Match fumed silica surface chemistry to your formulation solvent system, moisture sensitivity, and rheology targets.
Match fumed silica surface chemistry to your formulation solvent system, moisture sensitivity, and rheology targets.
3 Surface types50–380 BET range m²/g≤45 mN/m Treated surface energy
Hydrophilic fumed silica is the starting point for any water-based, alcohol-based, or polar resin system. Its untreated surface carries 2–3 silanol groups per nm², giving it strong hydrogen-bonding affinity with polar solvents. Grades like SEMISIL S200 (200 m²/g BET) or S380 (380 m²/g BET) disperse readily in waterborne coatings, adhesives, and personal care formulations at 1–5 wt% loading. Higher BET grades deliver stronger thixotropy at lower loading but require higher shear for dispersion. Choose hydrophilic when your continuous phase is water, glycol, or polyol — and when moisture pickup during storage is acceptable.
Hydrophobic fumed silica is surface-treated with dimethyldichlorosilane (DDS) or hexamethyldisilazane (HMDS) to replace surface silanols with methyl groups. This drops surface energy below 45 mN/m and cuts moisture uptake to under 0.5 wt% — critical for silicone sealants, epoxy systems, and powder coatings stored in humid climates. Typical grades offer 100–150 m²/g BET post-treatment. Use hydrophobic grades whenever your system is nonpolar (silicone oil, mineral oil, hydrocarbon resin) or when the formulation degrades on contact with moisture. HMDS-treated grades retain more residual silanols than DDS-treated, giving slightly better compatibility in semi-polar systems like polyester resins.
Polydimethylsiloxane (PDMS)-treated fumed silica is engineered specifically for antifoam compounds and silicone rubber reinforcement. The PDMS coating renders the surface strongly hydrophobic (contact angle \>110°) while providing compatibility with silicone oil carriers. In defoamer formulations, 3–8 wt% of PDMS-treated silica (100–150 m²/g BET) acts as the active particle that ruptures foam lamellae. For HTV and RTV silicone rubber, PDMS pre-treatment prevents the crepe-hardening effect that untreated silica causes over time. If your application is specifically defoaming or silicone rubber, PDMS-treated grades outperform generic hydrophobic silica because the silicone-silicone compatibility eliminates wetting issues.
Start with your continuous phase: if it is water or a polar solvent, hydrophilic is your default — see our hydrophilic fumed silica overview for grade detail. If the system is nonpolar or moisture-sensitive, move to hydrophobic; consult the HMDS treatment mechanism page to understand residual silanol trade-offs between DDS and HMDS routes. If the application is specifically antifoam or silicone rubber, PDMS-treated is the purpose-built choice. For borderline cases (e.g., UV-cure acrylates or hybrid polymer systems), run a hydrophilic-vs-hydrophobic compatibility test at lab scale — our comparison guide covers the key screening tests. Final grade selection within each chemistry depends on target BET surface area, which controls thixotropic efficiency and transparency.
The table below compares the three surface chemistries across the specifications that matter most for formulation…
The table below compares the three surface chemistries across the specifications that matter most for formulation decisions. BET values shown are typical ranges — exact values vary by manufacturer and grade series.
PropertyHydrophilicHydrophobic (DDS/HMDS)PDMS-Treated Surface silanols (OH/nm²)2–30.5–1.0BET surface area (m²/g)150–380100–150100–150Moisture uptake (wt%)1–2.5Carbon content (wt%)00.5–3.02–5pH (4% aqueous)3.7–4.55–8N/A (non-dispersible)Best solvent matchWater, glycols, alcoholsSilicone oils, hydrocarbons, epoxiesSilicone oils onlyTypical loading (wt%)1–80.3–63–25
Match surface chemistry to your solvent system first — hydrophilic for polar, hydrophobic for nonpolar or moisture-sensitive, PDMS-treated for defoamers and silicone rubber — then select BET grade for your target rheology.
Use hydrophilic fumed silica when your continuous phase is water, alcohol, glycol, or another polar solvent. The untreated silanol surface (2–3 OH groups/nm²) hydrogen-bonds with polar media, enabling efficient thixotropy at 1–5 wt% loading. Switch to hydrophobic only when moisture pickup or nonpolar solvents demand it.
DDS (dimethyldichlorosilane) replaces nearly all surface silanols, maximizing hydrophobicity. HMDS (hexamethyldisilazane) retains 0.5–1.0 residual silanols per nm², giving better wetting in semi-polar resins like polyesters. Choose DDS for silicone and hydrocarbon systems, HMDS for epoxy and polyester.
PDMS-treated silica has silicone-compatible surface chemistry that disperses homogeneously in silicone oil carriers. The high contact angle (\>110°) enables efficient foam lamella rupture. Generic hydrophobic grades can cause wetting defects because their methyl surface is not optimized for silicone oil compatibility.
Higher BET means more surface area per gram, which increases thixotropic efficiency — you need less material for the same viscosity build. A 380 m²/g grade may need only 1.5 wt% where a 150 m²/g grade needs 3 wt%. Trade-off: higher BET grades are harder to disperse and require higher shear energy.
Hydrophilic fumed silica works in epoxy resins but risks moisture absorption during storage, which can cause amine blushing or cure defects. For moisture-sensitive two-component epoxies, HMDS-treated hydrophobic grades at 2–3 wt% are safer. Hydrophilic grades are acceptable in one-pot, fast-cure epoxy systems with short shelf life.
Start at 1–2 wt% for thixotropy screening in liquid systems and 0.3–0.5 wt% for powder flow aid applications. Increase in 0.5 wt% increments until target viscosity or anti-settling performance is reached. Silicone rubber reinforcement requires much higher loading at 15–25 wt% of PDMS-treated grade.
Get Samples & TDS
Free samples for qualified buyers · reply within 24h. Tell us how you plan to use Select By Surface Chemistry.