using one ruler to measure something else.

Why "more concentration means more diffusion" also fails

The commenter who found lower doses more diffusive proposed that at high doses the material saturates itself. I found no literature for that mechanism so I will not repeat the explanation, though the phenomenon is common enough in formulation, and a more conservative statement is available:

What you smell is the concentration in the air, not the concentration in the bottle. Evaporation sits between them, and evaporation is not linear. Taking a material from 5% to 15% does not triple its partial pressure in the air, particularly in a mixture, where every material's evaporation is affected by the others.

Almeida and colleagues, in the International Journal of Cosmetic Science in 2021, did the measurement work on exactly this. They determined vapour pressures for fragrance molecules by thermogravimetric analysis, derived Antoine constants and Clarke and Glew parameters, and measured permeability coefficients for 14 fragrance molecules in ethanolic solution using Franz diffusion cells with porcine skin, analysing samples by GC/FID and HPLC/UV, then proposed a QSAR model (R² = 0.7786, standard relative deviation 0.190).

The authors state the limitation themselves: the dataset remains small compared with larger, more general QSAR models, though it is far more specific about solvent type and material class. The point stands: what happens after you spray is a question needing thermogravimetric analysis and diffusion cells, not intuition.

One more layer: what the nose is tuned to

The model so far is vapour pressure times potency. Where does potency itself come from?

Williams and Ringsdorf, in Philosophical Transactions of the Royal Society B in 2020, compared the odour thresholds and atmospheric lifetimes (with respect to the primary atmospheric oxidant, the OH radical) of a suite of volatile organic compounds.

The result is unexpected: within a given chemical family, odour threshold correlated with atmospheric lifetime better than with mass or vapour pressure. Molecules with short atmospheric lifetimes — those that react quickly in air — tend to be detected more sensitively by the human nose.

The authors examined outliers: diacetyl was an outlier among the ketones that fitted the trend once its more important photolysis lifetime was included, and the unusually low threshold of carbonyl sulfide was interpreted in terms of uptake by vegetation. They speculate that the nose's sensitivity to chemicals reactive in air is likely evolved rather than learned.

For someone composing, the implication is that "how strong is this material" is not a property you can derive from vapour pressure. It relates to how long the molecule survives in air, which is a different set of chemistry. So the strength table above is an observed association, not a mechanism.

What to actually do

One: for diffusion, put fast and potent material in the top and heart. That is the only route with physics behind it. Fixatives address a different problem.

Two: record direct impact and diffusion separately. Smell the strip close once, then leave it across the room, walk back ten minutes later and smell it again. Write both down.

Three: try lowering the dose first. The thread's observation is worth verifying yourself: build the same formula at 5% and at 15% and compare them across a room. That experiment is cheap.

Four: don't judge diffusion with a blotter. A strip is a direct-impact instrument, and it is the source of every misunderstanding in this article.

Five: add fixatives to extend, not to amplify. Both are worth doing; know which one you are doing.

What this doesn't establish

Most vapour pressure entries are marked as estimates. The field mixes measured and estimated values and I did not separate them. Read −0.625 as an order of magnitude.

Longevity hours are neat material on paper. That number does not measure skin performance, and 13.3% of materials sit at the 400-hour ceiling, which compresses variance at the top end.

"Recommended evaluation dilution" is not an odour threshold. It is a sensory recommendation from the data source, a proxy for strength rather than a measurement. The 0.01% group holds only 9 materials.

Correlation is not causation, and r = −0.625 leaves roughly sixty percent of the variance unexplained. Molecular weight, polarity and interaction with other materials all live in that sixty percent.

This does not measure diffusion itself. I measured vapour pressure and longevity and reasoned toward diffusion. Measuring diffusion properly needs headspace analysis, which is what studies like Almeida's do.

References

R. N. Almeida et al., Permeability coefficients and vapour pressure determination for fragrance materials, International Journal of Cosmetic Science, 43(2), 225–234 (2021). PMID 33452685. doi:10.1111/ics.12686

J. Williams, A. Ringsdorf, Human odour thresholds are tuned to atmospheric chemical lifetimes, Philosophical Transactions of the Royal Society B, 375(1800), 20190274 (2020). PMID 32306881. doi:10.1098/rstb.2019.0274

Related: Boiling point as a proxy, How long does this last, The 400-hour ceiling, Fixation.