Effect of Mixed Surfactant on Evaporation Driven Salt Crystallization Morphology in Sessile Droplets

Extensive studies have been conducted to manipulate the morphology of sodium chloride salt crystals to tailor their physical and chemical properties. Among the myriad factors considered, the effects of the substrate and additives have profound impacts on the types of salt depositions. Surface charge effects and various ionic surfactants influence ion movement, resulting in diverse crystal morphologies. This manuscript aims to provide a consolidated summary by concurrently studying multiple effects to uncover the salt crystal morphology under the influence of two oppositely charged ionic surfactants on charged and neutral surfaces. The cationic surfactant cetyltrimethylammonium bromide induces skeletal crystal growth by retarding salt precipitation until supersaturation is reached. Conversely, the anionic surfactant sodium dodecyl sulfate hinders ion diffusion at the three-phase contact line. Each surfactant effect is dominant at higher molar concentrations. Surface charge affects the amount of surface adsorption and free-moving ions within the saline surfactant droplets, greatly influencing the number of salt crystals formed on the neutral substrate. However, charge neutralization at the highest concentrations of both surfactants nullifies the surface charge effect, resulting in practically indistinguishable salt crystals with similar sizes and numbers, leading to only a small area difference of 1461 μm². This study provides insights into the kinetics of crystallization under the combined influence of anionic, cationic, and surface charge interactions. The findings can serve as a future reference for predicting and controlling ionic interactions and crystal morphology.