ramé-hart Technical Document

Technical Bulletin Regarding Water - Why use Deionized Water over Distilled Water

August 2025

Surface scientists prefer deionized (DI) water over distilled water for contact angle measurements primarily due to higher purity and better control of ionic contamination, which directly impacts the reliability and reproducibility of surface wettability measurements. Here's why in detail:

1. Higher Purity of Deionized Water

Deionized water has had almost all of its ions and mineral contaminants removed through ion exchange processes.
Distilled water, while free from many non-volatile impurities, may still contain dissolved gases, volatile organics, or traces of ions that distill over with the steam.
For sensitive surface measurements like contact angles, even trace ionic contamination can:
- Change surface tension.
- Alter the interaction with the surface.
- Skew contact angle results.

2. Reproducibility and Consistency

Contact angle measurements are highly sensitive to small changes in:
- Surface tension of the droplet.
- Surface energy of the substrate.
DI water (especially when freshly produced or from a high-grade purification system) provides more consistent surface tension, improving measurement reproducibility.

3. Lower Organic Contaminants

DI water systems often include organic filters or UV treatment to reduce total organic carbon (TOC).
Distillation can carry over volatile organics, which may:
- Interact with the surface.
- Form films or residues that alter the contact angle.

4. Standard Practice in Surface Science

Deionized water (especially 18.2 MΩ·cm resistivity, ultrapure grade) is a standard test liquid in surface science.
It's recommended for consistent comparison across:
- Different experiments.
- Different labs.

✅ Summary

Reason	Deionized Water	Distilled Water
Ionic purity	Very high	Moderate
Organic purity	Controlled (if TOC removal used)	May contain volatiles
Surface tension consistency	High	Variable
Standard in surface science	Yes	Less common

Conclusion:

Surface scientists use deionized water for contact angle measurements because it offers higher purity, better reproducibility, and is more chemically stable, ensuring more accurate assessments of surface wettability.