DPG vs IPM: two odorless solvents that share a catalogue entry, with all four numbers at opposite extremes

About a 4 minute read.

Too long, didn't read

• On paper they're twins: both odorless (strength field: none), both logged at 400 hours, both standard diluents
• Four numbers pull them apart: logP −0.60 vs +7.20, vapour pressure 0.0319 vs 0.000329 mmHg (97-fold), specific gravity 1.02 vs 0.84, and water: fully miscible vs barely soluble
• The 1,465 materials with odor profiles have a median logP of +3.0; DPG is too hydrophilic, IPM too lipophilic — each sits outside one edge of the distribution
• Dermatology's official name for IPM is penetration enhancer (Eichner 2017, neutron diffraction + NMR)
• Alcohol perfumes and water-based products: DPG. Oil-based products, roll-ons, candles: IPM
• Bonus: TEC's hydrophilicity sides with DPG, its vapour pressure with IPM — a hybrid

The catalogue files them in the same slot

Before writing this I assumed I could explain the difference between DPG and IPM. I opened the catalogue and found I couldn't: both strength fields say none, both longevity fields say 400 hours, both have thirty-to-fifty suppliers, both usage fields say "diluent, carrier". Like one material wearing two labels.

Only with the database fields side by side does it become clear they live at opposite ends of property space:

	DPG (#13450)	IPM (#33746)	TEC (#40008)
logP	−0.60	+7.20	0.10
Vapour pressure, mmHg @25 °C	0.0319	0.000329	0.000175
Water solubility	miscible	barely soluble	6.5 g/100 mL
Specific gravity	1.019–1.021	0.840–0.860	1.135–1.139
Longevity record	400 hr	400 hr	216 hr
GHS	H302	H302	H332

(Only DPG's vapour pressure is a measurement; IPM's and TEC's carry (est) — what that marker means has its own article.)

A logP gap of 7.8: neither resembles a fragrance material, in opposite directions

LogP is a log-scale measure of lipophilicity; a difference of 1 means tenfold. From DPG's −0.60 to IPM's +7.20 is 7.8 log units, roughly a sixty-million-fold difference in partitioning tendency.

That gap only means something against the distribution of fragrance materials themselves. This database holds 1,465 materials with both an odor classification and a logP: median +3.0, with 94.3% above +1. Fragrance materials are a crowd of moderately oil-loving molecules. DPG stands outside the hydrophilic edge, IPM outside the lipophilic one. Neither lives where the materials live.

In practice this decides who mixes with whom. DPG is fully miscible with water, so a DPG dilution folds straight into shower gels, room sprays, anything aqueous; IPM separates on contact. Flip it around and IPM is family to the oil phase in jojoba or coconut massage oils and candles, where DPG clouds and splits. The database's solubility field for DPG even lists isopropyl myristate itself — the two solvents dissolve each other, which is not the same as doing the same job for your formula.

The 97-fold vapour pressure gap: who stays behind

DPG's vapour pressure is 0.0319 mmHg; IPM's is a hundredth of that. This number already starred in the sprayer-clog article: after the alcohol flashes off, what stays on the nozzle as lubricant is IPM — DPG leaves too.

Reuse the number in another scene. Leave a blotter overnight and DPG has largely exited, while IPM is still on the paper alongside your base notes. Part of what you smell as "longevity" is the stage set by the solvent, not the material's own curtain call. Both longevity fields say 400 hours, but for something odorless, "lasting" and "smellable" are different things.

IPM's other identity: penetration enhancer

Search the cosmetic-science literature for isopropyl myristate and the phrase that comes back is penetration enhancer. In 2017 Eichner and colleagues used neutron diffraction and deuterium NMR to watch what IPM does to a stratum corneum lipid model membrane (PMID 28132900): IPM supported the formation of a single short-periodicity phase, and its molecules stayed isotropically mobile at every temperature tested. In plain terms, it wedges into the skin-lipid packing and loosens it, making things easier to pass through.

For perfumery the point is: the solvent shapes not just how materials sit in the bottle but how they move once on skin. Politano and colleagues measured human dermal irritation thresholds for the same fragrance esters across different vehicles in 2006 (PMID 16717034): in pure DEP the no-effect level was 0.33%; cut the DEP 1:3 with ethanol and it fell to 0.12%. Same material, different vehicle, a 2.75-fold shift in the safety boundary. Their vehicles did not include DPG or IPM, so treat this as directional evidence: the carrier is a safety parameter, not a backdrop.

TEC is a hybrid

Some people end up with a third odorless solvent, TEC (triethyl citrate). The table places it quickly: logP 0.10 puts its hydrophilicity on DPG's side; vapour pressure 0.000175 puts its staying power on IPM's side. At 1.14 it's the densest of the three.

In the 36-line crème brûlée formula, 405 of the 496 parts of solvent were TEC. It's the mainstream carrier in the flavour world; on the perfume side most people meet the other two first.

How to choose

If the formula ends up in anything watery or alcoholic — which covers most perfume work — use DPG.

If the base is oil (massage oils, roll-ons, candles), use IPM.

Anyone working both sides ends up with two 10% bottles of the same material that look identical, one of which will cloud the moment it hits alcohol. Write the solvent on the label. Labels are cheaper than memory.

What this article has not established

• "Odorless" is a catalogue statement (strength field: none). Industrial lots can carry faintly odorous impurities; I haven't verified batches.
• Eichner 2017 used a stratum corneum lipid model membrane, not living skin. Mechanistic evidence, not clinical numbers.
• Politano 2006 tested DEP and ethanol as vehicles, not DPG or IPM. The conclusion that carriers move the safety boundary extrapolates in direction, not in magnitude.
• IPM's and TEC's vapour pressures are estimates; the 97-fold and 182-fold gaps carry estimation error.