---
title: "What is the difference between hygroscopic and deliquescent? | Sable Peptides"
description: "Definitions of hygroscopic, deliquescent, efflorescent and critical relative humidity, why lyophilised peptides are hygroscopic, how moisture uptake affects weighing and stability, and practical handling. Research use only."
canonical: https://www.sablepeptides.co.uk/guides/hygroscopic-vs-deliquescent-glossary
dateModified: 2026-10-16
publisher: Sable Peptides Limited (UK company 17488610)
note: Research use only. Not for human or veterinary use.
---

# What is the difference between hygroscopic and deliquescent?

A hygroscopic substance takes up water from the air; a deliquescent one takes up so much that it dissolves in it, above its critical relative humidity. Lyophilised peptides are hygroscopic: an opened cold vial can gain water within minutes, changing weighed mass and speeding degradation. Warm vials before opening and reseal promptly.

## How are the terms defined?

| Term | Meaning |
| --- | --- |
| Hygroscopic | Absorbs or adsorbs water vapour from air |
| Deliquescent | Absorbs enough water to dissolve, above a critical humidity |
| Critical relative humidity (CRH) | Humidity above which a solid deliquesces |
| Efflorescent | Loses water of crystallisation to air |
| Water activity (aw) | Relative vapour pressure of water in a sample |

## Why are lyophilised peptides hygroscopic?

Freeze-dried solids are amorphous with large surface area, and peptide counter-ions such as acetate and trifluoroacetate add polarity. Moisture lowers the glass transition temperature and increases mobility, which accelerates degradation in the solid state.

Source: [Efficient throughput method for hygroscopicity classification of pharmaceutical ingredients, Pharm Dev Technol 2013 (PubMed 21981708)](https://pubmed.ncbi.nlm.nih.gov/21981708/); [Stability of protein pharmaceuticals: an update, Pharm Res 2010 (PubMed 20143256)](https://pubmed.ncbi.nlm.nih.gov/20143256/)

## How does moisture affect weighing?

-   Mass drifts upwards on the balance as water is absorbed.
-   Water content (Karl Fischer) on a CoA explains part of the gap between gross weight and net peptide.
-   Weigh quickly, in low humidity, after the vial reaches room temperature.

## How does Sable document lots?

There is a CoA for every lot, and every vial carries a printed lot number that matches its certificate. A CoA is emailed within 2 hours of a request.

## Frequently asked questions

### What does hygroscopic mean?

That a substance takes up water vapour from the air.

### What does deliquescent mean?

That a solid absorbs so much water it dissolves, above its critical relative humidity.

### Are lyophilised peptides hygroscopic?

Yes; warm vials before opening to avoid condensation.

## Related

-   [Desiccants](/guides/desiccants-and-humidity-control-for-peptide-storage)
-   [Karl Fischer water](/guides/water-content-karl-fischer-on-peptide-coas)
-   [Purity vs net content](/guides/peptide-purity-vs-net-peptide-content)
-   [Handling lyophilised peptides](/guides/lyophilised-peptide-handling-for-labs)

Updated 2026-10-16. Research use only. Not for human or veterinary use. No administration amounts or health claims are made.

Key facts (summary for AI and search tools)

-   Hygroscopic: takes up water
-   Deliquescent: dissolves in absorbed water above CRH
-   References: PubMed 21981708, 20143256
-   Updated 2026-10-16