Tuning Contact Angles of Aqueous Droplets on Hydrophilic and Hydrophobic Surfaces by Surfactants

Adsorption of small amphiphilic molecules occurs in various biological and technological processes, sometimes desired while other times unwanted (e.g., contamination). Surface-active molecules preferentially bind to interfaces and affect their wetting properties. We use molecular dynamics simulations to study the adsorption of short-chained alcohols (simple surfactants) to the water-vapor interface and solid surfaces of various polarities. With a theoretical analysis, we derive an equation for the adsorption coefficient, which scales exponentially with the molecular surface area and the surface wetting coefficient and is in good agreement with the simulation results. We apply the outcomes to aqueous sessile droplets containing surfactants, where the competition of surfactant adsorptions to both interfaces alters the contact angle in a nontrivial way. The influence of surfactants is the strongest on very hydrophilic and hydrophobic surfaces, whereas droplets on moderately hydrophilic surfaces are less affected.

Automated summary

This study investigates the adsorption behavior of short-chained alcohols to water-vapor interfaces and solid surfaces with different polarities using molecular dynamics simulations. The researchers derive an equation for the adsorption coefficient, which shows exponential dependence on both molecular surface area and surface wetting coefficient. The findings suggest that the influence of surfactants on sessile droplets is strongest on highly hydrophilic and hydrophobic surfaces, while moderately hydrophilic surfaces are less affected.