Kinetics and ionic regulation of surfactants desorption at the oil-water interfaces

The oil-water interface plays a crucial role in daily life and many industrial processes (such as oil extraction, emulsion preparation, material assembly, drug transportation, etc.). With the development of scientific research and engineering applications, the structures and properties of the oil-water interfaces are becoming the focus and the advanced subjects of multiphase flow, colloid and interface chemistry. microfluidics, material assembly and many other fields, and am more difficult to be predicted and described via the existing methods and theories. Surfactants. which are amphiphilic molecules, can spontaneously adsorb at the oil-water interfaces and have been proved to be effective materials for regulating the structures. mass transfer and mechanics of the oil-water interface. It is of great significance to investigate the surfactant kinetics at the oil-water interface for revealing the interfacial adsorption and regulation mechanisms of surfactants. The adsorption/desorption process is the rate-limiting step of surfactant kinetics at the oil-water interface. However, many studies focus on the relaxation process of surfactant adsorption, and the effects of ions in the aqueous phase on the surfactant adsorption/desorption kinetics are scarcely investigated. In this paper, by combing the coarse-grained molecular dynamics simulations and the umbrella sampling method, we study the desorption process of SDS surfactants from the decane-water interface and reveal the mechanisms of ions affecting the desorption kinetics and interfacial tension by analyzing the surfactant desorption kinetics via the desorption free energy and desoiption time. The results show that ionic species and concentration have great influence on the ionic number density and SDS-ion interaction near the interface. but show little effects on the number density and molecular conformation of surfactants at the interfaces. Based on the umbrella sampling simulation and Smoluchowski equation, we give an accurate and quantitative description of the surfactant desorption from the interfaces. and obtain the desoiption free energy and desorption time of surfactants. The desorption free energy and desorption time decrease with SDS adsorption number or interfacial concentration increasing, indicating that the enhanced interaction between SDS molecules promotes the desorption. Both the desorption free energy and the desorption time increase with ionic concentration increasing and we attribute this variation trend to the electrostatic shielding induced by ions. The ions shield the interaction between SDS molecules and thus inhibit the desorption of surfactants from the oil-water interfaces. Calcium ion with stronger electrostatic shielding has stronger inhibition effects. By analyzing the desorption kinetics of surfactants and oil-water interfacial tension, we obtain that the desorption ability of SDS is negatively correlated with its ability to reduce interfacial tension. The ions can weaken the ability of SDS to reduce interfacial tension by inhibiting the desorption kinetics of SDS from interfaces, and the weaken effect of calcium ion on SDS reducing interfacial tension is more significant. The results show that the adsorption/desorption kinetics of surfactants at interfaces as well as the interfacial tension can be effectively regulated by changing the ion species and concentrations properly. which provide a theoretical basis for the development of oil-water interfacial characteristics regulation technologies based on the coupling of ions and interfacial active components.