Getting a coating to spread evenly across a substrate — instead of beading up or leaving gaps — comes down to surface tension, and research on silicone-based wetting agents shows that lowering it isn’t a single-direction fix: what improves the final result can actually slow down how fast it happens.
What is it?
Wetting agents are additives used to reduce a coating’s surface tension so it spreads onto and adheres to a substrate, especially low-surface-energy or difficult substrates. The industry-standard way of measuring the surface tension involved is ASTM D7490, which uses contact angle measurements from two test liquids (one polar, one nonpolar) applied to a solid coating, substrate, or pigment sample, allowing the surface free energy and its polar and dispersive components to be calculated — the basis for diagnosing wetting, adhesion, and flow problems in a formulation.
Why should I use it?
- Silicone-based wetting agents can achieve substantially lower surface tension by adjusting the formulation’s pH, improving wetting on difficult substrates — see “How it’s used.”
- The pH that gives the lowest final surface tension isn’t the pH that gives the fastest wetting speed — these are two different things — see Limitations.
How it’s used
A 2025 peer-reviewed study synthesized four polyether- and tertiary amine-modified silicone surfactants and measured their dynamic surface tension across a pH range of 3.0 to 11.0. Increasing the pH substantially decreased both the equilibrium surface tension and the critical micelle concentration (CMC) — for example, one variant’s CMC dropped to 0.12 g/L at pH 11.0, compared to 0.42 g/L at pH 3.0 — meaning less surfactant was needed to reach its most effective concentration under alkaline conditions.
Limitations — when it isn’t the right choice
- Raising pH improves the final (equilibrium) surface tension, but it isn’t the way to get the fastest wetting speed — that trend actually runs in the opposite direction. The same 2025 study measured the rate at which surface tension drops over time (a parameter called R½, indicating how quickly a surfactant achieves rapid wetting) and found it was highest under acidic conditions (pH 3.0) for all four surfactant variants tested — the opposite direction from the equilibrium surface tension trend. This means a formulator optimizing purely for the lowest achievable surface tension by pushing pH higher could end up with a coating that wets more slowly than one formulated at a lower pH, if wetting speed matters for the application (such as fast-moving coating lines). Choosing the right pH depends on whether the priority is the lowest possible final surface tension or the fastest wetting response. This finding is specific to the silicone surfactant chemistries tested; other wetting-agent chemistries, such as fluorosurfactants or non-silicone hydrocarbon surfactants, would need separate verification.
