Surface Tension and Wettability
Surface tension is the force that makes a liquid tend to reduce its surface area; wettability is its ability to spread over a solid.
Molecules inside a liquid are surrounded by others that attract them in all directions; those at the surface are not, and that difference creates a tension that tends to minimise the area. Water has a high surface tension (about 72 mN/m at 20 °C), so on many surfaces it forms droplets instead of spreading. Wettability is measured by the contact angle between the droplet and the solid: the smaller the angle, the better the liquid wets. It depends on both the liquid and the surface; greasy or plastic surfaces wet less well than clean glass or metal.
Surfactants concentrate at the surface of the water and reduce its surface tension. As a result, the solution spreads better, penetrates pores and joints, reaches the interface between the soil and the substrate and helps the soil to come away and be emulsified. The same principle explains rinse additives in dishwashers: by lowering the surface tension, the water drains off in a thin film instead of remaining as droplets that leave marks when they dry.
Instaquim products whose technical data sheet refers to wetting or drying:
- Padex (pH 7.0; 1–5 g/kg of laundry): blend of anionic and non-ionic surfactants with «great wetting and emulsifying power» for wetting heavily soiled fabrics; boosts the action of the detergent.
- Prein (pH 1 %: 11.8; 10–20 g/kg of dry laundry): prewash detergent with «great wetting, saponifying and sequestering power», in water up to 20 ºHF.
- Deterdry (pH 2.0; 0.1–1 g/l at 65–80 °C): final rinse additive for dishwashers with drying agents; non-ionic surfactants 5–15 %.