Dynamic surface properties and dilational rheology of acidic and lactonic sophorolipids at the air-water interface

This study analyzes the equilibrium and dynamic surface tension curves of acidic and lactonic sophorolipids (SLs). It also investigates the dilational properties of the surface adsorptive film. Given their high hydrophobicity, lactonic SLs have lower surface tension and critical micelle concentration (CMC) than acidic SLs. As c_NaCl increases, the CMC values and the corresponding surface tension (γ_cmc) of acidic and lactonic SLs decrease gradually. For dynamic surface properties, lactonic SLs have a high diffusive rate from the bulk phase to the subsurface. At 0.05 CMC, the initial adsorption of acidic and lactonic SLs is diffusion-controlled. As c_surfactant increases, the values of diffusion coefficient (D) show a downward trend, and the mechanism is mixed kinetic diffusion. Adding NaCl increases the D values of acidic and lactonic SLs, and the influence degree for acidic SLs is more considerable than that for lactonic SLs. As frequency (ω) increases (0.005∼0.5 Hz), the dilational elasticity increases, and the phase angle decrease. The dilational elasticity of acidic and lactonic SLs shows a low-frequency dependence. Compared with acidic SLs, lactonic SLs have better dynamic surface properties, which decrease the gradient of interfacial tension because of the interface deformation. Consequently, the lactonic SLs exhibit a relatively small dilational elasticity. At 0.1 Hz, the dilational elasticity of acidic and lactonic SLs reaches the maximum values at 0.05CMC and 0.075CMC, respectively. When c_surfactant rises near CMC, the phase angle increases obviously, and the dilational elasticity further decreases. This result is attributed to the fast exchange of surfactant molecules between the interface and the micelles.