The Dynamic Adsorption of Charged Amphiphiles Characterized Using Maximum Bubble Pressure Tensiometry

Mass transfer of surfactant molecules from the bulk to the fluid interface determines time-dependent variation in surface tension known as dynamic surface tension. Many biological and industrial processes that involve the formation of drops or bubbles or free surface flows are influenced by dynamic surface tension. In particular foamability, printability and foam/emulsion stability are affected by the dynamic adsorption of surfactant molecules. In addition to bulk diffusion and adsorption-desorption fluxes that control the rate of mass transfer for non-ionic surfactants, the effects of the electrostatic potential created by adsorbed surfactants must be accounted for the mass transfer of ionic surfactants. In this study, the dynamic surface tension (DST) and dynamic adsorption of ionic surfactant cetyltrimethylammonium bromide (CTAB) and four bile salts are characterized and analyzed using maximum bubble pressure (MBP) tensiometry. The interfacial adsorption of CTAB and bile salts is a relatively fast process, and surface tension variation timescales (<50 ms) inaccessible in measurements with Du nouy ring, Wilhelmy plate, and pendant drop methods. A home-built MBP tensiometer is utilized for obtaining dynamic and quasi-equilibrium surface tension data. The time evolution of surface tension is analyzed using a full transient model for mass transfer of ionic surfactants that accounts for non-stationary interface, charge, and diffusion, and the quasi-equilibrium surface tension data are analyzed by using Frumkin and van der Waals isotherms. Surface concentration, surface potential, Gibbs elasticity, critical micelle concentration is obtained for CTAB and four bile salts. In addition, the CTAB data are analyzed to determine an apparatus constant that allows computation of the universal surface age, that corrects for the influence of non-stationary interface as bubble surface is younger than nominal surface age obtained from MBP method. The dynamic adsorption data for bile surfactants shows a more rapid adsorption to the interface, and the quasi-equilibrium datasets indicate that even though bile micelles are highly surface active, the effectiveness as surfactants for reducing surface tension values is quite limited.