Antisurfactant (Autophobic) Behavior of Superspreader Surfactant Solutions

Surfactants are often added to water to increase the wetting of hydrophobic surfaces. We previously showed that most surfactant solutions behave identically to simple liquids with the same surface tension, indicating that the surfactants do not change the wettability of the solid surface itself. Here, we show that the superspreading surfactant Silwet results in a systematically higher contact angle on a hydrophobic surface than other surfactant solutions of comparable liquid-vapor surface tension. We also experimentally observe this "antisurfactant" behavior for CTAB on hydrophilic substrates. Supported by sum-frequency generation spectroscopy results, we suggest that this effect is due to charge-binding of the surfactant with the substrate.