How Silicone Adjuvants Work
Water applied from a spray tank has a surface tension of 72 mN/m. At this tension, droplets bead up on leaf surfaces and roll off without spreading — a significant problem for contact herbicides and systemic pesticides that must penetrate the leaf cuticle. Trisiloxane adjuvants (heptamethyltrisiloxane with ethylene oxide/propylene oxide modification, equivalent to Silwet L-77) reduce spray solution surface tension to 20–23 mN/m. Most waxy leaf cuticles require a wetting agent capable of achieving <25 mN/m to overcome the hydrophobic barrier.
The result is the "superspreading" effect: a droplet landing on a leaf surface spreads 10–100× its original contact area within seconds, dramatically increasing coverage per unit of spray volume and delivering more active ingredient into the cuticle before evaporation.
Adjuvant Types
| Type | Surface Tension | Spread Factor | Best For |
|---|---|---|---|
| Trisiloxane (Silwet L-77 equiv.) | 20–22 mN/m | 100–500× | Herbicides, systemic pesticides |
| Polyether silicone (non-spreading) | 25–30 mN/m | 5–15× | Fungicides, contact pesticides |
| PDMS emulsion | 35–40 mN/m | 2–5× | Tank mix compatibility |
Dosage and Tank Mix Guidelines
Trisiloxane adjuvants are used at 0.05–0.1% v/v in the spray tank, equivalent to 500–1000 mL/ha at a typical 1000 L/ha application volume. Several compatibility constraints apply. Trisiloxane adjuvants are incompatible with anionic surfactants, producing heavy foaming and phase separation. They are also incompatible with copper-based fungicides, which cause precipitation.
pH is the most critical tank-mix parameter: trisiloxane hydrolyses at pH <4 or >8, destroying spreading activity before the spray is applied. Tank mix pH should be verified and adjuvant added last, after pH adjustment. Efficacy is best in the 15–30 °C temperature range; at temperatures above 35 °C, faster hydrolysis progressively reduces activity.
Registered Use and Environmental Notes
Trisiloxane adjuvants are generally classified as non-pesticidal materials. Registration requirements for their use as adjuvants vary by country, and formulators should confirm local regulatory status before commercialization. Biodegradation of EO-modified trisiloxanes in soil produces PDMS oligomers and silicic acid within 28 days under aerobic conditions. Aquatic ecotoxicity is a concern: trisiloxane is toxic to aquatic invertebrates at concentrations >1 ppm, and appropriate buffer zones near water bodies must be maintained during spray operations.
Related Reading
Polyether Silicone Oil · Precipitated Silica as Agrochemical Carrier