Boiling Point, Vapor Pressure & Key Physical Properties
of 2-Methyl-2,4-Pentanediol (CAS 107-41-5)
A complete physicochemical data guide for hexylene glycol - why each property value matters, its structural basis, and its direct impact on formulation performance.
📋 Table of Contents
- Master Properties Table
- Boiling Point: 197–198 °C
- Melting Point: −50 °C
- Vapour Pressure: <0.1 hPa at 20 °C
- Density: 0.921 g/cm³
- Viscosity: ~36 mPa·s at 25 °C
- Refractive Index: 1.4225 (nD²⁰)
- Flash Point: 88 °C & Flammability
- LogP (Octanol/Water Partition): 0.58
- Water Miscibility & Solubility
- Thermal & Spectroscopic Properties
- Comparative Properties vs Similar Glycols
- Frequently Asked Questions
1 📊 Master Properties Table
The table below consolidates all key physicochemical data for 2-methyl-2,4-pentanediol (CAS 107-41-5) in a single reference. Each property is discussed in detail in the sections that follow.
| Property | Value | Conditions | Significance |
|---|---|---|---|
| Boiling Point | 197–198 °C | 101.3 kPa (1 atm) | Low volatility; long open time in coatings |
| Melting Point | −50 °C | 101.3 kPa | Liquid at all normal storage temperatures |
| Density | 0.921 g/cm³ | 20 °C | Lighter than water; key for volume/weight conversion |
| Refractive Index (nD) | 1.4225 | 20 °C, sodium D line | Standard QC purity parameter |
| Dynamic Viscosity | ~36 mPa·s | 25 °C | Pumpable; moderate thickening in formulations |
| Vapour Pressure | <0.1 hPa | 20 °C | Negligible inhalation risk; slow evaporator |
| Flash Point | 88 °C (closed cup) | Pensky-Martens | Combustible, not flammable at ambient temperature |
| Auto-ignition Temperature | ~306 °C | 101.3 kPa | High self-ignition threshold; safe for heated processing |
| LogP (octanol/water) | 0.58 | 25 °C, experimental | Balanced amphiphilicity; partitions into polymer phase |
| Water Miscibility | Complete (all proportions) | 20 °C | Fully compatible with aqueous formulation systems |
| Surface Tension | ~29 mN/m | 25 °C | Moderate wetting; lower than water (72 mN/m) |
| Dielectric Constant (ε) | ~25 | 25 °C (estimated) | Moderate polarity; dissolves both ionic and non-ionic species |
| Evaporation Rate (vs. n-BuAc = 1) | <0.01 | 25 °C | Extremely slow evaporator; excellent open-time extender |
| Flammability Limits (LEL/UEL) | ~1.1% / ~8.0% v/v | Above flash point temp. | Not relevant at ambient temperature storage |
| Specific Heat Capacity | ~2.3 J/(g·K) | 25 °C (estimated) | Relevant for heat transfer calculations in process design |
📌 Data from NIST WebBook CAS 107-41-5, PubChem CID 11593, and Sinolook Chemical technical datasheet. For COA and specification, visit our product page →
2 🌡️ Boiling Point: 197–198 °C
The boiling point of 2-methyl-2,4-pentanediol is 197–198 °C at 101.3 kPa, placing it among the highest-boiling commonly-used glycol solvents while remaining well below the thermal decomposition temperatures encountered in standard industrial processing.
Why Is the Boiling Point So High?
Two structural features drive the elevated boiling point: the presence of two hydroxyl groups, each capable of forming intermolecular hydrogen bonds, requires considerably more thermal energy to disrupt the liquid structure and achieve vaporisation than would be expected from the molecular weight alone. For comparison, hexane (MW 86, no –OH groups) boils at just 69 °C - MPD's two –OH groups add approximately 130 °C to the boiling point.
| Compound | MW (g/mol) | –OH Groups | Boiling Point (°C) | Key Application |
|---|---|---|---|---|
| MPD (hexylene glycol) ⭐ | 118.17 | 2 | 197–198 | Coatings coalescent, cosmetics |
| Propylene glycol | 76.09 | 2 | 188 | Food, pharma, cosmetics |
| Butylene glycol | 90.12 | 2 | 207 | Cosmetics |
| Ethylene glycol | 62.07 | 2 | 197 | Antifreeze, polyester |
| Glycerin | 92.09 | 3 | 290 (dec.) | Humectant, pharmaceutical |
Practical Significance in Formulation
🔹 Coatings open time: The high boiling point is the primary reason MPD extends open time in waterborne architectural paints - it remains in the wet film as a "slow-evaporating reservoir" long after the bulk water has left.
🔹 Hot-process compatibility: MPD is thermally stable throughout standard emulsification and manufacturing processes (70–85 °C) with no meaningful evaporative loss.
🔹 Safe at elevated temperatures: Even in heated storage tanks and process vessels operating up to 80 °C, MPD remains well below its boiling point with no vapour pressure concerns.
3 ❄️ Melting Point: −50 °C
The melting point of MPD is approximately −50 °C, meaning it remains a liquid across the entire range of temperatures encountered in storage, transport, and use - from cold-climate winters to tropical warehouse conditions. This is a major practical advantage over related glycols with higher melting points.
💡 Why MPD Has Such a Low Melting Point
The branched molecular geometry of MPD disrupts the close packing of molecules in the solid state. Straight-chain molecules (like 1,6-hexanediol, mp ~41 °C) can pack efficiently into a crystalline lattice, releasing sufficient lattice energy to raise the melting point. MPD's methyl branch at C-2 prevents this efficient packing, resulting in a very low melting point despite the molecule's two hydrogen-bonding –OH groups. This same branching is why MPD remains a low-viscosity liquid even at sub-zero storage temperatures.
Contrast with related diols that are solids at room temperature: 1,6-hexanediol (mp ~41 °C) and TMPD (mp ~55 °C) both require heated tanks, traced pipes, and warming equipment before use in waterborne systems. MPD requires none of this - it can be pumped, metered, and handled at ambient temperature year-round, significantly reducing infrastructure and energy costs for large-scale users.
4 💨 Vapour Pressure: <0.1 hPa at 20 °C
The vapour pressure of MPD at 20 °C is below 0.1 hPa (Pascal) - effectively negligible under ambient conditions. This is one of MPD's most commercially important physical properties, with implications spanning workplace safety, formulation performance, and environmental compliance.
😷 Inhalation Risk
With VP <0.1 hPa, air concentrations above the estimated 50 ppm exposure guideline cannot be achieved at ambient temperature without heating. General ventilation is sufficient for most handling scenarios.
🎨 Coatings Open Time
The low VP is the mechanistic basis for MPD's open-time extension function in coatings - it simply does not evaporate quickly enough to leave the wet film during the workable application window.
🌿 VOC Contribution
Despite being classified as VOC under the EU Decopaint Directive (BP ≤250°C), MPD's actual atmospheric contribution from coatings is minimal due to its very slow evaporation rate vs. lighter solvents.
📦 Storage Stability
Drums and IBCs stored in warehouses experience negligible concentration drift from evaporation, even in hot climates. Long-term storage quality is preserved without special sealed-vessel requirements.
Vapour Pressure vs Temperature
While MPD's vapour pressure is negligible at 20 °C, it increases significantly when heated. The following approximate values illustrate the temperature dependence:
| Temperature (°C) | Approx. Vapour Pressure (hPa) | Practical Implication |
|---|---|---|
| 20 °C (ambient storage) | <0.1 | No inhalation concern; general ventilation sufficient |
| 50 °C (warm storage / summer) | ~0.5–1.0 | Still very low; slight odour may be detectable |
| 80 °C (hot-process manufacturing) | ~5–10 | Local exhaust ventilation recommended for open vessels |
| 120 °C (elevated process) | ~50–80 | Significant vapour generation; LEV required; approach flash point |
Reference thermodynamic data available from the NIST WebBook Antoine equation parameters for CAS 107-41-5 →
5 ⚖️ Density: 0.921 g/cm³ at 20 °C
The density of MPD at 20 °C is 0.921 g/cm³ - slightly lighter than water (0.998 g/cm³) and meaningfully different from other common glycol solvents which are typically denser than water (propylene glycol: 1.036 g/cm³; butylene glycol: 1.005 g/cm³).
🔹 Volume-to-weight conversion: 1 litre of MPD weighs 921 g, or ~0.921 kg. For drum sizing: a standard 200 kg drum contains approximately 217 litres of MPD.
🔹 Formulation density impact: When replacing propylene glycol (density 1.036) with MPD at equal weight, the formulation volume increases slightly - and when replacing at equal volume, the weight decreases. Formulators switching between glycols should recalculate batch weights accordingly.
🔹 Phase separation check: In systems where MPD might separate from an aqueous phase, it would float rather than sink - relevant for settling and stability assessment in crude blends.
Density shows a mild negative temperature coefficient: at 25 °C, density drops to approximately 0.917 g/cm³. At 40 °C, approximately 0.909 g/cm³.
6 🌊 Viscosity: ~36 mPa·s at 25 °C
The dynamic viscosity of MPD at 25 °C is approximately 36 mPa·s (centipoise) - considerably lower than glycerin (~950 mPa·s), similar to propylene glycol (~40 mPa·s), and much lower than butylene glycol (~71 mPa·s). This moderate viscosity profile is a significant practical advantage.
🔬 Structural Basis for Moderate Viscosity
Despite having two –OH groups capable of hydrogen bonding, MPD's viscosity is much lower than expected because the intramolecular hydrogen bond between the C-2 and C-4 hydroxyl groups (a six-membered pseudo-ring) partially satisfies the H-bonding capacity internally. This reduces the extent of intermolecular H-bonding that would otherwise build viscous networks, giving MPD its unusually low viscosity for a branched diol of this molecular weight.
🔹 Processing benefit: At 36 mPa·s, MPD is easily pumpable using standard centrifugal or gear pumps without heating, and meters accurately through flow meters and metering pumps at ambient temperature.
🔹 Formulation impact: Addition of MPD to aqueous formulations at 2–5% increases the viscosity of the water phase modestly (~5–15% increase on a water baseline), which can contribute positively to the body and application feel of cosmetic toners and essences.
🔹 Temperature dependence: Viscosity decreases significantly with temperature - at 50 °C, viscosity drops to approximately 12–15 mPa·s, making hot-process handling even easier.
7 🔭 Refractive Index: 1.4225 (nD²⁰)
The refractive index of MPD at 20 °C using the sodium D line is nD²⁰ = 1.4225. This is one of the primary quality control parameters used to verify the identity and purity of incoming MPD shipments in industrial and cosmetic QC laboratories.
| Grade | Purity (GC) | Refractive Index Spec (nD²⁰) | Typical Application |
|---|---|---|---|
| Industrial grade | ≥98.0% | 1.420–1.425 | Coatings, inks, hydraulic fluids |
| High-purity industrial | ≥99.0% | 1.421–1.424 | Synthesis, polyols |
| Cosmetic/pharma grade | ≥99.5% | 1.4215–1.4235 | Personal care, sensitive applications |
💡 A refractometer measurement takes less than 30 seconds and requires only 2–3 drops of sample - making refractive index the fastest, cheapest, and most practical routine QC test for incoming MPD shipments. Any value falling outside the 1.420–1.425 range at 20 °C warrants further investigation by GC analysis.
8 🔥 Flash Point: 88 °C & Flammability Data
MPD has a closed-cup flash point of 88 °C, classifying it as a GHS Flammable Liquid Category 4 (combustible liquid) under the Globally Harmonised System. The flash point is the minimum temperature at which the liquid produces sufficient vapour to form an ignitable mixture with air above its surface.
🔥 Flammability Classification Summary
🔸 Flash point: 88 °C (closed cup) - above typical ambient temperatures; NOT a flammable liquid at room temperature
🔸 GHS classification: Flammable Liquid Cat. 4 - H227 (Combustible liquid)
🔸 NFPA classification: Class IIIB (USA) - flash point above 60 °C
🔸 Storage classification: Not classified as flammable for most storage regulations at ambient temperature
🔸 Auto-ignition temperature: ~306 °C - well above normal process temperatures
🔸 Explosive limits: LEL ~1.1%, UEL ~8.0% v/v - only applicable when heated above flash point
The flash point of 88 °C provides a meaningful safety advantage over lower-flash-point glycol ethers (e.g. EGBE: 60 °C) that require more stringent storage and handling controls. For the complete safety profile: 2-Methyl-2,4-Pentanediol Safety: SDS, Handling & Regulatory Compliance →
9 ⚖️ LogP (Octanol/Water Partition Coefficient): 0.58
The LogP value of 0.58 for MPD is arguably its most strategically important physical property - it is the number that explains why MPD can simultaneously dissolve in water and interact with non-polar organic phases, and why it outperforms more hydrophilic glycols in several key applications.
LogP is defined as the logarithm of the ratio of the compound's concentration in 1-octanol versus water at equilibrium. A LogP of 0.58 means MPD slightly prefers the organic phase over the aqueous phase - it is more oil-like than water-like, despite being fully miscible with water in all proportions.
| Glycol | LogP | Implication for Formulation |
|---|---|---|
| MPD ⭐ | 0.58 | Partitions into polymer phase → effective coalescent; dissolves non-polar actives and fragrances |
| Propylene glycol | −0.92 | Strongly hydrophilic - stays in water phase; poor coalescent; weak fragrance solubiliser |
| Butylene glycol | −0.23 | Slightly hydrophilic; moderate fragrance solubilisation; weaker coalescent than MPD |
| Ethylene glycol | −1.36 | Very hydrophilic; minimal organic phase partitioning; antifreeze applications |
| 1,6-Hexanediol | 1.02 | Lipophilic; limited water solubility; excellent polyol for polymer synthesis |
10 💧 Water Miscibility & Solubility
MPD is completely miscible with water in all proportions at 20 °C. This is a fundamental enabling property for all aqueous formulation applications - it means MPD can be added to water phases at any desired concentration without causing haze, phase separation, or emulsification requirements.
Despite its LogP of 0.58 (suggesting mild lipophilicity), MPD achieves complete water miscibility because:
🔹 Two –OH groups provide sufficient hydrogen bond donor/acceptor capacity to maintain water miscibility even as the six-carbon chain provides hydrophobic character
🔹 The 1,3-diol geometry (vs. 1,2-diol in propylene glycol) actually slightly reduces intramolecular H-bonding strength, leaving more –OH groups available for H-bonding with water molecules
🔹 The branched structure prevents close molecular packing that might otherwise create hydrophobic domains
MPD is also miscible with most common organic solvents - ethanol, isopropanol, acetone, ethyl acetate, toluene, and chloroform - making it a versatile coupling agent in multi-solvent formulations.
11 🔭 Thermal & Spectroscopic Properties
| Property | Value | Notes / Source |
|---|---|---|
| Heat of Vaporisation (ΔHvap) | ~54 kJ/mol (estimated) | Calculated from Trouton's rule; high value consistent with strong H-bonding |
| IR: –OH stretch | 3200–3400 cm⁻¹ (broad) | Characteristic H-bonded diol absorption; NIST WebBook IR library |
| IR: C–O stretch | 1050–1150 cm⁻¹ | Two C–O bonds from primary/secondary alcohol groups |
| MS base peak (EI) | m/z 59 [(CH₃)₂C=OH⁺] | Diagnostic fragment from tertiary C-2 alcohol; m/z 100 (M−H₂O) also present |
| ¹H NMR: gem-dimethyl singlets | δ ~1.19 and ~1.21 ppm (CDCl₃) | The two methyl groups at C-2 - most diagnostic NMR signal |
| Thermal stability | Stable to >180 °C (prolonged); decomposition onset ~220–250 °C | Safe for all standard hot-process manufacturing (<100 °C) |
12 📊 Comparative Properties vs Similar Glycols
The following summary positions MPD's physical properties relative to five structurally related compounds, highlighting where it occupies a unique performance niche.
| Property | MPD ⭐ | PG | BG (1,3) | EG | Glycerin | 1,6-HDO |
|---|---|---|---|---|---|---|
| BP (°C) | 197–198 | 188 | 207 | 197 | 290 | 250 |
| MP (°C) | −50 | −60 | ~7 | −13 | 18 | ~41 ⚠️ |
| VP (hPa, 20°C) | <0.1 | 0.13 | <0.1 | 0.08 | <0.01 | <0.01 |
| Density (g/cm³) | 0.921 | 1.036 | 1.005 | 1.113 | 1.261 | ~0.96 (melt) |
| Viscosity (mPa·s, 25°C) | ~36 | ~40 | ~71 | ~16 | ~950 | ~22 (melt) |
| LogP | 0.58 | −0.92 | −0.23 | −1.36 | −1.76 | 1.02 |
| Water miscibility | ✅ Complete | ✅ Complete | ✅ Complete | ✅ Complete | ✅ Complete | ⚠️ Limited |
MPD's unique profile - liquid at room temperature, very low vapour pressure, complete water miscibility, balanced LogP, and moderate viscosity - is shared by no other single glycol in this comparison set. It is this combination that explains its adoption across such a diverse range of industries. For the full formulation comparison, see: 2-Methyl-2,4-Pentanediol vs Other Glycol Solvents →
13 ❓ Frequently Asked Questions
Q: What is the boiling point of hexylene glycol in Fahrenheit?
A: The boiling point of hexylene glycol (2-methyl-2,4-pentanediol, CAS 107-41-5) is 197–198 °C, which corresponds to 387–388 °F at atmospheric pressure (101.3 kPa). This high boiling point is responsible for the compound's low volatility and slow evaporation rate at ambient conditions.
Q: Does hexylene glycol freeze or solidify in cold storage?
A: No. With a melting point of −50 °C, hexylene glycol remains a liquid at all realistic storage and transport temperatures. Even in arctic cold-chain logistics, it will not freeze or solidify. This is a major practical advantage over 1,6-hexanediol (mp ~41 °C) and TMPD (mp ~55 °C), both of which are solid at room temperature and require heated handling systems.
Q: Why is the vapour pressure of hexylene glycol so low despite its relatively low molecular weight?
A: The low vapour pressure results from the strong intermolecular hydrogen bonding between the two –OH groups, which must be overcome before molecules can escape the liquid phase. Despite only having 118 g/mol molecular weight, MPD behaves much more like a high-MW compound in terms of volatility because the energy barrier to vaporisation (related to ΔHvap) is disproportionately high due to H-bonding. The partial intramolecular H-bond (C-2 –OH to C-4 –OH) also reduces the availability of free –OH for intermolecular bonding, giving the bulk liquid a slightly lower vapour pressure than would otherwise be expected.
Q: What refractive index should I expect from a genuine MPD sample?
A: A genuine, commercially pure MPD sample at ≥98% GC purity should show nD²⁰ = 1.420–1.425. The cosmetic/pharmaceutical grade (≥99.5%) should be within the tighter range of 1.421–1.424. Any reading outside 1.418–1.427 at 20 °C should be investigated - it may indicate water contamination (shifts reading down), heavy impurities (shifts up), or a wrong substance. Always calibrate your refractometer with distilled water (nD²⁰ = 1.3330) before measuring.
Q: How does the density of hexylene glycol affect dosage calculations in coatings?
A: Because MPD (density 0.921 g/cm³) is lighter than water and lighter than propylene glycol (1.036 g/cm³), switching from PG to MPD at equal weight percentage increases the volume contribution of the glycol component. If your formulation specifies "5% by weight PG", and you switch to MPD at "5% by weight", you will be adding a slightly larger volume of liquid. For most coatings this difference is negligible, but for precision formulations, always convert between glycols using weight percentages rather than volume measurements to ensure consistent formulation.
📚 Related Articles in This Series
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