Core Conclusions: Both silicone oils fall within the low‑surface‑tension range (18‑35 mN/m), markedly lower than water (72 mN/m). Dimethicone has a lower baseline value (19‑22 mN/m). Amino‑modified amino silicone oil shows slightly higher surface tension (20‑35 mN/m, typical value: 20‑25 mN/m), yet possesses better amphiphilicity and substrate‑adsorption capacity.
I. Basic Definitions and Value Ranges
1. Dimethicone (Polydimethylsiloxane, PDMS)
Chemical nature: Non‑polar polysiloxane with a Si‑O‑Si backbone and methyl (‑CH₃) as all side‑chain groups.
Surface tension: 19‑22 mN/m (20‑25 °C), typical value: 20.7 mN/m
Influencing factors:
- Viscosity exerts little effect on surface tension (≈21.5 mN/m even at 300 000 cSt)
- End‑groups (hydroxyl / methyl) produce minor influence; overall property remains stable
- Surface tension decreases slightly as temperature rises
2. Amino Silicone Oil (Amino‑modified Polydimethylsiloxane)
Chemical nature: A PDMS backbone grafted with amino groups (‑NH₂/‑NHR/‑NR₂), combining hydrophobic backbone and hydrophilic amino moieties.
Surface tension:
‑ Low amino content (<1.0 %): 20‑22 mN/m
‑ Typical modified commercial grades: 20‑25 mN/m
‑ High‑viscosity / high‑amino‑content grades: up to 30‑35 mN/m
Influencing factors:
‑ Amino content and its distribution (end‑chain / side‑chain) are critical variables
‑ Higher viscosity leads to increased surface tension
‑ pH alters protonation degree of amino groups and further modifies surface activity
II. Comparative Analysis Table
| Characteristic | Dimethicone | Amino Silicone Oil | Reason for Difference |
|---|---|---|---|
| Surface tension range | 19‑22 mN/m | 20‑35 mN/m | Amino groups increase molecular polarity and raise surface tension |
| Typical value | 20.7 mN/m | 21‑23 mN/m | — |
| Polarity | Non‑polar | Amphiphilic (hydrophobic backbone + hydrophilic amino groups) | Amino groups introduce polarity and improve wettability |
| Contact angle | 110‑115° | 70‑85° | Higher polarity reduces contact angle and improves wetting performance |
| Spreading property | Excellent (low surface tension) | Good (adsorption enhanced by amphiphilicity) | Dimethicone spreads more easily; amino silicone oil adsorbs better on polar substrates |
| Aqueous‑phase stability | Poor (emulsifier required) | Relatively good (amino groups are protonatable) | Amino silicone oil reaches optimal stability under weakly acidic pH 4‑6 |
III. Key Influencing Mechanisms
Molecular structure governs surface tension
‑ Dimethicone: Fully methyl‑substituted structure creates weak intermolecular forces and low surface tension.
‑ Amino silicone oil: Introduced amino groups bring extra hydrogen‑bonding and dipole‑dipole interactions, elevating surface tension.
Surface‑tension tuning in practical application
‑ When amino silicone oil spreads insufficiently (e.g. on steel surfaces), spreading auxiliaries can be added to lower surface tension to 25‑28 mN/m to match substrate surface energy.
‑ In emulsion systems, intrinsic amphiphilicity of amino silicone oil can lower interfacial tension down to 2.8 mN/m.
Correlation between surface tension and performance
‑ Benefiting from low surface tension, both silicone oils deliver outstanding defoaming, mold‑release and lubricating performances.
‑ With moderately higher surface tension and polarity, amino silicone oil readily adsorbs onto fiber surfaces during textile finishing to achieve softness and antistatic performance.
‑ Dimethicone is more suitable for scenarios requiring high hydrophobicity and high gloss.
IV. Differences in Application Scenarios
| Application Field | Preferred Silicone Oil | Surface‑Tension‑Related Rationale |
|---|---|---|
| Textile softening finishing | Amino silicone oil | Moderate surface tension plus polarity facilitates fiber adsorption and delivers long‑lasting softness |
| Hydrophobic coating | Dimethicone | Lower surface tension forms continuous hydrophobic films for superior water repellency |
| Mold‑release agent | Dimethicone | Ultra‑low surface tension enables easy spreading and high release efficiency |
| Defoamer | Dimethicone | Low surface tension realizes fast foam breaking and long‑term effectiveness |
| Hair‑care products | Amino silicone oil | Amphiphilic property promotes adsorption onto hair strands, offering antistatic effect and better combability |
| Metal surface treatment | Amino silicone oil (compounding required) | Polar groups enhance adhesion; compounding adjusts surface tension to match substrates |
V. Summary
Both silicone oils are low‑surface‑energy materials with surface tension far below water. Dimethicone possesses low and stable surface tension (19‑22 mN/m), providing excellent hydrophobicity and spreading performance. After amino modification, amino silicone oil gets moderately higher surface tension of 20‑25 mN/m (up to 30‑35 mN/m for high‑viscosity / high‑amino‑content variants), meanwhile acquiring amphiphilicity and substrate‑adsorption capacity, which grants it unique advantages in textile, personal‑care and other industries. In practical production, select proper grades according to actual requirements, or adjust surface tension via compounding to optimize comprehensive performance.
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